Long FBG sensor characterization of residual strains in AS4/PPS thermoplastic laminates

Abstract

The consolidation of thermoplastic composites produces internal residual strains due to the differences between the coefficients of thermal expansion of the component materials. In the case of AS4/PPS (carbon fibre-polyphenylene sulphide), where the melting/solidification temperature is 280°C, there exists a 255°C range wherein the various constituents will contract/expand to different degrees. A fibre Bragg grating (FBG) sensor may be embedded into this laminate with the goal of characterizing the residual strains; however, these strains may be non-uniform in the longitudinal and transverse directions, and may also vary depending on the laminate architecture. Non-uniform axial strains typically broaden and split the FBG sensor's spectral response, making it difficult to measure the strain distribution. Also, load-induced birefringence caused by the consolidation process will complicate the interpretation of the spectral response. This research is directed at understanding the residual strain state in FBG sensors due to the fabrication process. It is the aim of this study to experimentally investigate the residual strains in long and short gauge length FBG sensors embedded in the 0° plies of AS4/PPS unidirectional and cross-ply laminates (200 x 50 x 3.6 mm). Long gauge length sensors are monitored throughout the fabrication process, to provide insight into the development of the residual strains.