High resolution analysis of opening and sliding in fatigue crack growth

The use of digital image correlation analysis during fatigue crack growth (FCG) of polycrystalline and [1 1 1] oriented single crystal specimens of 316L stainless steel allows for the investigation of mixed mode crack propagation in the vicinity of the crack tip. This technique offers significant benefit in addressing crack closure at the microscale compared to the large body of work studying this phenomenon at the macroscale. Understanding of FCG behavior relies on the sliding (mode II) details which can be rather complicated. In this study, the mode I (opening) and mode II (sliding) mechanisms are differentiated within the single crystal specimens for slanted cracks. Further, crack opening displacement profiles are obtained in mode I and mode II, which are used to quantify crack closure in each specimen. Finally, the irreversible strain within the plastic zone ahead of the crack tip is measured during crack propagation. The results show that [1 1 1] oriented single crystal specimen fatigued at R = -1 display the most slip irreversibilities due to reverse dislocation motion leading to dislocation kinks/jogs. As a result, residual stress is diminished at the crack tip thereby resulting in earlier crack opening within the loading cycle.