Influence of dynamic-strain aging due to excess Mg on fatigue crack growth rate scatter in Al6061-T6 alloy

Abstract

Fatigue failure results in high industrial costs, and its mechanism requires close examination; however, current methods are costly and time-consuming due to the need for a large number of test specimens. The purposes of this study are to investigate the influence of dynamic strain aging on fatigue crack growth rate (FCGR) scatter in Al 6061-T6 alloys and to present a new approach to evaluate the scatter of FCGR using a limited number of the test specimen. Rotating bending fatigue tests of 6061-T6-based Al alloys with added Zr and excess Mg were performed under constant amplitude loading using smooth specimens. The scatter behavior of FCGR is investigated by examining the fatigue crack growth on the specimen surface and fractographic observation on the fracture surface. The accounting for the interaction effects of multiple surface cracks and fractographic examination on striation formation from previous findings revealed that excess Mg promoted small scatter in Mode I fatigue crack growth. This study showed that local plastic deformation affected the FCGR scatter of microstructurally-large fatigue cracks. These findings suggest that dynamic strain aging of Mg induces stable Mode I crack growth due to pinning of dislocation movement on slip planes during the crack growth process.