The influence of static mean stresses applied normal to the maximum shear planes in multiaxial fatigue

Abstract

Abstract Tubular specimens of two different hardnesses of SAE 1045 steel (BHN 456 and 203) were tested in axial tension-compression together with alternating internal/external pressure. For the SAE 1045 steel, BHN 456, static mean stresses ranging from -400 MPa to 740 MPa were applied normal to the maximum shear stress amplitude planes together with alternating shear stress amplitudes from 150 MPa to 1000 MPa. For the SAE 1045 steel, BHN 203, static mean stresses ranging from -60 MPa to 100 MPa were applied normal to the maximum shear stress amplitude planes together with alternating shear stress amplitudes from 125 MPa to 220 MPa. An approximately linear relationship was found between the applied normal static mean stress and the maximum cyclic shear stress on the critical shear planes for a given fatigue life. The fatigue life remained constant with increasing mean stress for tests with constant shear stress amplitude values and static tensile mean stresses larger than 500 MPa and 76 MPa for the hard and soft steel, respectively. It was assumed that, for static mean stresses larger than these values, the crack faces were fully separated thus allowing unhindered Mode II displacement, a condition defined as interference free crack growth by Bonnen and Topper [1]. A confocal scanning laser microscope was used to determine the validity of this assumption. With this apparatus, the crack depth profiles were measured as a function of the magnitude of the static mean stress applied normal to the shear plane. The 2-D line profiles of the fracture surfaces were obtained to investigate the impact of normal static mean stresses on asperity height and shape [2]. 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 gave the best fit to the experimentally obtained data. To include the interference free condition in the parameter, tensile static mean stresses larger than the interference free condition were replaced with the normal static mean stress that resulted in the interference free stress state in the modified Findley parameter.