Research On Fatigue Damage Accumulation Under Multiaxial Out-of-phase Loading

Fatigue failure is one of the main failure modes of actual engineering structures. The components on aircraft are subjected to multiaxial loading during working, so it is significant to research on multiaxial fatigue for aeronautic industry. Compared with uniaxial fatigue, multiaxial fatigue is more complicated no matter in mechanical analysis, experiment research or in physical mechanics. Until now, the multiaxial fatigue is still a hot and difficault problem in the field of fatigue.In this dissertation, the cyclic stress-strain relationship of LY12CZ aluminum alloy and the multiaxial fatigue damage accumulation rule are investigated.Fatigue experiments are conducted on LY12CZ aluminum alloy thin-walled tubular specimens under combined tension-torsion loading through strain control. Axial tension-compression, pure torsion and 90°out-of-phase paths are selected. Cyclic hardening is observed for LY12CZ under three paths and the degree of the hardening is higher with the improving of the amplitude of the loading equivalent strain. A little additional hardening is found under 90 degree out-of-phase loading compared to the stress responses for uniaxial cyclic loading. A parameter describing the extent of nonproportional additional hardening, the nonproportional additional hardening degree f, is proposed. Relation between 90°out of phase loading additional hardening degree f and fatigue lives is got according to data fit.Five popular damage accumulation rules are used to predicte the fatigue life for LY12CZ aluminum alloy and 304 stainless steel under two-level variable amplitude loading. Their fit field and the prediction ability are analyzed. Finally, a modified multiaxial fatigue damage accumulation model is proposed and tested, and the prediction results are acceptable.