Details of the Types of Failure in Materials

TABLE OF CONTENT

TOPIC Page

1. Introduction 2

2. Details of the types of failure in materials 3

2.1 Ductile failure 3

2.2 Brittle failure 3

2.3 Fatigue failure 4

3. How fatigue failure initiated from the point of physical behaviour 5

3.1 Crystallographic aspects 5

3.2 Crack initiation at inclusions 6

3.3 Small cracks, crack growth barriers, thresholds 6

3.4 Number of crack nuclei 7

3.5 Surface effects 7

4. How fatigue failure initiated from the point of physical microstructure 8

4.1 Fatigue failure initiated from the point of microstructure 8

4.2 Fatigue crack stages and fatigue crack propagation 10

5. Recommendation and discussions on some measure to prevent fatigue failure 12

6. Discussion 13

7. Conclusion 14

8. References 15

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1.0 INTRODUCTION

Fatigue is defined as a sudden fracture of a component after a period of cyclic loading

in the elastic regime. Failure is the end result of a process involving the initiation and growth

of a crack, usually at the site of a stress concentration on the surface. The fatigue life of a

component can be expressed as the number of loading cycles required to initiate a fatigue

crack and to propagate the crack to critical size. However, there is very little or warning

before failure if the crack is not noticed. Nevertheless, there is certain fatigue failure is that

there is no noticeable warning. The number of cycles required to cause fatigue failure at

certain peak stress is quite large and prone to failure as the stress increased. Thus, it can be

said that fatigue failure occurs in three stages which are crack initiation, slow, stable crack

growth and rapid fracture. One of the examples of fatigue failure is breaking a thin steel rod

or wire with hands after bending it back and forth several times in the same place. Another

example is an unbalanced pump impeller resulting in vibrations that cause fatigue failure. The

final fracture may occur in a ductile or brittle mode depending on the characteristics of the

material. Fatigue fractures have a characteristic appearance which has normal the initiation

site and start to progressive development of the crack before culminating in final overload

fracture. Fundamental requirements are necessary during designing and manufacturing of the

products to avoid fatigue failure and should be considered as critical in design phase.

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Figure 1: Metal fatigue failure

2.0 DETAILS OF THE TYPES OF FAILURE IN MATERIALS

The major types of failures to be encountered in materials are ductile, brittle and fatigue

failures.

2.1 Ductile failure

Ductile failure undergoes plastic deformation in the vicinity of an advancing crack, and is a

slow process. It is stable, and will not continue except there is an increase in the level of

applied stress. It normally occurs in a trans-granular manner (across the grains) in metals that

have good ductility and toughness. Regularly, amount of plastic deformation including

necking is observed in the failed component. This deformation happens before the final

fracture. Ductile failure are normally initiated by simple overloads or by applying too high a

stress to the material, and exhibit characteristic surface features with a significant portion of

the fracture surface having an irregular, fibrous face. It is also have a small shear lip, where

the fracture surface is at a 45° angle to the applied stress. The shear lip, indicating that slip

occurred, gives the fracture the cup-and-cone form.

2.2 Brittle failures

In brittle failures, cracks spread very quickly, with little or no plastic flow, and are so

unstable that crack propagation occurs without further increase in applied stress. It take place

in high strength metals, in metals with poor ductility and toughness, and in ceramics.

Even metals that are normally ductile may fail in a brittle manner at low temperatures, in

thick sections, at high strain rates (such as impact), or in existence of flaw.

Brittle fracture can be recognised by spotting the features on the failed surface. Generally, the

fracture surface is flat and perpendicular to the applied stress in a tensile test. If a failure

occurs by cleavage, each fractured grain is flat and differently oriented, giving a shiny,

crystalline appearance to the fracture surface.

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Ductile failure

Figure 2 : Ductile and brittle failure

2.3 Fatigue failure

Fatigue is a form of failure that occurs in materials due to repeated loading cycles. Fatigue

failure occurs at stresses below the yield strength of a material and no visible warning prior to

failure. The term ‘fatigue’ is used since this type of failure normally occurs after a lengthy

period of repeated stress cycling. It is the single largest cause of failure (approximately 90%)

of metallic materials, and polymers and ceramics are also at risk to this type of failure.

Failure usually start on the surface of an object at some discontinuity or imperfection in the

surface. This is the crack beginning. The crack propagates to a critical size without any

evidence of plastic deformation in the part. Once the crack reaches a critical size, the part

fails by sudden fracture. A main problem with fatigue is that it is dominated by design.

Although it is possible to assess the inherent fatigue resistance of a material, the effects of

stress-raisers such as surface irregularities and changes in cross-section, as well as the crucial

area of jointing ( welded) can be a major problem. Some factors that affect fatigue life is,

design factors, surface treatments, size effects, and environmental effects.

Design Factors – Geometric discontinuities provide areas for higher stress concentrations that

are more susceptible to fatigue.

Surface Treatments – The quality of a surface finish determines the number and size of

imperfections on the surface.

Size effects- The size of an object affects its fatigue. Larger objects have lower fatigue

strengths

Environmental effects- Temperature and corrosion may play are role in fatigue failure.

Constrained parts that are subject to fluctuating temperatures and repeated thermal

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Figure 3 : Fatigue failure process

3.0 HOW FATIGUE FAILURE INITIATED FROM THE POINT OF PHYSICAL

BEHAVIOUR

In case of fatigue life, it is split into a crack initiation period and a crack growth

period. The initiation period is supposed to include some microcrack growth, but the fatigue

cracks are still too small to be visible. Then, the crack is growing until complete failure.

Furthermore, it is technically consider the crack initiation and crack growth periods

separately due to the several practical conditions that have a large influence on the crack

initiation period, but a limited influence or no influence at all on the crack growth period.

For case of fatigue failure, it is consists of crystallographic nature of the material, crack

initiation at inclusions, crack growth barriers, small cracks, thresholds, number of crack

nuclei and surface effects.

3.1 Crystallographic aspects:

The initial growth of a microcrack shows a tendency to grow along a slip band. In this

case, it must be expected that the crystallography of a material has some influence on the

mechanistic behavior during the initiation period. The crystallographic properties vary from

one material to another. Thus, the initial microcracking depends on the material. A lot of

aspects of crystallographic are type of crystal lattice, elastic anisotropy, allotropy, slip

systems, ease of cross slip, and also grain size and shape.

Figure 4 : Microcracking Structure

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3.2 Crack initiation at inclusions:

In materials, a variety of inclusions can be present, such as impurities during the

melting production process of the alloys. Larger macroscopic inclusions are generally

regarded as material defects which should not be present such as slag streaks, weld defects,

major porosities. Large defects have occasionally caused disastrous failures in service, but

they are not considered in this section. Fatigue crack nucleation occurred at these inclusions

located at the material surface or slightly below the surface.

Figure 5 : Slip band microcrack

3.3 Small cracks, crack growth barriers, thresholds:

The cracks were nucleated which stopped growing at a small crack length. The cracks

encountered a type of a crack growth barrier and could not grow any further. The barrier was

a threshold for crack growth.

Figure 6 : Small crack

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3.4 Number of crack nuclei:

Figure 7 : Crack nuclei

Fatigue crack initiation is a surface phenomenon, but thousands of grains are found at

the material surface of unnotched specimens. In this case, nucleation depends on the

occurrence of cyclic slip. However, the stress is not equal in all surface grains due to the

anisotropy of the material.

3.5 Surface effects:

Surface effects include all conditions which can reduce the crack initiation period. In

this case, it is cover the phenomena which enhance the crack initiation mechanism. Surface

roughness and surface damage imply that the free surface is no longer perfectly flat. A small

sized stress concentrations along the material surface occur. Although the stress concentration

will rapidly fade away from the surface, it is still significant for promoting cyclic slip and

crack nucleation at the material surface. For example, rough machining causes deeper

circumferential grooves than fine machining.

Figure 8 : Surface effects on the S-N curve

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4.0 HOW FATIGUE FAILURE INITIATED FROM THE POINT OF PHYSICAL

MICROSTRUCTURE

4.1 Fatigue failure initiated from the point of microstructure

Fatigue is the most common type of fracture in engineered components. Fatigue fractures are

also particularly dangerous because they can occur under normal service conditions, with no

warning that a progressively growing crack is developing until the final catastrophic failure.

There are three stages of fatigue. The first is nucleation of a crack by small amounts of

inhomogeneous plastic deformation at a microscopic level. The second is the slow growth of

these cracks by cyclic stressing. Finally sudden fracture occurs when the cracks reach a

critical size. Often visual examination of a fatigue fracture surface will reveal clamshell or

beach markings. The initiation site of the crack can easily be located by examining these

marks. Microscopic examination of a fracture surface often reveals markings on a much finer

scale. These are called striations and they do represent the position of the crack front at each

cycle. The distance between striations is the distance advanced by the crack during one cycle.

Sometimes striations cannot be observed because they are damaged when the crack closes.

Figure 9 : Typical clamshell markings on a fatigue fracture surface of a shaft

The fracture started at the left side of the bar and progressed to the right, where final failure

occurred in a single cycle.A microscopic examination of the exterior surface of a specimen

after cyclic stressing will usually reveal a roughening even before any cracks have formed.

Under high magnification,intrusions and extrusions are often apparent. These intrusions and

extrusions are the result of slip on one set of planes during the compression half-cycle and

slip on a different set of planes during the tension half-cycle. Persistent slip bands beneath the

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surface are associated with these intrusions and extrusions. Fatigue cracks initiate at the

intrusions and grow inward along the persistent slip bands.

Figure 10 : SEM picture of fatigue striations

on a fracture surface of type 304 stainless

steel. From Metals Handbook, Vol. 9, eighth

ed., ASM, 1974.

Figure 11 : Intrusions and extrusions at

surface formed by cyclic deformation.

These correspond to persistent slip bands

beneath the surface. From A. Cottrell and

D. Hull, Proc. Roy.Soc. (London) Vol.

A242 (1957).

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4.2 Fatigue crack stages and fatigue crack propagation

STAGE 1

(a) static deformation (b) fatigue deformation leading to surface notch (intrusion) (c ) fatigue

deformation leading to slip-band extrusion

STAGE 2

Figure 12 : Plastic blunting process for growth of stage 2 fatigue crack

Figure 13: Showing how intrusions and extrusions can develop if slip occurs on different

planes during the tension and compression portions of loading.

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Figure 14: Dislocation Slip Crack Nucleation

Appearance and disappearance of slip bands at a various points of the hysteresis loop of the

first cycle. (low-cycle pull-push fatigue of aluminium alloy)

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5.0 RECOMMENDATION AND DISCUSSIONS ON SOME MEASURE TO

PREVENT FATIGUE FAILURE

To prevent fatigue, the stress in the material must not exceed a specified endurance or

fatigue limit. This value is determine from S-N diagram as the maximum stress the material

can resist when subjected to a specified number of cycles of loading. In order to specify a

safe strength for a metallic material under repeated loading, it is necessary to determine a

limit below which no evidence of failure can be detected after applying a load for specified

number of cycle. This limiting stress is called the endurance or fatigue limit. Using a testing

machine for this purpose a series of specimen are each subjected to a specified stress and

cycled to failure. The results are plotted as a graph representing the stress S (or σ) on the

vertical axis and the number of cycle-to-failure N on the horizontal axis. This graph is called

an S-N diagram or stress-cycle diagram.

5.1 Prevention of fatigue failure

Introducing a compressive stress into thin surface layer by “shot peening”, that is

firing small shot into surface to be treated. High tech solution that uses an ion

implementation, laser peening. In laser peening process high energy laser is fired at the

surface of a metal part to generate pressure pulses of one million pounds per square inch,

which send shocks waves through the part. Multiple firing of the laser in a pre-defined

surface pattern will impart a thick layer of residual compressive stress. The deeper the level

of compressive stress, the higher the resistance of the metal to fatigue and corrosion failure.

Carburizing is the addition of carbon to the surface of low-carbon steels at temperatures

generally between 850 °C and 950 °C, at which austenite is the stable crystal structure.

Hardening is accomplished when the high-carbon surface layer is quenched to form

martensite so that a high-carbon martensitic case with good wear and fatigue resistance is

created on a tough, low-carbon steel core. Fine austenite and tempered martensite prevent

nucleation of fatigue cracks and fatigue crack initiation until very high stress level is reached.

Next are the design and the geometry of the metal itself. Premature fatigue failure is

prevented by careful attention to detail at the design stage to ensure that cyclic stresses are

sufficiently low to achieve the required endurance. Stress concentrations should be avoided

where possible for example a design with smooth 'flowing' lines is usually the optimum.

Another thing is to optimize the geometry by avoiding internal corners and notches. Corners

should be avoided in a design as it causes high concentration of stress that will lead to fatigue

failure.12

6.0 DISCUSSION

Three basic factors are necessary to make fatigue cracks to initiate. First, large enough

variation or fluctuation with loading pattern must contain minimum and maximum peak

values. The reverse loading cycle must be sufficiently large for fatigue crack initiation even

though the peak values can be in tension or compression. Secondly, the peak stress levels

must be of sufficiently high enough to initiate the crack. Crack initiation will not exist if the

peak stresses are too low. Third, sufficient large number of cycles of the applied stress must

be experienced by the material. The number of cycles required to initiate and grow a crack is

largely are depend on the repeatedly maximum value of cycles.

Beside of these three factors, there are other variables that prone to fatigue failure

such as stress concentration, corrosion, temperature, overload, metallurgical structure, and

residual stresses. The surface condition of the material being experienced will have an effect

on its fatigue life when fatigue cracks generally initiate at a surface. The level and number of

stress concentrations on the surface is directly related to surface roughness. Thus, it is

important variable in fatigue failure. The crack is likely to nucleate if higher stress

concentration on the material. Smooth surfaces increase the time of crack nucleation.

Besides, fatigue life can be decrease if the material is in notches, scratches, and other stress

risers. Surface residual stress also has tremendous effect on fatigue life. Compressive residual

stresses from machining, cold working, heat treating will oppose a tensile load and therefore,

lower the amplitude of cyclic loading and decrease the failure.

To reduce or eliminate fatigue failure, redesign the parts can be done which requires

thorough education and supervised experience in structural engineering, mechanical

engineering, or materials science. There are four examples that keep the life assurance of the

parts which the parts must be design to keep below threshold of fatigue limit, inspection must

be done on the part periodically for cracks and to replace the part once a crack exceeds a

critical length, design safe-line design for a fixed life parts after which the user is instructed

to replace the part with a new one and design in with no single point of failure, thus, when

any one part completely fails, it does not lead to other failure and compromise the entire

system.

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7.0 CONCLUSION

Based on the characteristics of material fatigue failure, we can conclude that three

basic factors that promote the crack to initiate are large variation or fluctuation in loading

pattern, large peak stress applied on the material and sufficient large number of cycles

experienced by the material. There are also other factors that affect fatigue failure on the

material such as temperature, material type, quality, crack closure, residual stresses cause by

welding, casting, and drilling, and etc. Fatigue failure can be prevent by careful attention to

detail at the design stage to ensure that cyclic stresses are sufficiently low to achieve the

required endurance. The material parts can be changed to improve fatigue life which the parts

can be made from better fatigue rated metals. Complete replacement and redesign of parts

can also reduce the fatigue failure of the materials.

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8.0 REFERENCES

1. Fatigue Design Handbook, Society of Automotive Engineers, Warrendale PA, 1988.

2. Metal Fatigue, Frost N E, Marsh K J, & Pook L P, Clarendon Press, Oxford, 1974.

3. Fatigue Design of Welded Joints and Components, Recommendations of IIW Joint

Working Group XIII-XV, Abington Publishing, 1996.

4. BS 7608:1993 Fatigue Design and Assessment of Steel Structures. BSI London.

5. Fatigue Strength of Welded Structures, Maddox S J, Abington Publishing, Second Edition

1991.

6.

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