Probing and characterizing the early stages of cavitation in glassy polymers in molecular dynamics simulations

International audienceThis work focuses on a specific aspect of polymer fracture: the onset of cavitation during deformation. Failure in polymers involves plastic deformation by shear yielding and crazing. The competition between these two mechanisms is thought to govern the ductile versus brittle response of the material. The present molecular dynamics (MD) analysis shows that at a small scale, cavitation results from a transition between a homogeneous to a highly heterogeneous deformation field during loading. We characterize here these two regimes thanks to a scalar non-affine displacement probe, which displays a sharp transition at the onset of cavitation. Close scrutiny of cavitation allows for defining a stress-based cavitation criterion, the validity of which is checked for two temperatures in the glassy state. A mapping between the MD results and the corresponding estimates at the continuum scale indicates that the onset of cavitation at high deformation rates corresponds to a noticeably larger stress level as compared with that at low and intermediate loading rates. Since cavitation precedes failure in glassy polymers, this effect could be responsible for the marked increase in toughness reported experimentally under impact conditions