MICROMECHANISMS OF FRACTURE PROPAGATION IN GLASSY POLYMERS
While most glassy polymers are nominally brittle at macroscopic scales, they are known to exhibit plastic deformation in indentation, scratching, and microcutting when the loaded region is sufficiently small. The same applies to the micrometer size process zone at the tip of a propagating crack. While the presence and approximate size of this microscale plastic zone is well described by the Dugdale model, the prediction of the toughness of these materials is not possible without accounting for the details of the local large strain field and the work hardening behaviour of these polymers, which can be inferred from their response to compressive tests. Strain localization mechanisms such as crazing or shear banding should also be taken into account to properly model toughness. Finally, viscoplastic creep plays a major role in determining the dependence of the toughness on crack propagation velocity, as well as the important difference between the initiation and propagation toughness, which is responsible for the occurrence of a characteristic stick-slip propagation under some loading conditions.
We present here the important insights that can be obtained from an in-situ experimental investigation of the strain fields in the micrometric neighbourhood of a propagating crack. Atomic Force Microscopy combined with Digital Image Correlation reveals to be a very rich technique for our aim, although it limits our observations to the external surface of the sample and to very slow crack propagation (below nm/s).
The consequences of these observations on understanding the toughness of glassy polymers will be discussed. In particular a novel scenario is driven to explain the transition between the steady-state propagation (typically observed in thermoplastics) and the stick-slip behaviour (generally observed in epoxy resins). We finally discuss the implications of this scenario on the fracture behaviour of these polymers in very confined conditions such as when used as a matrix of a long fibre composite material.