Development of a process zone in rubber-modified epoxy polymers

摘要

The effects of a process zone on toughness and onR-curve behavior were investigated for a model, rubber-modifiedepoxy polymer. The system studied was one in which the bridgingmechanism of toughening does not operate. The characteristicfeatures of R-curve behavior, a rise in toughness with crackextension until an approximate steady-state is reached, wereobserved using double-cantilever-beam tests. The evolution ofthe process zone was studied using transmission-opticalmicroscopy. As the crack grew, the process zone appeared to fanout until it reached a steady-state thickness; it then remained auniform size upon further crack advance. The features of theexperimental R-curves were shown to be directly correlated tothe evolution of the process zone. Furthermore, the effect ofportion of the process zone in the crack wake was examined by aseries of experiments in which the wake was partially removed,and the R-curve re-established by subsequent loading. Theseexperiments demonstrated that removed of the crack wake causedthe crack-growth resistance to drop. The toughness then builtback up to the steady-state value as the crack wake re-developed.The unambiguously demonstrated a contribution to tougheningfrom the crack wake despite the absence of any observablebridging mechanism. These results support the accepted notionthat an extrinsic toughening mechanism is responsible for theincreased toughness observed upon adding rubber particles to an epoxy matrix.