Small punch testing and scanning electron microscopy analysis of damage evolution in dual-phase steel

Dual-phase (DP) steels are widely used in the automotive industry due to their excellent balance of strength and ductility. Enhancing their formability and crash performance requires a deeper understanding of microstructural deformation and damage mechanisms. This study presents an in-depth micromechanical investigation of damage initiation and propagation in DP1000 steel using a novel small-punch (SP) test setup combined with three-dimensional digital image correlation (3D DIC) and scanning electron microscopy (SEM). The test was strategically interrupted at multiple stages to capture the evolution of cracking. Initial cracks appeared at a punch displacement of approximately 1.12 mm, while the final stages revealed microstructural crack propagation resulting from deformation localized near the ferrite (F) – martensite (M) interface, leading to cracking of adjacent M islands or void growth within the F. 3D DIC measurements revealed maximum principal surface strains up to 23% before cracking. SEM analysis confirmed that the dominant through-thickness crack initiated at the punch sample interface and propagated toward the upper surface, aligning with surface strain localizations. M cracking was limited to the early stages, while subsequent damage growth was F-dominated. This integrated experimental approach offers new insights into the phase-specific damage behaviour and supports the development of predictive models for the formability and failure of advanced high-strength steels.