In-situ observation of ductile failure with high-speed X-ray phase contrast imaging

This study investigates the primary mechanisms contributing to damage accumulation in additively manufactured Scalmalloy (Al-Mg-Sc) subjected to tensile tests at low and high strain rates. High-speed X-ray Phase Contrast Imaging (XPCI) performed at beamline ID19 of the European Synchrotron Radiation Facility (ESRF) provide real-time, high-resolution through-volume visualization of internal void evolution processes, including void nucleation, growth, and coalescence. This advanced experimental approach facilitates the precise calibration of established physics-based fracture models, such as the Gurson-Tvergaard-Needleman (GTN) model, which otherwise relies on assumptions that are very difficult to verify experimentally. The 2D images captured during in-situ testing were segmented to identify and track individual voids using advanced computational techniques. The strain fields within the material were calculated via the Moving Least Squares (MLS) method, enabling accurate local strain estimation in materials with complex and evolving microstructures. The results show significant strain rate effects on the void evolution in Scalmalloy. At low strain rates, the void fraction increased steadily as a result of isolated void growth. In contrast, high strain rates demonstrated complex deformation behaviors, with slow initial void growth transitioning to rapid coalescence beyond a critical strain threshold, ultimately resulting in extensive internal damage. Moreover, the analysis of the time derivative of the apparent void fraction and its relationship with the local strain rate reveals proportional damage evolution at low strain rates, indicating progressive void growth. At high strain rates, the strong linear relationships observed between the rate of change of the apparent void fraction and the local strain rate, validate the applicability of the GTN model and demonstrate its ability to predict rapid void coalescence and ductile fracture under dynamic loading conditions.