Damage mechanisms in the dynamic fracture of nominally brittle polymers

TL;DR

This study investigates the damage and microcracking processes during dynamic fracture of PMMA, revealing a critical velocity where failure mode shifts and microcracks influence overall crack propagation speed.

Contribution

It uncovers the microcracking dynamics and the role of internal damage variables in fracture velocity selection in brittle polymers.

Findings

Identification of a critical velocity for failure mode transition

Microcracks can accelerate macro-scale crack propagation

Local crack front speed is limited and differs from apparent speed

Abstract

Linear Elastic Fracture Mechanics (LEFM) provides a consistent framework to evaluate quantitatively the energy flux released to the tip of a growing crack. Still, the way in which the crack selects its velocity in response to this energy flux remains far from completely understood. To uncover the underlying mechanisms, we experimentally studied damage and dissipation processes that develop during the dynamic failure of polymethylmethacrylate (PMMA), classically considered as the archetype of brittle amorphous materials. We evidenced a well-defined critical velocity along which failure switches from nominally-brittle to quasi-brittle, where crack propagation goes hand in hand with the nucleation and growth of microcracks. Via post-mortem analysis of the fracture surfaces, we were able to reconstruct the complete spatiotemporal microcracking dynamics with micrometer/nanosecond resolution. We demonstrated that the true local propagation speed of individual crack fronts is limited to a fairly low value, which can be much smaller than the apparent speed measured at the continuum-level scale. By coalescing with the main front, microcracks boost the macroscale velocity through an acceleration factor of geometrical origin. We discuss the key role of damage-related internal variables in the selection of macroscale fracture dynamics.