Overload induced fatigue crack growth and significance of retardation zone

The behaviour of fatigue crack propogation has been studied in an Al-alloy and a steel on application of an overloading cycle. The crack acceleration during overloading cycle has been measured using scanning electron microscope (SEM) and electrical potential difference (EPD) techniques. The effects of overloading ratio (OLR), stress ratio (R) and Delta K-I on crack acceleration have been investigated. The overload induced closure, K-op(oL) has been measured. The effects of various parameters on the size of retardation zones, a(D) and a(D)* have been further studied. The size scale of retardation zones with respect to the overload monotonic plastic zone (2r(m)(oL)) and the overload cyclic plastic zone (2r(C)(oL)) has been rationalized based on the findings from present research as well as reanalysis of data from literature.