Unraveling the Science of Variable Amplitude Fatigue

Conventional methods to estimate variable-amplitude fatigue life revolve either around cumulative damage analysis using the local stress-strain approach, or, around one of the crack growth load interaction models. Despite advances in modeling the mechanics of fatigue, none of these methods can faithfully reproduce the near-threshold variable amplitude fatigue response that determines the durability of machines and structures primarily because they fail to model the science behind the residual stress effect. Residual stress effects have a strong bearing on metal fatigue and owe their influence to the moderation of crack-tip surface chemistry and surface physics. This demands the treatment of threshold stress intensity as a variable, sensitive to load history. The correct estimation of crack closure is also crucial to determining the variable amplitude fatigue response and demands assessment of the cyclic plastic zone stress-strain response.