DELAY EFFECTS IN FATIGUE CRACK PROPAGATION.

The fatigue crack propagation behavior resulting from variations in load is examined for 2024-T3 aluminum alloy, from both a macroscopic and a fractographic point of view. A peak load is found to cause retardation of the crack growth rate, which becomes more pronounced as the percentage overload or baseline stress intensity level or both is increased. The delaying effect of the overload is observed to exist for a calculated crack length increment equivalent to the plastic zone size formed during the peak load. Multiple overloads and high-low block loading sequences are found to result in additional retardation. For a given DELTA K and percentage overload it is noted tthe greatest retardation occurs when lambda equals 1 ( lambda equals K//m//a//x/ DELTA K). It is observed that the macroscopic fracture surface appearance (that is, transition to plane stress) is a function of the crack growth rate. From fractographic examinations it is found that the initiation of microvoid coalescence during fatigue occurs when plane stress conditions are achieved.