J-INTEGRAL APPROACH TO MODE III FATIGUE CRACK PROPAGATION IN STEEL UNDER TORSIONAL LOADING

Mode III fatigue crack propagation tests were conducted on circumferentially cracked bars of a medium carbon steel under a constant value of the J-integral range. The ΔJ value was evaluated from the loading part of the hysteresis loop of the applied torque and the angle of twist. The fracture surface was macroscopically flat for all cases examined in the present study. The crack propagation rate decreased with crack extension, because of the shear contact of the crack faces. The crack propagation rate, without contact shielding, obtained by extrapolating the relation between the crack propagation rate and the crack length to the pre-crack length, was a power function of ΔJ irrespective of the initial notch depth.