Mean stress relaxation during cyclic straining of high strength aluminum alloys

Cycle-dependent relaxation may alter mean stress values and thus affect fatigue crack initiation life. This phenomenon is an issue both for accuracy of estimated fatigue lives and for the success of methods of intentionally introducing beneficial mean stresses. Although rigorous plasticity theories may describe cycle-dependent relaxation, it is not feasible to incorporate their complex mathematics into strain-based life estimates when loading histories contain millions of cycles. Empirical models are, on the other hand, more numerically efficient and, therefore, for the present purpose, constitute a better choice for modeling this transient phenomenon. Experiments have been performed to study mean stress relaxation in aluminum alloys 7075-T6511 and 7249-T76511. Preliminary tests were also conducted on alloy 7475-T651. Two empirical models are used to illustrate the experimentally observed cycle-dependent relaxation.