[PhD] Guillaume de Luca - Fractographic Investigation of the Failure Behavior of Brittle Polymers

This study aims to decipher fracture roughness to shed light on how toughening emerges in polymers and how it is piloted by local damage, specifically in PMMA (polymethyl methacrylate).

To do so, we generated a wide range of fracture surfaces under varied crack growth regimes, and employed optical measurements techniques (Digital Image Correlation - DIC) to compute crack velocity and fracture toughness. Our analysis identified two distinct surface features in the fast crack growth regime : highly reflective, optically smooth regions (mist) and light scattering regions displaying microcracks (conic marks). Transitioning from qualitative to quantitative analysis, we used profilometry to create topographic maps of fracture surfaces. These maps were analyzed to compute roughness metrics and damage length scale, a metric from statistical fractography correlated to the process zone size. The resulting database enabled the deployment of deep learning techniques to classify surface features, predict crack velocity and toughness from the roughness, as well as visualize the key structures of interest in the decision process.

Our findings highlight a critical velocity (~180 m/s) marking a transition from mist to conic marks, correlating with an increase in toughness and a sharp rise in the damage length scale. We interpret it as a phase transition that we model by a local temperature increase at the crack tip, reaching the glass transition temperature of PMMA. These insights contribute to a deeper understanding of the toughening mechanisms in polymers, the nature of the micro-processes taking place at the crack tip, and pave the way for direct applications in industrial failure analysis.