The study of fatigue in polycrystalline copper under various strain amplitude at stage I: crack initiation and propagation

Abstract

During the past two decades, several studies on the crack initiation have shown that the crack initiation at high plastic strain amplitude is mainly started at the grain boundaries(GB), and at low plastic strain amplitude crack initiation is mainly started at the persistent slip band(PSB). However, the GB is still considered to be the preferred locations for crack initiation. It has also been pointed out that crack initiation started at GB results from the interactions between grain boundaries and persistent slip bands (PSB–GB), but only few studies has been devoted to finding the starting places for the following crack propagation, which may start at either the PSB or the GB. Moreover, most of the observations on the PSB–GB are done by scanning electron microscopy (SEM) — yet, observation by transmission electron microscopy (TEM) is seldom reported. In this paper, pure copper square specimens are employed in low cycle fatigue testing under strain-controlled condition. Optical microscopy (OM), SEM and TEM are used to investigate the initiation, propagation and the accompanied dislocation structures of the cracks. The results reveal that the mechanisms of crack initiation are the same as those in earlier studies. The dislocation structures, for crack initiated at the PSB, are ladder-like dislocation wall; and for those initiated at the GB, are dislocation cell. With regard to the crack propagation, crack initiated at the GB is the majority — no matter the crack was initiated at the PSB or the GB at the beginning.