A THEORY OF STAGE II FATIGUE CRACK PROPAGATION

A continuum model of crack extension is used to derive a crack propagation law for the case of constant plastic strain amplitude fatigue. The rate of crack growth is found to be proportional to the square root of the crack length. Integration over the total number of cycles to failure, N sub f, yields an expression of the form N sub fdelta epsilon sub p bar to the n1 power fdelta epsilon sub p bar, where delta epsilon sub p bar is the applied plastic strain range, and f is a function which varies rapidly in the region of large strains, but approaches a constant as delta epsilon sub p bar becomes small. The strain hardening coefficient, n, and the fracture strain enter as material constants. Comparison with experimental data gives good agreement for delta epsilon sub p bar or 0.01, which is consistent with the assumptions used in the theory. A discussion is given which interprets the well known power law N sub f to the 12 power times delta epsilon sub p bar const. in terms of crack propagation.