Control of intergranular fatigue cracking by slip homogeneity in copper I: Effect of grain size

Abstract

Recent studies of long-life fatigue in polycrystalline copper have shown fractures to initiate primarily at grain boundaries. This is considered surprising in the light of early work where crack nucleation in persistent slip bands seemed to be the rule. To investigate the causes of this behavior, studies habe been carried out on polycrystalline copper with emphasis on the following factors: grain size (Part I), method of starting the test, frequency and mode of test control (Part II). The results show that the cyclic plasticity is the controlling factor in grain boundary initiation and propagation of fatigue cracks. It was found that transgranular cracking can be produced by factors which promote strain localization, and reducing the grain size of the specimens in constant-amplitude tests is one of them. In small-grain-sized metals, most of the applied plastic strain is concentrated in a few persistent slip bands, some of which develop into cracks with continued cycling. Use of coarse-grained structures causes strain homogenization and therefore intergranular cracking. For coarse-grained copper, a step is gradually built up at a sensitive grain boundary as a result of the “homogeneous” cyclic slip, at which point the cracking occurs in the same manner as proposed by Kim and Laird for high strain fatigue.