In situ elucidation of mechanisms governing crack transition to plasticity arrest
Despite extensive theoretical treatment of short- to long-crack transitions, direct experimental quantification of how elastic and plastic energy contributions evolve at the crack tip during arrest has remained absent. In this study, we present an in situ investigation of crack propagation in cold-worked AA-5052 using high-resolution scanning electron microscopy digital image correlation (SEM-DIC) and electron backscatter diffraction (EBSD). By reconstructing local crack-tip fields from measured displacement data, we extract mode I and II stress intensity factors and both elastic and elastoplastic energy release rates ({\Delta}JE and {\Delta}Jp). The results show that microstructure-sensitive cracks propagate in a mixed-mode manner at low driving force and transition to plasticity-dominated and load-aligned crack, arrested as the crack-tip process zone develops and expands multiple grains. This transition is identified through the divergence of elastic and elastoplastic energy measures ({\Delta}JE >= {\Delta}JP), crack-tip blunting, slip-band emission, and the emergence of localised plastic deformation. These findings demonstrate that crack arrest coincides with a measurable transition in crack-tip energy partitioning and with process-zone expansion beyond grain-scale dimensions. The results establish an experimentally measurable energy-partition criterion for crack arrest and demonstrate that fracture regime transition is governed by