An investigation of residual stresses induced by elevated temperature post-curing of glass reinforced plastic structures

Abstract

Performing an elevated temperature post-cure procedure on glass reinforced plastic vessels and components that are subjected to aggressive chemical environments enhances the mechanical and chemical resistant properties of a laminate. This is a result of the completion of the cross-link polymerisation reaction at an elevated temperature. The postcure process is however the cause of matrix embrittlement and possibly also contributes to crazing on the inner surface of the vessels or similar components. To determine the cause of crazing at the inner surface this work investigates the stress and strain development, resulting from post-cure, in chopped strand mat reinforced laminates in the form of small bore tubes, small bore full face flanges and small diameter vessels. The investigation consists of experimental measurements during post-cure procedures and numerical simulations of these procedures on specified test specimens. The numerical simulations consist of finite element analyses of models using commercially available finite element analysis software. Material properties for the analyses are determined using two different, but well-known approaches. The post-cure simulations are conducted by applying laminate shrinkages, by analogy, as coefficients of thermal expansion with a unit temperature difference in the finite element models. Initially only the structural layers of the laminates were considered and analysed for postcure shrinkage. However, the results obtained did not show any cause for crazing occurring on the inner surfaces. The resin rich corrosion barrier, that is usually applied as a first layer when manufacturing a vessel component, was added to the finite element models and analysed for post-cure stresses and strains. These results clearly indicate that the corrosion barrier causes stress conditions at the inner surfaces that should be unacceptable for the GRP vessel industry. The experimental and predicted results are compared to determine the accuracy of the finite element analyses. Factors influencing the accuracy of strain gauge measurements on laminates constructed of CSM are presented. The study investigates issues to be considered when finite element analyses are required on composite material components to determine the residual manufacturing stresses. The study's relevance to industry is discussed and topics for further and additional studies are recommended.