In situ elucidation of fracture mechanisms governing crack transition to plasticity arrest

• In situ SEM-DIC-based separation of elastic and plastic crack-tip energy • Energy-partition divergence marks crack arrest • Process-zone expansion governs fracture transition • Crack length alone does not control regime shift • Mixed-mode growth precedes plasticity-dominated 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 (Δ J E and Δ J P ). We show that microstructure-sensitive cracks propagate in a mixed-mode manner at low driving force and transition to a plasticity-dominated, load-aligned arrested state as the crack-tip process zone develops to span multiple grains. This transition is identified by the divergence of elastic and elastoplastic energy measures (Δ J E ≥ Δ J P ), together with crack-tip blunting, slip-band emission, and the emergence of localised plastic deformation. Under the present free-surface geometry and constraint conditions, these observations support the interpretation that crack arrest coincides with a measurable transition in crack-tip energy partitioning and correlates more directly with estimated surface process-zone expansion relative to the microstructure than with any single crack-length threshold.