Study on the Effects of Band Overload on Fatigue Crack Growth Retardation

Abstract

In the present work experiments were performed in Instron-8502 machine with a 250 kN load cell capacity. All tests were conducted in air and at room temperature. Tests were performed to investigate the effect of multiple consecutive overload cycles on constant amplitude fatigue crack propagation rate. For this purpose constant amplitude fatigue tests were carried out and overload was applied at an a/w ratio of 0.4 in all specimens, following which the same constant amplitude tests were continued. The overload ratio was maintained at 1.9 in all cases and the number of overload cycles applied was 1, 2, 5, 10 and 100. From experimental data it has been observed that a single spike overload (1 cycle) gives maximum retardation and the retardation effect decreases with the increase in number of overload cycles. The number of delay cycles (ND) and the retarded crack length (aD) were obtained for all the above cases. The values of both these parameters decreased with increase in number of overload cycles. Finally a model has been developed to predict the post-overload life of a specimen. The model was developed with experimental data of 1, 2, 5 and 100 overload cycles and was verified by comparing predicted data and experimental data for 10 overload cycles. The fractured surfaces of the test specimens were examined under SEM in order to investigate the nature of crack growth under different overload cycles. Fractogrpahs show that presence of micro voids and secondary cracks in the tear zone. This suggests development of a highly strained zone on application of overload cycles. It may be recalled that the extent of retardation reduces on application of multiple overload cycles. The overload tear zones are measured in different cases which show a maximum in case of 100 cycles. The width of the tear zones are nearly same for specimens subjected to overload cycles 2, 5 and 10. This suggests significant stress relaxation on application of large number of overload cycles. Thus the reduction in magnitude of retardation in case of multiple overloading may be due to stress relaxation and it becomes significant for large number of overload cycles.