Influence of matrix nature on the post-fire mechanical behaviour of notched polymer-based composite structures for high temperature applications

This study aims to investigate the influence of matrix nature (thermosetting – epoxy 914; and thermoplastic – Polyphenylene Sulfide PPS) on the tensile thermo-mechanical behaviour of both notched and unnotched laminates. Depending on variable prior fire-exposures, the purpose was to compare the changes on the residual tensile mechanical properties of carbon fibre reinforced epoxy 914- and PPS-based laminates subjected to stress concentration for aeronautical purposes (e.g., at service temperatures higher than glass transition temperature, Tg). With respect to the unnotched laminates, the area of the hole is an open access through the thickness for the heat flux causing more or less thermal degradation (depending on matrix nature) and resulting in variably decreasing the laminate tensile properties. Surprisingly, a low heat flux leads to virtually no decrease in the residual tensile strength of PPS-based laminates; however, in epoxy 914-based composites, the exposure to a low heat flux is more detrimental as exposure time is long. Fractography analyses were performed to investigate damage mechanisms in perforated laminates. The influence of fire-induced damages, the subsequent degradation of the mechanical properties due to fire exposure, combined with the overstresses near the hole all contribute to significantly decrease both stiffness and strength of C/epoxy notched laminates. In notched C/PPS laminates, the fire-induced degradation, resulting from the redistribution of melted PPS matrix within the fibre network, may compete with a relaxation effect in the overstressed 0° carbon fibres in the vicinity of the hole. These competing mechanisms are expected to reduce the influence of fire-exposure on the residual tensile strength of notched C/PPS laminates.