Fatigue testing

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Mechanical Testing 01C

An Introduction of mechanical Testing Pictorial Basic 机械性能试验图文简易教材 Part 3：Fatigue Testing (Descriptive approach) 疲劳试验


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Metal Fatigue

A phenomenon which results in the 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. Occasionally, a crack may initiate at a fault just below the

surface. Eventually the cross sectional area is so reduced that the component ruptures under a normal

service load, but one at a level which has been satisfactorily withstood on many previous occasions before

the crack propagated. 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 reflects the initiation

site and the progressive development of the crack front, culminating in an area of final overload fracture.

Initiation site(s).

Progressive of crack front characterise by beach marks.

Culminating in an area of final fracture.

Fig. la illustrates fatigue failure in a circular shaft. The initiation site is shown and the shell-like markings,

often referred to as beach markings because of their resemblance to the ridges left in the sand by retreating

waves, are caused by arrests in the crack front as it propagates through the section. The hatched region on

the opposite side to the initiation site is the final region of ductile fracture. Sometimes there may be more

than one initiation point and two or more cracks propagate. This produces features as in Fig. 1b with the the

final area of ductile fracture being a band across the middle. This type of fracture is typical of double bending

where a component is cyclically strained in one plane or where a second fatigue crack initiates at the

opposite side to a developing crack in a component subject to reverse bending. Some stress-induced fatigue

failures may show multiple initiation sites from which separate cracks spread towards a common meeting

point within the section.

Fig. 1

Fatigue strength is determined by applying different levels of cyclic stress to individual test specimens and

measuring the number of cycles to failure. Standard laboratory test use various methods for applying the

cyclic load, e.g. rotating bend, cantilever bend, axial push-pull and torsion. The data are plotted in the form of

a stress-number of cycles to failure (S-N) curve, fig 2. Owing to the statistical nature of the failure, several


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specimens have to be tested at each stress level. Some materials, notably low-carbon steels, exhibit a

flattening off at a particular stress level as at (a) in Fig.2 which is referred to as the fatigue limit. As a rough

guide, the fatigue limit is usually about 40% of the tensile strength. In principle, components designed so that

the applied stresses do not exceed this level should not fail in service. The difficulty is a localised stress

concentration may be present or introduced during service which leads to initiation, despite the design stress

being normally below the 'safe' limit. Most materials, however, exhibit a continually falling curve as at (b) and

the usual indicator of fatigue strength is to quote the stress below which failure will not be expected in less

than a given number of cycles which is referred to as the endurance limit.

Fig2


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Micro crack at grain boundary during fatigue test.


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Although fatigue data may be determined for different materials it is the shape of a component and the level

of applied stress which dictate whether a fatigue failure is to be expected under particular service conditions.

Surface condition is also important. Often complete components or assemblies, e.g. railway bogie frames or

aircraft fuselage, will be tested by subjecting them to an accelerated loading spectrum reproducing what they

are likely to experience over their entire service lifetime.

- Metal Fatigue -

Why Metal Parts Fail From Repeatedly-Applied Loads

Long ago, engineers discovered that if you repeatedly applied and then removed a nominal load to and from a

metal part (known as a "cyclic load"), the part would break after a certain number of load-unload cycles, even when

the maximum cyclic stress level applied was much lower than the UTS, and in fact, much lower than the Yield

Stress (UTS and YS are explained in Stress and Strain). These relationships were first published by A. Z. Wöhler

in 1858.

They discovered that as they reduced the magnitude of the cyclic stress, the part would survive more cycles before

breaking. This behavior became known as "FATIGUE" because it was originally thought that the metal got "tired".

When you bend a paper clip back and forth until it breaks, you are demonstrating fatigue behavior.

The following information on this page attempts to explain metal fatigue by answering several common questions:

1. What is fatigue loading?

2. How do you determine the fatigue strength of a material?

3. Does the strength of a material affect its fatigue properties?

4. Why is the surface of a part so important?

5. Is fatigue life an exact number?

6. Do real-world parts behave the same as laboratory tests?

7. Are fatigue cycles cumulative?

WHAT IS FATIGUE LOADING?

There are different types of fatigue loading. One type is zero-to-max-to zero, where a part which is carrying no

load is then subjected to a load, and later, the load is removed, so the part goes back to the no-load condition. An

example of this type of loading is a chain used to haul logs behind a tractor.


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Another type of fatigue loading is a varying load superimposed on a constant load. The suspension wires in a

railroad bridge are an example of this type. The wires have a constant static tensile load from the weight of the

bridge, and an additional tensile load when a train is on the bridge.

The worst case of fatigue loading is the case known as fully-reversing load. One cycle of this type of fatigue

loading occurs when a tensile stress of some value is applied to an unloaded part and then released, then a

compressive stress of the same value is applied and released.

Figure 1

A rotating shaft with a bending load applied to it is a good example of fully reversing load. In order to visualize the

fully-reversing nature of the load, picture the shaft in a fixed position (not rotating) but subjected to an applied

bending load (as shown here). The outermost fibers on the shaft surface on the convex side of the deflection

(upper surface in the picture) will be loaded in tension (upper green arrows), and the fibers on the opposite side will

be loaded in compression (lower green arrows). Now, rotate the shaft 180° in its bearings, with the loads remaining

the same. The shaft stress level is the same, but now the fibers which were loaded in compression before you

rotated it are now loaded in tension, and vice-versa.

In fact, the laboratory mechanism used to test the fatigue life of materials is a rotating shaft with an applied

bending load.

To illustrate how damaging fully-reversing load is, take a paper clip, bend it out straight, then pick a spot in the

middle, and bend the clip 90° back and forth at that spot (from straight to "L" shaped and back). Because you are

plastically-deforming the metal, you are, by definition, exceeding its yield stress. When you bend it in one direction,

you are applying a high tensile stress to the fibers on one side of the OD, and a high compressive stress on the

fibers on the opposite side. When you bend it the other way, you reverse the stresses (fully reversing fatigue). It

will break in about 25 cycles.

The number of cycles that a metal can endure before it breaks is a complex function of the static and cyclic stress

values, the alloy, heat-treatment and surface condition of the material, the hardness profile of the material,

impurities in the material, the type of load applied, the operating temperature, and several other factors.


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HOW IS THE FATIGUE STRENGTH OF A METAL DETERMINED?

The fatigue behavior of a specific material, heat-treated to a specific strength level, is determined by a series of

laboratory tests on a large number of apparently identical samples of that specific material.

This picture shows a laboratory fatigue specimen. These laboratory samples are optimized for fatigue life. They are

machined with shape characteristics which maximize the fatigue life of a metal, and are highly polished to provide

the surface characteristics which enable the best fatigue life.

Figure 2

A single test consists of applying a known, constant bending stress to a round sample of the material, and rotating

the sample around the bending stress axis until it fails. As the sample rotates, the stress applied to any fiber on the

outside surface of the sample varies from maximum-tensile to zero to maximum-compressive and back. The test

mechanism counts the number of rotations (cycles) until the specimen fails. A large number of tests is run at each

stress level of interest, and the results are statistically massaged to determine the expected number of cycles to

failure at that stress level.

The cyclic stress level of the first set of tests is some large percentage of the Ultimate Tensile Stress (UTS), which

produces failure in a relatively small number of cycles. Subsequent tests are run at lower cyclic stress values until

a level is found at which the samples will survive

10 million cycles without failure. The cyclic stress

level that the material can sustain for 10 million

cycles is called the Endurance Limit (EL).

In general, steel alloys which are subjected to a

cyclic stress level below the EL (properly

adjusted for the specifics of the application) will

not fail in fatigue. That property is commonly

known as "infinite life". Most steel alloys exhibit

the infinite life property, but it is interesting to

note that most aluminum alloys as well as steels

which have been case-hardened by carburizing,

do not exhibit an infinite-life cyclic stress level

(Endurance Limit).

Figure 3


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IS THERE ANY RELATIONSHIP BETWEEN UTS AND FATIGUE STRENGTH?

The endurance limit of steel displays some interesting properties. These are shown, in a general way, in this graph,

(Figure 3) and briefly discussed below.

It is a simplistic rule of thumb that, for steels having a UTS less than 160,000 psi, the endurance limit for the

material will be approximately 45 to 50% of the UTS if the surface of the test specimen is smooth and polished.

That relationship is shown by the line titled "50%". A very small number of special case materials can maintain that

approximate 50% relationship above the 160,000 psi level.

However, the EL of most steels begins to fall away from the 50% line above a UTS of about 160,000 psi, as shown

by the line titled "Polished".

For example, a specimen of SAE-4340 alloy steel, hardened to 32 Rockwell-C (HRc), will exhibit a UTS around

150,000 psi and an EL of about 75,000 psi, or 50% of the UTS. If you change the heat treatment process to

achieve a hardness of about 50 HRc, the UTS will be about 260,000 psi, and the EL will be about 85,000 psi,

which is only about 32% of the UTS.

Several other alloys known as "ultra-high-strength steels" (D-6AC, HP-9-4-30, AF-1410, and some maraging steels)

have been demonstrated to have an EL as high as 45% of UTS at strengths as high as 300,000 psi. Also note that

these values are EL numbers for fully-reversing bending fatigue. EL values for hertzian (contact) stress can be

substantially higher (over 300 ksi).

The line titled "Notched" shows the dramatic reduction in fatigue strength as a result of the concentration of stress

which occurs at sudden changes in cross-sectional area (sharp corners in grooves, fillets, etc.). The highest EL on

that curve is about 25% of the UTS (at around 160,000 psi).

The surface finish of a material has a dramatic effect on the fatigue life. That fact is clearly illustrated by the curve

titled "Corroded". It mirrors the shape of the "notched" curve, but is much lower. That curve shows that, for a badly

corroded surface (fretting, oxidation, galvanic, etc.) the endurance limit of the material starts at around 20 ksi for

materials of 40 ksi UTS (50%), increases to about 25 ksi for materials between 140 and 200 ksi UTS, then

decreases back toward 20 ksi as the material UTS increases above 200 ksi.

WHY IS THE SURFACE SO IMPORTANT?

Fatigue failures almost always begin at the surface of a material. The reasons are that (a) the most highly-stresses

fibers are located at the surface (bending fatigue) and (b) the intergranular flaws which precipitate tension failure

are more frequently found at the surface.

Suppose that a particular specimen is being fatigue tested (as described above). Now suppose the fatigue test is

halted after 20 to 25% of the expected life of the specimen and a small thickness of material is machined off the

outer surface of the specimen, and the surface condition is restored to its original state. Now the fatigue test is

resumed at the same stress level as before. The life of the part will be considerably longer than expected. If that


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process is repeated several times, the life of the part may be extended by several hundred percent, limited only by

the available cross section of the specimen. (ref-3:8:6) That proves fatigue failures originate at the surface of a

component.

IS THE ENDURANCE LIMIT AN EXACT NUMBER?

It is important to remember that the Endurance Limit of a material is not an absolute nor fully repeatable number. In

fact, several apparently identical samples, cut from adjacent sections in one bar of steel, will produce different EL

values (as well as different UTS and YS) when tested, as illustrated by the S-N diagram below. Each of those three

properties (UTS, YS, EL) is determined statistically, calculated from the (varying) results of a large number of

apparently identical tests done on a population of apparently identical samples.

The plot below shows the results of a battery of fatigue tests on a specific material. The tests at each stress level

form statistical clusters, as shown. a curve is fitted through the clusters of points, as shown below. The curve which

is fitted through these clusters, known as an "S-N Diagram" (Stress vs. Number), represents the statistical

behavior of the fatigue properties of that specific material at that specific strength level. The red points in the chart

represent the cyclic stress for each test and the number of cycles at which the specimen broke. The blue points

represent the stress levels and number of cycles applied to specimens which did not fail. This diagram clearly

demonstrates the statistical nature of metal fatigue failure.


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Figure 4

DO REAL-WORLD COMPONENTS

EXHIBIT THE "LABORATORY" EL?

Unfortunate experience has taught engineers

that the value of the Endurance Limit found in

laboratory tests of polished, optimized

samples does not really apply to real-world

components.

Because the EL values are statistical in

nature, and determined on optimized,

laboratory samples, good design practice

requires the determination of the actual EL

will be for each specific application, known as

the Application-Specific Endurance Limit

(ASEL).

In order to design for satisfactory fatigue life (prior to testing actual components), good practice requires that the

"laboratory" Endurance Limit value be reduced by several adjustment factors. These reductions are necessary to

account for:

(a) the differences between the application and the testing environments, and

(b) the known statistical variations of the material.

This procedure is to insure that both the known and the unpredictable factors in the application (including surface

condition, actual load, actual temperature, tolerances, impurities, alloy variations, heat-treatment variations, stress

concentrations, etc. etc. etc.) will not reduce the life of a part below the required value. Please read that paragraph again, and understand it well.

An accepted contemporary practice (ref-2:3:328) to estimate the maximum fatigue loading which a specific design

can survive is the Marin method, in which the laboratory test-determined EL of the particular material (tested on

optimized samples) is adjusted to estimate the maximum cyclic stress a particular part can survive (the ASEL).

This adjustment of the EL is the result of six fractional factors. Each of these six factors is calculated from known

data which describe the influence of a specific condition on fatigue life.


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Those factors are:

1. Surface Condition (ka): such as: polished, ground, machined, as-forged, corroded, etc. Surface is

perhaps the most important influence on fatigue life;

2. Size (kb): This factor accounts for changes which occur when the actual size of the part or the cross-

section differs from that of the test specimens;

3. Load (kc): This factor accounts for differences in loading (bending, axial, torsional) between the actual part

and the test specimens;

4. Temperature (kd): This factor accounts for reductions in fatigue life which occur when the operating

temperature of the part differs from room temperature (the testing temperature);

5. Reliability (ke): This factor accounts for the scatter of test data. For example, an 8% standard deviation in

the test data requires a ke value of 0.868 for 95% reliability, and 0.753 for 99.9% reliability.

6. Miscellaneous (kf): This factor accounts for reductions from all other effects, including residual stresses,

corrosion, plating, metal spraying, fretting, and others.

These six fractional factors are applied to the laboratory value of the material endurance limit to determine the

allowable cyclic stress for an actual part:

Real-World Allowable Cyclic Stress = ka * kb * kc * kd * ke * kf * EL

IS FATIGUE LOADING CUMULATIVE?

It is important to realize that fatigue cycles are accumulative. Suppose a part which has been in service is removed

and tested for cracks by a certified aircraft inspection station, a place where it is more likely that the subtleties

of Magnaflux inspection are well-understood. Suppose the part passes the inspection, (i.e., no cracks are found)

and the owner of the shaft puts it on the "good used parts" shelf.

Later, someone comes along looking for a bargain on such a part, and purchases this "inspected" part. The fact

that the part has passed the inspection only proves that there are no detectable cracks RIGHT NOW. It gives no

indication at all as to how many cycles remain until a crack forms. A part which has just passed a Magnaflux

inspection could crack in the next 100 cycles of operation and fail in the next 10000 cycles (which at 2000 RPM,

isn't very long!).


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Fatigue testing – Part 1

Fatigue as a specific failure mechanism has been recognised since the early part of the nineteenth century but it

was the development of rail travel that resulted in a major increase of interest in this type of fracture.

The premature failure of wagon axles led to Wohler in Germany investigating fatigue failure under rotating loading.

This led to the design of the first standardised test - a reversing stress rotating specimen, illustrated in Fig.1.

Fig.1. Wohler rotating fatigue test

There are many mechanisms that can lead to failure but fatigue is perhaps one of the most insidious since it can

lead to a catastrophic failure with little or no warning - one well known example being the failure of the Comet

aircraft in the 1950s.

Failure can occur at a fluctuating load well below the yield point of the metal and below the allowable static design

stress. The number of cycles at which failure occurs may vary from a couple of hundreds to millions. There will be

little or no deformation at failure and the fracture has a characteristic surface, as shown in Fig.2.

Fig.2. Typical fatigue crack fracture surface

The surface is smooth and shows concentric rings, known as beach marks, that radiate from the origin; these

beach marks becoming coarser as the crack propagation rate increases. Viewing the surface on a scanning

electron microscope at high magnification shows each cycle of stress causes a single ripple. The component finally

fails by a ductile or brittle overload.

Fatigue cracks generally start at changes in section or notches where the stress is raised locally and, as a general

rule, the sharper the notch the shorter the fatigue life - one reason why cracks are so damaging.


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There are two stages in the process of fatigue cracking - a period of time during which a fatigue crack is nucleated

and a second stage where the crack grows incrementally leaving the ripples described above. In an unwelded

component the bulk of the life is spent in initiating a fatigue crack with a shorter period spent in crack propagation.

An unwelded ferritic steel component exhibits an endurance limit - a stress below which fatigue cracking will not

initiate and failure will therefore not occur. This is not the case with most non-ferrous metals or with welded joints -

these have no clearly defined endurance limit.

The reason for this is that in arc welded joints there is an 'intrusion' - a small defect at the toe of the weld, perhaps

only some 0.1mm deep. Provided that the applied stress is sufficiently large a crack will begin to propagate within

an extremely short period of time. The endurance limit for a welded joint is therefore dependent on the intrusion

size that does not result in crack propagation at the applied stress range. In the case of a welded joint, therefore, a

fatigue limit - a 'safe life' is specified, often the stress to cause failure at 2x10 6 or 10 7 cycles.

During fatigue the stress may alternate about zero, may vary from zero to a maximum or may vary about some

value above - or below - zero.

To quantify the effect of these varying stresses fatigue testing is carried out by applying a particular stress range

and this is continued until the test piece fails. The number of cycles to failure is recorded and the test then

repeated at a variety of different stress ranges.

This enables an S/N curve, a graph of the applied stress range, S, against N, the number of cycles to failure, to be

plotted as illustrated in Fig.3. This graph shows the results of testing a plain specimen and a welded component.

The endurance limit of the plain specimen is shown as the horizontal line - if the stress is below this line the test

piece will last for an infinite number of cycles. The curve for the welded sample, however, continues to trend down

to a point where the stress range is insufficient to cause a crack to propagate from the intrusion.

Fig.3. S/N curves for welded and unwelded specimens

By testing a series of identical specimens it is possible to develop S/N curves. In service however, there will be

variations in stress range and frequency. The direction of the load may vary, the environment and the shape of the

component will all affect the fatigue life, as explained later in this article.

When designing a test to determine service performance it is therefore necessary to simulate as closely as

possible these conditions if an accurate life is to be determined. In order to enable the fatigue life to be calculated

when the stress range varies in this random manner, the Palmgren-Miners cumulative damage rule is used.


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This rule states that, if the life at a given stress is N and the number of cycles that the component has experienced

is a smaller number, n, then the fatigue life that has been used up is n/N.

If the number of cycles at the various stress ranges are then added together - n 1 /N 1 + n 2 /N 2 + n 3 /N 3 + n 4 /N 4

etc - the fatigue life is used up when the sum is of all these ratios is 1. Although this does not give a precise

estimate of fatigue life, Miners rule was generally regarded as being safe. This method, however, has now been

superceded with the far more accurate approach detailed in the British Standard BS 7608.

The design of a welded joint has a dominant effect on fatigue life. It is therefore necessary to ensure that a

structure that will experience fatigue loading in the individual joints has adequate strength. The commonest method

for determining fatigue life is to refer to S/N curves that have been produced for the relevant weld designs.

The design rules for this range of joint designs were first developed by TWI and incorporated with the bridge code

BS 5400 in 1980 and then into the industry design rules for offshore structures. Further refinements and

improvements finally resulted in the publication of BS 7608 Code of practice for fatigue design and assessment of

steel structures. This standard will be looked at in more detail in a future article.


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Fatigue testing - Part 2

The article in the September/October issue of Connect established some basic facts about fatigue and the

statement was made that a welded joint exhibited no clearly established fatigue limit as in an unwelded component.

In this article we will be looking at some of the reasons for this behaviour.

It should be mentioned that, in service, few structures experience purely static loads and that most will be

subjected to some fluctuations in applied stresses and may therefore be regarded as being fatigue loaded.

Motorway gantries, for example, are buffeted by the slipstream from large lorries and offshore oilrigs by wave

action. Process pressure vessels will experience pressure fluctuations and may also be thermally cycled.

If these loads are not accounted for in the design, fatigue failure may occur in as few as a couple of tens of cycles

or several million and the result may be catastrophic when it does.

Fatigue failures can occur in both welded and unwelded components, the failure usually initiating at any changes

in cross section - a machined groove, a ring machined onto a bar or at a weld. The sharper the notch the greater

will be its effect on fatigue life.

The effect of a change in section is illustrated in Fig.1, where it can be seen that the stress is locally raised at the

weld toe. The illustration shows a bead-on-plate run but a full penetration weld will show the same behaviour.

Fig.1. Stress concentrating effect of a change in thickness

In addition, misalignment and/or distortion of the joint will cause the applied stress to be further increased, perhaps

by introducing bending in the component, further reducing the expected fatigue life. A poorly shaped weld cap with

a sharp transition between the weld and the parent metal will also have an adverse effect on fatigue performance.

In addition to these geometrical features affecting fatigue life there is also the small intrusion at the weld toe,

mentioned in the last article and illustrated in Fig.2. In an unwelded component the bulk of the fatigue life is spent

in initiating the fatigue crack with a smaller proportion spent in the crack propagating through the structure. In a

welded component the bulk of the fatigue life is spent in propagating a crack. The consequences of this difference

in behaviour are illustrated in Fig.3.


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Fig.2. Weld toe intrusion

Fig.3. Effect of stress concentration on fatigue life

This shows that this small intrusion reduces the fatigue life of a fillet welded joint by a factor of perhaps 10

compared with that of an unwelded item and some eight times that of a sample with a machined hole. The other

consequence is that fatigue cracks in welded joints almost always initiate at the toe of a weld, either face or root.

It may be thought that the use of a higher strength material will be of benefit in increasing fatigue life. The rate of

crack propagation, however, is determined by Young's Modulus - a measure of the elastic behaviour of the metal -

and not simply by tensile strength.

Alloying or heat treatment to increase the strength of a metal has very little effect on Young's Modulus and

therefore very little effect on crack propagation rates. Since the bulk of a welded component's life is spent in

propagating a crack, strength has little or no influence on the fatigue life of a welded item. There is thus no benefit

to be gained by using high strength alloys if the design is fatigue limited. This is illustrated in Fig.4 which shows the

benefits of increasing the ultimate tensile strength of a steel if the component is unwelded or only machined but

how little effect this has on the life of a welded item.


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Fig.4. Effect of increase in tensile strength on fatigue life


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One additional feature in welded joints that set them apart from unwelded or machined items is the presence of

residual tensile stress.

In a welded component there will be stresses introduced into the structure by, for example, assembly stress. These

stresses are long range reaction stresses and from a fatigue point of view have little effect on fatigue life.

Of far greater significance with respect to fatigue are the short range stresses introduced into the structure by the

expansion and contraction of material close to and within the welded joint. Whilst the actual level of residual stress

will be affected by such factors as tensile strength, joint type and size and by run size and sequence, the peak

residual stress may be regarded as being of yield point magnitude. The implications of this are that it is the stress

range that determines fatigue life and not the magnitude of the nominal applied stress.

Even if the applied stress range is wholly compressive and there is apparently no fluctuating tensile stress to cause

a crack to form and grow, the effect of welding residual stress is to make the structure susceptible to fatigue failure.

This is illustrated in Fig.5, where it can be seen that, irrespective of the applied stress, the effective stress range is

up to the level of residual stress at the welded joint.

Fig.5. Effect of residual stress on stress range

It would seem reasonable, therefore, that a post-weld stress relief treatment would be of benefit to the fatigue life

by reducing the residual stresses to low levels. This is only true, however, where the applied stress range is partly

or wholly compressive. If the applied stress range is all tensile, research has shown that as-welded and stress

relieved components have almost identical fatigue performances with only a marginal improvement in the stress

relieved joints.

This is the result of the bulk of the fatigue life of a welded joint being spent in crack propagation where propagation

rates are only marginally affected by mean stress. It may be difficult therefore to justify the cost of stress relief if the

only criterion is that of improving fatigue life.

The methods of determining fatigue performance of welded joints, as detailed in BS 7608, and how fatigue

performance can be improved will be dealt with in the next Connect article.


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Fatigue testing Part 3

What will have become obvious from the previous two articles on fatigue is that a welded joint behaves in a

radically different way from an unwelded item, even if this item contains a significant stress raiser.

The last article, number 79, made the statement that a welded joint exhibits no clearly defined fatigue limit, the limit

varying dependent upon the joint type and weld quality. It is vitally important to understand this if fatigue analysis of

welded joints is to be carried out.

As mentioned earlier, rules for the design of components subject to fatigue loading were produced by TWI and

these were incorporated into the design rules in BS 5400, the British bridge design code. These rules were later

adopted by the offshore industry for offshore structures and adaptations of these rules now appear in many other

specifications such as BS PD 5500 Unfired pressure vessels and BS 8118 Structural use of aluminium.

The basis of all the rules is a system whereby various joint designs are assigned a 'classification' related to the

joint's fatigue performance. Fig.1 is an example of how this classification has been formalised in BS 7608 - the

same or similar methods will be found in other application standards.

Fig.1. Examples of joint classification from BS 7608

In BS 7608 each joint type is assigned a classification letter. For example, a plate butt weld with cap and root

ground flush is class 'C', an undressed plate butt weld class 'D' and a fillet weld class 'F' ( Fig.2).


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Fig.2. Effects of joint classification on fatigue life

For each classification a fatigue curve has been developed and from these curves the design life can be predicted.

This is obviously an over-simplification of what can be a very complicated task -the forces acting on a joint arising

from changes in temperature, changes in internal or external pressure, vibration, externally applied fluctuating

loads etc can be complex and difficult to determine.

Whilst the joint design has a major effect on design life and is the basis for calculating service performance, the

weld quality also has a decisive effect - any fatigue analysis assumes that the welds are of an acceptable quality

and comply with the inspection acceptance standards. However, in practice it is not always possible to guarantee a

'perfect' weld and cracks, lack of fusion, slag entrapment and other planar defects may be present, reducing the

fatigue life, perhaps catastrophically.

Other less obvious features will also have an adverse effect. Excessive cap height or a poorly shaped weld bead

will raise the stress locally and reduce the design life; misalignment may cause local bending with a similar effect.

Good welding practices, adherence to approved procedures and competent and experienced staff will all help in

mitigating these problems.

In some applications an as-welded joint will not have a sufficient design life and some method of improving the

fatigue performance needs to be found. There are a number of options available. The first and perhaps simplest is

to move the weld from the area of highest stress range, the next is to thicken up the component or increase the

weld size. Note that, as mentioned in the earlier article, using a higher strength alloy will not improve the fatigue life.

Local spot heating to induce compressive stresses at the weld toes will also help, although this needs very

accurate positioning of the heated area and very careful control of the temperature if an improvement is to be seen

and the strength of the metal is not to be affected. For these reasons, spot heating for fatigue improvements has

been virtually discontinued.

Hammer peening with a round nosed tool or needle gun peening gives very good results although the noise

produced may prevent their use. Shot peening can also be used to introduce compressive stresses at weld toes

with equally good results. Compressive stresses can be induced in a component by overstressing - a pressure test

of a pressure vessel is a good example of this - where local plastic deformation at stress raisers induces a

compressive stress when the load is released. This technique needs to be approached with some care as it may

cause permanent deformation and/or any defects to extend in an unstable manner resulting in failure.


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Although the next techniques described are not as beneficial as hammer peening of the weld toes they have the

advantage of being more consistent and easier to control. The techniques rely upon dressing the weld toes to

improve the shape and remove the intrusion mentioned in article 79. The dressing may be carried out using a TIG

or plasma-TIG torch which melts the region of the weld toe, providing a smooth blend between the weld face and

the parent metal.

Alternatively the toe may be dressed by the careful use of a disc grinder but for best results the toe should be

machined with a fine rotary burr as shown in Fig.3 and 4. Great care needs to be exercised to ensure that the

operator does not remove too much metal and reduce the component below its minimum design thickness and that

the machining marks are parallel to the axis of the main stress. Ideally the dressing should remove no more than

0.5m depth of material, sufficient to give a smooth blend and remove the toe intrusion. The results of these

improvement techniques are summarised in Fig.5.

Fig.3. Grinding tools Fig.4. Burr machining of weld toes

Fig.5. Improvement in fillet weld fatigue life

Whilst fatigue has resulted in some catastrophic and unexpected failures, the improvements in design life

calculation methods, particularly the use of powerful software packages allowing detailed finite element analyses to

be performed, has enabled engineers to approach the design of fatigue limited structures with far more confidence.

This still means, however, that the designer has to recognise the effect of welds in the structure and must consider

all possible sources of loading and ALL welds, even non-load carrying attachments that may be thought to be

unimportant to service performance.


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Samples failure

Bolt Failure

Steel bolt

This high tensile steel bolt failed

under low stress high cycle

conditions with a fatigue crack

running from 9 o'clock as shown

by the beach marks. The SEM

image of the fatigued surface

(shown left) is found to have no

striations due to the high yield

strength and high cycle

conditions.


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Stub Axle failure

Stub axle 1

This is the classic reverse

bending fatigue of a steel stub

axle from a road vehicle. Notice

cracks have grown from 8 o'clock

upwards and to a lesser extent

from 2 o'clock downwards. The

rough central region is the final

ductile rupture.

Bending fatigue fracture

Reversed bending fatigue fracture starting from keyway

This 100 mm diameter steel shaft

failed after a long period of service

on a large dumper truck. The

keyway terminated in a

circumferential groove

approximately half the depth of the

keyway. Fatigue cracks initiated at


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Fatigue cracks in steel cycle frame

Although steel has a fatigue

endurance limit certain parts of the

frame are stressed above this limit

and are prone to fatigue cracking.

The rear triangle, comprising the

chainstays and seatstays are

particularly vulnerable.

On the opposite side, the

associated cracked point had

caused surface corrosion. Notice

that the cracks had occurred close

to the brazed-on cross member.

The heat affected zone associated

with brazing and welding usually

reduces the endurance limit and

raises the chance of fatigue crack

initiation.

A metallurgical investigation of the extent of the heat affected zone is in

progress. The manufacturers did not fulfil their "lifetime" guarantee in

this instance by suggesting that damaged paintwork caused corrosion,

which in turn initiated fatigue.


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Method of revealing fatigue cracks:

PHOTOELASTICITY ; a method of examining transparent polymer models of structures

etc. to isolate stress concentrations and other weak zones. The model is placed between

crossed circular polarizing filters (eg Polaroid sheets) and a force applied. The technique

also enables residual stress to be shown in transparent articles.

The use of Photoelasticity

Stress fields (applied and

residual) can be exposed using

models of structures in

photosensitive material placed

between polarising filters in the

crossed polar position.

Here the stresses in a 7

member model bridge truss,

centrally loaded and simply

supported are shown.

These injection moulded safety

spectacles contain residual

moulding stresses shown here

using photoelastic viewing

techniques.


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Dye-penetrant testing

Dye-penetrant testing; a method of

examining components to detect surface-

breaking flaws, such as cracks. The

technique is based on the ability of a liquid to

be drawn into a "clean" surface-breaking flaw

by capillary action.

It is essential that the component to be

inspected is thoroughly cleaned to remove all

traces of dirt and grease.

It is then sprayed with a penetrating liquid,

usually a brightly coloured liquid or a

fluorescent dye, which penetrates any

surface-breaking cracks or cavities.

The liquid is allowed to soak into the

components surface. (Fig1 right)

After soaking, the excess liquid penetrant is

wiped from the surface and a developer

applied. The developer is usually a dry white

powder, which draws the penetrant out of any

cracks by reverse capillary action to produce

indications on the surface.(Fig2 right)

These (coloured) indications are broader

than the actual flaw and are therefore more

easily visible.

Fluorescent penetrants are normally used

with a UV lamp to enhance sensitvity.

These systems are often used to check weld

quality during fabrication.

Fig1

Fig2


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Another example of dye penetrant testing used on bicycle components. Select images below for larger picture.

This cycle crank arm was returned to

the supplier after a very short time in

use. The owner had seen a crack

coming from the square taper axle

attachment and suspected a smaller

crack close to the pedal thread

The suspect areas were sprayed with

red penetrant dye and left to soak. The

square hole location was clearly

cracked but the minor region may be a

surface scratch.

Such distinctions are very important in

the performance of engineering

components.

After the dye was cleaned off the

component was sprayed with chalk

developer. The crack running from the

square axle drive hole gave a very

distinct red indication at its precise

location, indicating it was clearly

cracked. The other feature showed no

red line on development indicating a

surface scratch not an embyronic crack.


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The crank spider arm of this chainset

fractured and unseated the cyclist in

heavy city traffic.

It was old but well looked after and

cleaned regularly The growing fatigue

crack was undetected until the

dangerous failure.

The cyclist was concerned if a similar

crack had been nucleated in the

matching plain crank arm shown

alongside.

The equivalent area was sprayed with

red penetrating dye which was then left

to soak into any cracks or fissures in

the component.

After several minutes of soaking the

dye was cleaned off the components

surface.

The pre-soaked and cleaned area was

then sprayed with developer spray

which is basically chalk powder in a

volatile carrier. Any defects present

show up as the red dye is pulled out of

any cracks or fissures in the

component.

It was concluded that none were

present . The only red marks were from

dye that had been retained in the

stamped product identification marks.


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Thermal Cracks on Commuter Wheel Tread

Fatigue Crack Profile


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Causes and Recognition of Fatigue Failures

General Causes of Material Failures:

• Design deficiencies

• Manufacturing deficiencies

• Improper and insufficient maintenance

• Operational overstressing

• Environmental factors (i.e. heat, corrosion, etc.)

• Secondary stresses not considered in the normal operating conditions

• Fatigue failures

Improper and insufficient maintenance seems to be one of the most contributing factors influenced by some

improper designs such as areas that are hard to inspect and maintain and the need for better maintenance

procedures. In many circumstances the true load is difficult to predict resulting in a structure being stressed

beyond its normal capabilities and structural limitations. When a structure is subject to cyclic loads, areas subject

to fatigue failure must be accurately identified. This is often very hard to analyze, especially in a highly composite

structure for which analysis has a high degree of uncertainty. Thus, in general, experimental structural fatigue

testing is frequently resorted to.

Recognition of Fatigue Failure

Two fatigue zones are evident when investigating a fracture surface due to fatigue, the fatigue zone and the

rupture zone. The fatigue zone is the area of the crack propagation. The area of final failure is called the rupture or

instantaneous zone. In investigation of a failed specimen, the rupture zone yields the ductility of the material, the

type of loading, and the direction of loading. The relative size of the rupture zone compared with the fatigue zone

relates the degree of overstress applied to the structure. The amount of overstressing can be determined from the

fatigue zone as follows: highly overstressed if the area of the fatigue zone is very small compared with the area of

the rupture zone; medium overstress if the size or area of both zones are nearly equal; low overstress if the area of

rupture zone is very small. Figure 1 describes these relations between the fatigue and rupture zones.

Figure 1 Fracture appearances of fatigue failures in Bending by Dr. Charles Lipson,"Why Machine Parts Fail,"

Machine Design, Penton, Cleveland 13, Ohio. From Metal Fatigue: Theory and Design, ed. A.F. Madayag, pg. 5.

The fatigue zone can be described as follows: a smooth rubbed, and velvety appearance, the presence of waves

known as "clam-shells" or "oyster-shells", "stop marks" and "beach marks," and the herringbone pattern or

granular trace which shows the origin of the crack. In general, stop marks indicate the variations in the rate of

crack propagation due to variations in stress amplitude in a cyclic application varying with time. Figure 2 is a

schematic representation of the fatigue zone.


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Figure 2 Typical fatigue zone with identifying marks. From Metal Fatigue: Theory and Design, ed. A.F. Madayag,

pg. 3.

Design Considerations

Even if careful attention to good design practices is constantly the goal of design engineers, fatigue problems are

sometimes introduced into the structure. Fatigue failures are often the result of geometrical or strain discontinuities,

poor workmanship or improper manufacture techniques, material defects, and the introduction of residual stresses

that may add to existing service stresses.

Typical factors affecting fatigue include the following: Stress raisers, usually in the form of a notch or inclusion;

most fatigue fractures may be attributed to notch effects, inclusion fatigue specimens are rare. High strength

materials are much more notch-sensitive than softer alloys. Corrosion is another factor that affects fatigue.

Corroded parts form pits that act like notches. Corrosion also reduces the amount of material which effectively

reduces the strength and increases the actual stress. Decarburization, the loss of carbon from the surface of the

material, is the next factor. Due to bending and torsion, stresses are highest at the surface; decarburization

weakens the surface by making it softer. Finally, residual stresses which add to the design stress; the combined

effect may easily exceed the limit stress as imposed in the initial design.

Influence of Processing and Metallurgical Factors on Fatigue

A myriad of factors affect the behavior of a material under fatigue loading. Obvious factors include the sign,

magnitude, and frequency of loading, the geometry and material strength level of the structure and the ambient

service temperature. However, processing and metallurgical factors are not often considered, but these factors

determine the homogeneity of materials, the sign and distribution of residual stresses, and the surface finish. Thus,

processing and metallurgical factors have an overriding influence on the performance of a structure.

Processing Factors

Stresses are normally highest at the surface of a structure, so it follows that fatigue usually initiates at the surface.

Stress raisers are more likely to be present as a result of surface irregularities introduced by the design of the

structure or produced in service or resulting from processing. Processing factors can introduce a detrimental or

beneficial effect into a structure, usually in the form of effect on strength level or residual stress condition of the

surface material. Therefore, the effect of processing on the mechanical properties of a material, especially the

surface of the material, directly affects fatigue properties. Processing factors that influence the fatigue life of a

structure include the following: the process by which a part is formed, such as die casting; the heat treatment of a

material, such as quenching, which builds up residual stresses and annealing, which relieves internal stress (see

Figure 3); case hardening, such as carburization or nitriding, which increases surface hardness and strength (see


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Figure 4); surface finish, such as polished smooth by electropolishing; cold working, which increases strength; also,

cladding, plating, chemical conversion coatings, and anodizing.

Figure 3 Effect of hardness on the fatigue life of threads rolled before and after heat treatment. From Metal

Fatigue: Theory and Design, ed. A.F. Madayag, pg. 82.

Figure 4 Bending fatigue test results on sections from crankshafts: endurance limit versus surface treatment. From

Metal Fatigue: Theory and Design, ed. A.F. Madayag, pg. 70.

Metallurgical Factors

Metallurgical factors refers to areas within the material, wither on the surface or in the core, which adversely affect

fatigue properties. These areas may arise from melting practices or primary or secondary working of the material

or may be characteristic of a particular alloy system. In virtually all instances the detriment to fatigue properties

results from a local stress-raising effect. Therefore, metallurgical factors affecting fatigue include the following:


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surface defects, sub-surface and core defects, inhomogeneity, anisotropy, improper heat treatment, localized

overheating, corrosion fatigue, and fretting corrosion.

Experimental Analysis of Fatigue

Fatigue Life Curves

Failure due to repeated loading is known as fatigue. A small crack, a scratch, or some other such minor defect

causes localized deformation. This deformation leads to a small crack if one was not initially present. After cyclic

loading, that is, loading in the same way multiple times, the crack grows, and eventually the material fails. A fatigue

life curve is a graphical representation of the cyclic loading. Simply, a fatigue life curve, also known as an S-N

curve is a plot of the stress amplitude versus the number of cycles the material goes through before it fails. That is,

for a certain stress, the material will fail within a certain number of cycles. Figure 5 is an example of a typical

fatigue life curve.

Figure 5 Typical Fatigue Life Curve. From Mechanical Behavior of Materials Laboratory, Prepared by Staff of

Engineering Science and Mechanics, Virginia Polytechnic Institute, pg. 8-3.

To help understand the concept of fatigue life curves, an experiment that may be performed easily by anyone is

presented here.

Fatigue Life Curves Experiment

What you need:

• 4 Jumbo Paper Clips

• A Copy of Figure 6

What to do:

1. Bend four paper clips at each of the four bending angles indicated in Figure 6 until each fails, counting the

number of times the paper clip is bent. For instance, bend a paper clip 45, then bend it to -45, then back to

zero. This is one cycle.

2. Record the data in Table 1.

3. Using a spreadsheet program, plot the number of cycles versus the angle. The angle represents the stress

amplitude.


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Figure 6 Bending angle guide. Place paper clip in a vertical plate and bend in the middle using this guide to judge

the angles. From Mechanical Behavior of Materials Laboratory, Prepared by Staff of Engineering Science and

Mechanics, Virginia Polytechnic Institute, pg. 8-6.

Table 1: Table For Experimental Data

Cycles to Failure Angle,

? 90

? 45

? 20

? 10

An example of this experiment:

Table 2: Experimental Data

Cycles to Failure Angle,

1.5 90

7 45

29 20

79 10


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Fatigue Crack Growth

If an engineering component contains a crack, and if a cyclic or repeated load is applied, the crack is likely to grow

slowly with increasing number of load cycles. This process is known as fatigue crack growth. In a fatigue crack

growth experiment, the progress of a crack growing under a cyclic load is measured, and the results are plotted as

a fatigue crack growth rate curve, da/dN versus K (that is, change in crack length divided by change in number of

cycles to failure versus change in fracture toughness). A typical fatigue crack growth curve is shown in Figure 7.

Figure 7 Crack growth rates obtained from adjacent pairs of a vs. N data points. From [Dowling 93] p. 465; ©1993

by Prentice Hall, Upper Saddle River, NJ. From Mechanical Behavior of Materials Laboratory, Prepared by Staff of

Engineering Science and Mechanics, Virginia Polytechnic Institute, pg. 9-3.


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In the simplest form of a fatigue crack growth rate test, a cyclic load is applied that has fixed maximum and

minimum loading levels. The test specimen is usually a plate of material in which a crack has already been started

at the end of a V-bottom machined slot. In a typical fatigue crack growth experiment, the sample is loaded in a

closed-loop servohydraulic testing machine and data for crack length, number of cycles to failure, and fracture

toughness is recorded. From this data the mechanical behavior for a certain material can be described under

fatigue crack growth loading by the fatigue crack growth rate curve. This sort of experiment is useful for materials

that would undergo high cyclic loading stresses such as an airplane wing or a helicopter rotor.

Low Cycle Fatigue

Low cycle fatigue is the repeated cyclic loadings that cause significant plastic deformation in a material and may

cause fatigue cracking after a relatively small number of cycles-hundreds or thousands. Low cycle fatigue typically

occurs as a result of repeated localized yielding near stress raisers, such as holes, fillets, and notches, despite the

elastic deformation occurring over the bulk of the component. Uniaxial testing is performed on several smooth

(unnotched) specimens under different cyclic deformation levels in a typical low cycle fatigue test. Each specimen

follows a given constant stress amplitude, completely reversed, cyclic strain. That is, the mode of testing is strain

control instead of stress control. Stress response is monitored during cyclic loading, and the number of cycles to

failure is recorded for these tests. The results from several tests are necessary to determine the cyclic stress-strain

curve and the strain life curve for the material. A schematic representation of a completely reversed controlled

strain test is shown in Figure 8.

Figure 8 Completely reversed controlled strain test and two possible stress responses, namely cycle-dependent

hardening and softening. From [Landgraf 70]; ©ASTM. From Mechanical Behavior of Materials Laboratory,

Prepared by Staff of Engineering Science and Mechanics, Virginia Polytechnic Institute, pg. 10-2.

If a sufficiently high strain level is reached, yielding may occur before the maximum strain is reached on each cycle

of loading. Stress amplitude usually varies; if it increases, the material is said to cyclically harden, if it decreases,


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the material is said to cyclically soften. However, this behavior tends to stabilize such that the variation in the

stress amplitude is small after an initial period of transient hardening or softening. Once the behavior is stabilized,

a closed stress-strain hysteresis loop is formed during each strain cycle. This hysteresis loop typically looks like

Figure 9. The area inside the hysteresis loop is the energy absorbed per unit volume of the material. This energy

mostly dissipates as heat. Fatigue failure results under repeated cycling and the life is measured by the number of

strain cycles to failure. A strain life curve may be plotted from the data contained in the hysteresis loop as is shown

in Figure 10. This sort of experiment is useful in analysis of parts where the material is strong enough to withstand

the cyclic loading it may go through, but fails due to fracture at a bolt hole or other such stress raiser. An example

of this is a structural member bolted to another structural member which fails due to a crack originating from the

bolt hole.

Figure 9 Stable stress-strain hysteresis loop. From Mechanical Behavior of Materials Laboratory, Prepared by

Staff of Engineering Science and Mechanics, Virginia Polytechnic Institute, pg. 10-2.

Figure 10 Elastic, plastic, and total strain vs. Life curves. Adapted from [Landgraf 70]; ©ASTM. From Mechanical

Behavior of Materials Laboratory, Prepared by Staff of Engineering Science and Mechanics, Virginia Polytechnic

Institute, pg. 10-5.


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Real Life-Design and Manufacturing Considerations

The following describes a relationship between factors that shape the S-N curves as they are influenced by design

and manufacturing conditions and the effects of such conditions on the fatigue properties of materials, components,

and structures.

Recommendations for Designs to Avoid Fatigue Failures

A designer can help to minimize the possibility of fatigue failure by proper design of structural components. Many

fatigue failures may be attributed to lack of sufficient consideration of design details or a lack of appreciation of

engineering principles. These principles, which are an integral part of good design of structures subject to fatigue

are well reported in literature, but this information has been scattered throughout sources and may be inaccessible

to a designer who needs to understand and utilize the principles. It is good design practice to seek out sources of

this information and to utilize the principles before, during and after the design process.

Fatigue Considerations in Helicopter Design and Service

The helicopter is subject to many fatigue considerations that fixed wing aircraft are not, such as the significant

oscillatory loads due to the harmonic content of aerodynamic loads from the combined rotational and translation

blade motion through the air, also the cyclic loading of the rotor start-stop cycles, transient maneuver loads, gust

loads, coriolis loads, and torque loads. These are just a small example of the many unique fatigue loading

considerations in helicopter design. Thus one may realize the complexity of helicopter design and service. The

helicopter is one of many vehicles and structures which undergo unique fatigue loading, therefore, fatigue

considerations are an important part of the design process.

Figure 11 A tension fatigue failure of a helicopter rotor blade flapping link. Fatigue crack originated at arrow B,

propagated to arrows. From "Metal Fatigue and Its Recognition," Civil Aeronautics Board, Bureau of Safety,

Bulletin No. 63-I, April 1963, by Frank R. Stone, Jr. From Metal Fatigue: Theory and Design, ed. A.F. Madayag, pg.

10.

Summary

In summary, fatigue plays an important role in all areas of the engineering process, from design to manufacture to

service during the life of a product or structure. All engineers must carefully study the effects and the many types of

fatigue, no matter what field of engineering and what part of the engineering process.


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Additional Scanning Electron Microscope Images

Scanning Electron Microscopy

Scanning Electron Microscope (SEM) image of intergrannular fracture indicative of hydrogen embrittlement. (Mag: 400X)

Scanning Electron Microscope (SEM) image of fatigue striations indicative of cyclic crack propagation. (Mag: 700X)

Scanning Electron Microscope (SEM) image of "beach marks" indicative of a progressive fatigue failure. The area of fatigue initiation is noted at the arrow. (Mag: 180X)


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Scanning Electron Microscope (SEM) image of the surface of a casting void in the fracture surface of an aluminum casting. (Mag: 1,000X)

Scanning Electron Microscope (SEM) image of a brittle fracture surface in an aluminum casting. The angular particles in the surface are silicon particles that contribute to the brittleness of the material. (Mag: 1,000X)

Scanning Electron Microscope (SEM) image of microbiological activity in a fire protection system piping. (Mag: 700X)

Metallography/Microstructure Evaluation

The properties of a material and its performance in a specific application depends on its microstructure. Our metallographs (light microscopes) are capable of examinations at magnifications from 15X to 1,000X. Analyses of microstructure and material defects in cross-sectioned samples determine material properties, flaw characteristics, and defect mechanisms.

Metallurgical Technologies, Inc. (MTi) has full metallographic preparation capabilities from sectioning and mounting the specimen through the grinding and polishing stages to proper selection and etching techniques of the tested material.


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View of intergranular stress corrosion cracking (IGSCC) in an Inconel heat exchanger tube. Note that the crack follows the grain boundaries. (Mag: 500X)

View of chloride stress corrosion cracking in a 316 stainless steel chemical processing piping system. Chloride stress corrosion cracking in austenitic stainless steel is characterized by the multi-branched "lightning bolt" transgranular crack pattern. (Mag: 300X)

Microstructure evaluation of the heat-affected zone of a welded stainless steel piping flange etched to reveal the carbide distribution. Fine carbide particles outline the grain boundaries, indicating a "sensitized" condition resulting in susceptibility to intergranular corrosion. (Mag: 600X)

A cross-section through a seam weld in a 400 series ferritic stainless steel tube. The seam exhibits a wide fusion zone and a large grain size contributing to brittleness of the weldment. (Mag: 25X)

Cyclic Fatigue Cracks Propagated by a Rust Pit (stress corrosion) Again, many of the high strength steel alloys are susceptible to stress corrosion. The photos illustrate such a failure. The first picture is a digital photo with an arrow pointing to the double origin of the fatigue cracks. The second photograph at 30X magnification shows a third arrow pointing to the juncture of the cracks propagating from the rust pits. L-19, H-11, 300M and Aeromet 100, are particularly susceptible to stress corrosion and must be kept well oiled and never


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Slide 1 : Typical fatigue failures in steel components.

Slide 2 : Striations in an aluminium alloy.


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Slide 3 : Fatigue failures in the Alexander L Kielland platform.

Slide 4 : Fatigue crack initiation at an inclusion in a high strength steel alloy.


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The s canning electron microscope operates by focussing a beam of electrons on a

Fractures are analyzed using the latest scanning electron microscopy (SEM) and other metal testing techniques. You receive a comprehensive written report with photographic documentation showing each stage of the laboratory analysis. We identify the cause of failure and recommend correction of material processing such as heat treatment, plating, machining, and/or design to prevent recurrence of the problem.


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FATIGUE FRACTURE Fatigue fractures are generally considered the most serious type of fracture in machinery parts simply because fatigue fractures can and do occur in normal service, without excessive overloads, and under normal operating conditions. Fatigue fractures are serious because they are insidious; that is, they are frequently "sneaky" and can occur without warning that anything is amiss. Obviously, if service is abnormal as a result of excessive overloading, corrosive environments, or other conditions, the possibility of fatigue fracture is increased. Let us consider the definition of fatigue that is commonly accepted:

The phenomenon leading to fracture under repeated or fluctuating stresses having a maximum value less than the tensile strength of the material. Fatigue fractures are progressive, beginning as minute cracks that grow under the action of the fluctuating stress.

Stages of fatigue fracture The definition above refers to fracture "under repeated or fluctuating stresses having a maximum value less than the tensile strength." (The final fracture may have either brittle or ductile characteristics, depending upon the metal involved and the circumstances of the stress and the environment.) There are three stages of fatigue fracture: initiation, propagation, and final rupture. Indeed, this is the way that most authors refer to fatigue fracture, for it helps to simplify a subject that can become exceedingly complex.

1. Stage 1- Initiation. Initiation is the most complex stage of fatigue fracture and is the stage most rigorously studied by researchers. The most significant factor about the initiation stage of fatigue fracture is that the irreversible changes in the metal are caused by repetitive shear stresses. The accumulation of microchanges over a large number of load applications, called "cumulative damage," has been the subject of study over the years." (Obviously, if this stage can be prevented, there can be no fatigue fracture.) The initiation site of a given fatigue fracture is very small, never extending for more than two to five grains around the origin. At the location of a severe stress concentration, the number depends on the geometry of the part as well as on environmental, stress, metallurgical, and strength conditions, as will become apparent.

2. Stage 2 - Propagation. The propagation stage of fatigue causes the microcrack to change direction and grow perpendicular to the tensile stress. The second, or propagation, stage of fatigue is usually the most readily identifiable area of a fatigue fracture.

3. Stage 3 - Final Rupture. As the propagation of the fatigue crack continues, gradually reducing the cross-sectional area of the part or test specimen, it eventually weakens the part so greatly that final, complete fracture can occur with only one more load application. The fracture mode may be either ductile (with a dimpled fracture surface) or brittle (with a cleavage, or perhaps even intergranular, fracture surface) or any combination thereof, depending upon the metal concerned, the stress level, the environment, etc. Stage 3 represents the "last straw" that broke the camel's back, to borrow a metaphor.


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Microscopic characteristics of fatigue fracture Striations are the most characteristic microscopic evidence of fatigue fracture, although striations are not always present on fatigue fracture surfaces, as will be seen. However, each time the crack is opened by a tensile stress of sufficient magnitude, creating a tiny ridge, or striation, on each of the mating fracture surfaces. If the maximum cyclic load remains constant, the striations near the fatigue origin are extremely small and closely spaced; the crack grows at a slow rate because the part is still quite strong. However, as the crack gradually propagates, the spacing between striations increases and the crack grows at an increasingly rapid rate because the crack greatly weakens the section. Eventually, complete final fracture

(stage 3) and separation occur. Unfortunately, striations are not always visible on fatigue fracture surfaces for a variety of reasons: On very hard or very soft metals. Artifacts caused by rubbing or other postfracture damage may produce parallel ridges that resemble striations. Certain lamellar microstructures in metals, resemble fatigue striations. However, careful study in the electron microscope will reveal that the orientation of the platelets varies randomly from one location to another, whereas true striations are generally concentric around the origin.

Macroscopic characteristics of fatigue fracture

Information can be learned about a fatigue fracture with only macroscopic examination. That is, study with the unaided eye and relatively low magnification - up to perhaps 25 to 50 times magnification - is usually the most important single way to study and analyze fatigue fractures. Lack of Deformation Since initiation of fatigue fracture does not require a high stress, there is usually little or no deformation in a part or specimen that has fractured by fatigue. If the maximum stress did not exceed the yield strength (actually the elastic limit), there can be no gross plastic, or permanent, deformation, although the final rupture region may have some obvious macroscopic deformation. The typical fatigue fracture that occurs in most load-bearing parts, which have relatively low-stress, high-cycle loading. Not only the fracture surface but the entire part should be examined for deformation. For example, if a unidirectional (one-way) bending fracture is observed, it is useful to carefully reassemble the pieces to determine if there was gross deformation in the part prior to fracture. Of course, the origin of the fracture would be on the convex side, which is the tension side in bending. As pointed out at the beginning of this section, in a "true" highcycle fatigue fracture, there will be no deformation in the fatigue region, provided that there has been no postfracture damage to the fracture surface. If the final rupture region (stage 3) is ductile, the resulting deformation will prevent close realignment of the fractured pieces; however, if the final rupture region is a truly brittle fracture, there should be no gross deformation, except for postfracture damage. A partially ductile/brittle final rupture region probably will show some degree of deformation. Beachmarks "Beachmarks" are a unique feature found in many fatigue fractures, and their presence is a positive means of identifying fatigue fractures. Beachmarks also have been called "stop marks," "arrest marks," "clamshell marks," and "conchoidal marks," all in an attempt to describe their origin or characteristic appearance. The term "beachmarks" is the most commonly used term but is not really as descriptive as some of the others. At any rate, this term is used to describe macroscopically visible marks or ridges that are characteristic of interruptions in the propagation periods (stage 2) of fatigue fractures in relatively ductile metals.


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Beachmarks must not be confused with striations, although they frequently are present on the same fracture surface; there may be many thousands of microscopic striations between each pair of macroscopic beachmarks.

Ratchet Marks The term "ratchet marks" is used to describe features that are very useful in identification of fatigue fractures and in locating and counting the number of fatigue origins. These marks are essentially perpendicular to the surface from which fatigue fractures originate. Therefore, in circular, shaft-like parts, the ratchet marks are essentially radial, pointing toward the center; in flat parts, such as leaf springs, they initially are perpendicular to the surface but may curve if the bending is unidirectional.

The ratchet marks are not the origins themselves; each ratchet mark separates two adjacent fatigue fractures. As the cracks become deeper, the cracks from each origin tend to grow together and become essentially one fatigue fracture that has numerous origins. The number of ratchet marks equals or is one less than the number of origins; thus recognition of the number of ratchet marks is important in determining the number of origins.

Similarities Between Striations and Beachmarks

• Both striations and beachmarks identify the position of the tip of the fatigue crack at a given point in time.

• Both striations and beachmarks expand from the fatigue origin or origins, often in a circular or semicircular fashion.

• Both striations and beachmarks are relatively parallel ridges which do not cross similar features from another origin.

Some fatigue fracture surfaces have neither striations nor beachmarks. Artifacts, or false features, can confuse observation of both striations and, beachmarks. Differences Between Striations and Beachmarks

• The most obvious difference between striations and beachmarks is size. Striations are extremely small ridges, visible only with an electron microscope. Beachmarks are much larger than striations. If they are present, they are normally visible to the unaided eye.

• The other difference between striations and beachmarks, as previously mentioned, is the factors that cause them. Striations represent the advance of the crack front by one load application in many ductile metals, whereas beachmarks locate the position of the crack front when repetitive, fluctuating loading was stopped for a period of time.

OTHER TYPES OF FATIGUE

Fatigue Under Compression Forces A seemingly puzzling type of fatigue crack is one that grows in a part, or a region of a part, that is stressed in compression when the part is under load. It may be recalled that the general principle of mechanically induced residual stresses is that "tensile yielding under an applied load results in compressive residual stresses when the load is released, and vice versa." This type of fatigue fracture can be illustrated with the example of the Belleville spring washer, a hardened spring steel washer that is cone-shaped in order to give high spring rates in limited space. When this type of spring washer is pressed flat, the upper surface and corner of the hole are heavily compressed


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in a triangular pattern around the hole. The compressive stress may be so high that the metal yields compressively around the hole. That is, the circumferential compressive stress on the upper side of the plate at the hole forces the metal to yield compressively when the spring washer is pressed flat. When the fatigue load is released, the washer springs back to its original position and the upper surface now is stressed in residual tension as a result of compressive yielding when the load was applied. Consequently, the fatigue action causes the necessary tensile stress at this location when the load is released, not when it is applied. With continued operation, radial fatigue cracks will form at the upper corner. The solution for this "compression" type of cracking often lies in the field of design, as well as in heat treatment to increase yield strength.

Thermal Fatigue A somewhat similar type of fatigue fracture is caused not by repetitive mechanical stresses but by cyclic thermal stresses. Review of the section on thermal residual stresses in Chapter 7 will help the analyst understand how thermal residual stresses are formed. The basic principle to recall is that the metal that cools last will have tensile residual stresses; also, for thermal residual stress to occur, it is necessary to have both heat and restraint. If, for example, a sharp edge of a part is repetitively heated and cooled while the restraining bulk of the part remains relatively cool, the sharp edge will expand when heated and contract when cooled. That is, the sharp edge will develop compressive forces and yield compressively when hot because of the lower strength and lower modulus of elasticity at the elevated temperature. When the edge is cooled, tensile residual stresses will form; if this action is repeated many times, thermal fatigue cracks will develop and will tend to grow each time the metal is cooled from the elevated temperature, for it is at that time that the tensile residual stress is again applied. When the metal is hot, the cracks will tend to close, although this will probably be resisted by an accumulation of oxide scale or other products of combustion from the high-temperature atmosphere. In fact, these products in the crack can act as a wedge and increase the compressive yielding when hot, so that the tensile residual stress that forms on cooling is increased. Since both heat and restraint are necessary for thermal residual stresses to occur and repetitive thermal cycling is necessary for thermal fatigue, the methods to prevent such fracture should become obvious. If restraint is internal within the part, as described above with the sharp edge, it may be possible to reduce the restraint by permitting the part to expand more uniformly when it is heated, instead of having just the sharp edge expand. If this is not practical, it may be possible to blunt the sharp edge by changing the shape, so that this action is not concentrated on a thin section. Or it may be possible to reduce the thermal gradient within the part by permitting more of the metal to be heated, or to design curves into the part so that expansion and contraction forces simply change the shape of the part instead of developing potentially destructive tensile residual stresses. This is the reason that expansion loops are designed into high-temperature piping, such as steam lines. It is also the reason that expansion joints are necessary in bridge and road design.

Corrosion Fatigue As if fatigue itself were not a complicated enough type of failure, it is further complicated by the addition of a corrosive environment. Corrosion fatigue is the combined simultaneous action of repeated or fluctuating stress and a corrosive environment to produce surfaceorigin fatigue cracking. In one sense, all fatigue fractures should be considered as corrosion fatigue fractures, for the action of the environment within the fatigue crack is critical to the success or failure of the part involved. The fatigue strength of a part or metal in' an inert environment, where there is no corrosion, is higher than that of the same part in an aggressive environment where corrosion may play a significant role.


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Fatigue Testing of Carbon Steels and Low–Alloy Steels

Carbon and low-alloy steels and austenitic stainless steels are used extensively in LWR steam supply systems as piping and pressure vessel materials. The environmentally related increase of fatigue crack growth rates in pressure vessel and piping steels in high-temperature water is well known. LWR coolant environments can also significantly affect the fatigue lives of these steels. Under certain conditions of loading and environment, the fatigue lives of these steels can be a factor of 70 lower in coolant environments than in air. Fatigue tests are being conducted in air and simulated LWR environments to obtain data under conditions where information is lacking in the existing fatigue data base. Additional studies have been undertaken to determine crack initiation and crack growth characteristics, and to better understand the actual mechanism of degradation. The results indicate that the decrease in fatigue life of pressure vessel and piping steels is caused primarily by the effect of the environment during the early stages of fatigue damage, i.e., growth of cracks that are <200 mm deep (Stage I crack growth). The mechanisms for the enhancement of the growth rates are slip dissolution/oxidation in carbon and low-alloy steels and hydrogen-induced cracking in austenitic stainless steels.

For carbon and low-alloy steels, the magnitude of the environmental effects on fatigue life increases as the level of dissolved oxygen (DO) in water is increased. In contrast, the fatigue lives of austenitic SSs are decreased significantly in low-DO water (<0.01 ppm DO). Also, the effect of environment is not influenced by the composition or heat treatment condition of the steel; the effect increases with decreasing strain rate and increasing temperature.

The effect of heat treatment can be quantified in terms of electrochemical potentiodynamic reactivation (EPR). The degree of sensitization of stainless steels is assessed by a potentiodynamic sweep over a range of potentials from passive to active and measures the amount of charge associated with the corrosion of chromium-depleted regions surrounding chromium carbide-precipitated particles. An EPR value less than 1.0 designates an unsensitized microstructure, while greater than 4 corresponds to a fully sensitized microstructure.

In high-DO water, the fatigue lives of austenitic SSs are influenced by the composition and heat treatment of the steel. As shown in the figure, fatigue life is lower for sensitized Type 304 steel in BWR water; the decrease in life appears to increase as the degree of sensitization is increased. Furthermore, the fracture mode is different in water. In air, irrespective of the degree of sensitization, the fracture mode for crack initiation and crack propagation is transgranular, most likely along crystallographic planes, leaving behind relatively smooth facets. In the high-DO water, the initial crack appeared intergranular for sensitized material, implying a weakening of the grain boundaries. By contrast, in low-DO environments, cracks initiate and propagate in a transgranular mode irrespective of the degree of sensitization.

(a)( See full-size image)


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(b)( See full-size image) The effect of material heat treatment on the fatigue life of Type 304 stainless steel in air, BWR, and PWR environments (a) and fracture surface morphology in high-dissolved oxygen environment (b)


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Fatigue

The concept of fatigue is very simple, when a motion is repeated, the object that is doing the work becomes weak. For example, when you run, your leg and other muscles of your body become weak, not always to the point where you can't move them anymore, but there is a noticeable decrease in quality output. This same principle is seen in materials. Fatigue occurs when a material is subject to alternating stresses, over a long period of time. Examples of where Fatigue may occur are: springs, turbine blades, airplane wings, bridges and bones.

Beach Marks - Also known as clam shell marks. Beachmarks represent the propagation of a crack through a material due to fatigue. These marks are visible to the eye, and may contain thousands of striations.

Boundary Conditions -The properties of a system which are externally imposed on it.

Brittle Fracture - Fracture that occurs by rapid crack propogation and without appreciable macroscopic deformation.

Case Hardening - This surface treatment may be accomplishes in many ways, most likely by exposing the material to a carbonaceous or nitrogen rich environment at elevated temperatures. The carbon or nitrogen then diffuses into the surface of the material, increasing surface strength, thus increasing resistance to scratches and other surface flaws.

Dispersion Phase - The part of he composite that is inside the matrix phase. e.g., steel rods in steel reinforced concrete.

Ductile Fracture - A mode of fracture that is attended by extensive gross plastic deformation.

Element -A section of a body obtained from dividing the body up into a finite number of regions.

Fatigue Life - The number of cycles the will cause failure at a certain stress level

Fatigue Limit - A characteristic of the material and its geometry. If a material is loaded below the fatigue limit, the material will not fail, regardless of the number of cycles it is subject to.

Fatigue Strength - The stress at which failure occurs for a given number of cycles.

Fracture toughness (Kc) - the critical value in which may be used to specify the conditions for brittle fracture.

Intergranular Fracture - Fracture of polycrystalline materials by crack prorogation along grain boundaries.

KC - The fracture toughness of a material that has thickness less than some minmum calculated value.

KIC - The fracture toughness of a material that has thickness greater than some minmum calculated value.

Matrix Phase - The part of the composite which completely encompasses the dispersion phase. 5

e.g., the matrix is the conrete in steel reinforced concrete.

Mean Stress - Average stress between maximum and minimum stress. For a reversed cyclic stress the mean stress would be zero.

Mesh -The grid of a finite element system

Node -Points at which different elements are jointed together; nodes are the locations where values of unknowns (usually displacements) are to be approximated.


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Plane Strain The condition, wherein, for tensile loading, there is zero strain perpendicular to both the stress axis and the direction of crack propagation; this condition is found in thick plates, and the zero-strain direction is perpendicular to the plate surface.

Plane strain fracture toughness (K1c) - The condition, wherein, for tensile loading, there is zero strain perpendicular to both the stress axis and the direction of crack propagation; this condition is found in thick plates, and the zero-strain direction is perpendicular to the plate surface.

Plane Stress - The state that a sample of material is in if a stress is applied to it and its thickness is less than some minimum value.

Plastic Deformation - Deformation that is permanent or nonrecoverable after release of the applied load. It is accompanied by permanent atomic displacements.

Porosity - A volume fraction of a material that is empty, or without material.

Repeatrd Stress Cycle - More commonly seen in engineering applications than the reversed stress cycle, the repeated stress cycle is a sine wave that is asymmetric about the x axis. The maximum and minimum stresses are NOT equal and opposite in sign.

Reversed Stress Cycle - This type of stress cycle has an amplitude symmetric about the x axis. The maximum and minimum stresses are equal, but opposite in sign.

Shot Peening - When a material is bombarded with small, round steel shot (i.e. bb's, buckshot) at high velocities, it is termed shot peening. This process will increase the resistance to fatigue failure by adding dislocations and hardening the surface, which is where a crack may begin due to surface flaws.

Stiffness Matrix -A matrix defining the geometric and material properties of a system.

Strain Hardening - The increase in hardness and strength of a ductile metal as it is plastically deformed below its recrystallization temperature.

Stress concentration (Kt) - is the measure of the degree to which an external stress is amplified at the tip of a small crack.

Stress intensity (K) - is a function of applied stress and the size and position of crack that specifies the intensity at the tip of a crack.

Striations - These markings are thought to be the advancement of a crack through a material. Unlike Beachmarks, Striations are thought to be the advancement of a crack due to one cycle of loading.

Transgranular Fracture - Fracture of polycrystalline materials by crack propagation through the grains.

Yield strength (sigma y) - is the stress at a 0.002 strain offset on the stress/strain curve of the material. Also, is the point of transition from elastic deformation to plastic deformation.


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Cyclic Stresses

There are three common ways in which stresses may be applied: axial, torsional, and flexural. Examples of these are seen in Fig. 1.

Figure 1

Visual examples of axial stress, torsional stress, and flexural stress. There are also three stress cycles with which loads may be applied to the sample. The simplest being the reversed stress cycle . This is merely a sine wave where the maximum stress and minimum stress differ by a negative sign. An example of this type of stress cycle would be in an axle, where every half turn or half period as in the case of the sine wave, the stress on a point would be reversed. The most common type of cycle found in engineering applications is where the maximum stress (σmax)and minimum stress (σmin) are asymmetric (the curve is a sine wave) not equal and opposite. This type of stress cycle is called repeated stress cycle. A final type of cycle mode is where stress and frequency vary randomly. An example of this would be automobile shocks, where the frequency magnitude of imperfections in the road will produce varying minimum and maximum stresses.

The S-N Curve

A very useful way to visualize time to failure for a specific material is with the S-N curve. The "S-N" means stress verse cycles to failure, which when plotted uses the stress amplitude, σa plotted on the vertical axis and the logarithm of the number of cycles to failure. An important characteristic to this plot as seen in Fig. 2 is the fatigue limit.

Figure 2

A S-N Plot for an aluminum alloy The significance of the fatigue limit is that if the material is loaded below this stress, then it will not fail, regardless of the number of times it is loaded. Material such as aluminum, copper and magnesium do not show a fatigue limit, therefor they will fail at any stress and number of cycles. Other important terms are fatigue strength and fatigue life. The stress at which failure occurs for a given number of cycles is the fatigue strength. The number of cycles required for a material to fail at a certain stress in fatigue life.


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Crack Initiation and Propagation

Failure of a material due to fatigue may be viewed on a microscopic level in three steps:

1. Crack Initiation: The initial crack occurs in this stage. The crack may be caused by surface scratches caused by handling, or tooling of the material; threads ( as in a screw or bolt); slip bands or dislocations intersecting the surface as a result of previous cyclic loading or work hardening.

2. Crack Propagation: The crack continues to grow during this stage as a result of continuously applied stresses

3. Failure: Failure occurs when the material that has not been affected by the crack cannot withstand the applied stress. This stage happens very quickly.

Figure 3

A diagram showing location of the three steps in a fatigue fracture under axial stress

One can determine that a material failed by fatigue by examining the fracture sight. A fatigue fracture will have two distinct regions; One being smooth or burnished as a result of the rubbing of the bottom and top of the crack( steps 1 & 2 ); The second is granular, due to the rapid failure of the material. These visual clues may be seen in Fig. 4:

Figure 4

A diagram showing the surface of a fatigue fracture. Notice that the rough surface indicates brittle failure, while the smooth surface represents crack propagation

Other features of a fatigue fracture are Beachmarks and Striations. Beachmarks, or clamshell marks, may be seen in fatigue failures of materials that are used for a period of time, allowed to rest for an equivalent time period and the loaded again as in factory usage. Striations are thought to be steps in crack propagation, were the distance depends on the stress range. Beachmarks may contain thousands of striations. Visual Examples of Beachmarks and Striations are seen below in Fig. 5 and 6:


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Figure 5

An example of beachmarks or "clamshell pattern" associated with stress cycles that vary in magnitude and time as in factory machinery

Figure 6

An example of the striations found in fatigue fracture. Each striation is thought to be the advancement of the crack. There may be thousands of striations in a beachmark

Demonstration of Crack Propagation Due to Fatigue


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The figure above illustrates the various ways in which cracks are initiated and the stages that occur after they start. This is extremely important since these cracks will ultimately lead to failure of the material if not detected and recognized. The material shown is pulled in tension with a cyclic stress in the y ,or horizontal, direction. Cracks can be initiated by several different causes, the three that will be discussed here are nucleating slip planes, notches. and internal flaws. This figure is an image map so all the crack types and stages are clickable. For more information on clickable maps and how to do them see the clickable map tutorial.

Propagation Rate

The rate at which a crack grows has considerable importance in determining the life of a material. The propagation of a crack occurs during the second step of fatigue failure. As a crack begins to propagate, the size of the crack also begins to grow. The rate at which the crack continues to grow depends on the stress level applied. The rate at which a crack grows can be seen mathematically in equation 8.16 in Callister by:

Eq. 1

The variables A and m are properties of the material, da is the change in crack length, and dN is the change in the number of cycles. K is the change in the stress intensity factor or by equation 8.17(a & b):

Eq. 2

Rearrangement and integration of Eq. 1 gives us the relation of the number of cycles of failure, Nf, to the size of the initial flaw length, ao, and the critical crack length, ac, and Eq. 2:

Note: Nf is an estimate of the number of cycles to failure

Eq. 3


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Factors That Affect Fatigue Life and Solutions

The Mean Stress, discussed in Callister, 8.8, is defined as:

Eq. 4

The Mean stress has the affect that as the mean stress is increased, fatigue life decreases. This occurs because the stress applies is greater.

I mentioned previously that scratches and other imperfections on the surface will cause a decrease in the life of a material. Therefore making an effort to reduce these imperfections by reducing sharp corners, eliminating unnecessary drilling and stamping, shot peening, and most of all careful fabrication and handling of the material.

Another Surface treatment is called case hardening, which increases surface hardness and fatigue life. This is achieved by exposing the component to a carbon-rich atmosphere at high temperatures. Carbon diffuses into the material filling interstisties and other vacancies in the material, up to 1 mm in depth.

Figure 7

A case hardened steel gear. Notice the effect of diffusion of Carbon into the material produces a "case" around the gear. Exposing a material to high temperatures is another cause of fatigue in materials. Thermal expansion, and contraction will weaken bonds in a material as well as bonds between two different materials. For example, in space shuttle heat shield tiles, the outer covering of silicon tetraboride (SiB4) has a different coefficient of thermal expansion than the Carbon-Carbon Composite. Upon re-entry into the earth's atmosphere, this thermal mismatch will cause the protective covering to weaken, and eventually fail with repeated cycles.

Another environmental affect on a material is chemical attack, or corrosion. Small pits may form on the surface of the material, similar to the effect etching has when trying to find dislocations.


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Figure 8

Example of pits formed by corrosion on the surface of LiF. The "chemical" attacks weak spots on the surface of the material, especially where dislocations intersect the surface.

This chemical attack on a material can be seen in unprotected surface of an automobile, whether it be by road salt in the winter time or exhaust fumes. This problem can be solved by adding protective coatings to the material to resist chemical attack.

Example problems

1. Consider a flat plate of some metal alloy that is to be exposed to repeated tensile-compressive cycling in which the mean stress is 25 MPa. If: ao = 0.25 mm, ac = 5.0 mm, m = 4.0, A = 5 * 10-15, Y = 2.0, and Nf =3.2 * 105 cycles .

Find: Estimate the maximum tensile stress to yield the fatigue life prescribed

Solution: Use Equations 3 above to solve for Δσ.

Eq. 3

More reading http://en.wikipedia.org/wiki/Tensile_strength


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Understanding common failure

ELASTIC FAILURE

Two types of failure occur due to unsuitable elastic behaviour in a material:

If the elastic modulus E is too low, the component may deflect too much in service, making it unusable. Examples are moving parts such as engines and structures such as bridges. Alternatively some components may not deflect as much as they should do, or may require too much force to deflect by the required amount. Examples are switches and springs. These problems will become apparent instantly as soon as the load is applied, except in viscoelastic materials such as polymers, where elastic strain can build up gradually over time. Elastic deflection can be distinguished from plastic strain and creep strain because it is totally recoverable on removal of the load.

Buckling: Though buckling leads to a failure by another mechanism (usually ductile failure), the process of buckling is essentially an elastic instability, which is controlled by the material's elastic modulus. A classic example is the failure of slender columns loaded in compression. This type of failure generally occurs instantaneously; it can be difficult to predict because it depends on the shape of the part and how it is loaded and restrained.

DUCTILE FAILURE

A failure is termed "ductile" if it occurs by plastic deformation, i.e. if the yield strength of the material is exceeded. This causes a permanent change in shape of the part, which becomes apparent if one attempts to reassemble broken components. Tensile stress causes necking, reducing the cross section of the part, as shown dramatically in this failed bolt:


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Shear stress causes deformation of the type shown below in this sketch of a failed rivet:

Some plastic deformation also occurs in other types of failure: for example fatigue failure always involves some plasticity, and brittle fracture may be preceded by plastic strain. However, in these cases the amount of plasticity is small: ductile fracture is characterised by gross plasticity which leads to an obvious change in shape of the component.

Microscopic Appearance: Below are two photos taken using a scanning electron microscope (SEM) showing the appearance of ductile fracture surfaces at high magnification. They have a distinctive 'dimpled' appearance, the surface being covered with small dimples or craters. These dimples result from the fracture of spherical pores which open up due to plastic deformation (see more about this


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under Brittle Fracture below). The dimples may contain inclusions and other small particles from which the pores nucleated.

Above: SEM photograph of part of a fracture surface, at high magnification. The total width of this picture is about 50µm (0.05mm)

SEM photograph of another ductile fracture surface. The dotted line (bottom left) shows the scale (in this case 10µm).

BRITTLE FAILURE

Brittle fracture is the term given to static failures caused by crack propagation. Crack propagation also figures in fatigue and creep failure, but brittle fracture occurs when a pre-existing crack propagates very rapidly through the material. Such fractures release a lot of energy and can be very loud, explosive and dangerous, as fragments of the material may be thrown long distances. In many cases, especially if the


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micromechanism of cracking is cleavage (see below), a macroscopic examination of the fracture surface may show lines (called "chevron marks") which lead back to the failure origin. Here's an example (the arrow shows the origin):

A big difference between brittle fracture and ductile fracture is that in brittle fracture there is no macroscopic deformation. So you can take the broken parts and put them back together again, like making a jigsaw. You can't do this after a ductile fracture because the shape of the pieces has changed. For example, compare the fracture of this bolt with the one shown above in the section on Ductile Fracture:


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Microscopic Appearance: The appearance of the fracture surface at high magnification depends on the mechanism of crack propagation. This can be either ductile or brittle. Ductile crack extension occurs if the crack grows by causing local plasticity ahead of the crack tip, which pulls the material apart. Pores form and grow in the material just ahead of the crack. The crack grow occurs by linking of these pores as shown in the sketch below.

This gives give a fracture surface identical to that described in the section above on Ductile Fracture (see the SEM photos in that section).

Brittle crack extension, also called "cleavage", occurs by the splitting apart of planes in the atomic lattice. Each grain cleaves like a gemstone, giving a perfectly flat surface, as shown in the sketch below:

Because the cleavage plane for each grain will be at a different angle, the final fracture surface will consist of a series of small flat planes, each corresponding to one grain. The following two pictures show what this looks like at high magnification, in the SEM:


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This photo shows cleavage fracture in a surface coating of Cadmium (applied to a steel component to prevent corrosion and wear); the total thickness of the coating is about 95µm.

The two mechanisms of crack extension described above are both "transgranular", i.e. the crack grows through each grain. If, alternatively, the crack propagates by travelling around the grains (i.e. along the grain boundaries), then the mechanism is termed "intergranular". Intergranular fractures can be either ductile or brittle; a brittle intergranular failure looks like this:


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Here you see cleavage planes again as in the pictures above, but now you are looking at the outsides of the grains. The existence of transgranular failure is often an indication that something is wrong with the material (e.g. an incorrect heat treatment causing a brittle phase to form on the grain boundaries). Most materials, in good condition, fail in the transgranular mode.

In a ductile intergranular failure you still see the outsides of the grains, but they have dimples on them as shown here:


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Ductile intergranular failures are quite rare - the only situation in which they commonly occur is Creep. Also certain incorrect heat treatments applied to certain metal alloys can cause them to fail in this way even at room temperature, but the phenomenon is rare.

The existence of the cleavage mode is an indication that the crack grew very easily in the material, needing very little energy to propagate. This is often a clue to something wrong with the material, such as an incorrect heat treatment or the use of a sub-standard material. Some metals actually change from ductile fracture to brittle fracture if you lower the temperature. However some materials are just inherently brittle and will always fail by this mode.

The terminology we use here is rather confusing, because we use the words 'ductile' and 'brittle' to mean two different things. In classifying the different types of failure we talk about Brittle Fracture (meaning fracture due to crack growth) and Ductile Fracture (sometimes called 'ductile failure') meaning fracture due to the yield strength being exceeded. But within the category of Brittle Fracture we also talk about cracks propagating by either ductile or brittle mechanisms, as you see above. Sorry for the confusion - that's just the way it is!

FATIGUE

Fatigue failures occur due to cyclic loading; the mechanism of failure is the initiation and propagation of cracks. When a crack has reached a critical length, final fast fracture will occur by either Brittle Fracture


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or Ductile Fracture. Fatigue fracture surfaces can usually be recognised because they have one or more of the following features (see also the sketch below):

• - a relatively flat, smooth region where fatigue crack growth took place, and a rougher, fast fracture region.

• - lines on the fracture surface, known as "beach marks" or "clam shell marks", which lie parallel to the crack front, perpendicular to the direction of crack growth.

• - corrosion (e.g. rusting) on the fatigue region of the fracture surface if the crack has been exposed to a source of corrosion during growth. This can be increased by fretting action if compressive stresses are involved.

• - wear on the fatigue region surface if compressive stresses are involved, causing the two surfaces of the crack to rub together.

The following two photographs show examples of fatigue failures, illustrating these features:


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Microscopic Appearance: The SEM photograph below is typical of a fatigue fracture surface. The failure is usually ductile in appearance (i.e. it does not show cleavage facets) but the scale of the ductility is small - not sufficient to create dimples as in Ductile Failure.

This type of fracture surface is quite difficult to recognise; the main thing to remember is that it does not show the features which are typical of fast fracture, i.e. Ductile Fracture or Brittle Fracture. In some


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cases - usually very ductile metals such as aluminium, a pattern of parallel lines known as "striations" occurs, marking the position of the crack front on successive load cycles, as shown here:

The total width of this SEM picture is only 12µm (0.012mm). This specimen was cycled with alternating blocks of 10 cycles at high stress followed by 10 cycles at low stress, creating groups of striations with two different spacings.

If you see striations like this then you know it is definitely a fatigue failure, but in many materials these striations don't occur.

CREEP

Creep failure is essentially plastic deformation, so its appearance is often similar to that of Ductile Fracture. Creep would be suspected if a component shows extensive plastic deformation before failure and if the material is operating at a temperature within its creep range, which is usually about 0.3 of its melting point (in degrees K). As with fatigue, the final failure may be ductile (by growth of voids) or brittle (by crack propagation).

Microscopic Appearance: Creep failures are usually of the intergranular ductile type, due to the formation of voids on the grain boundaries: see above under Brittle Fracture for an example of this kind of fracture surface. Failure is usually preceded by a lot of microscopic damage, which can be seen on polished sections as shown in the two photos below. This damage takes the form of voids (shown on the left) and/or small cracks (on the right), mostly located on the grain boundaries:


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STRESS-CORROSION CRACKING

Stress-corrosion cracking (SCC), also called Environmentally Assisted Cracking, is a process of crack initiation and growth which occurs in the presence of a corrosive environment; it can occur even if the stress is constant. If the stress is cyclic you can get a related phenomenon called 'Corrosion Fatigue'. Many different mechanisms are involved and this type of failure may be difficult to distinguish from other types, but some clues to the existence of a SCC failure are:

- Multiple crack initiation and crack branching, as shown here:


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- Corrosion which is present on the fracture surface even when the rest of the part is uncorroded.

- Intergranular fracture.

Technology

Fatigue testing