An experimental investigation of residual stress and strain in rapidly cured fibre reinforced composites

Carbon fibre reinforced composites are becoming increasingly prevalent in the aerospace and automotive industries for use in structural, and safety critical, applications due to their high specific strength and stiffness. However, residual stress is present in all fibre reinforced composites to some degree, caused by their inherent anisotropy and differential material properties between fibres and reinforcing matrix. Residual stress can cause premature failure in structures and must therefore be understood and accounted for, particularly in safety critical applications. In recent years, rapidly curing resins have been developed to reduce the processing time required to cure parts. However, the effect of shorter processing times on residual stress formation is still not well understood. Therefore, this thesis aims to investigate how rapidly curing a composite alters the residual stress and strain state of that composite and how this affects its mechanical performance. Fibre Bragg gratings were embedded into laminates during cure to offer a novel insight into the mechanisms causing residual strain in rapidly cured composites.

To ensure that there was sufficient bonding, and therefore strain transfer, between the embedded optical fibres and resin matrix, an investigation of the interface between the two was conducted. Single fibre fragmentation was used to quantify the strength of the interface. A novel sample preparation methodology was developed to allow for rapidly cured samples to be manufactured. A novel fragmentation length measuring technique was also developed to allow for instantaneous measurements of fragmentation length to be made. While it was possible to use this fibre fragmentation analysis technique with a slow curing resin system it was found that it was not possible with the rapid curing system as residual stress was too high and samples failed before meaningful measurements could be made. However, scanning electron microscopy and micro x-ray computed tomography were used to qualitatively evaluate the interface and it was determined that there was sufficiently good bonding between the embedded fibre and resin matrix in rapidly cured laminates.

Fibre Bragg gratings were embedded in laminates of varying thickness and cure temperatures to determine the effect of these parameters on the formation of residual strain during cure. It was found that the final residual strain of all samples was similar, but the route taken varied significantly. Samples cured at high (low) temperatures developed much of their strain at the end (beginning) of the cure which meant the final residual stress was thought to be higher (lower). A numerical analysis was then conducted to determine the residual stress state of the laminates cured with embedded sensors and the residual strain data captured with embedded FBGs was used to validate the model. A mechanical analysis of the transverse bending strength of variously cured laminates was conducted to further validate the numerical model and to experimentally link residual strain data to mechanical strength and by extension residual stress. It was found that it was not possible to accurately determine the effect of laminate thickness on bending strength due to their higher void content. Samples cured at a higher temperature had a higher bending strength which is indicative of a lower tensile residual stress. This was due to those samples only vitrifying upon cooling which allowed for a significant amount of residual stress to be relaxed during cure. Therefore, there is no clear trend between cure temperature and residual stress. However, it is hypothesised that higher curing temperatures will lead to higher levels of tensile residual stress if laminates are cured well below the T¬g of the cured laminate and not allowed to relax excessively.

This work has shown the applicability of using fibre Bragg gratings in measuring residual strain during the rapid curing of fibre reinforced composites. They offer a novel insight into the formation of residual strain during cure of these resin systems and are also a powerful validation tool for numerical work. The innovative tools developed in this work show great potential for furthering the adoption of rapidly cured composites for structural applications.