Rate and temperature effects on the fracture toughness of a rubber-modified epoxy

Abstract

A series of three-point bend fracture toughness tests was performed at various loading rates and temperatures on both rubber-modified and unmodified epoxy specimens. The fracture toughness of the modified samples was determined to be rate- and temperature-dependent. Using time-temperature superposition, an apparent activation energy was determined by shifting the fracture toughness data along the rate axis for each temperature tested. Surprisingly, this apparent activation energy calculated for the rubber-modified epoxy was found to be within 2% of the value of the activation energy determined for the [beta] relaxation peak found from small strain d.m.a. measurements of the unmodified epoxy matrix. This was an unexpected yet interesting correlation. Since shear yielding is the primary mechanism by which this epoxy system is toughened, it can be hypothesized the [beta] relaxation may significantly influence the kinetics of yielding and consequently the fracture tougness of the material. Plastic zone geometry and fracture mechanisms were investigated and were found to support this hypothesis.