AE2030 FATIGUE AND FRACTURE

AE2030FATIGUE AND FRACTURE1INTRODUCTIONFracture mechanics is the study of mechanical behavior of cracked material subjected to an applied loadFatigue is the weakening of a material caused by repeatedly applied loads. The process of progressive localized permanent structural changes occurring in a material subjected to conditions that produce fluctuating stresses at some point or points and that may culminate in cracks or complete fracture after a sufficient number of fluctuations.2WHY STRUCTURES FAIL?The cause of most structural failures generally falls into one of the following categories:Negligence during design, construction, or operation of the structure.Application of a new design or material, which produces an unexpected (and undesirable) result.3Common causes of Failure are:Yielding critical loading pointDeflection beyond a certain stageBucklingFatigueFractureCreepEnvironmental DegradationResonanceImpactWear4Fracture mechanismsDuctile fractureAccompanied by significant plastic deformationBrittle fractureLittle or no plastic deformationCatastrophic Usually strain is < 5%.5Brittle vs Ductile Materials

6FRACTURE MECHANICSStudy of crack propagation in bodiesMethodology used to aid in selecting materials and designing components to minimize the possibility of fracture.It begins with the assumption that all real materials contain cracks of some sizeeven if only submicroscopicallyBased on three types of displacement modes73 Modes of Crack Propagation

8FRACTURE TOUGHNESSFracture toughness is a property which describes the ability of a material containing a crack to resist fracture, and is one of the most important properties of any material for many design applications. The linear-elastic fracture toughness of a material is determined from the stress intensity factor (K) at which a thin crack in the material begins to grow.In fracture mechanics, one does not attempt to evaluate an effective stress concentration, rather a stress intensity factor KAfter obtaining K, it is compared with a limiting value of K that is necessary for crack propagation in that material, called KcThe limiting value Kc is characteristic of the material and is called fracture toughness910

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Fatigue Failure- Mechanism A fatigue failure begins with a small crack; the initial crack may be so minute and can not be detected. The crack usually develops at a point of localized stress concentration like discontinuity in the material, such as a change in cross section, a keyway or a hole. Once a crack is initiated, the stress concentration effect become greater and the crack propagates. Consequently the stressed area decreases in size, the stress increase in magnitude and the crack propagates more rapidly. Until finally, the remaining area is unable to sustain the load and the component fails suddenly. Thus fatigue loading results in sudden, unwarned failure. 12Four Different Stages of Fatigue FailureCrack initiation at points of stress concentrationCrack growthCrack propagationFinal rupture

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Factors Influencing Fatigue Loading Geometry Material Manufacturing Environment

14Stress Concentration FactorStress concentration factor (Kt), is a dimensionless factor which is used to quantify how concentrated the stress is in a material. It is defined as the ratio of the highest stress in the element to the nominal stress (reference stress )

15Characteristics of stress-concentration factors:Function of the geometry or shape of the part, but not its size or materialFunction of the type of loading applied to the part (axial, bending or torsional)Function of the specific geometric stress raiser in the part (such as fillet radius, notch, or hole) Always defined with respect to a particular nominal stressTypically assumes a linear elastic, homogeneous, isotropic material

16Stress Concentration Factor

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Fatigue Stress Concentration The existence of irregularities or discontinuities, such as holes, grooves, or notches, in a part increase the magnitude of stresses significantly in the immediate vicinity of the discontinuity .Fatigue failure mostly originates from such places. Hence its effect must be accounted and normally a fatigue stress-concentration factor Kf is applied when designing against fatigue, even if the materials behavior is ductile .

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Fatigue Stress Concentration Factor (Kf)

Miscellaneous-effect factor (Ke)19Notch Sensitivity (q)The value of q usually lies between 0 and 1. If q=0, Kf =1 and this indicates no notch sensitivity. If however q=1, then Kf = Kt and this indicates full notch sensitivity.

A measure of the reduction in strength of a metal caused by the presence of a notch

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21STRESS-LIFE DIAGRAM(Wohler S-N Curves)

Typical S-N curves for ferrous and non-ferrous metals.Steel, Ti. etcAl, Cu alloy, Mg, etc.,22Endurance Limit / Fatigue LimitThe fatigue life reduces with respect to increase in stress range and at a limiting value of stress, the curve flattens off. The point at which the S-N curve flattens off is called the endurance limit.Certain materials have a fatigue limit or endurance limit which represents a stress level below which the material does not fail and can be cycled infinitely.23Unlike steels, most nonferrous metals and alloys (Al, Mg, Cu alloy, etc.,) do not have a fatigue limit i.e. S-N curve continues to fall steadily with decreasing stress, though at a decreasing rate. Thus, fatigue will ultimately occur regardless of the magnitude of the applied stress. Fatigue response of these materials is specified for a number of stress cycles, normally 107, and is known as fatigue strength.An effective endurance limit for these materials is sometimes defined as the stress that causes failure at 1x108 to 5x108.24Metal fatigue is a significant engineering problem because it can occur due to repeated or cyclic stresses below the static yield strength, unexpected and catastrophic failure of a vital structural part may occur and rack initiation may start at discontinuities in highly stressed regions of the component. Fatigue failure may be due to discontinuities such as inadequate design, improper maintenance and so forth.25Fatigue failure can be prevented byAvoiding sharp surfaces caused by punching, stamping, shearing.Preventing the development of surface discontinuities during processing.Reducing or eliminating tensile residual stresses caused by manufacturing.Avoiding assembling errors, improper maintenance, manufacturing defects, design errorsUsing proper material and heat treatment proceduresEnvironmental Effects.2627

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32Goodman experimental observation are quite closer for brittle materials, but it is conservative for ductile alloys. For compressive mean stress, however it is non-conservative . To circumvent this problem, one may assume that compressive mean stress provide no beneficial effect on fatigue life.Gerber generally good for ductile material for mean tensile stress. But this does not distinguish between the fatigue life for tensile and compressive mean stresses.Soderberg provides a conservative estimation of fatigue life for most engineering alloys