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IN DEGREE PROJECT MATERIALS SCIENCE AND ENGINEERING,SECOND CYCLE, 30 CREDITS
, STOCKHOLM SWEDEN 2016
CORROSION-FATIGUE TESTINGON STEEL GRADES WITHDIFFERENT HEAT ANDSURFACE TREATMENTS USEDIN ROCK-DRILLING APPLICATIONS
LUIS MIGUEL BÉJAR INFANTE
KTH ROYAL INSTITUTE OF TECHNOLOGYSCHOOL OF INDUSTRIAL ENGINEERING AND MANAGEMENT
i
Abstract Corrosion fatigue is a common failure mechanism in rock drilling components and many other
mechanical parts subjected to cyclic loads in corrosive environments. A crucial part in the design of
such components resides in the selection of the right materials for the application, which ideally
involves testing and comparison of their performance under working conditions.
The present work was performed with the purpose of designing a corrosion-fatigue testing method
that would allow the designer to compare the performance of different materials exposed to corrosion
fatigue, permitting also the comparison with results from dry fatigue testing. The method was designed
for rotating-bending machines. Two different steel grades were used during the work, one through
hardened and one case hardened. The effect of these heat treatments and of shot peening over
corrosion-fatigue behaviour were studied using the proposed method.
It was proven that the testing speed has a strong impact on the fatigue life of steel. It was found that,
at a fixed stress level, the case hardened and shot peened steel reached 3X10^6 cycles at 2300 rpm,
while it failed at only 5X10^5 cycles with a testing speed of 500 rpm. A large beneficial influence of the
shot peening was demonstrated. It was also observed that, at fixed testing speed, the shot peening on
the through hardened steel can increase its fatigue strength from 190 MPa to 600 MPa under corrosion
fatigue. Many cracks were found at the surface of the shot peened parts, which are arrested near the
surface by the compressive stress layer from the shot peening. It was also found that, for the non-shot
peened parts, case hardening had a slightly higher corrosion-fatigue strength than the through
hardened. This might be a result of the compressive stresses from carburization, or due to the higher
core toughness of this steel grade.
Keywords: Corrosion fatigue, crack, rotating-bending, S-N curve, staircase method, fatigue strength,
shot peening, case hardening, through hardening
ii
Preface This work was performed to cover the thesis project in Mechanical Metallurgy as part of the master’s
degree in Engineering Materials Science at the Royal Institute of Technology in Stockholm, Sweden.
The project was conducted in cooperation with and financed by Atlas Copco Secoroc AB in Fagersta,
Sweden.
Very special thanks go to all the people who supported me throughout the whole project, without
whom it would not have been possible to complete this work. To Göran Stenberg for the right guidance
on every step of the project, and for providing me with scientific criteria to achieve a professional work.
To Richard Johanson for the invaluable support in several aspects, including all the laboratory training
I received, calibration of the instruments, production processes, metallographic sampling and analyses,
and for the interesting discussions that helped to shape the conclusions of this work. To Anders Olsson
who shared essential insights to the technical debates that guided the project to success. To Jonas
Falkestrom for the support I received within the company throughout the whole timeframe to perform
this project without any obstacle.
I also want to thank Alexander Beronius for the important technical contributions and interesting
discussions which had a vital impact on the outcome of the project, and Gabriella Brorson for the
guidance on essential technical information and valuable criteria.
Special thanks go to my supervisor in KTH, Stefan Jonsson, who followed my project in all technical
aspects and provided me with vital insights that helped me take decisive steps. And last but not least,
I want to thank my father, who introduced me to the fascinating world of mechanics and metals, for
his innumerable teachings in these subjects and in all others.
Luis Miguel Béjar
Fagersta, June 2016
iii
Abbreviations
CH Case hardened
TH Through hardened
SP Shot peened
NSP Non-shot peened
HCF High-cycle fatigue
LCF Low-cycle fatigue
iv
List of Tables Table 1. Different material scenarios used in the present work ........................................................... 28
Table 2. Metallographic samples selection ........................................................................................... 33
Table 3. Percent Replication .................................................................................................................. 34
Table 4. Average values for number of cycles to failure and time of test for frequency dependence of
corrosion fatigue on CH-SP in fresh water ............................................................................................ 35
Table 5. Average values for number of cycles to failure and time of test for frequency dependence of
corrosion fatigue on TH-NSP in fresh water .......................................................................................... 37
Table 6. Parameters for Proposed Corrosion-Fatigue Testing Method ................................................ 39
v
List of Figures Figure 1. Definition of cycles and reversals [3] ....................................................................................... 4
Figure 2. Load cycles for sinusoidal, square and triangular load paths. Middle figure: Same as above
but twice the frequency. Lower figure: A schematic spectrum load found in many applications. [2] ... 4
Figure 3. Definition of components in a stress cycle [2] ......................................................................... 5
Figure 4. S-N curve for AISI 4340 alloyed steel [7] .................................................................................. 6
Figure 5. Comparison of steel and aluminium fatigue behaviour [6] ..................................................... 6
Figure 6. Illustration of the steps of the fatigue crack evolution on radial and axial cross sections of a
cylindrical part [9] .................................................................................................................................... 7
Figure 7. Schematic illustration of the variation of fatigue-crack growth rate da/dN with alternating
stress intensity ΔK in steel, showing regimes of primary crack-growth mechanisms [8] ....................... 8
Figure 8. Development of extrusions and intrusions during fatigue [9] ................................................. 9
Figure 9. Schematic illustration of crack initiation, stable crack growth and fracture [2] ...................... 9
Figure 10. Fatigue crack propagation [6]............................................................................................... 10
Figure 11. Corrosion-fatigue and its general effect on the behaviour of steel [12] .............................. 11
Figure 12. Solubility of oxygen in water at different temperatures ...................................................... 12
Figure 13. Effect of NaCl concentration on the corrosion of Fe [18] .................................................... 13
Figure 14. Effect of NaCl concentration on crack initiation and crack propagation [19] ...................... 13
Figure 15. Effect of MnS inclusions in the film rupture/anodic dissolution process [20] ..................... 17
Figure 16. Hydrogen embrittlement ..................................................................................................... 19
Figure 17. The hydrogen embrittlement process [36] .......................................................................... 19
Figure 18. Shot peening compressive stress profile [21] ...................................................................... 21
Figure 19. Rotating-bending machine diagram ..................................................................................... 22
Figure 20. Example of staircase fatigue data [3]; suspensions are tests in which the specimens
survived ................................................................................................................................................. 23
Figure 21. S-N testing with a small sample size [3]; suspensions are tests in which the specimens
survived ................................................................................................................................................. 24
Figure 22. Rotating-bending fatigue testing machine - Atlas Copco Secoroc AB materials laboratory 25
Figure 23. Rotating-bending fatigue testing machine, rear side - Atlas Copco Secoroc AB materials
laboratory .............................................................................................................................................. 26
Figure 24. Specimen mounted on the machine and water collector underneath ................................ 26
Figure 25. Corrosion chamber ............................................................................................................... 27
Figure 26. Geometry and dimensions of specimen ............................................................................... 27
Figure 27. Laser scan micrometer ......................................................................................................... 28
Figure 28. Laser scan micrometer display ............................................................................................. 28
Figure 29. Specimens used. From left to right: (1) Case hardened, non-shot peened; (2) Case
hardened and shot peened; (3) Through hardened, non-shot peened; (4) Through hardening and
shot peened ........................................................................................................................................... 29
Figure 30. Residual stresses on CH-SP ................................................................................................... 30
Figure 31. Residual stresses on CH-NSP ................................................................................................ 30
Figure 32. Residual stresses on TH-SP ................................................................................................... 30
Figure 33. Residual stresses on TH-NSP ................................................................................................ 30
Figure 34. Corrosion-fatigue frequency dependence on CH-SP ............................................................ 34
Figure 35. Average time spent per test for different testing speeds on CH-SP in fresh water and brine
............................................................................................................................................................... 35
Figure 36. Corrosion-fatigue frequency dependence on TH-NSP ......................................................... 36
Figure 37. Average time spent per test for different testing speeds on TH-NSP in fresh water .......... 36
vi
Figure 38. Corrosion-fatigue frequency dependence comparison between CH-SP and TH-NSP .......... 37
Figure 39. Corrosion-fatigue frequency dependence of CH-SP at two load levels ............................... 38
Figure 40. Corrosion-fatigue frequency dependence of TH-NSP at two load levels ............................. 38
Figure 41. Testing speed selection ........................................................................................................ 39
Figure 42. S-N curve for CH-SP .............................................................................................................. 40
Figure 43. Corrosion-fatigue strength of CH-SP from staircase method ............................................... 40
Figure 44. S-N curve for CH-NSP ............................................................................................................ 41
Figure 45. Corrosion-fatigue strength of CH-NSP from staircase method ............................................ 41
Figure 46. S-N curve for TH-SP .............................................................................................................. 42
Figure 47. Corrosion-fatigue strength of TH-SP from staircase method ............................................... 42
Figure 48. S-N curve for TH-NSP ............................................................................................................ 43
Figure 49. Corrosion-fatigue strength of TH-NSP from staircase method ............................................ 43
Figure 50. Comparison of the four conditions (S-N curves and corrosion-fatigue strengths) .............. 44
Figure 51. S-N curve comparison between CH-SP and TH-SP ............................................................... 45
Figure 52. S-N curve comparison between CH-SP, CH-NSP, and dry tests ............................................ 46
Figure 53. S-N curve comparison between TH-SP, TH-NSP, and dry tests (on non-shot peened) ........ 46
Figure 54. Crack initiation and deformed propagation surface on TH-NSP @200 MPa, Specimen 00947
Figure 55. Crack initiation on TH-NSP @200 MPa, Specimen 009 ........................................................ 47
Figure 56. Surface major crack propagation on TH-NSP @744 MPa, Specimen 087 ............................ 47
Figure 57. Several cracks initiating on fracture surface at different planes near on TH-NSP @744 MPa,
Specimen 087 ........................................................................................................................................ 47
Figure 58. Crack initiation morphology on TH-NSP @744 MPa, Specimen 087 ................................... 47
Figure 59. CaS inclusion near crack initiation on TH-NSP @744 MPa, Specimen 087 .......................... 47
Figure 60. Possible crack initiation from (Ca, Si, Al)S inclusion on CH-NSP @200 MPa, Specimen 043 48
Figure 61. Cracking around initiation spot on CH-NSP @200 MPa, Specimen 043.............................. 48
Figure 62. Crack initiation on CH-NSP @725 MPa, Specimen 045 ........................................................ 48
Figure 63. Brittle crack propagation on CH-NSP @725 MPa, Specimen 045 ....................................... 48
Figure 64. Major rack initiation on TH-SP @600 MPa, Specimen 010 .................................................. 48
Figure 65. Crack propagation surface on TH-SP @600 MPa, Specimen 010 ......................................... 48
Figure 66. Several crack initiations on TH-SP @750 MPa, Specimen 034 ............................................. 49
Figure 67. Major crack initiation and sub-cracking on TH-SP @750 MPa, Specimen 034 .................... 49
Figure 68. Crack connecting residual fracture on TH-SP @750 MPa, Specimen 034 ............................ 49
Figure 69. Crack propagation surface TH-SP @750 MPa, Specimen 034 ............................................. 49
Figure 70. Crack propagation at 3mm depth from edge on CH-SP @550 MPa, Specimen 018 ............ 49
Figure 71. Intergranular crack propagation at 4mm from edge on CH-SP @550 MPa, Specimen 018 49
Figure 72. Brittle fracture surface on CH-SP @550 MPa, Specimen 018 .............................................. 50
Figure 73. Corrosion products at fracture surface on CH-SP @550 MPa, Specimen 018 ..................... 50
Figure 74. Main crack initiation and crack meeting facture surface on CH-SP @550 MPa, Specimen
018 ......................................................................................................................................................... 50
Figure 75. Main crack initiation and sub-cracking on CH-SP @550 MPa, Specimen 018 ..................... 50
Figure 76. Main crack initiation on CH-SP @725 MPa, Specimen 082 .................................................. 50
Figure 77. Intergranular crack propagation on CH-SP @725 MPa, Specimen 082 .............................. 50
Figure 78. Specimen 091. A) Overall surface view, B) General crack length, C) Cracks showing almost
no branching, D) Crack not following the microstructure ..................................................................... 51
Figure 79. Specimen 047. A) Overall surface view, B) General crack length, C) Cracks showing much
branching, D) Abundant branching of cracks ........................................................................................ 52
Figure 80. Specimen 045. A) Overall surface view, B) Tiny cracks at the decarburised layer, C)
Miniature cracks at the surface and one major crack growing into the material ................................. 52
vii
Figure 81. Specimen 095. A) Overall surface view, B) Almost no cracks at the surface, except for a few
major cracks that do not propagate to cause failure before 3X10^6 cycles ......................................... 53
Figure 82. Specimen 012. A) Overall surface view, B) General crack length, C) Cracks showing almost
no branching, D) Cracks magnification on etched sample .................................................................... 53
Figure 83. Specimen 013. A) Overall surface view, B) General crack length, C) Cracks showing
branching, D) Cracks magnification on etched sample ......................................................................... 54
Figure 84. Specimen 036. A) Overall, B) Big crack propagation ............................................................ 54
Figure 85. Specimen 016. A) Overall, B) Few large cracks can cause failure......................................... 55
Figure 86. Specimen 029. A) Overall cracking on the surface, B) Grown crack .................................... 55
Figure 87. Specimen 029. A) Overall surface cracking, B) Crack branching is eaten up by salty water, C)
General crack length, D) Few larger cracks that eventually propagate deep enough to cause failure 56
viii
Table of Contents Abstract .................................................................................................................................................... i
Preface ......................................................................................................................................................ii
Abbreviations .......................................................................................................................................... iii
1. Introduction ..................................................................................................................................... 1
1.1. Background .............................................................................................................................. 1
1.2. Purpose .................................................................................................................................... 1
1.3. Delimitations ........................................................................................................................... 1
1.4. Methodology ........................................................................................................................... 1
2. Theoretical Basis .............................................................................................................................. 3
2.1. Fatigue ..................................................................................................................................... 3
2.2. The S-N Curve .......................................................................................................................... 5
2.3. Fatigue Limit and Fatigue Strength ......................................................................................... 6
2.4. Crack Initiation and Crack Propagation ................................................................................... 7
2.4.1. Crack Initiation................................................................................................................. 8
2.4.2. Crack Growth ................................................................................................................... 9
2.5. Corrosion Fatigue .................................................................................................................. 10
2.6. Factors Influencing Corrosion Fatigue ................................................................................... 11
2.6.1. Environment .................................................................................................................. 11
2.6.1.1. Concentration of the Corrosion Species ................................................................ 12
2.6.1.2. Temperature .......................................................................................................... 12
2.6.1.3. Effect of NaCl ......................................................................................................... 12
2.6.1.4. Influence of pH ...................................................................................................... 14
2.6.2. Surface Finish................................................................................................................. 14
2.6.3. Stress Ratio .................................................................................................................... 14
2.6.4. Load Frequency ............................................................................................................. 14
2.6.5. Stress Intensity .............................................................................................................. 15
2.6.6. Size Effects and Hardness Gradients ............................................................................. 15
2.6.7. Metallurgical Variables .................................................................................................. 16
2.6.7.1. Inclusions ............................................................................................................... 16
2.6.8. Surface Residual Stresses .............................................................................................. 18
2.6.9. Crack Closure Effects ..................................................................................................... 18
2.7. Mechanisms of Corrosion Fatigue ......................................................................................... 18
2.7.1. Pitting Corrosion ............................................................................................................ 18
2.7.2. Corrosion at Preferential Locations ............................................................................... 19
2.7.3. Hydrogen Embrittlement............................................................................................... 19
ix
2.7.4. Rupture of Oxide Protective Film .................................................................................. 20
2.7.5. Surface Energy Reduction ............................................................................................. 20
2.8. Heat and Surface Treatments ................................................................................................ 20
2.8.1. Case Hardening .............................................................................................................. 20
2.8.2. Through Hardening ........................................................................................................ 20
2.8.3. Shot Peening .................................................................................................................. 21
2.9. Fatigue Testing ...................................................................................................................... 21
2.9.1. Rotating-Bending Test ................................................................................................... 22
2.9.2. The Staircase Method .................................................................................................... 22
2.9.3. The Median S-N Test Method for Small Sample Size .................................................... 24
3. Materials and Equipment .............................................................................................................. 25
3.1. Laboratory Equipment ........................................................................................................... 25
3.2. Specimens .............................................................................................................................. 27
3.2.1. Geometry and Dimensions ............................................................................................ 27
3.2.2. Material ......................................................................................................................... 28
3.2.3. Heat Treatment ............................................................................................................. 29
3.2.4. Shot Peening .................................................................................................................. 29
3.2.4.1. Residual Stress Analysis ......................................................................................... 29
4. Experimental Setup and Procedures ............................................................................................. 31
4.1. Corrosion-Fatigue Testing Method Proposal and Testing Parameters Selection ................. 31
4.2. Testing the Proposed Corrosion-Fatigue Method ................................................................. 32
4.3. Post-Test Analysis .................................................................................................................. 33
4.3.1. Microscope Analysis ...................................................................................................... 33
4.3.2. Metallographic Samples ................................................................................................ 33
5. Results ........................................................................................................................................... 34
5.1. Presentation of Corrosion-Fatigue Testing Method .............................................................. 34
5.1.1. Frequency Dependence Results .................................................................................... 34
5.1.2. Final Parameters for Proposed Testing Method ........................................................... 39
5.2. Testing the Proposed Corrosion-Fatigue Testing Method .................................................... 40
5.2.1. S-N curves and fatigue strength results ........................................................................ 40
5.2.2. Materials Comparison ................................................................................................... 44
5.3. Metallographic Results and Observations ............................................................................. 47
5.3.1. Fracture Surface Analysis from SEM ............................................................................. 47
5.3.2. Cracking Analysis from Optical Microscope .................................................................. 51
6. Discussion ...................................................................................................................................... 57
6.1. Discussion of Method and Parameters ................................................................................. 57
x
6.2. Discussion of Testing Results ................................................................................................. 59
6.3. Fracture Surface and Cracking Characteristics in Relation with the Results ......................... 60
7. Conclusions .................................................................................................................................... 64
8. Further Work ................................................................................................................................. 65
9. References ..................................................................................................................................... 66
1
1. Introduction In this section, the background, purpose of the project, delimitations of the work, and the methodology
followed by the author are presented.
1.1. Background Atlas Copco’s rock drilling components work in a tough environment. The drill string delivers an impact
pulse in the 250 MPa range, at a frequency that can reach above 40 Hz, while flushing aqueous media
with the purpose of transporting chippings together back to the hole entry. Often the flushing media
is water and can differ in pH, salt content and other contaminants. This environment can reduce the
service life and the reliability of this machinery.
Corrosion fatigue is found to be a highly recurrent cause of failure in drill string components. To
improve the reliability and service life of failing parts, it is vital to analyse the failure mechanisms. For
that reason, corrosion fatigue was studied by recreating in a laboratory the conditions that could be
encountered in underground applications, to be capable to understand the mechanisms involved in
the damage, and furthermore, to be able to predict failure.
1.2. Purpose The scope of this work was to design a testing method to compare the performance under corrosion
fatigue of different steel grades with different thermal and mechanical treatments used in drill string
components, which implicates cyclic loading and usually the exposure to corrosive environments. The
work comprehended proposing the method and characterizing the corrosion-fatigue tests by choosing
suitable parameters that enable the design engineer to discriminate between diverse steels
performing under cyclic loading and corrosive environments. Once the test was parametrized, the
comparison of four different material conditions was performed in order to test the veracity of the
method.
1.3. Delimitations The tests were limited to rotating-bending machines. Therefore, limitations regarding the detailed
study of crack growth rates must be considered.
The present work is delimitated tests in fresh water and salt water. The effect of different aqueous
media is not studied hereby.
The water temperature and oxygen concentration were not varied during the tests. The effects of
changes in these variables was not further studied.
Intensity of shot peening performed on the specimens was not varied within the same steel grade.
1.4. Methodology The work started with a thorough literature review to establish the conceptual basis of the project. A
literature survey was useful to study similar research that has been performed within the field of
corrosion-fatigue testing. Both literature surveys were performed using KTH’s digital library DiVA,
ScienceDirect database and Google Scholar. Atlas Copco’s current fatigue-testing methodology was
studied and the historical data was reviewed to set up the initial bases for further testing. The
corrosion-fatigue tests were performed using rotating-bending machines, and the results were
captured in an Excel spreadsheet. The statistical analyses of the gathered data were performed using
Excel’s statistical tools, and the calculations were performed within the Excel spreadsheet. The
preparation and evaluation of the specimens prior to testing were performed with special equipment
2
supplied by Atlas Copco, and it was all performed within the company’s facilities. Once the tests were
completed, metallographic samples were prepared from certain specimens and analysed in the optical
microscope. Some specimens’ fracture surfaces were analysed with a Scanning Electron Microscope.
The analyses of the specimens after testing, involving fracture surfaces, corrosion, degradation, and
metallography, were performed also with equipment supplied by Atlas Copco.
3
2. Theoretical Basis The purpose of the following section is to present, in general terms, the fundamental concepts involved
in the present work.
2.1. Fatigue Mechanical components are exposed to a great variety of stress modes and diverse loads during their
function. It is the role of the design engineer to decide the geometry, dimensions and material
characteristics of every component to ensure its desired performance and reliability. As it can be
supposed, the task of the design engineer when it comes to considering every kind of load the
component has to withstand is not a simple one; tension, compression, torsion, bending, or any of the
possible combinations of these could be stressing the material in complex ways.
Every material has a stress limit at which it will no longer withstand a static load and it will fail (either
fracture or deform permanently, depending on what is considered failure by the engineer), and it can
be inferred that under such limit, the static load will not be enough to cause the material to fail. In the
past this limit was considered alone for the design of machine components, and engineers started to
observe that, surprisingly, this limit was not respected by many of the failing parts. By the middle of
the XIX century, this behaviour started to concern engineers, who began to perform tests under cyclic
loads below the static load limits and found that the components would eventually fail at such low
load values. It was observed that failures happened after the component had been functioning
satisfactorily for a period of time, and the general opinion stated that the continuous cyclic load had
exhausted the ability of the material to carry load, or in other words, the fatigue of the material
occurred [1]. Fatigue is by far the most common cause of failure on mechanical components (about
80% of all the fractures of metals [2]), and it is the most feared one, since it “occurs suddenly without
any noticeable plastic deformation, i.e. without warning” [2].
Fatigue starts when a load is high enough to cause plastic deformation on a component, even when it
happens only at a small volume of the material, i.e. localized plastic deformation could occur at the
highest stress location. When such load is repeated cyclically, the damage accumulates and eventually
a crack is formed. If the cyclic load continues, the length of the crack increases until the stress on the
component is high enough to cause its sudden fracture [3]. It could be then assumed that the time to
fracture by fatigue is a purely cycle-dependent process, but this premise is not always true; when a
corrosive environment is present, both fatigue and corrosion have a synergetic effect on the material,
which is known as “corrosion fatigue” [2]. Since corrosion is mainly a time-dependent process, crack
initiation and crack growth become strongly time-dependent during corrosion fatigue, as it will be
explained more deeply in section 2.5.
The phenomenon of fatigue is generally divided into three steps: (1) crack initiation, (2) crack
growth/propagation and (3) final fracture. The first two steps of the process are not easy to
characterize and a great amount of work on research has been performed to try to understand the
mechanisms involved on each one of them; it is even hard to identify a sharp limit to divide when the
crack initiation ends and the crack propagation starts [14]. The general principles of such steps will be
described in section 2.4.
A load cycle can be applied in many different ways in terms of the load wave shape, magnitudes of the
loads, sign of the stress caused by the load and frequency of load application. In general, a load cycle
can be characterised by a maximum stress, σmax, and a minimum stress σmin [2]. It should be emphasised
that the stresses could be either positive or negative (tensile or compressive, respectively) depending
on the particular cycle. The step of going from the minimum to the maximum stress -or vice versa- is
4
one
reversal
one
reversal
one
cycle
called a reversal. A cycle is achieved when the stress is taken from the minimum to the maximum, and
then back to the minimum –or vice versa- completing two reversals [3]. Figure 1 shows how a cycle
and a reversal are defined graphically.
Figure 1. Definition of cycles and reversals [3]
Load cycles can be applied in a variety of wave shapes, from the sinusoidal shape shown above to
triangular, squared, or even an irregular shape, also called spectrum shape. The frequency of
application can also vary. Figure 2 shows the different loading wave shapes and frequencies. All those
factors are controlled during a fatigue test.
Figure 2. Load cycles for sinusoidal, square and triangular load paths. Middle figure: Same as above but twice the frequency. Lower figure: A schematic spectrum load found in many applications. [2]
Based on the stresses σmax and σmin explained above, a static component, σ0 (also known as mean
stress), and a dynamic component, σa (also known as stress amplitude), can be obtained for any
particular loading cycle. The mean stress is defined as the algebraic average of the maximum and
minimum stresses in one cycle. The stress amplitude is defined as the difference between the
maximum –or minimum- stress and the average stress. Another component that can be naturally
defined from the cycle is the stress range, σr, defined as the difference between the maximum and the
minimum stresses, i.e. two times the stress amplitude [5]. These components can be graphically seen
in Figure 3.
-300
-200
-100
0
100
200
300
0 1 2 3 4 5 6 7Stre
ss
5
Figure 3. Definition of components in a stress cycle [2]
From the image shown above, it is easy to imagine that the stress wave can be positioned anywhere
along the stress axis (without crossing the σUTS limit) depending on the way the load is applied. A fully
reversed cycle, for instance, will have the same magnitude for both the maximum and minimum
stresses but a different sign, i.e. the maximum stress will be tensile and the minimum will be
compressive; this will result in a mean stress with a magnitude of zero. This kind of stress cycle is
characteristic of rotating components that are also subjected to bending, like shafts, and it can be
simulated with a rotating-bending test [6]. This method will be further explained in section 2.9.1. If for
instance both σmax and σmin are positive, the static component of the cycle will also be positive. This
concept leads to an important parameter to characterise a fatigue test: the stress ratio. The stress ratio
is defined as the algebraic ratio of σmin to σmax (R = σmin/σmax). For a fully reversed cycle, R would be
equal to -1. If the cycle is partially reversed, R becomes negative with a value between -1 and 0. If the
load goes from 0 to σmax, R will be equal to 0, and if both σmin to σmax are positive, R will be also positive,
but less than 1 [6].
Depending on the load levels of the cycle, the stress in the component can be purely elastic or it could
jump into the plastic region. When the applied stress is elastic, the fatigue phenomenon is considered
to be high-cycle fatigue (HCF). When this happens, the component could reach a really long life (N >
105 cycles) [6]. Although the stress on the component is too low for initiating a general plastic
deformation, the crack tip undergoes plastic deformation, and that is the reason why it grows and
eventually causes failure. On the other hand, when the stress on the component is high enough to
cause plastic deformation, the phenomenon is known as low-cycle fatigue (LCF). Since plastic strain is
present in each cycle, the behaviour is more complex than in HCF, as the material undergoes plastic
deformation and forms a hysteresis loop on the stress-strain diagram. This phenomenon is well
explained elsewhere ([6], [7]).
2.2. The S-N Curve As explained in the previous section, materials will fail as an effect of fatigue when a cyclic load is
applied on them. The higher the load, the less number of loading cycles it will endure and vice versa.
This Stress-Life relation can be represented by the Wöhler method with the so-called S-N curve, which
is usually shown as a plot of the nominal stress amplitude versus the number of cycles to failure on a
logarithmic axis [5]. Figure 4 shows an illustration of the S-N curves for notched and unnotched AISI
4340 alloyed steel specimens at room temperature tested with a stress ratio of R = -1 [7].
6
Figure 4. S-N curve for AISI 4340 alloyed steel [7]
2.3. Fatigue Limit and Fatigue Strength The typical fatigue behaviour of most metals at dry conditions shows a stress level low enough at which
the metal will no longer fail under fatigue, and it is said that the material shows infinite life. For
instance, in Figure 4 a runout (no failure) is observed for each of the curves at certain stress level;
taking the unnotched curve as example, the runout is present at 345 MPa. This maximum stress that
the material can endure without failure is called fatigue (endurance) limit. Below this stress level there
is a 50% probability that no failure will occur on a material at the same conditions [5].
Most nonferrous metals and ferrous metals under corrosive environments do not exhibit an endurance
limit. In such cases, the S-N curve does not even out at certain stress value, but instead it continues
falling at a slow rate at high number of cycles [5]. When this happens, fatigue strength, rather than
fatigue limit, is reported. Fatigue strength is defined as the stress level that a material can withstand
to reach a defined number of cycles, generally enough be considered as ‘infinite life’. In these case, the
plot must specify the number of cycles at which the fatigue strength is reported [5]. A typical
endurance limit behaviour is shown in Figure 5 for AISI 1045 steel; in the same plot, the constant drop
in stress to reach longer life can be seen for aluminium 2024-T6.
Figure 5. Comparison of steel and aluminium fatigue behaviour [6]
7
2.4. Crack Initiation and Crack Propagation It is understood that, in order for the fatigue process to start, there is a requirement for certain plastic
strain on the material, as explained in section 2.1. There are three simultaneous conditions that are
required for fatigue damage to occur: cyclic stress, tensile stress and plastic strain [14]. The fatigue
phenomenon can be either studied as a stress-controlled or as a strain-controlled process. The process
of fatigue is generally divided into the following sequential steps [14]:
(1) Cyclic plastic deformation prior to fatigue crack initiation
(2) Initiation of one or more microcracks
(3) Propagation or coalescence of microcracks to form one or more macrocracks
(4) Propagation of one or more macrocracks
(5) Final failure
The fatigue process starts with the formation of a crack, which can be either nucleated during the cyclic
loading or present in the material from the beginning. As the loading cycles continue, the crack will
grow under a stable regime below the fracture toughness limit of the material. When the stress
intensity factor of the crack exceeds the fracture toughness of the material, instant fracture occurs [2].
Figure 6 shows a represnetation of the three steps conforming the fatigue process.
Figure 6. Illustration of the steps of the fatigue crack evolution on radial and axial cross sections of a cylindrical part [9]
Although the steps of the fatigue process are generally accepted, there is no general agreement on
when the crack initiation process ends and when the crack propagation starts, or at which crack length
the crack initiation turns into propagation [14]. In the past it was believed that as much as 95% of the
total life of a fatigue-failure was spent in crack initiation [4], however, better methods of crack
detection have appeared and nowadays it is possible to observe that cracks nucleate after the first
10% of the total life of the component [7]. In dry conditions, this is mainly dependent on the level of
applied stress. In general terms it is considered that “approximately 30 to 40% of the low-cycle fatigue
life and approximately 80 to 90% of the high-cycle fatigue life, measured by cycles to failure, involve
nucleation of the dominant fatigue crack that eventually causes failure” [5]. Since a large fraction of
the life is spent on crack growth on low cycle fatigue, it has become more interesting to study this
phenomenon in terms of the mechanical behaviour of the crack growth, i.e. by studying the crack
growth and characterizing it by linear elastic fracture mechanics, elastic-plastic fracture mechanics, or
fully plastic fracture mechanics. In such cases, the entire crack initiation process is taken as nucleation
phase, and the crack propagation is then expressed in terms of crack growth rates [14].
8
The linear elastic fracture mechanics approach assumes that every mechanical component contains
flaws and that cracks grow from an initial size, ao, until reaching a critical size, ac. The rate at which the
crack grows is expressed as the change in crack size per each cycle, da/dN. Figure 7 shows the three
regimes of crack evolution and the factors that influence each regime. During regime A, the crack
growth is very small, but still increases to get to a size large enough to enter regime B, where da/dN
can be modelled using the Paris law, which is represented as a straight line along the log ΔK axis. The
equation for the Paris law can be expressed as
𝑑𝑎
𝑑𝑁= 𝐶(∆𝐾)𝑚
(1)
where the constants C and m are material coefficients that can be obtained experimentally by plotting
log(da/dN) against log(ΔK) [2]. In regime C the crack growth rate grows faster without control, until it
reaches final failure [6].
Figure 7. Schematic illustration of the variation of fatigue-crack growth rate da/dN with alternating stress intensity ΔK in steel, showing regimes of primary crack-growth mechanisms [8]
For the present work, linear elastic fracture mechanics was not considered since the aim of this
investigation does not deal with crack growth rates or crack nucleation times. In fact, the interesting
event for this work was the final failure of the parts and the variable considered was the life reached
by the material at the final failure point. Nevertheless, the influence of different factors on each regime
shown in Figure 7 is interesting for understanding the behaviour of the materials investigated in the
present work.
2.4.1. Crack Initiation Usually cracks start on the surface of the material, where the highest stress is normally found [2].
Moreover, the surface usually contains defects such as machining marks, inclusions, “soft spots” from
carburisation, corrosion pits, etc. Below the surface, carbide clusters, inclusions or slag particles can
9
also have the same effect [2]. All those defects serve as crack initiation spots. However, if the material
is pure enough to be exempt of such defects and its surface is well polished, the crack nucleation
process can start from persistent slip bands. A slip band is a result of the systematic build-up of slip
movements on the order of 1 nm [6]. When the material is repeatedly stressed, the surface grains are
sheared in near 45° angle with the applied stress and slip bands are formed. Then persistent slip bands
will form extrusions and intrusions on the surface of the material, as shown in Figure 8. The intrusions
will act as nucleation sites for cracks to form and grow through the first grain following the shear band
at about 45° angle to the applied stress [2]. The crack starts as a featureless fracture surface parallel
to the slip bands, in what is referred to as Stage I. When the crack is large enough, the stress at the
crack tip becomes dominant and the overall crack plane is switched and becomes normal to the
principal stress [6].
Figure 8. Development of extrusions and intrusions during fatigue [9]
2.4.2. Crack Growth Once the stress at the crack tip becomes dominant, the crack changes plane and continues to grow
perpendicular to the principal stress. This could happen when the crack, growing at 45° relative to the
principal stress during the initial stage, reaches the next grain boundary on its way and it deflects. This
step could be considered as the start of the stable crack growth stage, referred to as Stage II. Figure 9
shows this phenomenon.
Figure 9. Schematic illustration of crack initiation, stable crack growth and fracture [2]
10
During Stage II, the crack will propagate as a result of its cyclic opening and closing on each stress cycle.
The tensile stress will open the crack, making it grow a tiny bit. The next reversal will relax the stress,
closing the crack. As this process is repeated, the crack sharpens and blunts leaving a very small
marking on the crack surface on every cycle. These marks, known as striations, leave a characteristic
pattern on the fracture surface of the material, with each striation representing one cycle of fatigue.
The steps of stable crack growth are illustrated in Figure 10. During step 1, the crack is “resting” while
no tensile stress is applied on it. On stage 2, tensile stress is applied and the crack opens and the crack
tip deforms and propagates a bit on step 3. On step 4 the crack starts to close again as the stress is
reduced, leaving a permanent deformation at the tip of the crack, i.e. the crack grew longer. On stage
5 the crack is completely closed again as the stress goes back to zero or to negative values, which are
harmless to crack propagation. A fatigue cycle covers from step 1 to step 5, so step 6 is the same as
step 2 but with a larger crack.
Figure 10. Fatigue crack propagation [6]
In order for a crack to grow, ΔK must exceed a threshold value. When this happens, crack growth can
be observed and the crack growth rate can be described by Paris law [2] (equation 1). However, since
compression is harmless, the minimum stress used to calculate ΔK is set as zero.
2.5. Corrosion Fatigue Having described the phenomenon of fatigue, it is easy to introduce the concept of corrosion fatigue.
Corrosion fatigue is, as its name suggests, the combined action of cyclic stress and a corrosive
environment on materials. “Corrosion fatigue is dependent on the interactions among loading,
environmental, and metallurgical factors” [4]. In general terms, the effect of corrosion will be
detrimental for the fatigue endurance of materials, as corrosion could cause a faster crack initiation, a
higher crack propagation rate, or both. The effect of the aggressive media on the fatigue performance
can vary widely, and is mainly dependent on the relation between the metal and the environment [5].
11
One of the main differences between corrosion fatigue and inert-environment fatigue is that there is
no “safe stress range” at which metal has infinite life. As the number of cycles to failure is increased,
the fatigue strength of the metal in corrosive environment continues to fall [10]. See Figure 11.
Figure 11. Corrosion-fatigue and its general effect on the behaviour of steel [12]
Since corrosion attacks the surface of the part, corrosion fatigue cracks will always originate at the
surface, unless the material presents defects underneath the surface, which would act as stress
concentration sites and initiate sub-surface cracks [4]. Corrosion-fatigue cracks are usually trans-
crystalline, although inter-crystalline cracks can nucleate if the media attacks the grain boundaries
preferentially [1]. Fatigue cracks will initiate and propagate in the material depending on the
metallurgical properties of each alloy; for carbon steels, for example, cracks often initiate from
corrosion pits and usually contain a lot of corrosion products, normally propagate trans-granularly and
present branching. However this is not a requisite; cracking in carbon steels can initiate without a
corrosion pit and follow grain boundaries [4].
Most of the times corrosion fatigue produces a characteristic failure on which corrosion products can
be observed on the fracture surface or at growing cracks. Nevertheless, such corrosion products could
be absent in some specific cases, for example, when high-strength steels are attacked by a hydrogen-
rich atmosphere. Also, when the frequency is high enough, the fracture surface produced by corrosion
fatigue might not differ significantly from fatigue fracture surfaces at non-aggressive environments [4].
2.6. Factors Influencing Corrosion Fatigue As it has been exposed in previous sections, crack initiation and crack propagation are two separate
processes driven by different phenomena. It is then important to notice that the corrosive media will
have a different effect on each of them. The following sections explain the factors that are known to
affect one or both of these steps.
2.6.1. Environment Corrosion fatigue will be enhanced by an increased chemical activity of the environment. Some of the
factors that have a strong influence on corrosion fatigue are temperature, pH, pressure of the gaseous
environment, and concentration of the corrosion species. There is no rule of thumb to generalize the
behaviour of materials when exposed to diverse environmental conditions since, as mentioned before,
the metallurgical characteristics of each material define its behaviour combined with the environment.
12
However, there are some commonly observed reactions to certain individual conditions. For example,
low pH, high pressure of a gaseous environment, high concentration of the corrosive species and high
temperature generally have a detrimental effect on the corrosion-fatigue resistance of the material.
In high-strength steels, crack growth rates are increased when the water vapour pressure increases
until saturation is reached [4].
2.6.1.1. Concentration of the Corrosion Species
The presence of oxygen is known to be very detrimental for the corrosion fatigue performance of many
metals, since it enhances many aggressive chemical processes. Therefore, if the atmosphere has free
access to the metal surface –allowing oxygen to have contact with the metal- the effect of corrosion
will be favoured. For this reason, corrosion fatigue strength will be lower if the specimen is sprayed or
dripped with the corrosive fluid than if it is totally submerged in it [1]. A similar effect is observed when
the corrosive fluid is aerated (contains dissolved oxygen) compared to when it is deaerated, being the
aerated solution the most aggressive one.
2.6.1.2. Temperature
The effect of temperature is complex and can highly vary depending on the material and the
temperature range. For instance, since water temperature decreases its oxygen solubility (see Figure
12), some materials could benefit from a lower oxygen level in the environment and therefore have a
better corrosion-fatigue endurance at high water temperatures. On the other hand, by decreasing the
oxygen content, the hydrogen evolution mechanism (typical in de-aerated solutions) might take place
and cause hydrogen embrittlement [10], for which the process is explained in section 2.7.3.
Figure 12. Solubility of oxygen in water at different temperatures
2.6.1.3. Effect of NaCl
It is well known that the presence of NaCl in the corrosive media causes the corrosion fatigue strength
to drop drastically. The corrosion fatigue rate has a maximum level when the concentration of NaCl is
around 3 – 4 wt% [15], as observed in Figure 13. This maximum is believed to be caused by the
combination of effects that the amount of NaCl have in electrical conductivity and on the oxygen
solubility, which are opposed. As the amount of salt is increased, the electrical conductivity increases
and the oxygen solubility decreases. The shape of the curve in Figure 13 is a result of the two effects
acting together in different intensities depending on the concentration of salt.
13
Figure 13. Effect of NaCl concentration on the corrosion of Fe [18]
Rollins et al [19] studied the influence of NaCl concentration in crack initiation and crack propagation
separately for steel. The findings are shown in Figure 14. It can be observed that the number of fatigue
cycles to initiate a crack decrease as the NaCl concentration increases from 0 to 4 wt%, has a minimum
at 4 wt% and then increases for higher NaCl concentrations. On the other hand, crack propagation
seems to be independent of the severity of the corrosive solution (which is maximum at 4 wt% NaCl),
otherwise the curve would be expected to have a shape similar to the initiation. The explanations
proposed by the authors was that the high conductivity at high NaCl concentrations allows cracks to
grow deeper by electrochemical cell action or that the adsorption of chloride ions on the crack tip
enhanced the crack propagation.
Figure 14. Effect of NaCl concentration on crack initiation and crack propagation [19]
14
2.6.1.4. Influence of pH
Previous studies [16] have shown that the effect of low pH (1.2 and 5.5) on steel (1.0 wt%C, 1.0 wt%Cr,
0.25 wt%Mo, 0.3 wt%Si, 0.5 wt%Mn) under rotating bending testing have a little influence on fatigue
crack initiation, whereas most of the reduction of endurance happened during crack propagation [15].
In other studies performed on the same steel in a 0.4% NaCl solution [16] it was proposed that in a pH
range of 4 – 10, the corrosion rate was determined by oxygen diffusion to the cathodes. At pH values
below 4, the corrosion rate increases due to hydrogen evolution at the cathodes and an increase in
conductivity. At pH values above 12, corrosion did not occur, and it was suggested that the metal was
protected by a film of metal hydroxide or adsorbed oxygen, preventing it from developing corrosion-
fatigue cracks [15].
2.6.2. Surface Finish Surface finish have an important effect on fatigue on both inert and corrosive environments. The
nucleation if microcracks becomes easier when the material surface presents a rough finish. The effect
of, for example, machining marks on the material surface can act as notches where cracks will start
forming. Such marks also make the material more vulnerable to corrosion, whereas a polished surface
is more resistant to this environmental attack.
Another possible negative effect of manufacturing on the specimen is that, besides the machining
marks, the surface could have suffered from hardening by cold-work or softening by decarburization.
Also, residual stresses could be introduced into the surface layers as a result of machining and
preparation [1]. A positive (tensile) residual stress is highly detrimental for fatigue, while a negative
(compressive) residual stress is beneficial, as it will be explained in section 2.8.
2.6.3. Stress Ratio Stress ratio, as described in previous sections, is the ratio between the minimum and the maximum
applied stress in the cycle. It indicates the way the part is being loaded in terms of magnitudes of the
uniaxial-stress-wave limits. Positive stress ratios indicate that both the maximum and minimum
stresses are positive, i.e. tensile, and the higher the stress ratio (closer to 1), the closer the magnitudes
are to each other. For dry-fatigue testing, a fully-reversed load (R = -1) is usually the most severe [7].
However, the opposite behaviour can be expected for corrosion fatigue; when the stress is positive,
the crack will open, leaving the fresh material in its interior exposed to the aggressive atmosphere. The
longer the time of exposure, the higher the corrosive damage suffered at the crack tip. Therefore, it
can be deduced that –in general- the rates of crack propagation are increased by high stress ratios [4].
If the stress ratio is increased while the stress intensity (ΔK) is held constant, the crack tip strain and
strain rate are increased. As a result, the passive film rupture is enhanced and the crack propagation
is therefore increased [5].
2.6.4. Load Frequency Frequency of the stress cycle has little effect on fatigue behaviour in nonaggressive environments [5].
However, it is the most important factor influencing corrosion fatigue for most material, environment
and stress intensity conditions [5]. This happens mainly because corrosion is a time-dependant
phenomenon. Then, if corrosion will have effect on the material after a certain fixed time, the
frequency will determine the amount of cycles the material will reach before this corrosion time lapse
is completed. Therefore, the higher the frequency, the longer the life reached by the component. In
fact, “frequencies exist above which corrosion fatigue is eliminated” [4]. On the other hand, at
frequencies below 10Hz the detrimental effect on the corrosion-fatigue strength is enhanced [5].
Another effect of frequency is the time the crack is open during each cycle. This is understood under
the basis of fresh material exposure by the open crack: when the crack opens due to the positive stress
15
applied, the crack tip exposes fresh material to the corrosive environment, increasing the corrosive
attack on each cycle. As a result, the crack growth rate is increased and the total life of the component
is reduced.
Load frequency also affects the temperature of the part, depending on its material and on the stress
level [1]. Generally, the higher stress levels and higher speeds, the heat generated by the deformation
increases and the rate of heat dissipation by the specimen decreases. Specimen overheating is less
common on rotating-bending tests than on other static tests since the specimen rotation helps the
cooling by forced convection. Moreover, in corrosion fatigue tests, liquid corrosive media can aid the
dissipation of the heat by the same principle.
2.6.5. Stress Intensity Although the correlation between the stress intensity and corrosion fatigue cracking varies noticeably,
the general tendency is that crack growth rates in corrosion fatigue increase when increasing the stress
intensity. However, the behaviour of the crack growth rate is different for each of the three different
regimes (near threshold, Paris law region, and final fracture), so it is incorrect to assume that the curve
in Figure 7 simply shifts to higher levels. This marked dependence becomes more pronounced for
materials that are extremely environment-sensitive, such as ultrahigh-strength steel in distilled water
[5]. Also in many cases, the combination of stress intensity with other factors such as stress-wave form,
cycle frequency and metallurgical characteristics have diverse and even more drastic effects on crack
growth rates.
In general terms, it has been commonly accepted that the fatigue behaviour of metals in aggressive
environments at high stress intensities is similar to that in air or inert atmospheres. The reason is that
the mechanics-controlled growth occurring in dry fatigue increases when the stress intensity factor is
augmented [15].
2.6.6. Size Effects and Hardness Gradients There are several ways to improve the hardness and toughness of a material, both generally having a
beneficial effect on its fatigue resistance. In the present work, the materials studied were subjected to
mechanical hardening by shot peening and to heat treatment by case hardening or through hardening.
Case hardening creates a layer of harder material on the part compared to its core, whereas through
hardening increases the overall hardness of the component. Further explanation on these methods is
found in section 2.8 of this document. The harder the material, the more fatigue-resistant it will be, so
it can be inferred that the through hardened material will generally perform better under fatigue than
a case hardened one.
In non-corrosive environments, it is generally observed that when the specimen diameter increases,
the fatigue limit of the material will decrease. This happens for diverse reasons related to the stress
distributions throughout the geometry, and it is greatly affected by the kind of test performed, either
reversed direct stress or rotating-bending [1]. For rotating-bending specifically, the maximum stress is
present at the surface of the specimen. If the surface of the specimen is hardened by case hardening
or cold working, surface cracks will not propagate as easy as they would without the treatment. The
thickness of this hardened layer depends on the severity of the applied treatment and on the
treatment itself, but it will not depend on the specimen size. Thus it can be implied that if the thickness
of the hard layer is held constant (produced by the same method), the ratio of the layer thickness to
the specimen diameter will decrease when the diameter increases. So a smaller specimen will have a
“thicker” layer in proportion to its softer core, increasing its fatigue limit. In general, as the layer/diam.
ratio increases, the closer the fatigue life approaches the one of the through hardened material [1].
16
Besides the general behavior of the induced hardness mentioned above, it is also important to consider
the residual stresses present in the material. When the specimen is polished, for example, compressive
stresses will be left on the surface, improving the fatigue and corrosion fatigue endurance of the
material. The same effect is achieved by shot peening (see section 2.8.3). A thorough explanation
about the relation of these variables is presented elsewhere ([1], p. 54-64).
In corrosive environments, on the other hand, the fatigue strength at a given life will increase when
the diameter increases [1]. The explanation for this is that fatigue crack growth rates are relatively not
affected by the corrosive media -depending on the stress level and crack length- opposite to in-air.
However, crack initiation time is dramatically reduced in corrosion fatigue. Therefore, a small part of
the total life is spent in nucleating a crack, and a bigger portion is spent on growth until failure. Since
the crack growth rate is unaffected, the critical crack will take a longer time to propagate through
larger specimens, increasing the total life at the same stress level, or the fatigue strength at the same
total-life value [1]. However, by increasing the specimen diameter, the surface area also increases,
leaving more material exposed to the aggressive environment, and as a result, making the specimen
more vulnerable to corrosion-crack initiations [5].
2.6.7. Metallurgical Variables Composition of the material also has an effect on fatigue, for example the addition of carbon to steel,
since it increases the hardness of the material, and as a result, the fatigue limit is increased. Grain size
of the steel has an indirect influence on its behaviour under fatigue since it has a close relation with
the material’s strength and fracture toughness. Thus, finer grain sizes have a better fatigue strength
than coarse-grained steels [7].
For steels with similar strength levels, microstructure can establish a marked difference in fatigue
behaviour. For instance, pearlitic structures have a poor fatigue resistance, whereas a tempered
martensitic structure provides the highest fatigue limit [7]. However, a research performed by Novak
has shown that, under salty conditions, crack initiation is insensitive to microstructure of a material
[10]. However, he concluded that tempered martensite and ferrite-pearlite structures have very poor
response to salt containing solutions, and its application under such conditions should be avoided [11].
Some other research has shown that AISI 4130 steel exposed to salty solutions presented crack
initiation sites at the grain boundaries, whereas in air environment the initiation sites were inclusions
[10].
The tensile strength of a material has a different influence on fatigue and on corrosion fatigue.
Generally speaking, the fatigue strength of materials will be higher if their tensile strength is higher;
however the corrosion fatigue limit is normally unaffected or even decreased when tensile strength is
increased [10].
Some of the factors increasing the brittle corrosion fatigue cracking are: impurities in the steel, such
as phosphorus and sulphur segregation at the grain boundaries, solute (e.g. chromium) depletion or
sensitization at the grain boundaries, planar deformation associated with precipitates, and large
inclusions (mainly MnS). Other important factors include the presence of a martensitic microstructure
in steels, carbide formation in stainless steels, and other inhomogeneities, for which a uniform
distribution can hardly be assumed [13].
2.6.7.1. Inclusions
Inclusions are extremely harmful for a metal exposed to corrosion fatigue conditions, and its impact is
markedly higher in corrosive environments than in inert conditions [13]. Most of the literature
mentions MnS inclusions as the most hazardous for steels under corrosion fatigue situations. Areas
17
with MnS inclusions create localized anodic regions, which will easily form corrosion pits [20]. A
detrimental effect of the presence of these inclusions must be expected on both crack initiation and
crack propagation, as explained hereby.
One of the biggest problems with MnS inclusions is that they easily dissolve in the presence of water.
When a MnS inclusion is present on the surface of a steel part subjected to corrosion fatigue, it will
dissolve, forming a tiny pit on the metal surface. This pit will potentially form a crack since it will act as
a stress concentrator. Furthermore, it is easier for this pit to corrode since the MnS will form a highly
acidic atmosphere rich in sulphides around the spot. This is associated with the total consumption of
oxygen inside the crack, which forms a differential aeration cell. A gradient in corrosion potential from
the surface to the crack tip is then formed, causing a shift in the pH within the crack, usually towards
acidic values [13].
When a crack intersects a MnS inclusion, it will dissolve and enrich the sulphide within the occluded
crack solution. These anions stimulate crack advance by increasing the anodic charge that promotes
film rupture and hydrogen embrittlement; the dissolution of MnS inclusions produces H2S and HS- at
the crack tip [20]. See Figure 15. In water, the local potential is critical for the hydrogen production
since it determines if the process is thermodynamically feasible; hydrogen can separate from water at
-0.6V [20]. As a result, a high concentration of hydrogen is present and adsorbed by the fresh metal at
the crack tip [20]. Figure 15 also shows a MnS inclusion with hydrogen trapped around it, which will
transform into molecular hydrogen at temperatures over 200 °C. This process increases the internal
pressure, and if the inclusion cavity is sharp, a crack will form ahead of it. This is common in medium-
to-low strength carbon steels [20].
Figure 15. Effect of MnS inclusions in the film rupture/anodic dissolution process [20]
18
2.6.8. Surface Residual Stresses Although there is no generalization that accurately predicts the effect of residual stresses on the
corrosion fatigue performance of metals, it is commonly seen that compressive residual stresses
improve fatigue strength, and tensile residual stresses do not [5]. The favourable effect of residual
compressive stresses on the fatigue strength is greater in harder materials, whereas softer materials
will have a better improvement when work-hardened [5].
The reason for the surface compressive residual stress being beneficial for the fatigue strength is that
it will lower –or even cancel- the general stress caused by the loading, slowing down the formation of
surface cracks. Plastic deformation can cause a gradual decrease in the compressive stress level [5].
On the contrary, tensile residual stresses will have the opposite effect. Since tensile residual stresses
could be produced on the surface by machining, it is of great importance to take special care of this
issue, which causes a highly harmful effect on fatigue –and corrosion fatigue- strength.
A typical way to effectively introduce surface compressive residual stresses on steel is by shot peening,
carburizing and nitriding. These processes will be further explained in section 2.8.
2.6.9. Crack Closure Effects When the material undergoes the unloading part of the cycle, the crack surfaces make contact with
each other, and the material relaxes. When this happens, the stress intensity at the crack tip is reduced,
decreasing the rate of crack growth. This phenomenon is particularly relevant for near-threshold crack
propagation, at large load applications, and when corrosive embrittlement is present [5].
Environmental embrittlement produces rough, intergranular crack surfaces, which promote crack
closure since “uniaxially loaded cracks open in complex three-dimensional mode, allowing for surface
interactions and load transfer” [5]. Moreover, the crack surface interactions become more relevant
when the crack opening displacement is less than the fractured grain size [5].
Corrosion products forming on the cracked surface also have a great impact on the crack growth rate,
and in some cases, it could slow down the growth rates to values even lower than the ones for air or
vacuum [5]. This effect depends highly on the stability of the corrosion products during complex
tension-compression loading and on the fluid conditions [5]. In a similar way, liquids can also enter the
crack cavity and act as a wedge for the crack [6].
2.7. Mechanisms of Corrosion Fatigue Mainly four mechanisms have been exposed in the attempt of explaining the process of corrosion
fatigue. Although every mechanism is able to cover some of the aspects of this complex phenomenon,
none of them is able to completely explain the whole process [10].
2.7.1. Pitting Corrosion One of the first mechanisms proposed as an explanation for the marked reduction in fatigue endurance
in corrosive environments was the formation of corrosion pits on the surface of the material, which
acted as stress concentration spots where cracks were nucleated easily, as McAdam observed in 1928
[10]. Cyclic-stressing the material accelerates the corrosion pitting and causes transverse extensions
of the pits, developing fissures or crevices [1].
Other studies have shown that the constant presence of the corrosive media causes the most
damaging effect, compared to situations at which the aggressive environment was only present until
the formation of corrosion pits. In the latter cases, the corrosion pits act as mechanical notches on the
specimen, presenting considerably higher fatigue strengths at long endurances compared to those
having the corrosive media applied continuously throughout the entire test [1].
19
Decreases in life of components under corrosion fatigue have also been observed on environments
and materials that do not exhibit the formation of corrosion pits. In 1971, Laird and Duquette claimed
that the formation of pits comes as a result of cracks previously formed [10]. As a result of such
discrepancies, other mechanisms have been proposed to explain these behaviours.
2.7.2. Corrosion at Preferential Locations This mechanism proposes that corrosion attacks spots where fresh metal is exposed to the aggressive
media, making the material vulnerable to corrosive damage. Such weak spots are created by intrusions
and extrusions caused by persistent slip planes (see section 2.4, Figure 8). The attack produces a stress
concentrator where the material was already highly strained, causing a decrease in the fatigue strength
of the material [10].
Some studies have shown that highly deformed spots on the material are anodic with respect to
undeformed areas, enhancing the electrochemical process [1]. Although this phenomenon is not
thermodynamically favoured, the fresh metal is believed to present a lower activation energy and
therefore is more prone to react with the corrosive environment [10].
2.7.3. Hydrogen Embrittlement When the steel is strained in the presence of hydrogen, the material microstructure undergoes a
dangerous process known as hydrogen embrittlement, which consists on the diffusion of dissolved
hydrogen atoms into the metal lattice, causing its dilation and weakening its atomic bonds. See Figure
16.
The process of hydrogen embrittlement is illustrated in Figure 17. (1) First, water molecules or
hydrogen ions diffuse between the crack walls to the crack tip. (2) There, electrons are discharged and
(3) hydrogen atoms are reduced and adsorb at the crack tip surface (reduction). (4) These atoms will
then diffuse to surface locations that are preferential for corrosion activity (surface diffusion). (5)
Finally, the hydrogen atoms absorb into the metal and (6) diffuse ahead of the crack tip into critical
locations such as grain boundaries (volume diffusion) [15]. This causes an embrittlement of the
material surrounding the crack tip, which promotes further propagation of the crack.
Figure 16. Hydrogen embrittlement
Figure 17. The hydrogen embrittlement process [36]
Hydrogen embrittlement is typical in high-strength alloys. Since this kind of damage depends on
diffusion and adsorption, it is considered to be a time-dependent mechanism. Contrary to the
dissolution mechanism, this process does not form any kind of passivating oxide layer, so the hydrogen
diffusion can occur during the whole load cycle [15]. The addition of oxygen in small amounts is known
to practically eliminate the brittle corrosion-fatigue mechanism in crack growth [13].
20
2.7.4. Rupture of Oxide Protective Film Some metals, such as aluminium, copper and stainless steel, form a protective oxide layer, which
prevents the material underneath from further corrosive attack. Therefore, this mechanism proposes
that corrosion fatigue occurs as this protective film is broken and corrosion fatigue cracks form easily.
Since the present work does not deal with metals forming protective oxide films, this mechanism will
not be considered here.
2.7.5. Surface Energy Reduction The reduction of surface energy due to the adsorption of environmental species is known as the
Rebinder mechanism, which initially stated that the surface-active agent adsorbed into the cracks,
increasing its internal pressure, resulting in the propagation of the crack. Later on, this theory was
modified to state that the adsorbing species actually reduced the surface energy of the material,
allowing for an easier formation of protrusions resulting from slipping bands [10]. Although these
theories seem reasonable, there is not enough data nowadays to prove its veracity.
2.8. Heat and Surface Treatments There exist some methods to selectively change the properties of a steel at the surface –or of the
whole body-, improving its fatigue resistance considerably. These methods can be divided into surface
hardening by heat treatment and mechanical working by inducing deformation on the surface.
Improvements are due to two different factors: the change in the microstructure and the residual
compressive stresses [21]. In surfaces hardened by heat treatments (quenching, for instance), the layer
must be deep enough to protect the core from operating forces, but thin enough to maximize the
effectiveness of the residual stresses [7]. This is controlled by the heating and cooling parameters, as
well as by the carbon content of the steel and its composition. In the case of mechanical working, shot
peening and skin rolling are very popular methods.
2.8.1. Case Hardening This thermo-chemical process consists on exposing the finished component to a carburizing
atmosphere at a high temperature, ranging between 850-950 °C. As a result, a layer rich in carbon is
formed at the surface of the material. After carburising, the component is quenched, which increases
its hardness. Quenching causes the carburized layer to transform into martensite, for which the
hardness will be in function of its carbon content. The resulting component will have a hard surface
and a softer-tougher core [24].
The carbon content and other alloying elements (such as Ni, Mn, Cr and Mo) in the steel will determine
its hardenability [24]. The carburised surface usually presents residual compressive stresses as a result
of the case hardening, ranging between -150 to -250 MPa [24]. The depth of hardening is chosen
depending on the function of the component, and it is achieved by adjusting the parameters of the
case hardening process.
2.8.2. Through Hardening Through hardening is a thermal process in which the component is subjected to quenching, i.e. it is
heated up and rapidly cooled, to achieve a martensitic or bainitic microstructure throughout its cross
section. If the cooling process is not fast enough, the resulting microstructure could result in ferrite,
pearlite, or upper-bainite [24]. For large components, the quenching might not be enough to cool down
the core of the piece, resulting in an incomplete martensite or bainite transformation in such regions.
This will cause a hardness gradient, decreasing from surface to core [24].
21
2.8.3. Shot Peening Shot peening is a kind of cold-working process that consists on bombarding the surface of the material
with small metal hard particles, such as metal, glass or ceramic spheres, or small cylinders made with
cut wire. As a result, the surface deforms and changes its properties. The hardness of the deformed
material increases, and a residual-compressive-stress layer is formed at the surface. The compressive
stress from shot peening forms a gradient with a characteristic shape, as seen in Figure 18, for which
the residual stress reaches a minimum (maximum compressive) underneath the surface, and then it
relaxes again.
Figure 18. Shot peening compressive stress profile [21]
The improvement in fatigue resistance on the material is caused by the combined effect of cold
working and compressive stresses. Cold working involves work hardening and the closure of pores and
surface cracks [21]. The results of the shot peening depend on the intensity of the process and on the
material being treated. The intensity of the shot peening process is affected by several variables, such
as the hardness and size of the balls, speed of the shots, coverage, etc. In general terms, the higher
the intensity, the greater the resultant residual compressive stresses and the deeper they go into the
material. The result of the shot peening also depends on the material properties. For instance, a higher
yield strength of the material will allow higher compressive stresses since any residual compressive
stress larger than the yield strength of the material will cause relaxation without any external load
applied [22].
2.9. Fatigue Testing Despite of the enormous amount of money, time and effort spent on studying fatigue and trying to
generate valuable data for preventing unpredicted fatigue failure in engineering applications, a
scientific basis for reliable estimates of fatigue life for all possible combinations of load and
environmental conditions remains intangible. Furthermore, existing approaches to standard test
development can result non-conservative in cases where two or more phenomena interact
synergistically against the performance of the material. Thus, a designer must rely on experience along
with research data to make decisions about the components in service [5].
Laboratory corrosion-fatigue tests can be classified as either cycles-to-failure or crack propagation
tests. The former consists on applying a sufficient number of cycles to cause complete fracture of the
22
specimen. With this kind of test it is difficult to identify crack initiation and propagation stages of the
fatigue process; nevertheless, the fraction of the total life spent in initiating a crack is generally
estimated in the ranges described in section 2.4 [5]. The result of these kinds of tests might not be
accurately representative of large components when the tests are performed on small specimens, but
the results can be effectively used to compare alloying additions, heat and surface treatments and
finishes, corrosion-control methods, etc., since they provide valuable “empirical data on the intrinsic
fatigue crack initiation behaviour of a metal or alloy” [5]. Therefore, in the present work, the cycles-
to-failure type of test was used for the experimentation.
Fatigue testing methods are firstly characterized by the loading mode of the specimen. Mainly three
different loading modes are applied: direct axial loading, plane-bending, and rotating-beam loading
[25]. Depending on the loading mode, the design of the machine and the shape of the specimen can
widely vary. In general, the testing machines are classified by the basic drive mechanism they use, and
the test parameter they allow to control [25]. In the present work, the rotating-bending test is used,
and therefore it will alone be described hereon.
2.9.1. Rotating-Bending Test The specimen used for this test method is generally a cylindrical rod with a waist at the centre of the
body to concentrate the stress. This kind of test consists on holding the specimen from both extremes
using special grips mounted on bearings that allow rotation. One of the extremes is rotated, generally
by an electric motor. The specimen is then bent by the application of death-weight loading. Thus, the
specimen surface is subjected to a sinusoidal stress loading, as shown in Figure 1, with maximum values
at the centre of the waist where the diameter is minimum. Rotating-bending tests generates a fully-
reversed loading cycle. Because of the characteristic loading method, it becomes impossible to run
mean-stress effect tests with this machine [25]. Since only a small part of the material at the surface
of the specimen will be loaded with the maximum stress at a time, the fatigue strengths and lives
obtained from rotating-bending tests will be typically higher than those obtained with axial-fatigue
tests [25].
Figure 19 shows an illustration of how a cantilever loading rotating-bending machine is designed.
Figure 19. Rotating-bending machine diagram
2.9.2. The Staircase Method One of the most widely used testing methods to determine the statistical distribution of the fatigue
strength of a material –usually providing conservative results [3]- is known as the staircase method. It
consists on running a set of tests around the estimated fatigue limit, and use the collected data to
calculate the fatigue strength statistically. The way to choose the load at which each individual test will
be conducted depends on the result of the previous test, as explain further on.
23
The first step is to select a certain number of cycles to be considered as the infinite life, for instance,
106 cycles. Then, a value for the fatigue limit is estimated from either experience or preliminary S-N
data [26], and a fatigue-life test is performed at a stress level just above the estimated value. If the
specimen fails before reaching the chosen infinite life, the next test is to be performed at a stress level
below the previous one. Else, if the specimen survives, the following test must be performed at a stress
level higher than the previous one. This process is then repeated as many times as needed, usually
between 20 and 30 [2, 3]. The stress increment –or reduction- is usually decided to be in a range from
half to twice the standard deviation of the fatigue limit [3].
The staircase method does not deal with the number of cycles to failure. Instead, it handles the data
in a “pass/fail” manner [26]. Once the data is collected, the results can be visualized in a plot as shown
in Figure 20. The Dixon-Mood method can then be applied to statistically calculate the mean, µS, and
the standard deviation, σe, of the fatigue limit. This method assumes that the fatigue limit follows a
normal distribution; if the data does not satisfy this requirement, a transformation (logarithmic,
power, squared, cubic, etc.) of the stress values can be applied [26].
Figure 20. Example of staircase fatigue data [3]; suspensions are tests in which the specimens survived
The Dixon-Mood method requires the use of the least frequent event (either survivals or failures) to
estimate the mean and standard deviation of the fatigue strength. In the plot shown in Figure 20, for
instance, the least frequent event is survivals (denoted as “suspensions”), represented by the white
circles. The Dixon-Mood method can be expressed in the following equations [26]:
𝐴 = ∑ 𝑚𝑖
𝑖𝑚𝑎𝑥
𝑖=0
, 𝐵 = ∑ 𝑖 ∙ 𝑚𝑖
𝑖𝑚𝑎𝑥
𝑖=0
, 𝐶 = ∑ 𝑖2 ∙ 𝑚𝑖
𝑖𝑚𝑎𝑥
𝑖=0
(2)
𝜇𝑆 = 𝑆0 + 𝑠 ∙ (𝐵
𝐴± 0.5)
(3)
𝜎𝑒 = 1.62 ∙ 𝑠 ∙ (𝐴 ∙ 𝐶 − 𝐵2
𝐴2+ 0.029) if
𝐴 ∙ 𝐶 − 𝐵2
𝐴2≥ 0.3
(4)
or
𝜎𝑒 = 0.53 ∙ 𝑠 if 𝐴 ∙ 𝐶 − 𝐵2
𝐴2< 0.3
(5)
24
where i is an integer of the stress level, and the parameter imax is the highest stress level in the staircase.
If the least frequent event is survivals, then S0 will be the lowest stress level at which a specimen
survived, and it will correspond to the i = 0 level; contrary, if the least frequent event is failures, S0 will
have the value of the lowest stress level at which a failure occurred, and it will correspond to the i = 0
level. The parameter mi is the number of specimens that failed at each stress level i. and s represents
the stress step size. In equation (3), the (+) sign is used when the least frequent event is survivals, and
the (–) sign is used when the least frequent event is failures [26].
2.9.3. The Median S-N Test Method for Small Sample Size This method was described by the Japan Society of Mechanical Engineers (1981), and it is used as a
guideline to determine the S-N curve with a reliability of 50% and a minimum sample size [3]. It requires
at least 14 specimens, 8 for the finite-life region (S-N curve) and 6 for the staircase method. The
arrangement and sequence of the tests is suggested as shown in Figure 21. Note that the value of the
stress levels and number of cycles in this illustration are mere examples.
Figure 21. S-N testing with a small sample size [3]; suspensions are tests in which the specimens survived
For the generation of the S-N curve, it is recommended that more than one specimen is tested at each
stress level in order to produce replicable data, which is necessary to estimate the statistical
distribution and variability of the fatigue life [3]. For preliminary and R&D tests, it is recommended to
use between 6 – 12 specimens to generate an S-N curve. The percent replication (PR) for R&D testing
is recommended to be between 33 – 50. This value indicates the portion of the sample size that can
be used to determine an estimate of the variability of replicate tests [3], and it is calculated from
equation 6 shown below:
𝑃𝑅 = 100 ∙ (1 −𝐿
𝑛𝑠)
(6)
where 𝑛𝑠 is the number of samples and 𝐿 is the number of stress levels.
25
3. Materials and Equipment The procedure followed in the present work was adapted to the equipment available in the materials
laboratory at Atlas Copco Secoroc AB in Fagersta. The fatigue tests were performed in rotating-bending
machines designed to test specimens with certain dimensions and geometries, which are detailed in
section 3.2.1. Two materials were tested, each one with different heat treatments. Shot peening was
applied on half the specimens of each of the materials, resulting in a total of four different material
conditions, as explained in section 3.2.2.
3.1. Laboratory Equipment The materials laboratory at Atlas Copco Secoroc AB has two rotating-bending fatigue testing machines.
A picture of these machines is presented in Figure 22. An electric motor drives one of the rotating grips,
while the other grip is pulled by a lever connected to dead-weights by pulleys and a cable (see Figure
23), creating the cantilever loading effect on the specimen. The corrosive conditions are created by
means of a water supply system, which consists of a stainless-steel water tank attached to the
machine, a water pump with maximum flow capacity of 12 l/min installed on one of the sides of the
tank, a closed chamber to cover the specimen, a hose connecting the pump with the chamber, and a
water collector at the bottom of the chamber which redirects the water into the tank.
Figure 22. Rotating-bending fatigue testing machine - Atlas Copco Secoroc AB materials laboratory
26
Figure 23. Rotating-bending fatigue testing machine, rear side - Atlas Copco Secoroc AB materials laboratory
Figure 24 shows a specimen mounted in the machine without the chamber housing installed. The two
white plastic washer-like parts are put around the specimen to prevent the water from flowing outside
the chamber. The water collector can be observed under the specimen. The chamber can be seen in
Figure 25.
Figure 24. Specimen mounted on the machine and water collector underneath
27
Figure 25. Corrosion chamber
The minimum weight that the machine can loaded with is 2.836 kg, which corresponds to the weight
of the basket on which the extra load is carried. This would mean that the basket would be left alone
with no extra weight added. For these machines, this minimum load corresponds to a stress of 83.6
MPa on a standard specimen with a waist diameter of 15 mm.
3.2. Specimens
3.2.1. Geometry and Dimensions As mentioned in section 2.9.1, the specimens used for the tests were cylindrical bars with a waist in
the centre to create a stress concentration. The geometry of the specimens is illustrated in Figure 26.
The surface roughness defined in the drawing is for the through hardened material –without shot
peening- after its final polishing. Nonetheless, all the other dimensions apply for the case hardened
specimens as well. The minimum diameter of the waist is 15 mm and the tolerance for this dimension
was always respected, even on the shot peened specimens, which have a rougher surface and
generally tend to have a slightly larger diameter.
Figure 26. Geometry and dimensions of specimen
28
Before testing, the diameter of each specimen was measured using a Mitutoyo laser scan micrometer,
which is shown in Figure 27 and Figure 28. With this device, the exact diameter and the ovality of the
waist was measured.
Figure 27. Laser scan micrometer
Figure 28. Laser scan micrometer display
3.2.2. Material The two steels used during the present work were a case hardened (which in the present document
will be referred to as CH) and a through hardened (which will be referred to as TH) steel grades. Half
of the population of each steel was shot peened, resulting in four different material conditions, as
shown in Table 1.
Table 1. Different material scenarios used in the present work
Shot peened Non-shot peened
Case Hardened (CH) CH-SP CH-NSP
Through Hardened (TH) TH-SP TH-NSP
29
Figure 29 shows a picture of one specimen of each material condition studied in the present work.
Figure 29. Specimens used. From left to right: (1) Case hardened, non-shot peened; (2) Case hardened and shot peened; (3) Through hardened, non-shot peened; (4) Through hardening and shot peened
3.2.3. Heat Treatment Case hardening in CH steel was performed within Atlas Copco’s facilities in Fagersta. After the
treatment, the surface hardness was HRC 57, and the core hardness was HRC 43. The hardening depth
was 1.4 – 1.2 mm. As a result of the heat treatment, the case hardened parts will have a hard case and
a tough core.
The through hardening process was not performed by Atlas Copco. The steel grade was heat treated
by the supplier before delivery. The hardness of this material is, by standard, HRC 49 - 51 and is
constant throughout the cross section of the parts.
3.2.4. Shot Peening The CH steel (CH-SP) was shot peened with an intensity of 0.58-0.59 mm A. The TH steel (TH-SP) was
shot peened with an intensity of 0.66 mm A. The difference in the intensity was due to a maintenance
adjustment made on the shot peening machine between the two processes, which were performed in
different days. The residual stresses achieved by the shot peening are presented in section 3.2.4.1.
3.2.4.1. Residual Stress Analysis
Specimens from the four different conditions were analysed for residual stresses on the surface using
the X-Ray diffraction method, which is explained elsewhere ([27]). The measurements were not
performed within the Atlas Copco laboratories. The results are shown in the following figures. In Figure
33, Phi=0 corresponds to axial direction and Phi=90 to circumferential direction. The measurement on
the axial direction is the one to be taken into consideration, since it is the one affected by the bending
moment of the test. The circumferential direction is not relevant for the stress state during the test as
long as it has the same sign as for the axial direction. Then, it will not be part of the biggest Mohr’s
circle being responsible for the maximum shear stress in the material.
30
Figure 30. Residual stresses on CH-SP
Figure 31. Residual stresses on CH-NSP
Figure 32. Residual stresses on TH-SP
Figure 33. Residual stresses on TH-NSP
From Figure 30 and Figure 32, it can be observed that the residual stresses on both shot peened
materials is negative with a value of around -400 MPa at the surface, going down to around -900 MPa
at 0.07 mm for CH-SP and down to -800 MPa at 0.2 mm for TH-SP.
For CH-NSP, shown in Figure 31, it can be seen that a tensile residual stress is present at the surface,
but then it decreases suddenly down to approximately -150 MPa, and it remains negative as the
measurement goes deep inside the specimen for more than 1 mm. In the case of TH-NSP, shown in
Figure 33, the residual stress is negative on the surface in the axial direction, and then it goes close to
zero as the depth increases.
31
4. Experimental Setup and Procedures The present work was divided into three main steps. First, a corrosion-fatigue testing method was
proposed, and the testing parameters were defined. The second step consisted on using the method
to test different material conditions and make a comparison to prove the validity of the proposed
method. The third step comprised a set of metallographic examinations of the tested specimens to
observe the cracking phenomena. The details of these three steps are presented in this section.
4.1. Corrosion-Fatigue Testing Method Proposal and Testing Parameters Selection It was decided that the corrosion-fatigue testing method would cover the materials’ behaviour on both
finite and infinite life ranges, i.e. it would involve the S-N curve and the fatigue strength. To be able to
compare fatigue results from wet and dry conditions, one key parameter taken from the dry fatigue
method is the infinite life value, which is considered to be at 3X106 cycles. The proposed method is
shown in section 5.1.
The results obtained with the proposed testing method should permit the visualization of well-defined
differences between the materials and scenarios. Since the results from fatigue tests are strongly
dependent on the testing parameters (as explained in section 2.6), a right set of testing parameters is
required to achieve such a good method. It was crucial to select the main factors influencing corrosion
fatigue -based on their relevance on the real-product application and on the capabilities of the
laboratory equipment- and perform a series of tests varying the values of such factors to observe their
effect on the results. From these observations, the values for these parameters were selected for the
final method.
It was decided that frequency was the main parameter defining the new corrosion-fatigue testing
method. Therefore, frequency dependence of corrosion fatigue on CH-SP and TH-NSP was studied in
order to choose a frequency that would allow a proper comparison of the materials and a good
visualization of the data in the proposed testing method. The machines allow a rotating speed in the
range of 350 – 3000 rpm. Currently the company performs dry-fatigue tests at 2300 rpm. The chosen
range for testing frequency dependence in the present work was therefore 500 – 2300 rpm.
The frequency dependence analysis consisted of a set of tests at fixed load but different rotating
speeds: 500 rpm, 1000 rpm, 1600 rpm, and 2300 rpm. The load for these tests was the R95C90 lower
bound from the dry fatigue limit, which means that there is a 95% reliability with a 90% confidence
interval that the specimens would survive at this load in dry conditions [3]. The one-side lower-bound
value can be calculated with equation 7 [3]. The calculations are shown below:
𝑆𝑒,𝑅,𝐶 = 𝜇𝑆 − 𝐾 ∙ 𝜎𝑒
(7)
For CH-SP:
Dry fatigue data: µS = 725 MPa ; σe = 26.5 MPa
The K factor has a value of 2,894 for R95C90 lower bound and n = 7 [3]
From equation 7, R95C90 = 650 MPa
For TH-NSP:
Dry fatigue data: µS = 744 MPa ; σe = 26.5 MPa
The K factor has a value of 2,755 for R95C90 lower bound and n = 8 [3]
From equation 7, R95C90 = 670 MPa
The results from the frequency dependence tests are presented in section 5.1.1.
32
Regarding the loading characteristics, the stress ratio R was -1, with a sinusoidal wave shape. These
variables are fixed for rotating-bending fatigue testing machines.
In terms of the environment, water conditions were kept constant except for the NaCl content. Water
was taken from the tap found in the laboratory, and it was expected to have the same characteristics
throughout the whole experimental work timeframe. It is important to mention that the water was
renewed every test without exception. Temperature of the water was not varied, and was found to be
between 20 and 23 °C, sometimes increasing up to 25.6 °C during long-lasting tests and warmer days.
Such variations were neglected since they are considered to be too small to have a significant effect
on the results, for instance in the oxygen content, as seen in Figure 12. The pH was also kept constant
in a value of 7, as coming from the tap. Oxygen concentration in the water was not controlled with
detail, but during the experiments, continuous stirring of the water was ensured to allow constant
dissolution of oxygen. The level of oxygen dissolution is not of vital importance, since all the tests –
during this work and in the future- are to be performed in the same way, which ensures equal water
characteristics on every test, allowing to fairly compare the results. The dependence on NaCl content
in the water was tested, performing a set of tests with water containing 4 wt% NaCl.
Further tests of frequency dependence were carried out at a load level corresponding to each
material’s dry fatigue limit with the purpose of comparing the results with the ones from R95C90 lower
bound stress level. The intention was to prove that the frequency dependence results showed a
different trend for each load level, i.e. that the frequency dependence response changes depending
on the applied stress. However, these tests were not extensive, and only one specimen was tested at
each test speed. The results of these tests are shown in Figure 39 and Figure 40.
4.2. Testing the Proposed Corrosion-Fatigue Method Once the testing method and the parameters were decided, the four material conditions presented in
Table 1 were tested using this method in order to prove its functionality. The results for these tests are
shown in 5.2. As well as for the frequency dependence tests, the water was renewed for every test.
The approach in the present work was to run corrosion fatigue tests at the same load levels as the dry
tests, even though a lower fatigue strength value was expected from corrosion fatigue testing; this
way, the S-N curve for corrosion fatigue could be generated in the load range going from the dry fatigue
limit down to the corrosion fatigue strength. The separation between the load levels used in the S-N
curves was chosen based on the results obtained in the first tests. For instance, if the life of the
specimen in the first level was low, the decrement in load for the next level should be large, and vice
versa. The fatigue strength for every condition was found using the staircase method.
The first material condition tested was CH-SP. The load for the first test was chosen to be the dry
fatigue limit of that material condition, which is 725 MPa. The results for this material condition are
shown in Figure 42 and Figure 43. TH-NSP was tested in the same way. For this material condition, the
dry fatigue limit is 745 MPa, and the first test was also performed at this load level. The results for TH-
NSP are shown in Figure 48 and Figure 49. Then, CH-NSP was tested. Since the steel in CH-NSP is the
same as in CH-SP, and only shot peening differentiates these two conditions, the load for CH-NSP was
chosen as 725 MPa for the first test, even though the dry fatigue limit is 625 MPa. This would allow a
comparison between conditions at high loads. The plots for CH-NSP are presented in Figure 44 and
Figure 45. Finally, TH-SP was tested following the same steps. Since this material condition has not
been tested in dry fatigue yet, the dry fatigue limit was taken from its partner condition (TH-NSP), so
the load for the first test of TH-SP was 750 MPa (745 MPa + 5 MPa to round up the value). The results
for TH-SP are presented in Figure 46 and Figure 47.
33
4.3. Post-Test Analysis After the tests were performed, a set of specimens was selected based on the testing parameters and
on the quality of the fracture surface, to be later analysed in SEM and optical microscope. The purpose
of these analyses was to characterize the fracture surface, and also to try to understand the cracking
process by observing the morphology of the cracks.
4.3.1. Microscope Analysis A set of specimens was selected to be prepared as metallographic samples in order to examine the
cracks on a cross-sectional view using a metallographic microscope available in the company’s
laboratory in Fagersta. The selection of the specimens is described in section 4.3.2. The pictures
obtained in this analysis are shown in section 5.3.2.
Some specimens which failed during the test were also selected in order to analyse the fracture surface
on a Scanning Electron Microscope (SEM). The purpose of these observations was to show the crack
initiation sites, the morphology of the crack-propagation surface and inclusions on the cracking path.
The results of this analysis is presented in section 5.3.1
4.3.2. Metallographic Samples The selection of the specimens was based on the conditions of the test they represent. In general, two
specimens of each material condition were chosen: one for the highest load and one for the lowest
load tested for the condition. Some other specimens were also selected based on the relevance of the
resulting observations, for instance, TH-NSP on low speed values or CH-SP in salty water. Table 2 shows
the specimens selected and prepared as metallographic samples.
Table 2. Metallographic samples selection
Material Condition Specimen Number Stress Level during Test
[MPa]
Testing Speed
[rpm]
TH-NPS
093 750 1000
016 200 1000
029 680 1000
036 680 500
TH-SP 012 800 1000
013 550 1000
CH-NSP 045 725 1000
095 200 1000
CH-SP 091 800 1000
047 500 1000
CH-SP in
4.0 wt% NaCl water
028 650 500
032 650 2300
34
5. Results This section is divided into three parts: the first subsection (section 5.1) presents the frequency
dependence plots and the final selection of testing parameters for the proposed method; the second
subsection (section 5.2) presents the corrosion-fatigue test results obtained with the proposed
method; the third subsection (section 5.3) offers a group of metallographic analyses performed on the
already-tested specimens with the intention of understanding the mechanisms involved in cracking
and fracture of the materials compared with the proposed method.
5.1. Presentation of Corrosion-Fatigue Testing Method The proposed method for testing corrosion fatigue was the “Median S-N Test Method for Small Sample
Size”, described in section 2.9.3. It consists in building the S-N curve with 8 samples (ns = 8) and 4 stress
levels (2 samples on each stress level), and the staircase with 6 samples. The calculated percent
replication (PR) from equation 6 is shown in Table 3.
Table 3. Percent Replication
5.1.1. Frequency Dependence Results The frequency dependence results for CH-SP and TH-NSP are presented below. Figure 34 shows the
corrosion-fatigue frequency dependence of CH-SP in fresh water and brine (water with 4 wt% NaCl).
For the values obtained in fresh water, the averages are also shown. Trendlines are included in the
plot.
Figure 34. Corrosion-fatigue frequency dependence on CH-SP
y = 128.51x + 525843R² = 0.5451
0.0E+00
5.0E+05
1.0E+06
1.5E+06
2.0E+06
2.5E+06
3.0E+06
3.5E+06
4.0E+06
0 500 1000 1500 2000 2500
Nu
mb
er o
f cy
cles
to
fai
lure
Test speed [rpm]
0% NaCl
4% NaCl
Average
Parameter Value Comment
𝒏𝒔 8 OK for preliminary and R&D tests (ASTM, 1998) Stress levels 4 From Nakazawa and Kodama, 1987
PR 50 OK for R&D tests (ASTM, 1998)
35
The chart shown in Figure 35 shows the average time spent on the tests performed for the frequency
dependence study, so it represents the data shown in Figure 34.
Figure 35. Average time spent per test for different testing speeds on CH-SP in fresh water and brine
The values of time for 0% NaCl in Figure 35 are shown in Table 4 below, including the average number
of cycles from the fresh water tests plotted in Figure 34.
Table 4. Average values for number of cycles to failure and time of test for frequency dependence of corrosion fatigue on CH-SP in fresh water
Speed [rpm]
Average number of cycles
Number of tests
Average time of test [hrs]
500 382 450 2 12.75
1000 1 165 266 3 19.42
1600 1 878 466 3 19.57
2300 2 988 566 3 21.65
Frequency dependence for TH-NSP was also proven, and the results are shown in Figure 36. The effect
of NaCl in the water was not tested on this steel grade. The plot in Figure 37 shows the average time
spent by test on each testing speed, and Table 5 shows the corresponding values.
0
5
10
15
20
25
500 1000 1600 2300
Tim
e [h
ou
rs ]
Speed [rpm]
0% NaCl
4% NaCl
36
Figure 36. Corrosion-fatigue frequency dependence on TH-NSP
Figure 37. Average time spent per test for different testing speeds on TH-NSP in fresh water
y = 3442.4x0.4364
R² = 0.8839
0.0E+00
2.0E+04
4.0E+04
6.0E+04
8.0E+04
1.0E+05
1.2E+05
1.4E+05
0 500 1000 1500 2000 2500
Nu
mb
er o
f cy
cles
to
fai
lure
Test speed [rpm]
0% NaCl
Average
0
0.5
1
1.5
2
500 1000 1600 2300
Tim
e [h
ou
rs ]
Speed [rpm]
0% NaCl
37
Table 5. Average values for number of cycles to failure and time of test for frequency dependence of corrosion fatigue on TH-NSP in fresh water
Speed [rpm]
Average number of cycles
Number of tests
Average time of test [hrs]
500 50 433 3 1.68
1000 73 933 3 1.23
1600 87 366 3 0.91
2300 98 566 3 0.71
A side-to-side comparison of corrosion-fatigue frequency dependence between CH-SP and TH-NSP is
shown in Figure 38.
Figure 38. Corrosion-fatigue frequency dependence comparison between CH-SP and TH-NSP
y = 124.28x1.3015
R² = 0.9292
y = 3442.4x0.4364
R² = 0.8839
1.0E+04
1.0E+05
1.0E+06
1.0E+07
0 500 1000 1500 2000 2500
Nu
mb
er o
f C
ycle
s to
fai
lure
Test speed [rpm]
CH-SP
TH-NSP
Potencial (CH-SP)
Potencial (TH-NSP)
38
The plots in Figure 39 and Figure 40 show the results of the corrosion fatigue tests performed at a
load level corresponding to the dry fatigue limit of each material. For CH-SP, this limit is found at 725
MPa, and the R95C90 value is 650 MPa. For TH-NSP, the dry fatigue limit is 744 MPa and its R95C90
value is 670 MPa.
Figure 39. Corrosion-fatigue frequency dependence of CH-SP at two load levels
Figure 40. Corrosion-fatigue frequency dependence of TH-NSP at two load levels
y = 124.28x1.3015
R² = 0.9292
y = 470245ln(x) - 2E+06R² = 0,9333
0.0E+00
5.0E+05
1.0E+06
1.5E+06
2.0E+06
2.5E+06
3.0E+06
3.5E+06
4.0E+06
0 500 1000 1500 2000 2500
Nu
mb
er o
f C
ycle
s to
Fai
lure
Test speed [rpm]
R95C90
Average N
Dry fatigue limit
y = 3442.4x0.4364
R² = 0.8839
y = 2114,4x0,4584
R² = 0,9607
0.0E+00
2.0E+04
4.0E+04
6.0E+04
8.0E+04
1.0E+05
1.2E+05
1.4E+05
0 500 1000 1500 2000 2500
Nu
mb
er o
f C
ycle
s to
Fai
lure
Test speed [rpm]
R95C90
Average N
Dry fatigue limit
39
5.1.2. Final Parameters for Proposed Testing Method Figure 41 shows the same frequency dependence results for CH-SP in fresh water presented in Figure
34. It can be noticed that a testing speed of 1000 rpm fits the needs of the proposed method in the
best way, as it is further discussed in section 6. Thus, this value can be found in the final parameter
selection shown in Table 6.
Figure 41. Testing speed selection
The proposed testing parameters are presented in Table 6.
Table 6. Parameters for Proposed Corrosion-Fatigue Testing Method
0.0E+00
5.0E+05
1.0E+06
1.5E+06
2.0E+06
2.5E+06
3.0E+06
3.5E+06
4.0E+06
0 500 1000 1500 2000 2500
Nu
mb
er o
f cy
cles
to
fai
lure
Test speed [rpm]
0% NaCl
Average
Infinite Life
Marked Difference
Small Spread
Parameter Value Comment
Loading Characteristics
Rotating speed 1000 rpm From frequency dependence
Stress ratio -1 For rotating-bending test
Stress amplitude variable Dependent on stress level
Stress-wave shape sinusoidal For rotating-bending test
Water Characteristics
pH 7 Tap water
NaCl content 0 wt% NaCl Tap water
Oxygen content Full Saturation Constant stirring
Temperature 20 – 23 °C Room temperature
Other Parameters
Infinite life 3X10^6 cycles Same as for dry fatigue tests
Staircase load increment 50 MPa Same as for dry fatigue tests
40
5.2. Testing the Proposed Corrosion-Fatigue Testing Method
5.2.1. S-N curves and fatigue strength results The proposed corrosion-fatigue testing method was performed on the four material conditions using
the selected parameters presented in Table 6 in order to prove its functionality. With this method, the
S-N curves and fatigue strengths for each of the four conditions were generated.
For CH-SP, the data forming the finite life curve adjusted properly to a logarithmic regression, as it can
be seen in Figure 42. The highest stress level tested for this condition was 800 MPa, and the mean life
expected is 3X10^5 cycles. As the load decreases, this material condition reaches infinite life at a mean
stress of 525 MPa, as shown in Figure 43.
Figure 42. S-N curve for CH-SP
Figure 43. Corrosion-fatigue strength of CH-SP from staircase method
y = -114.2ln(x) + 2240.3R² = 0.9173
400
450
500
550
600
650
700
750
800
850
1.0E+05 1.0E+06 1.0E+07
Stre
ss [
MP
a]
Number of Cycles
Failures
Run outs
400
450
500
550
600
650
0 1 2 3 4 5 6 7 8 9 10
Stre
ss [
MP
a]
Test number
Runouts
Failures
Fatigue strength@ 3X10^6 cycles
525 ± 42.8 MPa
41
For the case of CH-NSP, the data for the S-N curve, shown in Figure 44, was better adjusted with a
power regression. The highest stress tested for this condition was 725 MPa and the expected life at
this load is around 4X10^4 cycles, significantly lower than the shot peened version. The corrosion-
fatigue strength also dropped considerably without shot peening, reaching a value of 208.3 MPa,
shown in the staircase plot in Figure 45.
Figure 44. S-N curve for CH-NSP
Figure 45. Corrosion-fatigue strength of CH-NSP from staircase method
y = 18930x-0.308
R² = 0.9438
0.00
100.00
200.00
300.00
400.00
500.00
600.00
700.00
800.00
1.0E+04 1.0E+05 1.0E+06 1.0E+07
Stre
ss [
MP
a]
Number of cycles
Failures
Run outs
100
150
200
250
300
0 1 2 3 4 5 6 7 8 9 10
Stre
ss [
MP
a]
Test number
Runouts
Failures
Fatigue strength@ 3X10^6 cycles
208.3 ± 26.5 MPa
42
For the case of TH-SP, the maximum stress level tested was also 800 MPa, and for this material
condition, the life expected at this load is around 1.6X10^5 cycles, which is a bit lower than for CH-SP.
The S-N curve for this condition is shown in Figure 46. The data for the finite life was also well adjusted
to a logarithmic regression. The staircase method, shown in Figure 47, determined a corrosion fatigue
strength of 600 MPa.
Figure 46. S-N curve for TH-SP
Figure 47. Corrosion-fatigue strength of TH-SP from staircase method
y = -80.77ln(x) + 1767.7R² = 0.9298
500
550
600
650
700
750
800
850
1.00E+05 1.00E+06 1.00E+07
Stre
ss [
Mp
a]
Number of cycles
Failures
Run outs
500
550
600
650
700
0 1 2 3 4 5 6 7 8 9 10
Stre
ss [
MP
a]
Test number
Runouts
Failures
Fatigue strength@ 3X10^6 cycles
600 ± 26.5 MPa
43
Lastly, the S-N curve for and staircase for TH-NSP is presented in Figure 48 and Figure 49, respectively.
The highest stress level tested for this condition was 750 MPa, for which the expected life is 5X10^4,
which is slightly higher than the CH-NSP. The corrosion fatigue strength was found to be 191.6 MPa.
Figure 48. S-N curve for TH-NSP
Figure 49. Corrosion-fatigue strength of TH-NSP from staircase method
y = -165ln(x) + 2523.7R² = 0.9843
0
100
200
300
400
500
600
700
800
1.0E+04 1.0E+05 1.0E+06 1.0E+07
Stre
ss [
Mp
a]
Number of cycles
Failures
Run outs
100
150
200
250
300
0 1 2 3 4 5 6 7 8 9 10
Stre
ss [
MP
a]
Test number
Runouts
Failures
Fatigue strength@ 3X10^6 cycles
191.6 ± 26.5 MPa
44
5.2.2. Materials Comparison The following plots present a comparison between the four material conditions tested. Different
combinations of conditions are also shown in an attempt to expose important observations regarding
the performance of each condition.
First, a comparison between the four material conditions is presented in Figure 50, containing the finite
life predicted by the S-N curves and the corrosion-fatigue strengths of each one of the conditions.
Figure 50. Comparison of the four conditions (S-N curves and corrosion-fatigue strengths)
Figure 51 shows a magnification of the shot peened conditions, since they appear small in Figure 50.
100
200
300
400
500
600
700
800
900
1.0E+04 1.0E+05 1.0E+06 1.0E+07
Stre
ss (
Mp
a)
Number of Cycles
CH-SP
TH-SP
CH-NSP
TH-NSP
Infinite Life - 3X10^6 cycles
600 MPa
525 MPa
208.3 MPa
191.6 MPa
45
Figure 51. S-N curve comparison between CH-SP and TH-SP
Finally, a comparison between the corrosion fatigue and dry fatigue results is presented. It is important
to remember that the latter were obtained only for staircase testing, but they are easily included in
these plots since a record of the number of cycles achieved by the fractured specimens is kept by the
company.
Figure 52 shows the two versions of CH under corrosion fatigue, and also the results from dry testing,
for both shot peened and non-shot peened specimens of the same steel. Figure 53 presents TH-SP and
TH-NSP under corrosion fatigue, and the dry fatigue data of the TH-NSP condition generated for the
staircase. As mentioned earlier, the TH-SP condition has not been tested in dry conditions yet.
500
550
600
650
700
750
800
850
1.0E+05 1.0E+06
Stre
ss [
MP
a]
Number of cycles
CH-SP
TH-SP
CF Strength CH-SP
CF Strength TH-SP
Infinite Life - 3X10^6 cycles
600 MPa
525 MPa
46
Figure 52. S-N curve comparison between CH-SP, CH-NSP, and dry tests
Figure 53. S-N curve comparison between TH-SP, TH-NSP, and dry tests (on non-shot peened)
0
100
200
300
400
500
600
700
800
900
1.0E+04 1.0E+05 1.0E+06
Stre
ss [
MP
a]
Number of cycles
Shot Peened
Non-Shot Peened
Dry tests (shot peened)
Dry tests (Non-shot peened)
CH
0
100
200
300
400
500
600
700
800
900
1.0E+04 1.0E+05 1.0E+06
Stre
ss [
MP
a]
Number of cycles
Shot Peened
Non-Shot Peened
Dry tests (non-shot peened)
TH
47
5.3. Metallographic Results and Observations This section presents a series of pictures as part of a post-test analysis in the attempt of explaining the
failure behaviour observed in the plots shown in the previous section.
5.3.1. Fracture Surface Analysis from SEM Pictures for TH-NSP are shown below.
Figure 54. Crack initiation and deformed propagation
surface on TH-NSP @200 MPa, Specimen 009
Figure 55. Crack initiation on TH-NSP @200 MPa, Specimen
009
Figure 56. Surface major crack propagation on TH-NSP
@744 MPa, Specimen 087
Figure 57. Several cracks initiating on fracture surface at
different planes near on TH-NSP @744 MPa, Specimen 087
Figure 58. Crack initiation morphology on TH-NSP
@744 MPa, Specimen 087
Figure 59. CaS inclusion near crack initiation on TH-NSP
@744 MPa, Specimen 087
48
The following pictures show CH-NSP. A large inclusion containing Ca, Si, Al and S was found near a crack
initiation of a specimen tested under 200 MPa stress. The fracture surface at low load seems generally
deformed after the repetitive cycling of the test. For high loading, the fracture surface shows an
intergranular crack propagation, characteristic of brittle fracture.
The pictures shown below correspond to TH-SP. Several crack are observed on the fracture surface in
shot-peened conditions. Propagation seems rather ductile.
Figure 60. Possible crack initiation from (Ca, Si, Al)S
inclusion on CH-NSP @200 MPa, Specimen 043
Figure 61. Cracking around initiation spot on CH-NSP
@200 MPa, Specimen 043
Figure 62. Crack initiation on CH-NSP @725 MPa,
Specimen 045
Figure 63. Brittle crack propagation on CH-NSP @725
MPa, Specimen 045
Figure 64. Major rack initiation on TH-SP @600 MPa,
Specimen 010
Figure 65. Crack propagation surface on TH-SP @600 MPa,
Specimen 010
49
Next, pictures for CH-SP are shown. Several cracks meet the fracture plain near the surface. The crack
propagation is intergranular, a possible sign of brittle fracture.
Figure 66. Several crack initiations on TH-SP @750 MPa,
Specimen 034
Figure 67. Major crack initiation and sub-cracking on TH-SP
@750 MPa, Specimen 034
Figure 68. Crack connecting residual fracture on TH-SP
@750 MPa, Specimen 034
Figure 69. Crack propagation surface TH-SP @750 MPa,
Specimen 034
Figure 70. Crack propagation at 3mm depth from edge on
CH-SP @550 MPa, Specimen 018
Figure 71. Intergranular crack propagation at 4mm from
edge on CH-SP @550 MPa, Specimen 018
50
Figure 72. Brittle fracture surface on CH-SP @550 MPa,
Specimen 018
Figure 73. Corrosion products at fracture surface on CH-SP
@550 MPa, Specimen 018
Figure 74. Main crack initiation and crack meeting facture
surface on CH-SP @550 MPa, Specimen 018
Figure 75. Main crack initiation and sub-cracking on CH-SP
@550 MPa, Specimen 018
Figure 76. Main crack initiation on CH-SP @725 MPa,
Specimen 082
Figure 77. Intergranular crack propagation on CH-SP
@725 MPa, Specimen 082
51
5.3.2. Cracking Analysis from Optical Microscope Pictures of samples from the specimens in Table 2 are presented in this section.
CH-SP in fresh water
Specimen 091 – 800 MPa @ 1000 rpm
Many micro cracks at the surface, presenting little branching. Corrosion pits are formed at the crack
base.
Figure 78. Specimen 091. A) Overall surface view, B) General crack length, C) Cracks showing almost no branching, D) Crack not following the microstructure
Specimen 047 – 500 MPa @ 1000 rpm
Many cracks at the surface with profuse branching. Corrosion pit formed at the crack base.
A B
C D
A B
52
Figure 79. Specimen 047. A) Overall surface view, B) General crack length, C) Cracks showing much branching, D) Abundant branching of cracks
CH-NSP in fresh water
Specimen 045 – 725 MPa @ 1000 rpm
Tiny cracks at the surface of the specimen, growing through the decarburised layer, as can be seen in
C. Eventually one or more of those cracks propagate to become catastrophic, as seen in C. The crack
in C was not the cause of final fracture, but a similar one made the specimen fail.
Figure 80. Specimen 045. A) Overall surface view, B) Tiny cracks at the decarburised layer, C) Miniature cracks at the surface and one major crack growing into the material
C D
A B
C
53
Specimen 095 – 200 MPa @ 1000 rpm
Virtually no cracks form at the surface, only slight cracking at the decarburised layer, as seen in A. A
few cracks will nucleate and grow, as seen in B. This specimen survived.
Figure 81. Specimen 095. A) Overall surface view, B) Almost no cracks at the surface, except for a few major cracks that do not propagate to cause failure before 3X10^6 cycles
TH-SP in fresh water
Specimen 012 – 800 MPa @ 1000 rpm
Several cracks form on the surface, and show little or no branching. No corrosion pit at the crack bases.
Figure 82. Specimen 012. A) Overall surface view, B) General crack length, C) Cracks showing almost no branching, D) Cracks magnification on etched sample
A B
A B
C D
54
Specimen 013 – 550 MPa @ 1000 rpm
Plenty of cracks form at the surface, and most of them branch out. Little crack deep propagation was
observed. This specimen survived.
Figure 83. Specimen 013. A) Overall surface view, B) General crack length, C) Cracks showing branching, D) Cracks magnification on etched sample
TH-NSP in fresh water
Specimen 036 – 680 MPa @ 500
A few cracks present on the surface, and several of them seem to be able to propagate simultaneously.
The cracks in B grew very deep into the specimen, but another one was the cause of failure.
Figure 84. Specimen 036. A) Overall, B) Big crack propagation
A B
A B
C D
55
Specimen 016 – 200 MPa @ 1000 rpm
At this low load, the surface is almost free of cracks, and only a few will nucleate an grow, in some
cases deep enough to cause failure. A corrosion pit can be seen at the base of the cracks.
Figure 85. Specimen 016. A) Overall, B) Few large cracks can cause failure
CH-SP in brine (4.0 wt% NaCl)
When running in brine, the crack branching seems to be eaten up by the more aggressive environment.
Specimen 028 – 650 MPa @ 500 rpm
Figure 86. Specimen 029. A) Overall cracking on the surface, B) Grown crack
Specimen 032 – 650 MPa @ 2300 rpm
A B
A B
A B
56
Figure 87. Specimen 029. A) Overall surface cracking, B) Crack branching is eaten up by salty water, C) General crack length, D) Few larger cracks that eventually propagate deep enough to cause failure
C D
57
6. Discussion This section is divided in three parts. First, a discussion about the frequency dependence results and
the testing parameters selection is presented. Then, the validity of the proposed testing method, as
well as of the parameters chosen, is evaluated by discussing results obtained through its application.
Finally, the fracture surfaces and micrographs are examined to discuss the apparent cause of the
observed failure behaviour.
6.1. Discussion of Method and Parameters Before the present work, the company compared materials in terms of fatigue only by means of the
fatigue strength calculated with the staircase method. No study about finite life at higher stresses was
performed, and the construction of a Wöhler or S-N curve was not made. The comparison of fatigue
results between dry and corrosive environments might lack of valuable insight apart from the
observable decrease in fatigue resistance that the aqueous media causes. Therefore, the use of fatigue
results at stress levels above the corrosion fatigue strength was proposed in the present work. This
way, the lives of materials under corrosion fatigue could be compared with the ones in inert
environments at high stresses. However, this implicates more extensive tests, which might come along
with increased testing times and costs. For that reason, the Median S-N Test Method for Small Sample
Size is proposed; since it does not require a large number of tests to build a valid S-N curve and to find
the fatigue strength, a decent number of tests can be carried out in a short time and with a relatively
low budget.
Regarding the selection of the water characteristics, a recent analysis of the water used with the
company’s equipment in several mines around the world was taken into account to select the
parameters of the testing method. Most of the parameters were kept as unaffected as possible –
meaning that the water was used as it was taken from the tap- to keep the characteristics closest to
the product application. The water flushed while drilling usually does not contain any salt, its
temperature ranges around 20 °C, and the pH is generally kept around neutral values. In some rare
cases, the water contains extreme levels of salt contaminants, but since that goes out of what is
considered “normal” for the application, it was not further studied for the method besides the four
tests ran for the frequency dependence analysis shown in Figure 34. Other reasons to not continue
testing on salty water involve the harm that the testing machines take, and the lack of simplicity in the
test preparation. It takes longer time to prepare the water before every test if a precise measure of
the salt content is desired. The oxygen level was not controlled in detail, but it was decided to be kept
as highest as possible by constant stirring of the water. This way, the method stays simple and with
consistent parameters.
As for the selection of a suitable testing speed, the frequency dependence on CH-SP is discussed. From
Figure 34, it can be observed that corrosion fatigue in fresh water is markedly frequency dependent,
whereas the life of the specimens tested in brine tends to stay almost constant along the tested
frequency range. Looking at the results for fresh water conditions in Figure 41, one can appreciate how
the number of cycles reached at 2300 rpm averages infinite life. Although the applied load is lower
than the dry fatigue limit, two of the three specimens reached more than 3X10^6 cycles, and the
average life is similar to the one achieved without corrosion, which means that this speed would not
be good for the new corrosion-fatigue testing method. The difference compared to dry fatigue
becomes larger as the testing speed decreases. Also the spread of the data is smaller for lower speeds
and largest at 2300 rpm. Note that the trend shows how the life tends to a value of zero at a speed of
0 rpm, which would mean that the specimen would not rotate, and the failure phenomenon would be
stress corrosion cracking. On the other hand, the values for brine show that life that tends to stay
almost constant regardless of the testing speed. This means that corrosion fatigue for this material in
58
salty environments is virtually frequency independent within the tested range. Since a life tending to
0 rpm speed is also expected for this condition, a strong frequency dependence would be expected in
the speed range of 0 – 500 rpm.
In Figure 35, a fraction of the time of each test was occupied by crack initiation, and the rest by
propagation. In the present work, no effort was made to identify these two separate cracking steps,
and thus these fractions were not studied. However, it is known that time plays an important role on
corrosion-fatigue cracking, and that both crack initiation and propagation are partly time dependent
and partly cycles dependent, as proven by Rollins et.al ([16]) and discussed nu Jonsson ([15]).
Therefore, the fraction of time spent on crack initiation is not expected to be the same for all test
speeds.
What is desired from the proposed testing method is that a noticeable difference between different
materials and conditions can be achieved. Also, that the testing time is not excessive. From the plot in
Figure 41 it can be seen that the lower the frequency, the larger the difference is with respect to dry
fatigue results, which is desired in the new method. However, the lower the testing frequency, the
more time the test takes. For instance, if a test is ran till runout (at 3X106 cycles), it will take double
the time at 500 rpm than at 1000 rpm. Therefore, since a marked difference can be observed already
at 1000 rpm, this frequency was chosen for the new corrosion-fatigue testing method. Thus, the main
reasons for this decision are:
Frequency independence on dry testing was proven at 1000 rpm
Significant difference in fatigue life exists between wet and dry conditions at this frequency
Testing time is not excessive
Good number of tests can be carried, resulting in solid data generation
Data spread at this frequency is not large
The machine works fine (avoids natural frequencies) at chosen frequency, resulting in reliable
results and lower maintenance
For the staircase method, the load increment used corresponded to 50 MPa. The reason for this is that
larger load steps result in a fewer number of tests required to find a limit or strength value, reducing
the testing time and making the process cheaper. Also, it was chosen since it is the value used on the
dry fatigue tests already performed by the company, and its validity has been proven. Furthermore,
this value lays between half and twice the standard deviation of the fatigue strength of each of the
material conditions, as the method indicates it should be (see section 2.9.2).
It was desired to prove the effect of the load on the frquency dependence response of the materials.
From Figure 39 it is clear that the load has a strong effect on the frequency dependence response on
CH-SP, while Figure 40 shows that such a load increment does not affect the frequency dependence
response significantly. Perhaps the difference is an effect of the shot peening or the lack of it. However,
it must be noted that the results from loading with the dry limits were obtained by testing only one
specimen at each speed. Thus, the results only show a rough estimation on how the general trend at
this load lever would look like.
The proposed method was tested, and valuable results were obtained, proving that the method works
fine as designed in the present work. However, it is worth mentioning that, although the Median S-N
Test Method for Small Sample Size suggests four load levels with two tests on each load level for the
S-N curve, more than two tests were performed at some load levels and in some cases more than four
levels were tested for generating the S-N curves. The same happened for the staircase method tests,
where more than the suggested number of tests were performed in some cases. The reason for this is
59
that more reliable results can be obtained with a greater sample size, and enough time and specimens
were available during the project to carry out more tests. In the case of the staircase method
conducted for CH-SP (see Figure 43), it was necessary to conduct more tests than planned, since the
spread in the data was rather large.
6.2. Discussion of Testing Results The tests were performed following the methodology of the proposed method. The sequential order
of the tests was decided using the schema in Figure 21. The values of the fatigue life reached by every
material condition were presented in section 5.2.1, and a description of the testing procedure was
presented in section 4.2. As mentioned there, the load level for the first test of the S-N curve
corresponded to the dry fatigue limit of each material condition, stepping down and upwards for the
other levels. Nevertheless, for CH-NSP and TH-NSP, a load higher than their corresponding dry fatigue
limits were tested since the life reached at the first test was already too low (see Figure 44 and Figure
48).
A fifth load level was included in the S-N curve of CH-NSP (Figure 44) since the points on the top level
of the staircase (Figure 45) were incorporated as data in the finite life region. For Conditions 1 and 3,
a greater amount of tests in the staircase were required (Figure 43 and Figure 47) since the spread
around the strength was large, and even four stress levels had to be tested to find the corrosion fatigue
strength for CH-SP. On the other hand, the staircase method for TH-NSP (Figure 49) was applied using
only the suggested number of tests since the trend was stable enough, compared to the previous
conditions.
For TH-SP only two tests per load level were performed, and the results show an even spread along
the mean life in plot with a logarithmic x-axis. The staircase method, shown in Figure 47, shows a fairly
good distribution of the data around the fatigue strength. This value is substantially higher than the
one found for the other shot peened condition, with a difference of 75 MPa.
For the TH-NSP, the life at high stress is slightly higher than the CH-NSP. However, its corrosion fatigue
strength also drops enormously compared to its shot peened version, reaching a value of 191.6 MPa,
which is even lower than the other non-shot peened condition. The spread of the values conforming
the finite life is narrow in almost every stress level, becoming a bit bigger at 250 MPa, where the stress
is closer to the strength value. Yet, the spread is smaller than on the other material conditions.
Concerning the comparison shown in Figure 50, the first thing that can be observed is the large gap
between the shot peened and the non-shot peened versions of the steel grades. The gap between TH-
SP and TH-NSP is 408.4 MPa, and between CH-SP and CH-NSP is 316.7 MPa. It is also noticeable that
at high loads (around 800 MPa), the difference in expected life between shot peened and non-shot
peened conditions is around 1.7X10^5 cycles. It can be seen how both shot peened and non-shot
peened versions cross each other at certain points. For the case of the shot peened conditions, TH-SP
has a lower life expectancy than CH-SP at high stresses. However, as the load decreases, the expected
lives for these conditions get closer together and intersect at around 640 MPa and 1.2X10^6 cycles.
Finally, the corrosion-fatigue strength of TH-SP was found to be higher than the one of CH-SP. A similar
behaviour can be observed for the non-shot peened versions. In this case, both material conditions
have a close life expectancy of around 4X10^4 – 5X10^4 cycles at 725 – 750 MPa. As the load is
reduced, CH-NSP life starts decreasing more drastically than for TH-NSP. Then it evens out, intersecting
the other condition at around 7X10^5 cycles and 300 MPa, to end up having a slightly higher corrosion-
fatigue strength.
60
The comparison between fatigue in dry and wet environments (Figure 52) shows a noticeable
reduction in fatigue strength in wet environments, even for the shot peened condition. The dry-fatigue
strength (shot peened) is 725 MPa, whereas its corrosive equivalent is only 525 MPa. It should be
mentioned that, for the shot peened version under corrosion fatigue, a few new data points appear in
this chart. The reason why they were not presented before is that they correspond to the data used
for the staircase, and the proposed method establishes this separation (see Figure 21). Regarding
Figure 53, the drop of fatigue strength is quite large when exposing the non-shot peened version to
wet environments, going from 745 MPa in dry conditions down to 191.6 MPa in corrosive media. This
plot also shows data that is not contained in the previous graphs, and the reason is the same as for the
other steel grade. Note how, in both cases, the dry results show a spread in the data that is
considerably larger than the one found on the corrosion-fatigue tests. For some stress levels, the data
points are quite far away from each other. This can be explained with the fact that the mechanisms of
crack initiation might be different in corrosion fatigue and in dry fatigue. While the time to initiate a
crack in dry conditions is variable due to inclusions and surface defects, the corrosive environment will
generally enhance the nucleation of a crack within a short period of time. It is also important to
consider the fact that the dry data is very close to the fatigue strength, which generally causes a large
spread between runouts and failures, a behaviour also observed in some of the corrosion-fatigue tests.
6.3. Fracture Surface and Cracking Characteristics in Relation with the Results The post-testing analysis of the fracture surfaces by the use of SEM, presented in 5.3.1, was performed
as an attempt to observe the crack initiation sites and identify the characteristics of the crack
propagation process. This task was challenging since the rotating-bending test damages the fracture
surfaces in the process, and usually the violent final fracture of the specimen results in scratched
surfaces which are practically unusable for the analysis. Nevertheless, a few specimens were collected
and successfully observed in the SEM. An important note is that all the cracks were found initiated at
the surface of the material, in accordance with the theory presented in the previous parts of this work.
In the case of TH-NSP at 200 MPa, shown in Figure 54 and Figure 55, only one crack was found initiated
on the fracture surface, which was the cause of failure. It should be remarked that at this low stress
level, some specimens of this material condition survived and some others –like the one analysed in
SEM- failed. Some corrosion attack can be observed where the crack meets the surface of the
specimen. As discussed in the literature review in this document, it is not feasible to conclude if the
crack initiated from the corrosion pit, or if the latter was formed after the crack nucleated. Some tiny
cracks can be seen on the surface in Figure 55, and also what seems to be the formation of thin flakes.
Propagation is not easily appreciated since the surface appears very flat, perhaps because the crack
closure during repetitive cycling had deformed the crack surface. For the same material condition at
744 MPa, presented in Figure 56 to Figure 59, several crack initiations were found on the fracture
surface. One major crack was leading the propagation, and the majority of the fracture surface is found
on the plane of this crack; however, when the cracking process had advanced enough, the fracture
surface was pulled on diverse directions by the other simultaneously growing cracks, since the energy
to fracture the material in those directions was lower than the one required to continue propagating
in the main plane.
As for the CH-NSP at 200 MPa, shown in Figure 60 and Figure 61, the fracture surface also showed one
single crack initiation, with non-metallic inclusion containing Ca, Al and Si sulphides found at this site.
Perhaps this was the cause of the crack nucleation in this case. Figure 62 and Figure 63 show the same
material condition at 725 MPa. The crack initiation looks severely damaged, perhaps by corrosion, and
it could be due to the decarburised layer material being released. The crack propagation seen in Figure
63 is totally intergranular, which indicates brittle fracture that could be caused by hydrogen
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embrittlement. Apparently the high stress state enhances the brittle propagation since the plastic
strain is higher at the crack tip, allowing more hydrogen absorption into the steel.
The TH-SP at 600 MPa presented a ductile crack propagation surface, observable in Figure 65. It was
difficult to identify if the crack initiation occurred at the surface of the specimen, as seen in Figure 64,
but it seems to be that the propagation lines flow already from the edge of the fracture surface
inwards. A few corrosion pits are also appreciable along the edge. Again, it cannot be defined if the
corrosion pit is the cause of the crack, or if it is the other way around. The same material condition
stressed at 750 MPa, on the other hand, presented many simultaneous crack initiations on the fracture
surface, as seen in Figure 66. For that reason, the fracture surface was shaped on different planes.
Some cracks that initiated on planes further away from the predominant ones were reached by the
shear lip of the residual fracture, as seen in Figure 68. From Figure 67, it is visible that the major crack
initiation presented several cracks around it, perhaps initiating from the fracture plane or possibly
initiated elsewhere and intersecting it.
In the case of CH-SP at 550 MPa, two different fracture surface characteristics were observed at
different depths, as shown in Figure 70 and Figure 71. At 3 mm, the fracture seems more transgranular
than at 4 mm, where the propagation looks mainly intergranular. This could be a cause of the crack
growth rate, as discussed by Ritchie [8], who stated that when the cyclic plastic-zone size –which
increases as the crack grows- is near the grain size, the facets on the surface will form. The crack growth
rate is unknown in this study, and therefore this statement is not conclusive. Nevertheless, the
intergranular crack propagation is an indicator brittle fracture, possibly caused by the hydrogen-
embrittlement mechanism. Some corrosion products were found on the fracture surface, as seen in
Figure 73. These could have formed during or after the test, but they seem to be occupying the place
of a rounded body, possibly an inclusion that was dissolved by the water. The crack initiation happened
also on the surface of the specimen boosted by the corrosive media, as shown in Figure 74. Several
cracks were found on the fracture surface surrounding the crack, and equal to the case of TH-SP, these
cracks could have either intersected the fracture surface or grown from it. For the same material
condition at 725 MPa, multiple crack initiation sites were found around the fracture edge, and the
crack propagation was also intergranular, as Figure 77 shows.
The purpose of analysing the cracks on a cross section of the different material conditions, presented
in section 5.3.2, was to relate the variables of each condition to its cracking characteristics. This study
provided useful information in terms of the ease of crack propagation mainly as a function of the
residual stresses present on the materials. It is important to remember that each material condition
presented different residual stress scenarios. CH-NSP has a relatively low compressive residual stress
due to the carburisation, which reaches -150 MPa in the first 0.02 mm of depth and stays negative
further in. On the other hand, TH-NSP has a negative residual stress on the very surface, but quickly
reaches a value of zero at 0.05 mm of depth. TH-SP has a highest compressive stress of around -800
MPa at approximately 0.2 mm of depth, being considerably high already at 0.1 mm with a value of
around -750 MPa, resulting in a thick layer of compressive stresses below the surface. CH-SP reached
a highest compressive stress of around -900 MPa at 0.07 mm, staying high on -840 MPa at 0.14 mm
and then abruptly decreasing. It can be highlighted how the compressive stress level in both materials
reaches a similar value, but the depth of penetration is higher –approximately double- on the TH-SP.
The reason for this could be that the surface hardness of CH is higher than the one of TH, restricting
the compressive stress from going deep into the material. However, this hypothesis is not conclusive,
since there is a considerably big gap between the measurement at 0.14 mm and the measurement at
0.48 mm, and no data about the compressive stress between these two points is existent. Although
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the trend seems to go up from the 3rd to the 4th measurement depth, it is not valid to conclude what
happens at the gap.
In general terms, the micrographs of shot peened specimens showed abundant cracking at the surface,
growing in to reach a certain common depth, before being arrested by the compressive stresses. As
this happens, the cracks start forming branches and spreading laterally instead of propagating straight
forward radially. Observe for instance the cracks for CH-SP at 500 MPa showed in Figure 79. The cracks
grow to a general depth of around 72.4 µm, branching out from around half of their length. The
explanation for this phenomenon has not been totally clarified in the present work, but one feasible
explanation is that the energy to initiate a crack is lower than the one required to propagate it, and as
the cracks are stopped by the compressive stress layer, many cracks nucleate from the initial cracks,
fanning out simultaneously. It can be seen from Figure 78-D that the branches do not follow the
microstructure. A general behaviour observed in these cases is that, for lower loads, the cracks branch
out more than for higher loads. Compare for example CH-SP at 800 MPa and 500 MPa, shown in Figure
78 and Figure 79 respectively. In both cases the cracks reach approximately the same general depth,
but at higher load, the cracks have a leading branch with sharp tip that pass the compressive stress
barrier and cause failure. This difference is more evident for TH-SP, presented in Figure 82 and Figure
83. The reason for the greater branching on lower stresses could be then that the specimen runs for
longer time, allowing for more branches to form, without having enough energy to overcome the
compressive stress layer. Additionally, at lower loads, corrosion pits can be seen at the base of the
cracks, also explained by the longer run time allowing more exposure to the corrosive media. It is
important to mention that the compressive stress around the crack and between the branches is lost,
otherwise the branches would close as a result of the compression. It is also suggested that the
compressive stress in the material between the cracks is decreased due to the same effect. However,
compressive stresses are kept ahead of the cracks front, preventing the cracks to grow forward. The
residual stresses were not measured on the specimens post-test, so there are no values to show the
extent of this fact.
The shot peening on both conditions arrested the growing cracks at a depth that appears to match the
peak of compressive stresses, or close to it. For CH-SP, this value averages 73 µm, for which the highest
compressive stress is around 75 µm. For TH-SP, the range is larger since the compressive stress layer
is thicker, but already reaches high values at about 100 µm, so the cracks are considerably stopped
around that depth. However, the crack front profile is more irregular on this material for this reason,
as shown in Figure 82. This information can be used to propose an explanation for the stress-life
behaviour seen in the S-N curve in Figure 51. First, it is observed that TH-SP has a higher corrosion-
fatigue strength, and its life at low stress levels will be longer, compared to CH-SP. This could be
because it results more difficult for the cracks to pass a thicker compressive stress layer. Therefore,
TH-SP can hold larger cracks than CH-SP. On the other hand, TH-SP has a lower life at higher stress
levels. The reason for this could be that, in both cases the cracks eventually cross the compressive
stress layer, and this probably takes longer time in TH-SP since the layer is thicker, but once they grow
further in, the propagation is slower in CH-SP since the material core is tougher than in TH-SP.
For the case of the non-shot peened versions, the higher the stress levels, the more cracks that
nucleate on the surface of the specimen, as seen for example in TH-NSP at 500 MPa and 200 MPa,
presented in Figure 84 and Figure 85, respectively. At high loads, many small cracks were found
growing radially in parallel, and some larger cracks were already rather advanced into the material
body. At the end, the final fracture surface united several predominant cracks, as discussed in the SEM
analysis. Oppositely, at low stresses, only a small number of cracks will nucleate, and as the load
approaches the fatigue strength, these cracks might not grow large enough to cause failure before
63
3X10^6 cycles are reached. It was observed that, for low load cases, CH-NSP presented more cracks
than TH-NSP. The reason for this is that, unlike TH-NSP, the compressive stresses from the carburised
layer on CH-NSP stop the cracks from propagating, allowing for more cracks to form without becoming
critical. Additionally, a corrosion pit is observed at low stress levels in both steels, since the tests run
for longer times, allowing more attack from the environment. In the case of CH-NSP at high stresses,
the microcracks grow through the decarburised layer, and seem to follow the weaker phases, as seen
in Figure 80-B. Same as for TH-NSP, a few predominant cracks grow and cause failure. At low loads,
the surface is practically free of critical cracks, and only a few nucleate after many cycles and grow
slowly due to the low stress level, as seen in Figure 81-B.
The addition of 4 wt% NaCl in the water for the tests in CH-SP resulted in micrographs similar to the
ones from the fresh water tests in the same material condition. Yet, a few differences can be observed.
See Figure 86 and Figure 87. Both show specimens tested at 650 MPa, a relatively medium stress for
this material condition, but at different test speeds (500 rpm and 2300 rpm). The surface is also
covered with branched-out cracks partially arrested by the compressive stress. However, the
branching is not clearly defined, but rather eaten up by the extremely aggressive media, as seen in
Figure 87-B. Some cracks appreciably crossed through the compressive stress layer evidently easier
than in fresh water. It is not possible to know to which extent the salty water affected the cracking
process in terms of initiation and propagation, since this information was not generated for any of the
situations. Nonetheless, it can be supposed that the aggressiveness of the salty water caused a rather
similar crack initiation time regardless of the testing speed, and that the propagation was dominated
by the cycles. Then, from Figure 35, it can be deduced that the length of the bands represent the time
that the propagation takes at each speed. At 500 rpm, the time is long since less revolutions happen
on a given period of time, compared to the opposite extreme, for which the time is shorter since more
load cycles are covered in the same amount of time. This explanation encloses the general assumed
behaviours, but as detailed by Jonsson ([15]), in reality both initiation and propagation stages have
time-dependence and a cycles-dependence components.
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7. Conclusions This work investigated the main variables affecting the corrosion fatigue on TH and CH, and offered
parameters for a corrosion-fatigue testing method hereon proposed. Rotating-bending machines were
used, resulting in a fully-reversed sinusoidal stress wave. Frequency dependence tests were performed
to study the response of corrosion-fatigue life as a function of the testing speed, in both fresh water
and brine (4.0 wt% NaCl water). The testing method was used on shot peened and non-shot peened
specimens of both steel grades, and its validity was proven. The following conclusions were found in
relation to the objectives of this project:
1. The Median S-N Test Method for Small Sample Size is proposed as a suitable method for
testing corrosion fatigue in martensitic steel grades with different heat and surface
treatments, since it requires a relatively small sample size, resulting in relatively cheap studies
with low testing times. However, a slightly greater sample size is was used in the present work
since the number of tests suggested by the method could not be used to build concrete
conclusions. Thus, whenever time and resources are available, a greater sample size should
be used to generate more reliable results.
2. The generation of S-N curves allows the study of different behaviours of the materials and
their comparison at a broader stress range, opposite to the comparison of the fatigue
strengths alone. In the present work, TH-SP was found to have a higher corrosion-fatigue
strength than CH-SP, although its expected life is lower at higher loads.
3. A testing speed of 1000 rpm is the best option from all the four speeds tested. The reason for
this is that it results in a marked difference between the dry and the wet fatigue results, which
is desired for the proposed testing method. The lower the testing speed, the bigger the
difference is. However, as the speed is decreased, the testing time increases. The proposed
testing speed results in a decent balance between testing time and comparison capabilities.
4. The addition of salt in the water for testing is discarded since it is not a common characteristic
in real rock-drilling applications, and the damage mechanisms are different to the ones caused
by fresh water. Additionally, the corrosive attack was observed to also affect the test
machines, which is not desired by any means.
5. The compressive residual stresses are extremely beneficial for the corrosion-fatigue
performance of steel since it arrests the cracks and stops them from growing. Shot peening
was found to enhance the resistance of the materials against cracking. A great amount of
cracks nucleate on the surface and start growing up to a certain general distance close to the
peak of the compressive stress layer. Moreover, the thicker this layer is, the more resistance
it builds for the cracks to propagate.
6. The crack growth rate appeared to be smaller in the tough core of CH than in the hard core of
TH. In general, it is well accepted that the higher the toughness of a material, the better fatigue
properties it has.
The present study provided a useful method to test corrosion fatigue on steel grades used in rock-
drilling applications, and generated valuable information about the behaviour of TH and CH. The
mechanisms of cracking and failure were discussed and provided explanations for the results obtained
from the tests. Nevertheless, it is worth to mention that the phenomenon of corrosion fatigue is not
yet fully understood. Considerable discrepancy is occasionally found in literature regarding the failure
mechanisms of different materials at different conditions. Additional data generation by testing
specific desired conditions results of high importance for R&D activities.
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8. Further Work This section presents several recommendations for future work related to the present study.
Conduct a study to identify the fraction of total life spent in crack initiation and in crack
propagation. Since the tests are performed with rotating-bending machines, the use of 2-stage
testing method is suggested.
Generate S-N curves for fatigue at dry conditions to be able to compare the behavior of the
materials at stresses higher than the fatigue limit.
Apply the proposed testing method to generate data for different combinations of materials
and treatments. For instance, test CH steel grade without case hardening treatment, but only
with shot peening. If the material performs well, the company could consider removing the
heat treatment process for this steel grade.
Evaluate of the effect of different shot peening parameters on the performance of steels
subjected to corrosion fatigue. Changing the shot peening intensity could result in different
compressive stress profiles, improving the performance of the steel at corrosion fatigue.
Address the problem of the curved specimens after shot peening. The residual stresses from
the shot peening usually cause a slight curvature rod-like components. For this reason, the
shot peened products generally require a straightening process. The same thing could apply
for the testing specimens, which also suffer from this post-peening deformation.
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