Process-induced matrix defects: Post-gelation

Formation of resin matrix defects is common during composites manufacturing. The mechanisms behind these defects depend on the state of the resin at the time of their formation. Process-induced matrix defects are commonly studied in the form of porosity before gelation, or matrix cracking after vitrification. The present study investigates defect formation when the thermoset resin is between gelation and vitrification. After gelation, the resin is in a rubbery state and does not flow. M this state, cure in a constrained geometry leads to internal stress development and potential formation of defects. An experimental setup was used which allowed direct visual observation of the resin behavior after gelation. It was found that post-gelation defect formation is driven by hydrostatic stress and the defect morphology is cure rate-dependent. An analytical approach to avoid post-gelation defects based on comprehensive material characterization and internal stress evolution calculations is proposed, and experimentally validated.