The Effects of Mean Stresses Normal to the Maximum Shear Planes in Multiaxial Fatigue

摘要

Tubular specimens of SAE 1045 steel, RC 48 were tested in axial tension-compression with alternating internal/external pressure. Static mean stresses ranging from ―400 MPa to 740 MPa were applied normal to the maximum shear planes with alternating shear stress amplitudes from 100 MPa to 1000 MPa. An approximately linear relationship was found between the applied normal static mean stresses and the maximum cyclic shear stresses on the critical shear planes for a given fatigue life. For a constant shear stress, the fatigue life for tests with static tensile mean stresses larger than 420 MPa remained constant with increasing mean stress. It was assumed that for static mean stresses larger than 420 MPa, the crack faces were fully separated thus allowing unhindered Mode Ⅱ displacement, a condition defined as an interference-free crack state by Bonnen and Topper. Current critical plane theories were investigated to determine their ability to predict fatigue life for alternating shear stresses and static mean stresses normal to the maximum shear planes. A modified Findley parameter was developed to provide the best fit for the experimentally obtained data. The empirical constant modifying the contribution of the static mean stresses was determined from the slope of the maximum permissible shear stress versus static mean stress curve for a given fatigue life. To include the interference free condition in the parameter, tensile static mean stresses larger than the interference free condition (420 MPa), were replaced with the normal static mean stress that resulted in the interference free stress state in the modified Findley parameter.