Failure mechanics: Fatigue Failure

Mumbai University.
Mechanical Engineering
SEM III
Material Technology
Module 2.2

Fatigue Failure:
Definition of fatigue and significance of cyclic stress, Mechanism of fatigue and theories of fatigue failure, Fatigue testing, Test data presentation and statistical evolution, S-N Curve and its interpretation, Influence of important factors on fatigue, Notch effect, surface effect, Effect of pre-stressing, corrosion fatigue, Thermal fatigue.

1. Module 2 FAILURE MECHANISM Fracture Fatigue failure creep Keval K. Patil, M.E.-DESIGN, ([email protected]) 1

2. Fatigue Failure Keval K. Patil, M.E.-DESIGN, ([email protected]) Part 2 2

3. Fatigue Failure Fatigue is a form of failure that occurs in structures subjected to dynamic & fluctuating stresses (e.g., bridges, aircraft, & machine components). Under these circumstances it is possible for failure to occur at a stress level considerably lower than the tensile or yield strength for a static load. The term “fatigue” is used because this type of failure normally occurs after a lengthy period of repeated stress or strain cycling. Fatigue is important inasmuch as it is the single largest cause of failure in metals, estimated to comprise approximately 90% of all metallic failures; polymers & ceramics (except for glasses) are also susceptible to this type of failure. Keval K. Patil, M.E.-DESIGN, ([email protected]) 3

4. Fatigue Failure Furthermore, fatigue is catastrophic & insidious, occurring very suddenly & without warning. Fatigue failure is brittle like in nature even in normally ductile metals, in that there is very little, if any, gross plastic deformation associated with failure. The process occurs by the initiation & propagation of cracks, & ordinarily the fracture surface is perpendicular to the direction of an applied tensile stress. Keval K. Patil, M.E.-DESIGN, ([email protected]) 4

5. STRESS Cycles Cyclic stress distribution of forces (a stress) that changes over time in a repetitive fashion. When cyclic stresses are applied to a material, even though the stresses do not cause plastic deformation, the material may fail due to fatigue. Fatigue failure is typically modeled by decomposing cyclic stresses into mean & alternating components. Mean stress is the time average of the principal stress. The definition of alternating stress varies between different sources. It is either defined as the difference between the minimum & the maximum stress, or the difference between the mean & maximum stress. Keval K. Patil, M.E.-DESIGN, ([email protected]) 5

6. Mechanism of fatigue It can be developed in four stages. Crack nucleation; Crack-growth; Ultimate failure. Crack Nucleation : Fatigue failure begins with formation of a small crack, generally at some point on the external surface Keval K. Patil, M.E.-DESIGN, ([email protected]) 6

7. Crack Growth : The crack formed on surface is then develops slowly into the material in a direction roughly perpendicular to the main tensile axis. Ultimately the cross- sectional area of the member will have been so reduced that it can no longer withstand the applied load & ordinary tensile fracture will result path which the crack has followed Fracture : A fatigue crack ‘front’ advances a small amount during each stress cycle & each increment of advance is shown on the fracture surface as a minute ripple line. These ripple lines radiate out from the origin of fracture as a series of approximately concentric arcs. These individual ripples are visible only by very high-powered metallographic methods. Few ripples much larger than the rest & shows the general Keval K. Patil, M.E.-DESIGN, ([email protected]) Mechanism of fatigue 7

8. The figure shows mechanism of fatigue failure in three stages. The crack propagates slowly from the source, the fracture surface rub together due to pulsating nature of the stress & so the surface become burnished. Fatigue failure in metals is very easy to identify. The fatigue cracks are not result of brittle fracture but of plastic slip. Keval K. Patil, M.E.-DESIGN, ([email protected]) Mechanism of fatigue 8

9. Theories of fatigue failure A number of theories (or mechanism) proposed for fatigue failure are as follow Orowan’s theory Wood’s theory Cottrell & hull theory Mott theory Keval K. Patil, M.E.-DESIGN, ([email protected]) 9

10. Theories of fatigue failure Orowan’s theory The metal is considered to contain small, weak regions, which may be areas of favourable orientation for slip or areas of high stress concentration due to metallurgical notches such as inclusions. If the loading or the stress is such that the total plastic strain in the weak region exceeds the critical value, a crack is formed. Mott theory Mott suggested a model involving the cross slip of screw dislocations & as a result a column of metal is extruded from the surface & a cavity is left behind in the interior of the crystal which is source of fatigue crack & ultimately the fatigue fracture occurs Keval K. Patil, M.E.-DESIGN, ([email protected]) 10

11. Wood interprets microscopic observation of slip produced by fatigue as indicated that slip bands are the result of a systematic build-up of fine slip movement, corresponding movements of the order of 10^-5 to 10^-4 cm, which are observed for static slip bands. According to wood, the back & fort fine slip movement of fatigue could build up notches or ridges at the surface as shown in figure. These notches act as stress raisers & this way starts a fatigue crack which ultimately leads to fatigue fracture. Keval K. Patil, M.E.-DESIGN, ([email protected]) Wood’s theory: 11

12. Cottrell & Hull Theory: A model involving the interaction of edge dislocation on slip systems was suggested by Cottrell & Hull. Two different slip systems when work with different directions & planes of slip produce slip step at surface, thereby forming intrusions & extrusions as shown in figure Fatigue crack starts from intrusions at the surface. Keval K. Patil, M.E.-DESIGN, ([email protected]) 12

13. Fatigue testing The method of testing the metal for fatigue was developed by Wohler. The figure shows a typical experimental set up for Wohler’s setup. The specimen is in the form of a cantilever & loaded at one end through ball bearing. Keval K. Patil, M.E.-DESIGN, ([email protected]) 13 tension on bottom compression on top counter motor flex coupling specimen bearing bearing

14. Fatigue testing It is rotated by means of a high speed motor to which a counter is attached to count the number of rotations. At any instant, the upper surface of the specimen is under tension & lower surface is under compression, with the neutral axis. In one rotation, the specimen undergoes two cyclic fluctuations of stress. The number of cycles to cause failure will vary with the applied stress. Higher the stress, lower will be the cycles to cause failure. Similarly, if the stress is lowered, more number of cycles required to cause failure. Keval K. Patil, M.E.-DESIGN, ([email protected]) 14

15. S-N Curve & its interpretation The basic method of presenting engineering fatigue data is by means of the S-N curve, a plot of stress ‘S’ against the number of cycles to failure ‘N’. The value of stress that is plotted can be alternating stress, maximum stress, or minimum stress. Most determinations of the fatigue properties of materials have been made in completed reverse bending. Keval K. Patil, M.E.-DESIGN, ([email protected]) 15

16. S-N Curve & its interpretation Keval K. Patil, M.E.-DESIGN, ([email protected]) 16

17. S-N Curve & its interpretation For a few important engineering materials such as steel & titanium, the S-N curve becomes horizontal at a certain limiting stress. Below this limiting stress, which is called the fatigue limit or endurance limit, the material can endure an infinite number of cycles without failure. Most non ferrous metals, like aluminium, magnesium & copper alloys, have an S-N curve which slopes gradually downward with increasing number of cycles. These materials do not have a true fatigue limit because the S-N curve never becomes horizontal. In in such cases it is common practice to characterize the fatigue properties of material by giving the fatigue strength at an arbitrary number of cycles, e.g. 10^8 cycles. Keval K. Patil, M.E.-DESIGN, ([email protected]) 17

18. Influence of Important factors on FATIGUE Fatigue strength is seriously reduced by following factors: Notch effect Surface effect Corrosion fatigue Thermal fatigue Pre-stressing Keval K. Patil, M.E.-DESIGN, ([email protected]) 18

19. Notch effect: Fatigue strength of material is reduced by presence of notch in the material. E.g. Machine element it contains fillets, keyways, screw threads and holes. Fatigue cracks in structure parts usually starts at such geometrical irregularities. The effect of fatigue is generally studied by specimens containing a ‘V’ notch or a circular notch. The effect of notches on fatigue strength is determined by comparing the S-N curves of notched and un-notched specimens. Keval K. Patil, M.E.-DESIGN, ([email protected]) 19

20. Thermal fatigue Fatigue failure can be produced by fluctuating thermal stress under conditions where no stress are produced by mechanical causes. Thermal stress result when the change in dimensions of a member as the result of a temperature change. For the simple case of a bar with fixed end supports, the thermal stress developed by a temperature change ‘ dT ’ is σ = α *E* dT where , α = Linear thermal coefficient of expansion. E = Elastic modulus If failure occurs by one application of thermal stress, the condition is called thermal shock. However, if failure occurs after repeated application of thermal stress it is called as Thermal fatigue. Keval K. Patil, M.E.-DESIGN, ([email protected]) 20

21. Surface effect All fatigue failures start at the surface. The factors which affect on the surface of a fatigue specimen are roughly divided into 3 categories: Surface roughness Changes in surface properties Surface residual stress Surface roughness: Smoothly polished specimen in which the fine scratches are oriented parallel with the direction of the principle tensile stress, give the highest values in fatigue tests. Such polished specimen are usually used in laboratory fatigue tests and are known as ‘par bars’. Keval K. Patil, M.E.-DESIGN, ([email protected]) 21

22. Change in surface properties : Fatigue failure is dependent on the surface conditions. Decarburization of the surface of heat-treated steel reduces fatigue performance. Carburizing and nitriding makes steel surface stronger and harder, which improves fatigue properties. The fatigue performance is improved when notched fatigue specimens are nitrided . Electroplating of surface generally decreases the fatigue limit of steel. Surface residual stress : Residual stresses are stresses that exist in a part independent of any external force. Nearly every manufacturing operation will result in residual stresses in varying degrees. Residual stresses are beneficial when they are opposite to the applied load. Keval K. Patil, M.E.-DESIGN, ([email protected]) 22

23. Corrosion fatigue : The simultaneous action of cyclic stress and chemical attack is known as corrosion fatigue. Corrosion attack without superimposed stress often produces pitting of metal surfaces. The pits acts as notches and produce a reduction in fatigue strength. When corrosion and fatigue occur simultaneously, the chemical attack greatly accelerates the rate at which fatigue cracks propagate. Material which shows a definite fatigue limit when tested in air at room temperature shows no indication of a fatigue limit when the test is carried out in a corrosive environment. Keval K. Patil, M.E.-DESIGN, ([email protected]) 23

24. Corrosion fatigue : When fatigue test is carried out in air not affected by the speed of testing, over a range from about 10 to 200 Hz, when test is carried out in corrosive environment there is a definite dependence on testing speed. Since corrosive attack is a time-dependent phenomenon, the higher the testing speed, the smaller the damage due to corrosion. In usual method, corrosion-fatigue test is carried out by continuously subjecting specimen in combined influences of corrosion and cyclic stress until failure occurs. A number of method are available for minimizing corrosion-fatigue damage. In general material is protected from corrosive environment by metallic and non-metallic coating is successful method. Nitriding is effective in minimizing corrosion fatigue. Keval K. Patil, M.E.-DESIGN, ([email protected]) 24

25. Effect of Pre- stressing: Pre-stressing is the process of loading an engineering component under controlled conditions to a cyclic stress for a fixed number of cycles prior to any possibility of fatigue failure. When magnitude of pre-stressing is lower than the operating stress level then it is known as under-stressing. Under controlled conditions when the magnitude of the stress is higher than operating stress level the condition is called overstressing. The number of pre-stressing cycles is always lesser than the number of cycles needed to cause fatigue failure. Pre-stressing is desirable under conditions of under-stressing. Keval K. Patil, M.E.-DESIGN, ([email protected]) 25

26. Pre- stressing: An under stressed components always exhibits best fatigue resistance. For an over-stressed component the failure resistance is lesser compared to a components that has been pre-stresses. Thus under stressing cause significant improvement in fatigue behaviour by strengthening the weak regions and enhancing their dynamic response to operating stress. Keval K. Patil, M.E.-DESIGN, ([email protected]) 26

27. Conclusion Keval K. Patil, M.E.-DESIGN, ([email protected]) 27