Stress Ratio Dependence of Fatigue Crack Initiation and Small Crack Growth in Cast Aluminium Alloy AC4CH

摘要

This paper describes the stress ratio dependence of fatigue behaviour in a cast aluminium alloy, AC4CH. Axial fatigue tests have been conducted using smooth specimens at three different stress ratios, R, of -1, 0.05 and 0.4. Particular attention has been paid to crack initiation and small crack growth behaviour. The Goodman relationship gave fairly good or slightly conservative estimation of fatigue strength at 10~7 cycles. Cracks initiated from a casting porosity or Sb-segregated defect and the latter defect acted more predominantly as crack initiation site. Regardless of stress ratio, small cracks initially grew faster than the corresponding large cracks characterized in terms of the effective stress intensity factor range, DELTA K_(eff), then gradually approached the da/dN- DELTA K_(eff) relationship for large cracks and coincided with it at a DELTA K value of 2 approx 3MPa m~(1/2). Based on detailed fractographic analyses, shear-type growth region following the crack initiation was clearly discerned, the size of which showed remarkable stress ratio dependence, with increasing crack size with decreasing stress ratio, particularly very large at R = -1. This shear-type crack growth was responsible for the acceleration of small cracks because of a different growth mechanism from large cracks.