Effect of inclusions on fatigue behaviour of hardened spring steel

Abstract

The fatigue lifetimes of hourglass shaped specimens of a hardened spring steel were studied. The failure probability was determined experimentally at one loading level and causes of fatigue failure were identified on fracture surfaces. The depth profile of residual stresses after fatigue testing was determined using X-ray techniques. Cyclic flow data, long crack growth data, and the threshold for crack propagation were determined. Inclusion size distributions of the steel were obtained using different techniques. A model for the probability of fatigue failure of the hourglass specimens was formulated. Microcracks are assumed to exist at all inclusions and specimen failure is controlled by those cracks which can propagate to failure. Two different models based on linear fracture mechanics were used to determine critical inclusion sizes for crack propagation. The models take into account all the above independent experimental data, i.e. residual stresses, cyclic flow data, threshold for crack propagation, inclusion distribution, etc. Experimental failure probabilities were satisfactorily reproduced by the model.

MST/1648