Mechanism investigation of scratch-induced ductile damage in brittle polylactic acid revealed by in situ 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.