A Study On The Ratcheting And Low-cycle Fatigue Behavior Of Aluminum Alloy

Aluminum alloy, one of the main materials used in the manufacture of high-speed trains, always undertakes a kind of symmetric or asymmetric cyclic loading condition. Under asymmetric stress-controlled cyclic loading conditions, ratcheting behavior will occur in the materials. However, the accumulation of ratcheting strain will result in a large deformation of structure members, which influences their normal usage. Thus, the ratcheting behavior of the materials needs to be taken into consideration in the practice of engineering design. Under the strain-controlled cyclic loading conditions, fatigue damage occurs in the materials. Furthermore, the process of rolling will lead to the anisotropy in the material, which affects the fatigue life of the materials in the different loading directions. Thus, it is significant to investigate the effects of ratcheting behavior and anisotropic responses on the fatigue life of aluminum alloy.In this paper, extruded6005aluminum alloy and rolled5083H111aluminum alloy plate are used. At first, the uniaxial cyclic deformation behavior of6005aluminum alloy materials is experimentally researched at room temperature, and the effects of different strain amplitude and mean strain on the responding stress amplitude, and that of mean stress, stress amplitude, loading rate, hold time of peak/valley value on the ratcheting behavior of the material are discussed. Then, the fatigue behavior of rolled5083H111aluminum alloy plate in the different directions is investigated under the uniaxial symmetric strain-controlled cyclic loading conditions, and the differences between the cyclic deformation behavior and fatigue life occurred in three typical directions of the rolling direction (RD), the direction at45°angle to the rolling direction (denotes as MD) and the transverse direction (TD) of aluminum alloy plate are studied. The results show that6005aluminum alloy is a cyclic stable material, and under the strain-controlled cyclic loading conditions, the responding stress amplitude increases with the increase of strain amplitude. Under the stress-controlled cyclic loading conditions, the ratcheting behavior occurs in6005aluminum alloy and depends not only on the mean stress and stress amplitude, but also on the loading rate and the peak/valley hold. The rolled5083H111plate characterizes obvious cyclic hardening feature in three prescribed directions, i.e., the responding stress amplitude increases progressively with the number of cycles during the symmetrical strain-controlled cyclic loading. The larger the applied strain amplitude is, the larger the responding stress amplitude. However, the differences between the ε-N curves in three directions are not so obvious.Also, in this paper, some existing fatigue failure models are assessed by predicting the low-cycle fatigue life of5083H111rolled aluminum alloy plate in three directions. The results show that both Ostergren’s method and three-parameter power function model are suitable to predict the fatigue life of the plates, but the Manson-Coffin model is not suitable.