Load determines fatigue crack growth rates

Crack growth rates determined from constant-amplitude cyclic loading tests are approx-innately the same as for random loading tests in which the maximum stress is held constant but mean and range of stress vary randomly. However, in random loading tests where the maximum stress is also allowed to vary, the sequence of loading cycles may have a marked effect on crack growth rate, with the overall crack growth being significantly higher for random loading spectra.
Many investigations have shown a significant delay in crack propagation following in- termittent application of high stresses. That is, fatigue damage and crack extension are dependent on preceding cyclic load history. This dependence of crack extension on preceding history and the effects upon future damage increments are referred to as interaction effects.
Most of the interaction studies conducted have dealt with retardation of crack growth as a result of the application of occasional tensile overload cycles. Retardation may be characterized as a period of reduced crack growth rate following the application of a peak load or loads higher and in the same direction as those peaks that follow.
The modeling of interaction effects requires consideration of crack-tip plasticity and its subsequent influence. In metals of all types, cracks will remain closed or partially closed for a portion of the applied cyclic load as a consequence of plastically deformed material left in the wake of the growing crack. Under cyclic loading, crack growth will occur during the loading portion of the cycle. Given that a plastic zone exists at the crack tip prior to crack extension, as the material at the crack tip separates, the newly formed crack surfaces will exhibit a layer of plastically deformed material along the newly formed crack faces. Subsequent unloading will compress this plastically deformed material, closing the crack while the applied stress remains tensile. This phenomenon is known as plasticity-induced fatigue crack closure and was first discussed by Ether.” Upon reloading during the following cycle, crack growth will not continue unless the applied load is sufficiently large such that the compressive stresses acting along the crack surfaces are overcome and the crack is fully opened. This load is known as the crack opening load and has been demonstrated to be a key parameter in determining fatigue crack growth rates under both constant amplitude and spectrum loading.
 

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