What is Fatigue in Mechanics Of Material .
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Fatigue is the progressive structural damage that occurs in materials when they are subjected to fluctuating or repetitive loads. It results in failure at stress levels that are much lower than the material's tensile strength. Fatigue failures occur without warning through plastic deformation and result in a smooth fracture surface. The factors that influence fatigue failure include the maximum stress level, the amount of stress fluctuation, the number of stress cycles, stress concentrations, temperature, microstructure, and residual stresses. Fatigue behavior is represented using an S-N curve, which plots the cyclic stress amplitude against the number of cycles to failure.
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What is Fatigue in Mechanics Of Material .
- 1. Fatigue Fatigue is theweakening of a material caused by repeatedly applied loads. It is the progressive and localised structural damage that occurs when a material is subjected to cyclic loading. Characteristics .Fatigue failures occur when metal is subjected to a repetitive or fluctuating stressand will fail at a stress much lower than its tensile strength. •Fatigue failures occurwithout any plastic deformation(no warning). •Fatigue surface appears as a smooth region, showing beach markor origin of fatigue crack. ( Fig.) Fatigue in crankshaft
- 2. Factors causing fatiguefailure Basic factors 1) A maximum tensile stress of sufficiently high value. 2) A large amount of variation or fluctuation in the applied stress. 3) A sufficiently large number of cycles of the applied stress Additional factors • Stress concentration • Corrosion • Temperature • Overload • Metallurgical structure • Residual stress • Combined stress The S-N curve Engineering fatigue datais normally represented by means of S-N curve, a plot of stressS against the number of cycle, N. •Stress can be→ σ a, σ max, σ min .σ m, R or A should be mentioned. •S-N curveis concerned chiefly with fatigue failure at high numbers of cycles (N > 105cycles) →high cycle fatigue (HCF). •N< 104 or 105 cycles →low cycle fatigue (LCF). •N increases with decreasing stress level. Fatigue of materials
- 3. In materials science,fatigue is the progressive and localised structural damage that occurs when a material is subjected to cyclic loading. The maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material. Characteristics of fatigue ●The process starts with dislocationmovements, eventually forming persistent slip bands that nucleate short cracks. ●The greater the applied stress, the shorter the life. ●Damage is cumulative. Materials do not recover when rested. ●Fatigue is a stochasticprocess, often showing considerable scatter. Fatigue life scatter tends to increase for longer fatigue lives. ●Fatigue life is influenced by a variety of factors, such as temperature, surface finish, presence of oxidizingor inertchemicals, residual stresses, contact (fretting), etc. Fatigue strength Number of cycles of stress of a specific character that a specimen of material can withstand before failure of a specific nature occurs. Number of cycles required (usually 10 million) to cause a failure decreases as the level of stress increases. Also called fatigue limit, it is affected by the environmental factors such as corrosion.