Notch fatigue behaviour in cast irons explained using a fracture mechanics approach

Abstract

Many aspects of the fatigue behaviour of cast iron materials are difficult to explain. For example, they display a very low notch sensitivity, and notched specimens are subject to a strong size effect. These effects can be explained if a fracture mechanics approach is adopted, which has two elements. First, the notch itself is modelled as a crack: this approach is shown to be valid for a wide range of notches. Second, the notch depth a n is augmented by a crack length a c, thus modelling the notch as a crack of length ( a n + a c). This recognizes the fact that cracks initiate and grow from the notch root, even at stresses below the fatigue limit. Thus the fatigue limit is characterized by the presence of a crack of length a c. It is shown that the value of a c is directly related to the short crack parameter a o defined by ElHaddad et al. The method is tested on three cast iron materials: a grey cast iron, a ferritic SG iron and a pearlitic SG iron. The effect of section size is explained by this analysis, making it possible to use these specimen results to predict the behaviour of components. An incidental benefit is that accurate values of the threshold parameter ΔK th can be obtained from notch fatigue data.