S. Xinhong et alii, Frattura ed Integrità Strutturale, 35 (2016) 441-448; DOI: 10.3221/IGF-ESIS.35.50 441 Focussed on Crack Paths Crack initiation and early propagation plane orientation of 2A12-T4 aluminum alloy under tension-torsion fatigue loading including mean tensile stress Shi Xinhong Institute of Solid Mechanics, Beihang University, Beijing 100191, PR China shixinhong@buaa.edu.cn Zhang Jianyu College of Aerospace Engineering, Chongqing University, Chongqing 400044, PR China jyzhang@cqu.edu.cn , jyzhang@buaa.edu.cn Xiao Qingshan, Liu Tianqi Institute of Solid Mechanics, Beihang University, Beijing 100191, PR China qingshanwhut@163.com, tqliu1227@foxmail.com A BSTRACT . The specimen fractures appearances are analyzed to investigate the effects of mean tensile stress on the fatigue crack initiation and early propagation plane orientation under axial-torsion fatigue loading for 2A12- T4 aluminum alloy. The fatigue crack initiation and early propagation plane orientations are measured by optical microscope, the results of macro-analysis show that both the maximum shear stress amplitude and normal mean stress have effects on the orientations of crack initiation and propagation plane orientation, which are close to the plane of the maximum shear stress amplitude plane. With increasing the mean tensile stress, more cracks are inclined to initial and propagate on or near the maximum shear stress amplitude plane with larger normal mean stress, and the angle of deviation from the plane of maximum shear stress amplitude increases. The predicted plane orientations based on critical plane methods are compared with experimental measured results. K EYWORDS . Multiaxial fatigue; Mean tensile stress; Crack initiation and propagation plane; Critical plane orientation; Aluminum alloy. I NTRODUCTION n practice many engineering structures are subjected to fatigue loading, especially multiaxial fatigue loading, and that many of the structures undergo tensile mean stress due to external static load, weight or residual stress [1].So many multiaxial fatigue failure criteria based on critical plane approach have been proposed and applied to determine the I

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