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

During the past two decades, several studies on the crack initiation have s hown that the crack initiation at high plastic strain amplitude is mainly s tarted at the grain boundaries(GB), and at low plastic strain amplitude cra ck initiation is mainly started at the persistent slip band(PSB). However, the GB is still considered to be the preferred locations for crack initiati on. It has also been pointed out that crack initiation started at GB result s from the interactions between grain boundaries and persistent slip bands (PSB-GB), but only few studies has been devoted to finding the starting pla ces 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 sca nning electron microscopy (SEM) - yet, observation by transmission electron microscopy (TEM) is seldom reported. In this paper, pure copper square spe cimens are employed in low cycle fatigue testing under strain-controlled co ndition. Optical microscopy (OM), SEM and TEM are used to investigate the i nitiation, propagation and the accompanied dislocation structures of the cr acks. The results reveal that the mechanisms of crack initiation are the sa me as those in earlier studies. The dislocation structures, for crack initi ated at the PSB, are ladder-like dislocation wall; and for those initiated at the GB, are dislocation cell. With regard to the crack propagation, crac k initiated at the GB is the majority - no matter the crack was initiated a t the PSB or the GB at the beginning. (C) 2000 Elsevier Science S.A. All ri ghts reserved.