On the dependence of the rate of fatigue crack growth on the σna(2a) parameter

An empirical fatigue crack growth law, d(2a)/dN=Cσna(2a), where σa is the stress amplitude, n is a material-dependent exponent, N is the number of cycles and 2a is the length of a surface crack, has been used to describe the rate of fatigue crack growth in steels in the physically short-crack, elastic—plastic range. The present paper demonstrates that the rate of fatigue crack growth can also be predicted by a more physically based, modified linear-elastic approach. The modifications involve: a material constant that provides a transition from stress control of fatigue crack growth for small cracks to stress intensity factor control for large cracks, a plastic zone size correction, and consideration of crack closure in the wake of a newly formed crack. The modified linear-elastic fracture mechanics approach also predicts that in the very early stages of crack growth, i.e., up to a crack size of 300 μm for the steels investigated, the empirical relation cited above is not valid.