Mechanism investigation of scratch-induced ductile damage in brittle polylactic acid revealed by <i>in situ</i> observation

Polylactic acid (PLA) is widely utilized across various fields for its excellent biocompatibility and mechanical properties. However, inevitable sharp-object contacts cause surface scratches, hindering performance optimization, owing to the poorly characterized in situ damage processes and internal mechanisms. Our research addresses this gap through a novel integration of in situ observation and polarized light imaging, systematically investigating PLA scratch behavior under linearly increasing normal loads. The brittle PLA exhibits unexpected ductile damage characteristics under scratching: pronounced plastic deformation beneath the scratch tip at low normal loads, periodic 30°-angled crack propagation under moderate loads linked to scratch-induced high temperature rise, and lateral tearing with circumferential debris formation at high loads. These findings reveal a ductile-dominated scratch mechanism of brittle PLA, providing important guidance for understanding the damage mechanism of PLA and enhancing its practical application in diverse industries.