Abstract
Surface fatigue crack growth tests under constant amplitude loading were carried out on three different materials of a medium carbon steel, JIS S38C, namely quenched and tempered material, quenched material and induction surface hardened material. It was found that the crack opening stress at the surface of hardened material was higher than those of the other materials, and the fatigue crack growth resistance in terms of the stress intensity range of surface hardned material was superior to the other materials. These differences resulted from the surface residual stress and there found little difference among three materials in the relationship between fatigue crack resistance and the effective stress intensity range. The higher crack opening stress intensity factor of the surface hardned material, Kop, could be well estimated by considering the actual stress ratio at the fatigue crack tip, R', which is defined as R'=(Kmin+Kr)/(Kmax+Kr), where Kmax and Kmin are the maximum and minimum applied stress intensity factors and Kr is the stress intensity factor induced by residual stress.
