BURST PHENOMENA IN FATIGUE OF 70-30 ALPHA-BRASS AT ROOM-TEMPERATURE

Strain bursts in 70-30 alpha-brass were studied under both strain control and load control. In strain control, the influence of strain amplitude and strain rate on the frequency of strain bursts were determined. Burst frequency passed through a maximum as the first cycle plastic-resolved shear strain, gamma(p/2), increased, while burst frequency decreased with increased strain rate for a given strain amplitude. Strain bursts were mainly associated with new slip which formed immediately adjacent to the previously operating slip. A model was proposed which incorporates Zn diffusion to mobile dislocations during those periods in a fatigue cycle where they are essentially immobile, i.e. during elastic loading and unloading. In load control, there are two types of strain burst: large strain bursts associated with load increases, and small strain bursts occurring without load increases. The operative mechanism for load-control strain bursts was considered to be the same as the mechanism producing increased strain under cyclic creep conditions where loading is continually interrupted. For comparison purposes, the burst behavior vas also examined for pure Cu under strain control. No strain bursts were found.