Fatigue Behavior And Fatigue Life Prediction Of Metals Under Multiaxial Cyclic Loading

Many components used in the aircraft, power generation industries and automobiles usually undergo multiaxial loading. However, multiaxial fatigue is more complicate than uniaxial fatigue. Up to now, little experimental work has been done for multiaxial loading, and many problems have not been solved completely. Thus the systematical studies on the multiaxial fatigue damage and the method of life prediction have not only the important significance, but also a practical value. Proportional and nonproportional tension-torsion fatigue tests were conducted on LY12CZ aluminum alloy. Two types of tubular specimens were used: the first one is a thin-walled tubular specimen while the other is a notched thin-walled tubular specimen. Axial tension-compression, torsion, in-phase, 45°out-of-phase and 90°out-of-phase loading paths were selected. Another series of biaxial two-level variable amplitude loading tests and two-path variable mode loading tests were conducted on smooth tubular specimens. The loading sequences of axial/torsion, torsion/axial, in-phase/ 90°out-of-phase, 90°out-of-phase/in-phase, in-phase/45°out-of-phase were selected. Several multiaxial fatigue criteria are reviewed. The criteria are divided into three groups, according to the parameters used to describe the fatigue life or fatigue strength of materials. They are stress criteria, strain criteria and energy criteria. Their predictive capabilities are checked against the experimental data of four materials under proportional and nonproportional loading. The stresses, strains and damage parameters on multiaxial fatigue critical planes were analyzed. A new multiaxial fatigue damage criterion based on critical plane concept was proposed. The capability of fatigue life prediction for the proposed fatigue damage model was checked against the experimental data of LY12CZ aluminum alloy and two other metals under proportional and nonproportional loading, and the predicted results were compared with results from a common multiaxial fatigue model. It is demonstrated that the proposed criterion gives better satisfactory results for all the three checked materials. Several popular damage accumulation rules are examined with the test data obtained in this study. A new nonlinear damage mode was proposed. The proposed nonlinear damage mode was applied to the life prediction of LY12CZ aluminum alloy under two-lever variable amplitude and two-mode variable mode multiaxial loading. The predicted results show good agreement with experimental results.