Effect of cure regime on internal strain and stress development in a filled epoxy resin assessed by fiber Bragg-grating optical strain and normal force measurements

Abstract

The influence of thermal treatment on the reaction-induced strain and stress states of an anhydride-hardened bisphenol A-based epoxy (EP) resin containing 50 wt.% inorganic filler particles was studied. Two different cure regimes were selected for the EP curing and their effect on the strain and force development were assessed by Bragg-grated fiber optical and normal force detection techniques. Surface cracking of the specimens was also visualized optically. During long isothermal treatment in the dwelling phase of the curing process, tensile forces evolved in the cross-linking EP. When vitrification occurred already at a low degree of conversion in a given cure regime, high internal forces were generated, causing early cracking of the EP. A cure regime (temperature profile) compensating the cure-induced volume shrinkage by additional thermal expansion was proposed as most beneficial to delay the vitrification and thus the build-up of damage-causing high internal stresses.