Issue 35

S. Xinhong et alii, Frattura ed Integrità Strutturale, 35 (2016) 441-448; DOI: 10.3221/IGF-ESIS.35.50 447 measured angles are in the other side of the maximum shear stress amplitude plane. The predicted angles made by Findley parameter are close to coincident with predictions made by Fatemi-Socie parameter. And most of the predicted crack angles are in the middle part of the measured angles and hollow dots corresponding to the maximum shear stress amplitude. The measured angles and predicted angles based on three parameters under tension-torsion loading for ,  x m =150MPa are shown in Fig. 8(d).In this situation all of the measured crack angles are in the side of maximum shear stress amplitude plane with larger normal mean stress. The direction of this plane is about -10 ° apart with the direction of specimen axial, and is smaller 5 ° than predicted angles corresponding to the maximum shear stress amplitude. All of the predicted crack angles based on Findley and Fatemi-Socie parameters are close to the hollow dots corresponding to the maximum shear stress amplitude. With increasing of normal mean stress on both maximum shear stress amplitude planes, more cracks are becoming to initial and propagate from the side of maximum shear stress amplitude plane to the other side of maximum shear stress amplitude plane with larger normal mean stress. And the angle of deviation from the plane of maximum shear stress amplitude increases. C ONCLUSION he experiment and analysis of stress components on plane orientations under tension-torsion fatigue with different axial mean tensile stress for 2A12-T4 aluminum alloy were conducted, the plane orientation in macroscopic fracture are studied. The following conclusions can be drawn: (1) Under the tension-torsion loading, both of the maximum shear stress amplitude and normal mean stress have effect on the crack initiation and early propagation plane orientation. Fatigue cracks initial and early propagate on the plane which is close to the plane of the maximum shear stress amplitude plane. This characteristic is not changed by the presence of mean stress. With increasing of 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. (2) When axial mean stress is  x,m =0MPa, there are two predicted plane angles separated by 90  but only one crack plane exists. When axial mean stress is  x,m =50MPa, the predicted angles are not coincident with the crack plane orientations which are not the planes with larger normal mean stress. When axial mean stress is  x,m =100MPa, the differences between most of predicted angles and crack plane orientations with larger normal mean stress is nearly 5 ° . When axial mean stress is  x,m =150MPa, the predicted angles are close to the crack plane orientations. N OMENCLATURE   x axial stress  ,  x a axial stress amplitude  ,  x m axial mean stress   x shear stress  ,  xy a shear stress amplitude  ,  xy m mean shear stress   a shear stress amplitude on the critical plane  ,  n a normal stress amplitude on the critical plane  ,max  n maximum normal stress on the critical plane  ,  n m mean normal stress on the critical plane    shear strain range   y yield tensile strength  G shear modulus T