Theoretical assessment of fatigue life of metal materials at constant cycle stress range

Abstract

A computational model is proposed for theoretical evaluation of fatigue life of samples made of metallic materials at a constant applied cycle stress range. The total fatigue life is divided into two stages: the stage of crack nucleation, i.e., the number of loading cycles before its initiation (stage 1) and the stage of crack growth (stage 2). Different approaches are used to estimate the duration of each stage. For stage 1, the power law fatigue life equation is obtained from the condition that the accumulated strain energy is equal to the specific cracking energy, and for stage 2, the number of cycles is obtained by integrating the crack growth rate equation, where the effective range of the stress intensity factor (SIF), defined as the difference between the applied "external" range of the SIF and the "internal" range of the SIF, which characterizes the material's resistance to crack growth, is taken as its driving force. The initial data for the calculation, in addition to the loading parameters, are the characteristics of the static strength and microstructure of the initial material. Calculations based on the proposed fatigue life model for steel 0.45%C specimens show good agreement with experimental results.