laboratory 9 fractographyeng.sut.ac.th/metal/images/stories/pdf/lab_9fracto_eng.pdf · laboratory...

Laboratory 9: Fractography

Mechanical Metallurgy Laboratory 431303 1

T. Udomphol

LLaabboorraattoorryy 99

Fractography

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Objectives

• Students are required to achieve basic fractographic observation skills. The samples

failed under tensile, torsion, impact or fatigue will be investigated using visual

inspection and equipments such as stereoscope, optical microscope and electron

microscope at various magnifications.

• Students should be able to determine types of specimen failure or failed

components. Students are required to identify cause and factors affecting such

failures and give solutions in order to prevent undesirable failures.

• Students can explain influences of forces acting on specimens, microstructures and

test environments on types of fracture modes

• Students should be able to use interpreted data obtained to improve material

properties or to select the right materials for intended applications and to avoid

undesirable failures.



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1. Literature Review

Failures of components in services are undesirable in any engineering applications and should

be avoided in any cases. For example, the failure of the Minnesota Bridge in August 2007, USA has

caused terrible losses in life and required high cost of reconstructions, see figure 1. The cause of the

failure might be plausibly due to insufficient maintenance of the Minnesota Bridge. Failures in aero-

engines, auto parts, or machines during service operations are also undesirable in any cases. In order

to tackle or to avoid any imminent accidents, attempts have been made to understand the causes of

such failures.

Figure 1: Failure of the bridge in Minnesota, USA, August 2007.

Failure analysis deals with studying the causes of failures, analyzing factors influencing

failures and finally solving or preventing such failures that might occur in the future. In order to study

the cause of failure, fractographic observation or fractography has been a powerful tool used for the

determination of failure causes in materials or components. In general, a scanning electron

microscope (SEM) is commonly utilized because it provides a practical range of magnifications

suitable for the investigation. The information obtained from fracture surfaces is used for the analysis

of the failure causes and factors affecting such failures and thus leads to the prevention of failures in

the future. The whole process of failure analysis requires a handful of skills and experiences to come

out with an accurate judgment or corrective preventions. Failure analysis is currently becoming an



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important skill for engineers whose work is relevant to operating machines, components, or

engineering structures.

For a short learning period, this laboratory will provide you the fractographic skills involving

visual inspection, stereo microscope and SEM techniques. These require a good understanding of

different types of fracture in materials. Therefore different fracture modes in metallic materials

should be first introduced here to give you some important background about fractographic

investigation.

1.1 Types of fracture in metals

Types of fracture can be roughly divided into two categories, which are brittle and ductile

fractures. The former has however gained a lot of interests due to its catastrophic results whenever

happens. Limited amount of plastic deformation occurring during brittle fracture, promotes a sudden

failure without warning. Ductile failure on the contrary exhibits rough and dull fracture surfaces with

gross plastic deformation, therefore allowing more time to correct or prevent such failures. This

failure type is not too catastrophic and has gained less interest from many researchers as compared to

brittle failure. Nevertheless, a true understanding of all types of fracture is of importance such that

causes of failure can be accurately determined and can lead to the suitable means of material

selection. Types of fracture are described as follows

• Transgranular fracture, which can be classified into

- Brittle cleavage fracture

- Ductile fracture

- Fatigue fracture

• Intergranular fracture, which can be classified into

- Intergranular fracture without microvoid

- Intergranular fracture with microvoid

1.1.1 Brittle cleavage fracture

Figures 2 (a) and (b) illustrate fractographs of tensile specimens, which have been failed in

ductile and brittle manners respectively. It is noticed in figure 2 (b) that flat surfaces with limited

plastic deformation are belonged to tensile specimens that have been failed in a brittle failure mode.



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Fracture surfaces of the tensile specimen failed in a ductile mode on the other hand exhibit a so called

cup and cone type fracture, showing gross plastic deformation on both half of the fracture surfaces.

Brittle materials normally exhibit flat fracture surfaces consisting of transgranular cleavage

facets. This type of fracture give small areas of fracture surface; hence, requiring low specific surface

energy (γ) to produce two new fresh fracture surfaces. The fracture energy is therefore low in this

case. Figure 2 (b) shows a classic characteristic of brittle facets with their size similar to the grain size

of the material. The brittle facets were caused by a fast propagation of the crack transgranularly

across the grains. Investigated at higher magnifications using a scanning electron microscope (SEM),

the fracture surface show a river line pattern or stress line pattern in which its direction pointing

toward the origin of the crack. A fracture surface of a broken piece of glass might be seen quite

similar to that observed from cleavage fracture. When the crack is initiated possibly at the inclusion,

the metal grains then readily cleave along the low-fracture-energy crystallographic plane. Figure 3 (a)

depicts a transgranular cleavage fracture of ferrite grains in carbon steel. This feature is a true

cleavage fracture. Nevertheless, if some areas of ductile tearing is observed, for example, in the case

of a fracture surface observed in in 17-4 PH stainless steel of a tempered martensite microstructure as

shown in figure 3 (b), this type of fracture surface is called quasi-cleavage fracture [1].

Figure 2 Ductile fracture and brittle cleavage fracture observed from tensile fracture surfaces.



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Figure 3: Fracture surfaces showing (a) cleavage facets of ferrite in carbon steel and b) Quasi

cleavage facets of tempered martensite observed in 17-4 PH stainless steel [1].

The river lines or the stress lines are steps between cleavage or parallel planes, which are

always converged in the direction of local crack propagation as seen from figure 4. This direction is

normally observed pointing to inclusion, porosity, crack or second phase particle, which create stress

concentration. Transgranular cleavage fracture is usually associated with defects such as cracks,

porosity, inclusions or second phase particles in which dislocations movement is obstructed. Stress is

therefore concentrated in front of these defects, initiating a crack of a critical size. The propagation of

this crack then finally causes the global failure with very little plastic deformation as illustrated in

figure 5. As the defects are known as the cause of the failure, minimizing these defects is considered

to avoid stress concentration, hence to increase fracture resistance of the materials. This can be done

by controlling the quality of the products in manufacturing processes.

Figure 4: River line pattern observed from brittle cleavage fracture surfaces [3].



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The crack initiation site can be traced back by studying fracture surfaces of the specimens or

components by visual inspection or by means of using microscopes at different magnifications. An

example of the crack initiation which are obviously observed by naked eyes from fracture surfaces of

metal plates are illustrated in figure 6. There are apparent stress lines pointing to one end of the plate

on both halves as depicted in figure 6 (a) and an initiation site located in the middle of the top surface

as seen in figure 6 (b). However if the identification of sites for crack initiation is not easily carried

out by visual inspection such similar to these cases, SEM investigation is then becoming more

practical. For instance, a fracture surface of a steel fracture toughness specimen was visually

investigated first and the result has shown a possible crack initiation site at point S, according to the

indicating stress lines as demonstrated in figure 5. Investigating at higher magnifications using the

SEM technique reveals areas of cleavage facets with the crack initiated at an inclusion as determined

by the direction of the river line pattern. The inclusion observed was subsequently determined as a

Mg2S particle, which are typically found in steels. Minimizing of these inclusions leads to a

significant improvement in mechanical properties and fracture resistance of the steel.

Figure 5: Fractographic observation indicating initiation site at an inclusion[4].



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Figure 6: Fracture surfaces showing crack initiation sites eventually leading to failures [3].

1.1.2 Ductile fracture

If we now consider a tensile sample which is loaded beyond the maximum tensile strength as

shown in figure 7, we can see necking occurring along the specimen gauge length. Within the

necking area, microvoids are formed probably around the particles. As the load continues, these

microvoids expand and join together to create a crack with its plane perpendicular to the axis of the

tensile force applied. As this crack grows, the shear plane of approximately 45o to the tensile

direction develops around specimen edge and merges with the existing crack, resulting in the classic

cup and cone fracture surfaces as demonstrated in figure 7.

Ductile materials normally exhibit gross plastic deformation during fracture, providing rough

fracture surfaces with relatively high surface areas as has been illustrated in figure 1. Ductile fracture

therefore requires higher energy to create two new fresh fracture surfaces in comparison to energy

required to cleave flat brittle surfaces. Investigation under higher magnifications shows ductile

fracture surface consisting of copious of ductile dimples or microvoids. The mechanism of microvoid

formation is demonstrated in figure 8, showing the influence of inclusions or second phase particles as

microvoid initiation sites. During loading, these particles decohere from the matrix, forming a

microvoid around each particle. Continuing the applied load results in the expansion of these



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microvoids and finally leads to microvoid coalescence, giving the complete failure. Generally, these

microvoids are normally observed to be centred on inclusions or second phase particles as shown in

figure 8 (d). Materials such as ductile cast iron, aluminium or alloys operating at high temperature

exhibit this type of fracture surface.

Figure 7: Cup and cone fracture.

Furthermore, the shape of the microvoids observed in a microscopic scale can be used to

determine the type of the force applied on to the specimen. If the microvoids are in equiaxed shape,

as shown in figure 9 (a), the applied force are uniaxial tensile loading. Parabolic-shaped microvoids

with its tip pointing in the opposite direction when seen on both halves of fracture surface are due to

shear force, see figure 9 (b). If the microvoids appear in the parabolic shape with the tip of each

microvoid orientated in the same direction on both fracture surface halves, the tensile tearing is

applied in this case as shown in figure 9 (c).



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Figure 8: Microvoid coalescence in ductile fracture[4].

Figure 9: Different characteristics of microvoids observed under (a) Uniaxial tensile loading, (b)

shear and (c) tensile tearing[].



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1.1.3 Intergranular fracture

Intergranular fracture is normally associated with service conditions which are corrosive or

high temperature. These conditions resulting in precipitation of the second phase particles along the

weakened grain boundaries. Figure 10 illustrates two types of intergranular fracture; a) intergranular

fracture with microvoid coalescence and b) without microvoid coalescence. Although the former

exhibits areas of ductile dimples similar to those observed from typical ductile fracture surfaces, these

ductile dimples are relatively shallow and require much less energy involved in the fracture process.

These ductile dimples or microvoids are formed around particles precipitated in some cases as a

network along the grain boundaries. This considerably reduces the bonding between the particles and

the matrix, which in turn allows microvoid coalescence to occur more readily. As a result, the

material has significantly reduced fracture energy which promotes a rapid crack propagation path

along the grain boundaries.

Figure 10: Fracture surfaces of (a) Intergranular fracture with microvoid coalescence and (b)

without microvoid coalescence.



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Temperature is one of the most important factors influencing intergranular fracture. High

temperature not only facilitates ductile failure (increased amount of plastic deformation) but also

promotes metallurgical changes. If the bonding between the new phases and the matrix is quite poor,

responses of materials to external loads will be significantly affected, which can subsequently alter the

fracture mode of the materials. In general, oxidation or combustion reactions are involved when

materials are subjected to high temperatures, such as in the case of turbine blades. Oxide scales on the

metal surface are normally brittle and heterogeneously formed, leading to a crack formation on the

surface. If the load continues at high temperature, oxidation would progress even further into material

interior through the existing surface cracks. This might result in the formation of brittle oxidation

products within the grain and especially along the grain boundaries. If these grain boundary oxidation

products are interconnected and poorly bonded with the matrix, intergranular fracture will result.

Therefore, it can be seen that although high temperature promotes ductile failure, metallurgical

changes might alter the fracture mode from transgranular ductile fracture to intergranular fracture due

to poorly bonded grain boundary particles. This embrittlement effect results in noticeable reductions

in tensile strength and ductility. The utilization of the materials at high temperatures or strong

oxidation will considerably affect mechanical properties of the materials. Material selection therefore

becomes significantly important in this case. Nickel based alloys are a good alternative for service

conditions at high temperature and oxidation. Furthermore, material which is subjected to corrosive

environment where corrosion products are observed along the grain boundaries will give comparable

results.

1.1.4 Fatigue fracture

Fatigue fracture surfaces possess quite a unique characteristic of flat surfaces with limited

plastic deformation and in some cases show visible beach mark as shown in the gear teeth subjected to

fatigue failure in figure 12. Surface condition is a prime factor in controlling surface crack initiation.

Crack or defect-free surfaces greatly help to improve fatigue life of the components. Rough surfaces

are also attractive to stress concentration, leading to easy fatigue crack initiation.

In general, initiation and propagation of the fatigue crack can be divided into three stages, 1)

Fatigue crack initiation Bstage I, 2) stable fatigue crack propagation-stage II and 3) Unstable fatigue

crack initiation B stage III. The initiation of the fatigue crack is sensitive to surface conditions.

Figure 12 shows mechanisms of extrusion and intrusion on the top surface of sample subjected to

fatigue loading. Persistent slip planes caused by back and forth motion of fatigue produce a number



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of extrusions and intrusions as illustrated in the schematic diagram. The latter is considered as an

atomic scale notch and acts as a stress raiser to eventually become the initial fatigue crack on such a

surface. The plane of the fatigue crack at this stage is along the shear plane or slip plane. When the

fatigue crack grows a little further, the fatigue crack is now influenced by the tensile stresses which

are perpendicular to the crack propagation direction. The fatigue crack is now said to be in the stage

II region, and propagates at a relatively stable rate. The fatigue propagation rate in this region

therefore represents the fatigue crack growth of the specimen. The final stage is the unstable fatigue

crack propagation which takes place when the fatigue crack length exceeds the critical length for

stable crack growth, leading eventually to fatigue failure.

If we study the stage II fatigue fracture surface using SEM technique, striations orientated

normal to the fatigue crack growth direction as shown in figure 14 (a) will be observed. Each striation

is due to plastic blunting process as demonstrated in figure 14 (b). When the tensile loading

progresses, the crack opens and allows the slips to operate at the top and bottom ends to produce local

plastic deformation as shown by the arrows. At a higher tensile loading, plastic blunting occurs,

which leads to an increase in the fatigue crack length. During unloading, the fatigue crack is then

closed and the slips are now operating in the opposite direction. Therefore, after one cycle, the fatigue

crack now arrives at the original stage of crack closing with an increase in one fatigue striation.

Figure 12: Beach mark observed on a fatigue fracture fracture surface of gear.



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Figure 13: Mechanism of extrusion and intrusion on the top surface due to persistent slip bands [1].

Figure 14: a) Fatigue striations in aluminum and b) Fatigue striation due to plastic blunting process.

Furthermore, the fatigue failure can be found to occur in conjunction with corrosive and high

temperature environment, which are called corrosion fatigue and thermal fatigue respectively.

Corrosion and high temperature accelerate the rate of the fatigue crack propagation and promote

severe fatigue failures. An example of corrosion fatigue shows obvious areas of rust on the fatigue

surface observed in an automobile shaft as depicted in figure 15 (a). This significantly reduces the

fatigue life of the automobile shaft. Thermal fatigue illustrated in figure 15 (b) shows a beach mark

which indicates the fatigue crack initiation to have started at the top surface.



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Figure 15: Fracture surfaces of specimens failed in a) corrosion-fatigue and b) thermal fatigue.

1.2 Fractography techniques

Sample preparation is an important step to ensure accurate information achieved from

fracture surfaces. Failed specimens or components should be kept in an environment that can protect

them from moisture and not to produce oxide scales on the surfaces. Broken halves of fracture

surfaces should not be rubbed against each other. The sample should be cleaned using an ultrasonic

vibration technique and then washed by acetone prior to dry blowing for further investigation.

The techniques and the microscopes used for fractographic investigation come in various

magnifications as schematically shown in figure 16. If the fracture surfaces are too big to put under

any microscopes, visual inspection is very practical and the origin of the failure can be roughly

indicated. Optical microscope and SEM are employed for investigating at higher magnifications to

determine the fracture mode and the crack initiation site on a microscopic scale. Transmission

electron microscope (TEM) is utilized when lattice parameter are required to identify the existing

phases. However sample preparation for TEM technique is a time consuming process and the

interpretation of the acquired data requires adequate understanding.

Figure 16: Equipment used for fractographic observation ranging according to magnification [5].



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The most powerful technique commonly used for fractographic investigation is the SEM

technique. This technique utilizes electron as a source to create images as illustrated in figure 17.

The electron beam released from the electron gun travels pass through the electromagnetic field and

electromagnetic lenses where it is focused prior to hitting on the specimen surface. Then the X-ray,

back scattered electrons, secondary electron and auger electrons are produced. These signals are

subsequently collected to produce images and information important for the interpretation of failure

analysis. Sample preparation for SEM investigation is a critical stage, which requires a proper

cleaning in order to obtain free- moisture surfaces. The sample should also be conductive. However

in the case of non-conductive materials, gold sputtering is normally used to produce conductive

coating on their surfaces. Specimen should be carefully griped onto the specimen holder to avoid

sliding off in the vacuum chamber especially when sample is tilted.

Figure 17: Schematic diagram of SEM operation [Lowa State University].



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2. Experimental procedure

2.1 Broken samples achieved from mechanical tests such as tensile, impact, torsion or fatigue

tests.

2.2 Cleaning equipment (Ultrasonic cleaning equipment)

2.3 Scanning electron microscope coupled with energy dispersive spectroscopy

3. Experimental procedure

3.1 Study the principle of the scanning electron microscope from the skilled operators/technicians

responsible for the machine.

3.2 Examine the fracture surfaces of the broken specimens provided by naked eyes before

gripping them on to a specimen holder and putting it into the chamber. The specimens to be

explored have been failed in brittle cleavage, ductile, intergranular and fatigue failure.

3.3 Examine the specimen surfaces at a low magnification first to locate the sample and then

change to higher magnifications. Determine the nature of each fracture surface such as

phases, facets, dimples etc.

3.4 Sketch the fracture surfaces observed in the table provided and provide descriptions.

3.5 Discuss on the issues such as what type of failure mode of each specimen, which type of

force influencing the failure and factors which might affect the failure. Give conclusions.



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4. Results

Specimen Sketch Description

1

2

3

4

Table 1 Sketches of fracture surfaces of tested specimens.



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Figure 18: Fractographs observed from fractured specimens.



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5. Discussion

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6. Conclusions

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7. Questions

7.1 Describe the characteristics of cleavage facets, microvoids and fatigue striations.

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7.2 Which types of fracture mode do require the most fracture energy? Why?

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7.3 Predict the fracture mode of compacted graphite, aluminium and white cast iron at room

temperature under tensile loading.

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8. References

8.1 Dieter, G.E., Mechanical metallurgy, 1988, SI metric edition, McGraw-Hill, ISBN 0-07-

100406-8.

8.2 Hashemi, S. Foundations of materials science and engineering, 2006, 4th edition, McGraw-

Hill, ISBN 007-125690-3.

8.3 W.D. Callister, Fundamental of materials science and engineering/an interactive e. text,

2001, John Willey & Sons, Inc., New York, ISBN 0-471-39551-x.

8.4 Hull, D., Bacon, D.J., Introduction to dislocations, 2001, Forth edition, Butterworth-

Heinemann, ISBN 0-7506-4681-0.

8.5 Ewaksm H.L., Wanhill, R.J.H., Fracture mechanics, 1986, Edward Arnold, ISBN- 07131

3515 8.

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