Monitoring the fatigue-induced damage evolution in ultrafine-grained interstitial-free steel utilizing digital image correlation

The digital image correlation (DIC) technique was successfully utilized to detect fatigue-induced damage and monitor its evolution in ultrafine-grained interstitial-free steels of three different microstructures in the low-cycle fatigue regime. Specifically, visualization of strain localization with DIC allows for detecting the crack initiation sites after only a few cycles into the deformation. Furthermore, optical microscopy, atomic force microscopy and electron backscatter diffraction analyses revealed a direct correlation between elongated grains and the crack initiation sites. The results of a crystal plasticity model demonstrated that higher overall stresses are prevalent in the microstructures with elongated grains, which is attributed to strain localizations, and corresponding stress concentrations responsible for crack initiation. Overall, the current findings show that DIC is a novel and promising non-destructive technique for determining the crack initiation sites at the very early stages of cyclic deformation.