Differences in the Local Strains and Crack Initiation Processes between Plain Specimens of Age-Hardened and Annealed Al Alloys in Low Cycle Torsional Fatigue

Abstract

The local strain concentration plays an important roll in low cycle fatigue of plain specimens. It is controlled by the strain hardening and strain softening properties of materials. In this study, low cycle torsional fatigue tests of age-hardened, strain softening Al alloy and annealed, strain hardening Al alloy were carried out on plain specimens having the fine lines drawn on the surface by which local strain s were evaluated. The results obtained are summarized as follows: (1) The crack nucleation due to the strain concentration near the grain boundaries is one of the representative mechanisms of micro-crack initiation in both age-hardened and annealed Al alloys. (2) Since an age-hardened Al alloy is not necessarily as strain softening type in a macroscopic scale but is strain softening type in a microscopic slop, the tendency of strain concentration in any location is strong. (3) The relation between the number of cycles and the amount of slip in the location where it becomes a crack in the future depends on the strain hardening-softening properties of the two materials. However, the relations in slop band crack and grain boundary crack are similar in each material.