Development of a process zone in rubber-modified epoxy polymers

The effects of a process zone on toughness and on R-curve behavior were investigated for a model, rubber-modified epoxy polymer. The system studied was one in which the bridging mechanism of toughening does not operate. The characteristic features of R-curve behavior, a rise in toughness with crack extension until an approximate steady-state is reached, were observed using double-cantilever-beam tests. The evolution of the process zone was studied using transmission-optical microscopy. As the crack grew, the process zone appeared to fan out until it reached a steady-state thickness; it then remained a uniform size upon further crack advance. The features of the experimental R-curves were shown to be directly correlated to the evolution of the process zone. Furthermore, the effect of the portion of the process zone in the crack wake was examined by a series of experiments in which the wake was partially removed, and the R-curve re-established by subsequent loading. These experiments demonstrated that removal of the crack wake caused the crack-growth resistance to drop. The toughness then built back up to the steady-state value as the crack wake re-developed. This unambiguously demonstrated a contribution to toughening from the crack wake despite the absence of any observable bridging mechanism. These results support the accepted notion that an extrinsic toughening mechanism is responsible for the increased toughness observed upon adding rubber particles to an epoxy matrix.