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 cr ack surfaces of several steels and Al-alloys under different loading condit ions, is presented. The plastic zone sizes of fatigue cracks, for plane str ain conditions, are measured from the in-depth distribution of residual str esses and X-ray diffraction peak broadening. In addition to the usual monot onic plastic zone size determination methodology, a model for the estimatio n of the reverse plastic zone size was established in the case of fatigue s oftening 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 mon otonic plastic zone size equation, increases as the yield strength of the m aterial increases, following the relationship alpha = 0.196 . [sigma(ys)/(1 29 + 0.928 sigma(ys))](2). The alpha-value versus sigma(ys) evolution has b een understood through the influence of the hardening rate of materials on the plastic zone size. X-ray fractography has been applied to actual failur e analyses to predict some aspects of the actual loadings.