Strain bursts in pure copper subjected to various forms of static and cyclic loading at ambient temperature

Abstract

The phenomenon of strain bursts has been discovered in pure polycrystalline copper subjected to cyclic or static creep, cyclic incremental tension and cyclic tension-compression. In cyclic creep, bursts appear over the stress range 0.5–0.8 of the ultimate tensile strength. The threshold stress for cyclic creep is much lower than this, however, and the lower limit for the appearance of bursts is believed to be dictated by the sensitivity of the apparatus used to detect them. Bursts can be produced in exaggerated form by slight increments of stress when the material has reached a saturated state. The bursts were usually observed to be more pronounced the higher the stress level. The mechanism of the strain bursts is discussed. They are shown not to be caused by dynamic strain aging (due to an impurity, say, such as oxygen). Their mechanism is also different in detail from that of the bursts observed by Neumann because the dislocation structures are different. We conclude that they are caused purely by dislocation locking and the subsequent sudden collapse of the locking mechanism.