Effect of Mean Stress on Torsional Fatigue Behavior of a Medium Carbon Steel
Xu CHENG, Isao OHKAWA and Masaaki MISUMI
Abstract:Load-controlled fatigue tests were conducted on smooth tubular specimens of an annealed medium carbon steel under fully-reversed torsion with superimposed static torque or tension. Effect of mean stress on initiation and growth of short crack was studied in the intermediate life regime. Microcracks initiated around the maximum shear stress amplitude planes rather than the planes of maximum shear stress, irrespective of mean stress conditions. The dominant crack grew accompanying coalescence of the shear cracks initiated in the early stage of life and the preferential growth direction was different depending on the type of mean stress. The increase of the crack growth rate and the reduction of the fatigue life with application of mean stress were slight in the present material. The relatively smaller mean stress effect appeared to be resulted from the decrease of the plastic shear strain amplitude with static loading. Therefore, two-stage short crack growth model proposed by Hobson and Brown was modified incorporating the effects of mean stress acting on the crack planes and the hardening/softening of material. The model showed a satisfactory estimation of fatigue lives in torsion under various mean stress conditions. Key Words:Torsion, Mean stress, Carbon steel, Stress-strain response, Short crack, Crack growth rate, Fatigue life