X-ray diffraction studies on fatigue crack plastic zones developed under plane strain state conditions

An overview of the X-ray fractography technique, as performed on fatigue crack surfaces of several steels and Al-alloys under different loading conditions, is presented. The plastic zone sizes of fatigue cracks, for plane strain conditions, are measured from the in-depth distribution of residual stresses and X-ray diffraction peak broadening. In addition to the usual monotonic plastic zone size determination methodology, a model for the estimation of the reverse plastic zone size was established in the case of fatigue softening materials. Monotonic and cyclic plastic zone sizes are related to the stress intensity by, respectively, r(pm) = alpha . (K-max/sigma(ys))(2) and r(pc) = alpha . (Delta K/2 sigma(ys)')(2). The alpha-value, in the monotonic plastic zone size equation, increases as the yield strength of the material increases, following the relationship alpha = 0.196 . [sigma(ys)/(129 + 0.928 sigma(ys))](2). The alpha-value versus sigma(ys) evolution has been understood through the influence of the hardening rate of materials on the plastic zone size. X-ray fractography has been applied to actual failure analyses to predict some aspects of the actual loadings.