Calculation of S-N curves for low alloy steels, based on crack length and an inclusive crack propagation model

In 1991 Miller explained that the accumulation of fatigue damage in the whole fatigue domain can be understood as a process of propagation of cracks, starting with microstructurally short ones, rind later on with large cracks, and that appropriate calculation procedures do exist for each fatigue regime. This statement has been largely accepted, but not too many studies since have shown the correlation of the crack concept with the traditional fatigue parameters, and the S-N curve in particular. A two parts model that is useful in the prediction of crack propagation over the whole fatigue domain, when the stress amplitude is above the endurance limit and the stress intensity factor range is below or above the threshold value, has been recently proposed by one of the authors. The proposed model has been modified now and used to calculate S-N curves of a family of AISI 4340 low alloy steels, for different alternating stress amplitudes, in mode I loading, with R = -1. The calculated S-N lines were found to be in good agreement with the experimental results from four different sources. A more general shape of the model is claimed to have a semi-theoretical justification and is proposed to serve as a basis for an extensive prediction rule.