Fatigue testing

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Fatigue testing

  1. 1. www.cmmok.xinwen520.com 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RIG/PLANTIn house training Radiographic, Dye Penetrant, Magnetic Particle, Eddy Current, Ultrasonic Testing. Post weld heat CMM NDT & Inspection ADDITIONAL CODE SDRL CODE TOTAL PGS treatment, infra red tomography. 21 Thickness gauging, Underwater inspection. FRPREMARKS Inspection.MAIN TAG NUMBER DISCIPLINE MechanicalCLIENT PO NUMBER DOCUMENT NUMBER REVCLIENT DOCUMENT NUMBER Mech-Course-001C 01 CMM NDT Services www.cmmok.xinwen520.com Pg: 1/ 76
  2. 2. www.cmmok.xinwen520.comREVISION HISTORY 01 10.05.2008 For Approval CMM Rev Date (dd.mm.yyyy) Reason for issue Prep Check ApprCHANGE DESCRIPTIONRevision Change description 01 For Approval Pg: 2/ 76
  3. 3. www.cmmok.xinwen520.com Metal FatigueA phenomenon which results in the sudden fracture of a component after a period of cyclic loading in theelastic regime. Failure is the end result of a process involving the initiation and growth of a crack, usually atthe site of a stress concentration on the surface. Occasionally, a crack may initiate at a fault just below thesurface. Eventually the cross sectional area is so reduced that the component ruptures under a normalservice load, but one at a level which has been satisfactorily withstood on many previous occasions beforethe crack propagated. The final fracture may occur in a ductile or brittle mode depending on thecharacteristics of the material. Fatigue fractures have a characteristic appearance which reflects the initiationsite 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 retreatingwaves, are caused by arrests in the crack front as it propagates through the section. The hatched region onthe opposite side to the initiation site is the final region of ductile fracture. Sometimes there may be morethan one initiation point and two or more cracks propagate. This produces features as in Fig. 1b with the thefinal area of ductile fracture being a band across the middle. This type of fracture is typical of double bendingwhere a component is cyclically strained in one plane or where a second fatigue crack initiates at theopposite side to a developing crack in a component subject to reverse bending. Some stress-induced fatiguefailures may show multiple initiation sites from which separate cracks spread towards a common meetingpoint within the section. Fig. 1Fatigue strength is determined by applying different levels of cyclic stress to individual test specimens andmeasuring the number of cycles to failure. Standard laboratory test use various methods for applying thecyclic load, e.g. rotating bend, cantilever bend, axial push-pull and torsion. The data are plotted in the form ofa stress-number of cycles to failure (S-N) curve, fig 2. Owing to the statistical nature of the failure, several Pg: 3/ 76
  4. 4. www.cmmok.xinwen520.comspecimens have to be tested at each stress level. Some materials, notably low-carbon steels, exhibit aflattening off at a particular stress level as at (a) in Fig.2 which is referred to as the fatigue limit. As a roughguide, the fatigue limit is usually about 40% of the tensile strength. In principle, components designed so thatthe applied stresses do not exceed this level should not fail in service. The difficulty is a localised stressconcentration may be present or introduced during service which leads to initiation, despite the design stressbeing normally below the safe limit. Most materials, however, exhibit a continually falling curve as at (b) andthe usual indicator of fatigue strength is to quote the stress below which failure will not be expected in lessthan a given number of cycles which is referred to as the endurance limit. Fig2 Pg: 4/ 76
  5. 5. www.cmmok.xinwen520.com Pg: 5/ 76
  6. 6. www.cmmok.xinwen520.comMicro crack at grain boundary during fatigue test. Pg: 6/ 76
  7. 7. www.cmmok.xinwen520.com 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 LoadsLong ago, engineers discovered that if you repeatedly applied and then removed a nominal load to and from ametal part (known as a "cyclic load"), the part would break after a certain number of load-unload cycles, even whenthe maximum cyclic stress level applied was much lower than the UTS, and in fact, much lower than the YieldStress (UTS and YS are explained in Stress and Strain). These relationships were first published by A. Z. Wöhlerin 1858.They discovered that as they reduced the magnitude of the cyclic stress, the part would survive more cycles beforebreaking. 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 noload is then subjected to a load, and later, the load is removed, so the part goes back to the no-load condition. Anexample of this type of loading is a chain used to haul logs behind a tractor. Pg: 7/ 76
  8. 8. www.cmmok.xinwen520.comAnother type of fatigue loading is a varying load superimposed on a constant load. The suspension wires in arailroad bridge are an example of this type. The wires have a constant static tensile load from the weight of thebridge, 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 fatigueloading occurs when a tensile stress of some value is applied to an unloaded part and then released, then acompressive stress of the same value is applied and released.Figure 1A rotating shaft with a bending load applied to it is a good example of fully reversing load. In order to visualize thefully-reversing nature of the load, picture the shaft in a fixed position (not rotating) but subjected to an appliedbending 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 willbe loaded in compression (lower green arrows). Now, rotate the shaft 180° in its bearings, with the loads remainingthe same. The shaft stress level is the same, but now the fibers which were loaded in compression before yourotated 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 appliedbending load.To illustrate how damaging fully-reversing load is, take a paper clip, bend it out straight, then pick a spot in themiddle, and bend the clip 90° back and forth at that spot (from straight to "L" shaped and back). Because you areplastically-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 thefibers on the opposite side. When you bend it the other way, you reverse the stresses (fully reversing fatigue). Itwill 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 stressvalues, 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. Pg: 8/ 76
  9. 9. www.cmmok.xinwen520.comHOW 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 oflaboratory 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 aremachined with shape characteristics which maximize the fatigue life of a metal, and are highly polished to providethe surface characteristics which enable the best fatigue life.Figure 2A single test consists of applying a known, constant bending stress to a round sample of the material, and rotatingthe sample around the bending stress axis until it fails. As the sample rotates, the stress applied to any fiber on theoutside surface of the sample varies from maximum-tensile to zero to maximum-compressive and back. The testmechanism counts the number of rotations (cycles) until the specimen fails. A large number of tests is run at eachstress level of interest, and the results are statistically massaged to determine the expected number of cycles tofailure at that stress level.The cyclic stress level of the first set of tests is some large percentage of the Ultimate Tensile Stress (UTS), whichproduces 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 Pg: 9/ 76
  10. 10. www.cmmok.xinwen520.comIS 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 thematerial 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 thatapproximate 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 shownby the line titled "Polished".For example, a specimen of SAE-4340 alloy steel, hardened to 32 Rockwell-C (HRc), will exhibit a UTS around150,000 psi and an EL of about 75,000 psi, or 50% of the UTS. If you change the heat treatment process toachieve 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 thatthese values are EL numbers for fully-reversing bending fatigue. EL values for hertzian (contact) stress can besubstantially higher (over 300 ksi).The line titled "Notched" shows the dramatic reduction in fatigue strength as a result of the concentration of stresswhich occurs at sudden changes in cross-sectional area (sharp corners in grooves, fillets, etc.). The highest EL onthat 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 curvetitled "Corroded". It mirrors the shape of the "notched" curve, but is much lower. That curve shows that, for a badlycorroded surface (fretting, oxidation, galvanic, etc.) the endurance limit of the material starts at around 20 ksi formaterials of 40 ksi UTS (50%), increases to about 25 ksi for materials between 140 and 200 ksi UTS, thendecreases 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-stressesfibers are located at the surface (bending fatigue) and (b) the intergranular flaws which precipitate tension failureare more frequently found at the surface.Suppose that a particular specimen is being fatigue tested (as described above). Now suppose the fatigue test ishalted after 20 to 25% of the expected life of the specimen and a small thickness of material is machined off theouter surface of the specimen, and the surface condition is restored to its original state. Now the fatigue test isresumed at the same stress level as before. The life of the part will be considerably longer than expected. If that Pg: 10/ 76
  11. 11. www.cmmok.xinwen520.comprocess is repeated several times, the life of the part may be extended by several hundred percent, limited only bythe available cross section of the specimen. (ref-3:8:6) That proves fatigue failures originate at the surface of acomponent.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. Infact, several apparently identical samples, cut from adjacent sections in one bar of steel, will produce different ELvalues (as well as different UTS and YS) when tested, as illustrated by the S-N diagram below. Each of those threeproperties (UTS, YS, EL) is determined statistically, calculated from the (varying) results of a large number ofapparently 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 levelform statistical clusters, as shown. a curve is fitted through the clusters of points, as shown below. The curve whichis fitted through these clusters, known as an "S-N Diagram" (Stress vs. Number), represents the statisticalbehavior of the fatigue properties of that specific material at that specific strength level. The red points in the chartrepresent the cyclic stress for each test and the number of cycles at which the specimen broke. The blue pointsrepresent the stress levels and number of cycles applied to specimens which did not fail. This diagram clearlydemonstrates the statistical nature of metal fatigue failure. Pg: 11/ 76
  12. 12. www.cmmok.xinwen520.comFigure 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 toaccount 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 surfacecondition, actual load, actual temperature, tolerances, impurities, alloy variations, heat-treatment variations, stressconcentrations, etc. etc. etc.) will not reduce the life of a part below the required value. Please read thatparagraph again, and understand it well.An accepted contemporary practice (ref-2:3:328) to estimate the maximum fatigue loading which a specific designcan survive is the Marin method, in which the laboratory test-determined EL of the particular material (tested onoptimized 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 knowndata which describe the influence of a specific condition on fatigue life. Pg: 12/ 76
  13. 13. www.cmmok.xinwen520.comThose 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 theallowable cyclic stress for an actual part:Real-World Allowable Cyclic Stress = ka * kb * kc * kd * ke * kf * ELIS FATIGUE LOADING CUMULATIVE?It is important to realize that fatigue cycles are accumulative. Suppose a part which has been in service is removedand tested for cracks by a certified aircraft inspection station, a place where it is more likely that the subtletiesof 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 factthat the part has passed the inspection only proves that there are no detectable cracks RIGHT NOW. It gives noindication at all as to how many cycles remain until a crack forms. A part which has just passed a Magnafluxinspection could crack in the next 100 cycles of operation and fail in the next 10000 cycles (which at 2000 RPM,isnt very long!). Pg: 13/ 76
  14. 14. www.cmmok.xinwen520.comFatigue testing – Part 1Fatigue as a specific failure mechanism has been recognised since the early part of the nineteenth century but itwas 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 testThere are many mechanisms that can lead to failure but fatigue is perhaps one of the most insidious since it canlead to a catastrophic failure with little or no warning - one well known example being the failure of the Cometaircraft in the 1950s.Failure can occur at a fluctuating load well below the yield point of the metal and below the allowable static designstress. The number of cycles at which failure occurs may vary from a couple of hundreds to millions. There will belittle or no deformation at failure and the fracture has a characteristic surface, as shown in Fig.2. Fig.2. Typical fatigue crack fracture surfaceThe surface is smooth and shows concentric rings, known as beach marks, that radiate from the origin; thesebeach marks becoming coarser as the crack propagation rate increases. Viewing the surface on a scanningelectron microscope at high magnification shows each cycle of stress causes a single ripple. The component finallyfails 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 generalrule, the sharper the notch the shorter the fatigue life - one reason why cracks are so damaging. Pg: 14/ 76
  15. 15. www.cmmok.xinwen520.comThere are two stages in the process of fatigue cracking - a period of time during which a fatigue crack is nucleatedand a second stage where the crack grows incrementally leaving the ripples described above. In an unweldedcomponent 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 notinitiate 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, perhapsonly some 0.1mm deep. Provided that the applied stress is sufficiently large a crack will begin to propagate withinan extremely short period of time. The endurance limit for a welded joint is therefore dependent on the intrusionsize that does not result in crack propagation at the applied stress range. In the case of a welded joint, therefore, afatigue 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 somevalue above - or below - zero.To quantify the effect of these varying stresses fatigue testing is carried out by applying a particular stress rangeand this is continued until the test piece fails. The number of cycles to failure is recorded and the test thenrepeated 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 beplotted 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 testpiece will last for an infinite number of cycles. The curve for the welded sample, however, continues to trend downto 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 specimensBy testing a series of identical specimens it is possible to develop S/N curves. In service however, there will bevariations in stress range and frequency. The direction of the load may vary, the environment and the shape of thecomponent 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 aspossible these conditions if an accurate life is to be determined. In order to enable the fatigue life to be calculatedwhen the stress range varies in this random manner, the Palmgren-Miners cumulative damage rule is used. Pg: 15/ 76
  16. 16. www.cmmok.xinwen520.comThis rule states that, if the life at a given stress is N and the number of cycles that the component has experiencedis 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 4etc - the fatigue life is used up when the sum is of all these ratios is 1. Although this does not give a preciseestimate of fatigue life, Miners rule was generally regarded as being safe. This method, however, has now beensuperceded 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 astructure that will experience fatigue loading in the individual joints has adequate strength. The commonest methodfor 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 codeBS 5400 in 1980 and then into the industry design rules for offshore structures. Further refinements andimprovements finally resulted in the publication of BS 7608 Code of practice for fatigue design and assessment ofsteel structures. This standard will be looked at in more detail in a future article. Pg: 16/ 76
  17. 17. www.cmmok.xinwen520.comFatigue testing - Part 2The article in the September/October issue of Connect established some basic facts about fatigue and thestatement 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 besubjected 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 waveaction. 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 cyclesor 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 changesin cross section - a machined groove, a ring machined onto a bar or at a weld. The sharper the notch the greaterwill 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 theweld 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 thicknessIn addition, misalignment and/or distortion of the joint will cause the applied stress to be further increased, perhapsby introducing bending in the component, further reducing the expected fatigue life. A poorly shaped weld cap witha 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 spentin initiating the fatigue crack with a smaller proportion spent in the crack propagating through the structure. In awelded component the bulk of the fatigue life is spent in propagating a crack. The consequences of this differencein behaviour are illustrated in Fig.3. Pg: 17/ 76
  18. 18. www.cmmok.xinwen520.com Fig.2. Weld toe intrusion Fig.3. Effect of stress concentration on fatigue lifeThis shows that this small intrusion reduces the fatigue life of a fillet welded joint by a factor of perhaps 10compared with that of an unwelded item and some eight times that of a sample with a machined hole. The otherconsequence 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 ofcrack propagation, however, is determined by Youngs 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 Youngs Modulus andtherefore very little effect on crack propagation rates. Since the bulk of a welded components life is spent inpropagating a crack, strength has little or no influence on the fatigue life of a welded item. There is thus no benefitto be gained by using high strength alloys if the design is fatigue limited. This is illustrated in Fig.4 which shows thebenefits of increasing the ultimate tensile strength of a steel if the component is unwelded or only machined buthow little effect this has on the life of a welded item. Pg: 18/ 76
  19. 19. www.cmmok.xinwen520.com Fig.4. Effect of increase in tensile strength on fatigue life Pg: 19/ 76
  20. 20. www.cmmok.xinwen520.comOne additional feature in welded joints that set them apart from unwelded or machined items is the presence ofresidual tensile stress.In a welded component there will be stresses introduced into the structure by, for example, assembly stress. Thesestresses 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 theexpansion and contraction of material close to and within the welded joint. Whilst the actual level of residual stresswill be affected by such factors as tensile strength, joint type and size and by run size and sequence, the peakresidual stress may be regarded as being of yield point magnitude. The implications of this are that it is the stressrange 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 causea 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 isup to the level of residual stress at the welded joint. Fig.5. Effect of residual stress on stress rangeIt would seem reasonable, therefore, that a post-weld stress relief treatment would be of benefit to the fatigue lifeby reducing the residual stresses to low levels. This is only true, however, where the applied stress range is partlyor wholly compressive. If the applied stress range is all tensile, research has shown that as-welded and stressrelieved components have almost identical fatigue performances with only a marginal improvement in the stressrelieved joints.This is the result of the bulk of the fatigue life of a welded joint being spent in crack propagation where propagationrates are only marginally affected by mean stress. It may be difficult therefore to justify the cost of stress relief if theonly criterion is that of improving fatigue life.The methods of determining fatigue performance of welded joints, as detailed in BS 7608, and how fatigueperformance can be improved will be dealt with in the next Connect article. Pg: 20/ 76
  21. 21. www.cmmok.xinwen520.comFatigue testing Part 3What will have become obvious from the previous two articles on fatigue is that a welded joint behaves in aradically 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 limitvarying dependent upon the joint type and weld quality. It is vitally important to understand this if fatigue analysis ofwelded joints is to be carried out.As mentioned earlier, rules for the design of components subject to fatigue loading were produced by TWI andthese were incorporated into the design rules in BS 5400, the British bridge design code. These rules were lateradopted by the offshore industry for offshore structures and adaptations of these rules now appear in many otherspecifications 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 thejoints fatigue performance. Fig.1 is an example of how this classification has been formalised in BS 7608 - thesame or similar methods will be found in other application standards. Fig.1. Examples of joint classification from BS 7608In BS 7608 each joint type is assigned a classification letter. For example, a plate butt weld with cap and rootground flush is class C, an undressed plate butt weld class D and a fillet weld class F ( Fig.2). Pg: 21/ 76
  22. 22. www.cmmok.xinwen520.com Fig.2. Effects of joint classification on fatigue lifeFor 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 arisingfrom changes in temperature, changes in internal or external pressure, vibration, externally applied fluctuatingloads 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, theweld quality also has a decisive effect - any fatigue analysis assumes that the welds are of an acceptable qualityand comply with the inspection acceptance standards. However, in practice it is not always possible to guarantee aperfect weld and cracks, lack of fusion, slag entrapment and other planar defects may be present, reducing thefatigue life, perhaps catastrophically.Other less obvious features will also have an adverse effect. Excessive cap height or a poorly shaped weld beadwill 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 inmitigating these problems.In some applications an as-welded joint will not have a sufficient design life and some method of improving thefatigue performance needs to be found. There are a number of options available. The first and perhaps simplest isto move the weld from the area of highest stress range, the next is to thicken up the component or increase theweld 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 veryaccurate positioning of the heated area and very careful control of the temperature if an improvement is to be seenand the strength of the metal is not to be affected. For these reasons, spot heating for fatigue improvements hasbeen virtually discontinued.Hammer peening with a round nosed tool or needle gun peening gives very good results although the noiseproduced may prevent their use. Shot peening can also be used to introduce compressive stresses at weld toeswith equally good results. Compressive stresses can be induced in a component by overstressing - a pressure testof a pressure vessel is a good example of this - where local plastic deformation at stress raisers induces acompressive stress when the load is released. This technique needs to be approached with some care as it maycause permanent deformation and/or any defects to extend in an unstable manner resulting in failure. Pg: 22/ 76
  23. 23. www.cmmok.xinwen520.comAlthough the next techniques described are not as beneficial as hammer peening of the weld toes they have theadvantage of being more consistent and easier to control. The techniques rely upon dressing the weld toes toimprove the shape and remove the intrusion mentioned in article 79. The dressing may be carried out using a TIGor plasma-TIG torch which melts the region of the weld toe, providing a smooth blend between the weld face andthe parent metal.Alternatively the toe may be dressed by the careful use of a disc grinder but for best results the toe should bemachined with a fine rotary burr as shown in Fig.3 and 4. Great care needs to be exercised to ensure that theoperator does not remove too much metal and reduce the component below its minimum design thickness and thatthe machining marks are parallel to the axis of the main stress. Ideally the dressing should remove no more than0.5m depth of material, sufficient to give a smooth blend and remove the toe intrusion. The results of theseimprovement techniques are summarised in Fig.5.Fig.3. Grinding tools Fig.4. Burr machining of weld toes Fig.5. Improvement in fillet weld fatigue lifeWhilst fatigue has resulted in some catastrophic and unexpected failures, the improvements in design lifecalculation methods, particularly the use of powerful software packages allowing detailed finite element analyses tobe 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 considerall possible sources of loading and ALL welds, even non-load carrying attachments that may be thought to beunimportant to service performance. Pg: 23/ 76
  24. 24. www.cmmok.xinwen520.comSamples failure Bolt Failure Steel bolt This high tensile steel bolt failed under low stress high cycle conditions with a fatigue crack running from 9 oclock 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. Pg: 24/ 76
  25. 25. www.cmmok.xinwen520.com 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 oclock upwards and to a lesser extent from 2 oclock 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 Pg: 25/ 76
  26. 26. www.cmmok.xinwen520.com 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. Pg: 26/ 76
  27. 27. www.cmmok.xinwen520.comMethod 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. Pg: 27/ 76
  28. 28. www.cmmok.xinwen520.com 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 Fig1 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 Fig2 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. Pg: 28/ 76
  29. 29. www.cmmok.xinwen520.com 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. Pg: 29/ 76
  30. 30. www.cmmok.xinwen520.com 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. Pg: 30/ 76
  31. 31. www.cmmok.xinwen520.comThermal Cracks on Commuter WheelTread Fatigue Crack Profile Pg: 31/ 76
  32. 32. www.cmmok.xinwen520.com Causes and Recognition of Fatigue FailuresGeneral 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 failuresImproper and insufficient maintenance seems to be one of the most contributing factors influenced by someimproper designs such as areas that are hard to inspect and maintain and the need for better maintenanceprocedures. In many circumstances the true load is difficult to predict resulting in a structure being stressedbeyond its normal capabilities and structural limitations. When a structure is subject to cyclic loads, areas subjectto fatigue failure must be accurately identified. This is often very hard to analyze, especially in a highly compositestructure for which analysis has a high degree of uncertainty. Thus, in general, experimental structural fatiguetesting is frequently resorted to.Recognition of Fatigue FailureTwo fatigue zones are evident when investigating a fracture surface due to fatigue, the fatigue zone and therupture zone. The fatigue zone is the area of the crack propagation. The area of final failure is called the rupture orinstantaneous zone. In investigation of a failed specimen, the rupture zone yields the ductility of the material, thetype of loading, and the direction of loading. The relative size of the rupture zone compared with the fatigue zonerelates the degree of overstress applied to the structure. The amount of overstressing can be determined from thefatigue zone as follows: highly overstressed if the area of the fatigue zone is very small compared with the area ofthe rupture zone; medium overstress if the size or area of both zones are nearly equal; low overstress if the area ofrupture 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 wavesknown as "clam-shells" or "oyster-shells", "stop marks" and "beach marks," and the herringbone pattern orgranular trace which shows the origin of the crack. In general, stop marks indicate the variations in the rate ofcrack propagation due to variations in stress amplitude in a cyclic application varying with time. Figure 2 is aschematic representation of the fatigue zone. Pg: 32/ 76
  33. 33. www.cmmok.xinwen520.comFigure 2 Typical fatigue zone with identifying marks. From Metal Fatigue: Theory and Design, ed. A.F. Madayag,pg. 3.Design ConsiderationsEven if careful attention to good design practices is constantly the goal of design engineers, fatigue problems aresometimes 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 stressesthat 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 strengthmaterials 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 effectivelyreduces the strength and increases the actual stress. Decarburization, the loss of carbon from the surface of thematerial, is the next factor. Due to bending and torsion, stresses are highest at the surface; decarburizationweakens the surface by making it softer. Finally, residual stresses which add to the design stress; the combinedeffect may easily exceed the limit stress as imposed in the initial design.Influence of Processing and Metallurgical Factors on FatigueA 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 ambientservice temperature. However, processing and metallurgical factors are not often considered, but these factorsdetermine 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 FactorsStresses 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 thestructure or produced in service or resulting from processing. Processing factors can introduce a detrimental orbeneficial effect into a structure, usually in the form of effect on strength level or residual stress condition of thesurface material. Therefore, the effect of processing on the mechanical properties of a material, especially thesurface of the material, directly affects fatigue properties. Processing factors that influence the fatigue life of astructure include the following: the process by which a part is formed, such as die casting; the heat treatment of amaterial, such as quenching, which builds up residual stresses and annealing, which relieves internal stress (seeFigure 3); case hardening, such as carburization or nitriding, which increases surface hardness and strength (see Pg: 33/ 76
  34. 34. www.cmmok.xinwen520.comFigure 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 MetalFatigue: Theory and Design, ed. A.F. Madayag, pg. 82.Figure 4 Bending fatigue test results on sections from crankshafts: endurance limit versus surface treatment. FromMetal Fatigue: Theory and Design, ed. A.F. Madayag, pg. 70.Metallurgical FactorsMetallurgical factors refers to areas within the material, wither on the surface or in the core, which adversely affectfatigue properties. These areas may arise from melting practices or primary or secondary working of the materialor may be characteristic of a particular alloy system. In virtually all instances the detriment to fatigue propertiesresults from a local stress-raising effect. Therefore, metallurgical factors affecting fatigue include the following: Pg: 34/ 76
  35. 35. www.cmmok.xinwen520.comsurface defects, sub-surface and core defects, inhomogeneity, anisotropy, improper heat treatment, localizedoverheating, corrosion fatigue, and fretting corrosion. Experimental Analysis of FatigueFatigue Life CurvesFailure due to repeated loading is known as fatigue. A small crack, a scratch, or some other such minor defectcauses localized deformation. This deformation leads to a small crack if one was not initially present. After cyclicloading, that is, loading in the same way multiple times, the crack grows, and eventually the material fails. A fatiguelife curve is a graphical representation of the cyclic loading. Simply, a fatigue life curve, also known as an S-Ncurve 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 typicalfatigue life curve.Figure 5 Typical Fatigue Life Curve. From Mechanical Behavior of Materials Laboratory, Prepared by Staff ofEngineering 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 ispresented here.Fatigue Life Curves ExperimentWhat you need: • 4 Jumbo Paper Clips • A Copy of Figure 6What 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. Pg: 35/ 76
  36. 36. www.cmmok.xinwen520.comFigure 6 Bending angle guide. Place paper clip in a vertical plate and bend in the middle using this guide to judgethe angles. From Mechanical Behavior of Materials Laboratory, Prepared by Staff of Engineering Science andMechanics, Virginia Polytechnic Institute, pg. 8-6. Table 1: Table For Experimental Data Cycles to Failure Angle, ? 90 ? 45 ? 20 ? 10An example of this experiment: Table 2: Experimental Data Cycles to Failure Angle, 1.5 90 7 45 29 20 79 10 Pg: 36/ 76
  37. 37. www.cmmok.xinwen520.comFatigue Crack GrowthIf an engineering component contains a crack, and if a cyclic or repeated load is applied, the crack is likely to growslowly with increasing number of load cycles. This process is known as fatigue crack growth. In a fatigue crackgrowth experiment, the progress of a crack growing under a cyclic load is measured, and the results are plotted asa fatigue crack growth rate curve, da/dN versus K (that is, change in crack length divided by change in number ofcycles 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; ©1993by Prentice Hall, Upper Saddle River, NJ. From Mechanical Behavior of Materials Laboratory, Prepared by Staff ofEngineering Science and Mechanics, Virginia Polytechnic Institute, pg. 9-3. Pg: 37/ 76
  38. 38. www.cmmok.xinwen520.comIn the simplest form of a fatigue crack growth rate test, a cyclic load is applied that has fixed maximum andminimum loading levels. The test specimen is usually a plate of material in which a crack has already been startedat the end of a V-bottom machined slot. In a typical fatigue crack growth experiment, the sample is loaded in aclosed-loop servohydraulic testing machine and data for crack length, number of cycles to failure, and fracturetoughness is recorded. From this data the mechanical behavior for a certain material can be described underfatigue crack growth loading by the fatigue crack growth rate curve. This sort of experiment is useful for materialsthat would undergo high cyclic loading stresses such as an airplane wing or a helicopter rotor.Low Cycle FatigueLow cycle fatigue is the repeated cyclic loadings that cause significant plastic deformation in a material and maycause fatigue cracking after a relatively small number of cycles-hundreds or thousands. Low cycle fatigue typicallyoccurs as a result of repeated localized yielding near stress raisers, such as holes, fillets, and notches, despite theelastic 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 specimenfollows a given constant stress amplitude, completely reversed, cyclic strain. That is, the mode of testing is straincontrol instead of stress control. Stress response is monitored during cyclic loading, and the number of cycles tofailure is recorded for these tests. The results from several tests are necessary to determine the cyclic stress-straincurve and the strain life curve for the material. A schematic representation of a completely reversed controlledstrain test is shown in Figure 8.Figure 8 Completely reversed controlled strain test and two possible stress responses, namely cycle-dependenthardening 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 cycleof loading. Stress amplitude usually varies; if it increases, the material is said to cyclically harden, if it decreases, Pg: 38/ 76
  39. 39. www.cmmok.xinwen520.comthe material is said to cyclically soften. However, this behavior tends to stabilize such that the variation in thestress 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 likeFigure 9. The area inside the hysteresis loop is the energy absorbed per unit volume of the material. This energymostly dissipates as heat. Fatigue failure results under repeated cycling and the life is measured by the number ofstrain cycles to failure. A strain life curve may be plotted from the data contained in the hysteresis loop as is shownin Figure 10. This sort of experiment is useful in analysis of parts where the material is strong enough to withstandthe cyclic loading it may go through, but fails due to fracture at a bolt hole or other such stress raiser. An exampleof this is a structural member bolted to another structural member which fails due to a crack originating from thebolt hole.Figure 9 Stable stress-strain hysteresis loop. From Mechanical Behavior of Materials Laboratory, Prepared byStaff 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 MechanicalBehavior of Materials Laboratory, Prepared by Staff of Engineering Science and Mechanics, Virginia PolytechnicInstitute, pg. 10-5. Pg: 39/ 76
  40. 40. www.cmmok.xinwen520.com Real Life-Design and Manufacturing ConsiderationsThe following describes a relationship between factors that shape the S-N curves as they are influenced by designand manufacturing conditions and the effects of such conditions on the fatigue properties of materials, components,and structures.Recommendations for Designs to Avoid Fatigue FailuresA designer can help to minimize the possibility of fatigue failure by proper design of structural components. Manyfatigue failures may be attributed to lack of sufficient consideration of design details or a lack of appreciation ofengineering principles. These principles, which are an integral part of good design of structures subject to fatigueare well reported in literature, but this information has been scattered throughout sources and may be inaccessibleto a designer who needs to understand and utilize the principles. It is good design practice to seek out sources ofthis information and to utilize the principles before, during and after the design process.Fatigue Considerations in Helicopter Design and ServiceThe helicopter is subject to many fatigue considerations that fixed wing aircraft are not, such as the significantoscillatory loads due to the harmonic content of aerodynamic loads from the combined rotational and translationblade motion through the air, also the cyclic loading of the rotor start-stop cycles, transient maneuver loads, gustloads, coriolis loads, and torque loads. These are just a small example of the many unique fatigue loadingconsiderations in helicopter design. Thus one may realize the complexity of helicopter design and service. Thehelicopter is one of many vehicles and structures which undergo unique fatigue loading, therefore, fatigueconsiderations 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.SummaryIn summary, fatigue plays an important role in all areas of the engineering process, from design to manufacture toservice during the life of a product or structure. All engineers must carefully study the effects and the many types offatigue, no matter what field of engineering and what part of the engineering process. Pg: 40/ 76
  41. 41. www.cmmok.xinwen520.comAdditional Scanning Electron Microscope ImagesScanning Electron Microscopy Scanning Electron Microscope (SEM) image of intergrannular fracture indicative of hydrogen embrittlement. (Mag: 400X)Scanning Electron Microscope (SEM) image offatigue striations indicative of cyclic crackpropagation. (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) Pg: 41/ 76
  42. 42. www.cmmok.xinwen520.comScanning Electron Microscope (SEM) image of thesurface of a casting void in the fracture surface of analuminum 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 ofmicrobiological activity in a fire protection systempiping. (Mag: 700X)Metallography/Microstructure EvaluationThe properties of a material and its performance in a specific application depends on its microstructure. Ourmetallographs (light microscopes) are capable of examinations at magnifications from 15X to 1,000X. Analyses ofmicrostructure 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 mountingthe specimen through the grinding and polishing stages to proper selection and etching techniques of the testedmaterial. Pg: 42/ 76
  43. 43. www.cmmok.xinwen520.comView of intergranular stress corrosion cracking (IGSCC) in an Inconelheat exchanger tube. Note that the crack follows the grainboundaries. (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 awelded stainless steel piping flange etched to reveal thecarbide distribution. Fine carbide particles outline the grainboundaries, indicating a "sensitized" condition resulting insusceptibility 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. Thefirst picture is a digital photo with an arrow pointing to the double origin of the fatigue cracks. The second photograph at30X 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 Pg: 43/ 76
  44. 44. www.cmmok.xinwen520.com Pg: 44/ 76
  45. 45. www.cmmok.xinwen520.com Pg: 45/ 76
  46. 46. www.cmmok.xinwen520.com Slide 1 : Typical fatigue failures in steel components. Slide 2 : Striations in an aluminium alloy. Pg: 46/ 76
  47. 47. www.cmmok.xinwen520.com Slide 3 : Fatigue failures in the Alexander L Kielland platform. Slide 4 : Fatigue crack initiation at an inclusion in a high strength steel alloy. Pg: 47/ 76
  48. 48. www.cmmok.xinwen520.com The s canning electron microscopeoperates 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. Pg: 48/ 76
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  50. 50. www.cmmok.xinwen520.comFATIGUE FRACTUREFatigue fractures are generally considered the most serioustype of fracture in machinery parts simply because fatiguefractures can and do occur in normal service, withoutexcessive overloads, and under normal operating conditions.Fatigue fractures are serious because they are insidious;that is, they are frequently "sneaky" and can occur withoutwarning that anything is amiss. Obviously, if service isabnormal as a result of excessive overloading, corrosiveenvironments, or other conditions, the possibility of fatiguefracture is increased.Let us consider the definition of fatigue that is commonlyaccepted:The phenomenon leading to fracture under repeated orfluctuating stresses having a maximum value less than the tensile strength of the material. Fatigue fractures areprogressive, beginning as minute cracks that grow under the action of the fluctuating stress.Stages of fatigue fractureThe definition above refers to fracture "under repeated or fluctuating stresses having a maximum value less thanthe tensile strength." (The final fracture may have either brittle or ductile characteristics, depending upon the metalinvolved 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 thatmost 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 camels back, to borrow a metaphor. Pg: 50/ 76
  51. 51. www.cmmok.xinwen520.comMicroscopic 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 electronmicroscope will reveal that the orientation of the platelets varies randomly from one location to another, whereas truestriations are generally concentric around the origin.Macroscopic characteristics of fatigue fractureInformation can be learned about a fatigue fracture with only macroscopic examination. That is, study with theunaided eye and relatively low magnification - up to perhaps 25 to 50 times magnification - is usually the mostimportant single way to study and analyze fatigue fractures.Lack of DeformationSince initiation of fatigue fracture does not require a high stress, there is usually little orno deformation in a part or specimen that has fractured by fatigue. If the maximumstress did not exceed the yield strength (actually the elastic limit), there can be no grossplastic, or permanent, deformation, although the final rupture region may have someobvious 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 aunidirectional (one-way) bending fracture is observed, it is useful to carefully reassemble the pieces to determine ifthere was gross deformation in the part prior to fracture. Of course, the origin of the fracture would be on theconvex 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 inthe fatigue region, provided that there has been no postfracture damage to the fracture surface. If the final ruptureregion (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 postfracturedamage. 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 presenceis a positive means of identifying fatigue fractures. Beachmarks also have been called"stop marks," "arrest marks," "clamshell marks," and "conchoidal marks," all in anattempt to describe their origin or characteristic appearance. The term "beachmarks" isthe 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 arecharacteristic of interruptions in the propagation periods (stage 2) of fatigue fractures in relatively ductile metals. Pg: 51/ 76
  52. 52. www.cmmok.xinwen520.comBeachmarks must not be confused with striations, although they frequently are present on the same fracturesurface; 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 fatiguefractures. As the cracks become deeper, the cracks from each origin tend to grow together and becomeessentially one fatigue fracture that has numerous origins. The number of ratchet marks equals or is one less thanthe number of origins; thus recognition of the number of ratchet marks is important in determining the number oforigins.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 FATIGUEFatigue Under Compression ForcesA seemingly puzzling type of fatigue crack is one that grows in a part, or a region of a part, that is stressed incompression when the part is under load. It may be recalled that the general principle of mechanically inducedresidual stresses is that "tensile yielding under an applied load results in compressive residual stresses when theload is released, and vice versa." This type of fatigue fracture can be illustrated with the example of the Bellevillespring washer, a hardened spring steel washer that is cone-shaped in order to give high spring rates in limitedspace.When this type of spring washer is pressed flat, the upper surface and corner of the hole are heavily compressed Pg: 52/ 76

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