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A Brittle Fracture-Based Experimental Methodology For Ductile Damage Analysis
MT07.39
Mickaël Pradelle
Student: Mickaël A.P. Pradelle
Supervisors: C.Cem Tasan, J.P.M.Hoefnagels, M.G.D.Geers
Department of Mechanical Engineering, TU/e, Eindhoven, the Netherlands
Department of Materials, Polytech’Grenoble, Grenoble, France
Period from the 2/05/07 to the 22/08/07
Sciences et
Génie des
Matériaux
Sciences et
Génie des
Matériaux
1
ABSTRACT
One of the great motivations for research in the automotive industry is the weight
reduction of the components. Several strategies were studied to reach this objective. The
replacement of the conventional steels (HSLA steel) by Dual-Phase steels promises good
results in terms of weight reduction and strength/weight ratio. However, the increase in
specific strength is done in expense of ductility, and unexpected ruptures were observed
during working operations of these new steels. It’s believed that these ruptures depend on the
progressive damage evolution in these materials. The main goals of the project “Forming the
limits of damages prediction” which is carried out in TU/e are to understand the physical and
mechanical mechanisms responsible of the evolution in ductile damages and to develop new
experimental methods that are able to provide the necessary parameters for the modeling
efforts. In this paper a new experimental method is proposed to observe the progression of the
damage inside some sheet of steels. A brittle fracture is done in the length of our samples to
characterize the ductile fracture due to tensile tests. The goal of this work is to determine the
evolution of voids, and their volume, generated during a deformation in a Dual-Phase steel
(DP600) and in an Interstitial Free (IF) steel using a new method that involves the use of
experimental tools such as SEM, confocal microscope and a new device adapted for the
Charpy impact test machine.
2
CONTENTS
Introduction p.3
Materials and Experimental Methodology p.4
Results p.10
Discussion p.14
Conclusions p.15
Recommendation p.16
Acknowledgments p.17
Appendix A p.18
Appendix B p.23
References p.25
3
Introduction:
The recent decades witnessed the introduction of materials with higher specific strength
(such as dual phase (DP) or transformation-induced plasticity (TRIP) steels) into the
automotive industry. In despite of their higher strength, these materials are also susceptible to
develop unexpected failures during forming operations, mainly due to the microvoid
evolution as they are being deformed. This weakness requires a better understanding of
physical mechanisms of ductile damage evolution to obtain numerical tools with higher
predictive capabilities.
Observation of ductile damage mechanisms is an experimentally difficult task due to the
small scale in which the process takes place. Nevertheless, there are several different methods
that are used for this purpose. Lemaitre classified these methods into two groups: direct and
indirect methods [1]. Direct methods such as micro-tomography and electron microscopy
allow the visualization of damage. Whereas indirect methods aim to determine a damage
variable through the measurement of change in young’s modulus, hardness, electrical
potential or other physical properties of matter.
The most commonly used direct method is electron microscopy due to its ease of specimen
preparation and observation capabilities. However, the specimen preparation methods for this
type of analysis may also cause some unwanted artifacts. For example, a detailed examination
of microvoids requires the use of consecutive grinding and polishing steps which may cause a
smearing effect even if the final polishing step is as fine as 1mu. This is due to the mechanical
character of this type of polishing procedure. An example of such a smearing effect is shown
in figure 1.
Figure 1: SEM picture of a smeared surface
To overcome this smearing effect, an electrochemical polishing is commonly used.
Electropolishing removes mechanically deformed layer on the surface of the material
revealing out the microvoids successfully. However, this method also has its limitations. One
of these limitations is the fact that there is an almost unavoidable edge rounding effect. More
importantly, the size of the microvoids may be exaggerated due to the material removal
around the void (Figures 2a, 2b).
4
Figure 2a: SEM image of an Figure 2b: SEM picture of
electropolished (smooth) surface an exaggerated void
One of the suggested ideas to overcome these problems is to develop a methodology that
does not involve any plastic deformation in examination of the voids [2]. This can be
achieved through brittle fracture of deformed specimens into two parts. Shi et.al. in their work
compared many of the possible specimen preparation methods for SEM evaluation and
concluded that this method was the most suitable. However, they do not present detailed
examples of void morphologies obtained by the use of this method. The critical point of such
a strategy is to differentiate between the deformation due to the tension test and that due to the
brittle fracture itself.
In this work, a microscopical and topographical characterization of brittle fracture surfaces
of IF and DP600 steels are carried out for the evaluation of ductile damage evolution in these
materials. In the following section the details of the set-up and the materials are explained.
Afterwards, obtained results are analyzed and discussed. Some fundamental information
about brittle and ductile fracture and fracture mechanisms of IF and DP steels are provided in
Appendix A.
Materials and Experimental Methodology:
I. Materials:
IF steel is ferritic steel with a very low carbon concentration. It is used for the body
structure of cars. The IF steel has no carbide precipitates at the grain boundaries. Its
microstructure is given in figure 3. It’s only composed of ferrite grains (their size is roughly
equal). It has second phase particles of AlO and TiN (example figure 4).
Figure 3: (SEM picture) Microstructure of IF steel
5
Figure 4: (SEM picture) Second phase particle in IF steel
DP600 steel is composed of two phases: martensite and ferrite. These type of steels are
produced by annealing at the austenite plus ferrite phase field, followed by cooling at a
sufficient rate to transform the optimum amount of austenite to martensite. The microstructure
is presented in figure 5. Figure 6 shows that a high concentration of martensite formation has
occurred in the center of the sheets.
Figure 5: (SEM picture) Microstructure of DP600 steel. The dark part is the ferrite and the brighter part the
martensite
Figure 6: (SEM picture) Concentration of martensite
The samples have a classic geometry. The specimen dimensions for both of these steel are
given in figure 7a and 7b consecutively.
6
Figure 7a: dimensions of IF steel specimen Figure 7b: dimensions of DP600 steel specimen
II. Experimental Methodology:
a) Tensile tests:
The objective of the tensile tests is to obtain the elastic and plastic properties of the steels of
interest. The tests are done with a tensile stage at a strain rate of 20 µm/s, in rolling direction.
b) The longitudinal brittle fracture:
The goal is to observe the voids caused by the tensile test inside the DP and IF steel
samples. For this objective, specimens have to be fractured in the longitudinal direction. The
goal is to observe only these types of voids, so the longitudinal fracture is required not to
create dislocation slip leading to plastic deformation. The idea is to develop a new adaptable
tool on the Charpy impact test machine. The Charpy machine is traditionally used for testing
of samples in the transverse direction, and requires no clamping. But for this project this setup
is modified, as it is necessary to cut the samples in the longitudinal direction (figure 8a, 8b).
Figure 8a: Direction of cutting after the tensile stage Figure 8b: Observation sense
� The design of the tool:
A massive block is designed to be fixed on the Charpy impact test machine (figure 9). On
the right side there are two cylinders to slip a "removable clamp". This component (figure10)
is divided into two parts. The first is inserted and maintained by the massive block thanks to
two holes for the cylinders. The other is to tighten the sample with the first by three screw.
Two pieces allow doing the same experiment for samples having various thicknesses. Each
piece of the clamp is serrated for better tightening.
With the values which are obtained by the tensile tests, it is possible to determine what
maximum weight our samples can bear without plastic deformation (calculation shown in
appendix B). Consequently, the idea is to develop a clamp which will be maintained in the air
only by tightening on the specimen. The "block in the air" is based on the same principle that
the mobile block: two grooved parts to tighten the sample by four screws (figure11).
7
Figure 9: Massive block for the Charpy machine
Figure 10: Schema of the “removable” clamp
8
Figure 11: Schema of the bloc just tightened on the sample
� Principle and explanations:
After the fracture by tensile test, the samples are classified and are grooved in their center to
facilitate the crack propagation without shear.
The strong block is fixed on the Charpy machine. A specimen is placed between the two
clamps then it is tightened.
The system {mobile clamp / sample / bloc "in the air"} is plunged in a container in which
there is liquid nitrogen. After 25 minutes the sample is cooled at a sufficiently low
temperature to obtain brittle fracture, the removable clamp is slipped quickly on the massive
block by the two cylinders. Once installed, the hammer is launched and hits the block in the
air to break the specimen into two parts.
The design of the complete system (without the sample) is represented by the figure 12. The
cross represents the impact point with the Charpy hammer. It is not located at the center of the
rectangular surface but it shifted towards the left to bring more energy during the break and to
avoid the bending of the sample.
9
Figure 12: Schema of the complete device for the Charpy impact test machine
c) Fractography :
For general characterization of fracture surfaces an optical microscope is used. For detailed
microstructural analysis the device which is used is a scanning electron microscope. The
observations are carried out using a 30kV electron beam in secondary and backscattering
electron modes.
d) Topography:
The confocal microscope is a measuring microscope for topographical analysis of various
different applications. It allows characterizing both very smooth and rough surfaces. The size
of the surface to be analyzed is determined by the objective used. For big surfaces, “extended
topography” mode is used: it breaks up the entire surface on several small surfaces. This
mode enables to join several topographies to represent the total surface.
The final representation is done thanks to the stitching method (assembly of images). First
the total depth of analysis is defined. Then the definition is regulated, i.e. the number of plans
which will be photographed. The software of the confocal microscope saves data files with
the values of the topography’s points. The values can be imported to Matlab or Excel. Figure
13 shows an example of a surface from the topography data to Matlab representation.
Figure 13: Final Matlab representation
of a rough tensile fracture surface
10
Results:
� Tensile tests:
Obtained tensile test results are given in figure 14. It is seen that IF steel is much more
formable than the DP600 with a maximum elongation of 41% versus 25%. Whereas the
DP600 steel, due to the martensite content in its microstructure, has higher yield (σe) and
tensile (σp) strengths of σe = 348.75 MPa and σp = 620.67 MPa compared to σe = 130.17
MPa and σp = 294.57 MPa of IF steel.
Figure 14: Curves obtained by tensile tests on IF steel and DP600 steel samples
� Longitudinal Brittle Fracture of Tensile Test Specimens
To see whether it is possible to obtain complete brittle fracture in both of these steels,
undeformed specimens are fractured in the Charpy impact test setup explained earlier. The
shiny microstructures obtained (figure 15a and figure 15b) show that brittle fracture could be
obtained successfully.
Tensile test on DP600 and IF steels
-100,00
0,00
100,00
200,00
300,00
400,00
500,00
600,00
700,00
-5,00 0,00 5,00 10,00 15,00 20,00 25,00 30,00 35,00 40,00 45,00
Strain (%)
Str
ess (
MP
a)
IF steel (2,8 mm^2)
DP600 steel (4 mm 2̂)
On the left, Figure 15a presents an optical microscope photography of the fracture surface after
the Charpy fracture. On the right, Figure 15b shows a SEM fractography of the shiny surface
which is a cleavage surface.
11
� Microstructural Characterization of Fracture Surfaces :
As explained in the experimental methodology part, the brittle fractured cross sections of IF
and DP600 steels are examined using a scanning electron microscope.
The micro examination of IF and DP600 steels revealed that:
- In different parts of the specimens both transgranular and intergranular fracture
morphologies are observed. However, for IF steel, the mechanism of brittle fracture is
generally in transgranular manner (figures 16 and 17) away from the neck, in the
undeformed part of tensile specimen.
Figure 16: SEM image of transgranular crack propagations
in the undeformed part of an IF steel sample.
Figure 17: SEM image of transgranular crack propagations
in the undeformed part of a DP600 steel sample.
Nevertheless, near to the neck of the tensile test specimens intergranular crack
propagation among the severely deformed and elongated grains is observed to be more
favourable (figure 18 and 19).
12
Figure 18: SEM image of intergranular crack propagations
in the neck region of an IF steel sample.
Figure 19: SEM image of intergranular crack propagations
in the neck region of an IF steel sample.
- A number of microvoids are also observed on the grain boundaries that separate the
brittlely fractured grains (figure 20 and 21).
Figure 20: SEM image of voids on the grain boundaries
in an IF steel specimen.
13
Figure 21: SEM image of voids on the grain boundaries
in a DP600 steel specimen.
- The other half of these microvoids could also be observed in the opposite part of the
specimen (figure 22).
Figure 22: Two SEM fractographies of the exact opposite
brittle fracture surfaces of an IF steel samples.
Open like a “book”.
14
� Topographical Analysis of Longitudinal Brittle Fracture Surfaces:
The first idea was to make a topographical representation of each opposite brittle fracture
surfaces from the profilometry data to a Matlab representation and superposition.
But the profilometer has some limits. The size of the voids is around 1 and 2 micrometers
what causes problems of resolution. Moreover, the light is scattered by the rough and irregular
sides of the voids. Consequently, the laser light which allows drawing the surface gives
unreliable analysis in depth. Figure 23 is a SEM photography of the neck region of a DP600
steel sample after the tensile fracture. Figures 24 and 25 show two topographies of this
fractography with different enlargements. Black parts are dispersed on the topographical
surface. These last represent the lack of data for the topography.
Figure 23 Figure 24: Topography 20X of enlargement
Figure 25: Topography 50X of enlargement
Discussion:
An inter and transgranular crack propagation modes are observed during the longitudinal
brittle fracture. IF steel has a quasi total intergranular fracture in the neck and only
transgranular away from the neck region. DP600 steel, also, has inter and transgranular modes
of crack propagation in the neck and only transgranular in the undeformed part.
Two sides of voids are found on the grain boundaries. These cannot be due to the brittle
fracture. This may provide to be a nice way to observe void growth. Figure 26 shows a
smooth surface and some voids along the grain boundaries after an electropolishing. The
figure 27 shows voids after the brittle longitudinal fracture. This last reveals that voids follow
the grain boundaries too and presents the mechanisms of the brittle fracture and the damage
due to the tensile deformation.
15
Figure 26: SEM image of an IF specimen after
Mechanical and electrochemical
polishing preparation.
Figure 27: SEM image of an IF specimen after
a longitudinal brittle fracture preparation.
Conclusions:
• A new method to characterize ductile damage is tested which produced promising
results. Longitudinal brittle fracture, of IF and DP600 steel samples, is done to
characterize the ductile fracture due to tensile tests. Thanks to a SEM analysis, a
clear observation of voids caused by a deformation is realized and it proves that the
longitudinal brittle fracture is achieved.
• Inter and transgranular propagation modes are observed together in the brittle
fracture. Their evolution with the deformation in the material can be studied.
• A topographical representation of brittle fracture surfaces is limited by the scattering
of the light in deep voids. These last are very small compared to the roughness of the
surfaces.
16
Recommendations:
A hypothesis could be submitted: the deformation decreases the interaction force of grain
boundaries so intergranular crack propagation is favorised during brittle fracture giving the
possibility to observe the voids situated along the grain boundaries.
To check it, the Nano-Identation would be used to determine the evolution of the hardness
at the grain boundaries and the tomography to determine the volume of voids.
The future works would allow developing a model of the evolution of the voids in function
of the stress (and the strain) and checking if the crack propagation modes depend of the
deformation.
17
Acknowledgments
This project has been possible thanks to the Mechanical Engineering department of the
Technische Universiteit Eindhoven. I would like to thank Prof.dr.ir. M.G.D. Geers for
accepting me to work in his laboratory, dr.ir. J.P.M.Hoefnagels for his eye of supervisor and
ir. Cem Tasan for his time and his humanity.
18
Appendix A
1. Fractures:
a- Ductile fracture: [3]-[4]
Ductile fracture occurs in formable metals that goes through severe plastic deformation.
Generally the FCC materials are more ductile than others. During the tensile test of such
materials, fracture occurs in five steps:
Figure 28: Steps of a ductile fracture [5]
The observed microvoids can be generated from non-metallic inclusions. For example,
voids on the surface may be initiated by sulphide inclusions or are nucleated at precipitated
carbide.
Figure 29: SEM image presenting a void
due to an inclusion.
b- Brittle fracture: [3]-[4]-[6]
The brittle fracture occurs without significant plastic deformation. This type of fracture
appears generally in BCC or HCP metals. The crack starts through or along the grain
boundaries. The propagation of the crack is fast and the fracture is a cleavage fracture.
There are different conditions to obtain a brittle fracture: a triaxial deformation, the low
temperatures or a very fast stress.
19
Figure 30: SEM image of bainitic steel’s fracture
by cleavage [6]
The figure 30 shows a cleavage initiation facet by inclusion of another phase in the steel
(aimed by the white arrow).
c- Ductile-to-Brittle Transition: [3]-[4]-[7]-[8]
The D-to-B transition is defined by the temperature of transition Td (or DTBT) under
which material starts to behave in a brittle way. To determine Td, Charpy impact tests are
carried out at different temperatures and the fracture energy (J.cm-²) as a function of the
temperature is obtained.
The Td is determined when the curve increases a lot from the low energies (brittle) to the
high energies (ductile) as it’s shown on the figure 31:
Figure 31 [9]
For DP steels, the DTBT is situated around -100°C [8]. DTBT temperature is influenced by
several factors:
• Chemical components:
The main chemical components which affect the Td in steels are carbon and manganese. It
can increase of 25°F for each 0.1% in more of C and it can decrease of 10°F for each 0,1% in
more of Mn. Other components are presented in the next picture with their effects:
20
Components Effect on Td Comments
P increase of 13°F per 0,01%
to avoid Bessemer
process
N
difficult to evaluate his
effect harmful for the toughness
Ni
decrease the Td
good for toughness if
more than 2%
Mo
increase the Td
Almost the same effects
than C
O
Increase the Td, effect of
deoxidation
Td=5°F for 0,001%,
Td=650°F for 0,057%
Si increase the Td from 0,25% used in killed steel
Consequently, more there is C in the steel and more the steel is brittle. The Charpy test
curves take different forms with the different concentrations:
Figure 32: Curves evolution in function of the Carbon concentration. [4]
• The grain’s size:
ASTM is the number of grains by surface units. If the ASTM raises (consequently the
diameter of grains decreases), the Td decreases.
For example, the fall of the grain’s size from ASTM = 5 to ASTM = 10 in the middle steel
changes the Td from 70°F to -60°F.
• The microstructure:
Steel which is composed only by martensite has the best toughness whereas a pearlite
composition has the worst. A bainitic structure which has a good toughness is easy to work.
• Production process:
Rolling at high temperature and the des-oxidation by Al decrease the DTBT
21
2. Damages in DP and IF steels:
- Microstructure and failure of DP steel: [7]-[10]-[11]
This steel is composed of two parts, so different zones are observed on the fracture surfaces
after a tensile test:
Figure 33: SEM image, cleavage facets on the fracture surface [11]
Figure 34: Fractography showing voids on the surface fracture [11]
The figure 33 presents the cleavage zone due to the brittle part (martensite) of the material
and the figure 34 refers to the ductile (ferrite) part which is defined by the voids.
There are two mechanisms for the voids formation:
• Separation of the ferrite and the martensite phases
• Fracture of the martensite: Before the fracture, the martensite is elongated and a
neck is formed.
Figure 35: SEM image of a sample during a fracture
by tensile test. [11]
Cleavage
Voids
Void formation by
martensite fracture
22
Dual Phase steel is noted with the next manner: DPxxx, with x a number. More xxx is important and
more the martensite concentration rises in the steel.
- Microstructure and failure of IF steel:[12]-[13]-[14]
Analyse of an IF steel sample:
Figure 36: SEM images of fracture surfaces
of an IF steel sample. [12]
The fractography (figure 36) shows the fracture surface is partly ductile and partly brittle.
The voids refer to the ductile part. The size of the voids depends of the toughness of the
samples: when the toughness increases the average voids size decreases.
The lack of carbides at the grain boundaries causes intergranular brittle fractures. The
micro-cracks are nucleated preferentially at random boundaries. The fracture occurs in a
typical intergranular fracture mode when a high fraction of random boundaries exits and when
they are connected to each other.
The combined process of intergranular and transgranular fracture occurs in a ductile manner
when the fraction of random boundaries is low.
: Figure 37: Schematic representation of grain boundary structure-dependent
fracture process in polycrystal.
Path A: combined process of intergranular
and transgranular fractures.
Path B: typical intergranular fracture. [14]
Brittle Ductile
23
Appendix B
RESOLUTENESS OF THE MAXIMUM WEIGHT WHICH CAN BEAR THE STEEL SAMPLES
The goal of this proof is to calculate the limit mass that can bear our samples without plastic
deformation.
After the tensile test, the values of the limit elastic and plastic stresses of steel are known.
The lowest elastic stress will be taken to get just one bloc which will be used for all the
samples.
The results obtained are:
e
σ (IF steel) = 130.17 MPa et e
σ (DP 600 steel) = 342.41 MPa.
Consequently, the e
σ of the IF steel will be used for the calculation.
Representation of the bloc’s influence on our samples:
First relation : ( ) ( / ) *
:
ff w w wf
with
σ σ= =
Fbloc = m * g
fσ
w
0
wf
24
0 0
0 0
0
bloc
Calculation of the moment caused by the bloc:
* *
* * *
* *( / ) * *
*( / ) ² *
* ( / ) * ( ^ 3) / 3
(1)
Then : * . . (2) with m the mas
. . ² / 3
wfh
wfh
w
M w dA
M w dw dh
M dh w w wf f dw
M h f wf w dw
M h f w
M h f
f wf
M F l m g l
wf
σ
σ
σ
σ
σ
σ
=
⇔ =
⇔ =
⇔ =
⇔ =
⇒ ⇔
= =
=
∫∫
∫ ∫
∫ ∫
∫
s of the bloc
Equalizing (1) and (2) :
. . ( . . ²) / 3
Numerical mapping with h = 10^-3 m ; / ; g = 10 m.s-² ; l = 3 cm ; 4 mm.
After the calculation, the result is m = 2,3
( . . ²) /(3. . )m g l h f wf
f F A wf
m h f wf g lσσ
σ
= ⇔
= =
=
1 kg.
25
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[9] Google images source
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