MICRO-MACRO RELATIONS FOR FATIGUE-CRACK GROWTH

A fatigue crack growth model for the threshold and the power regimes i s presented. The fatigue crack growth rate equation is postulated by i dentifying the crack growth driving force with the difference between the calculated strain intensity at the crack tip for an ideal material (HRR-type macro solution) and the actual yield strain. The model is b ased on four micromechanic material parameters: (i,ii) a characteristi c slip distance (Neumann's type) and it's statistical distribution coe fficient, (iii) a crack tip shape parameter, and (iv) a crack growth c ompliance, which represents the materials response (crack growth) to t he above driving force. Since the effective crack tip radius of curvat ure cannot be smaller than one slip distance, a natural stress intensi ty threshold value exists, which is statistically distributed. A direc t connection is found between the above microscale coefficients and th e characteristics of the fatigue crack growth curve: the Paris law coe fficients, and the near threshold response. The model is examined thro ugh direct comparisons with experimental data for three different meta ls. Good agreement is found between the experiments and the model in b oth the Paris and the threshold regimes, including a statistical scatt er for very low stress intensities.