Understanding the influence of water absorption and fibre architecture on matrix dominated properties of carbon fibre reinforced composite

The increased demand for and usage of Carbon Fibre Reinforced Polymers (CFRP)composites in many industries across sectors such as in aerospace, automotive and marine applications require further scientific attention leading to greater understanding of these materials which are expected to operate for long periods under harsh environments. This thesis discusses a key parameter ‘fibre architecture’ and shows that it plays a major role in the performance of CFRP composites when subjected to water absorption. Unidirectional (UD), and 2D woven (plain and twill weaves) of CFRP specimens were immersed in water. An investigation on the directional moisture diffusion coefficients of this materials has been conducted with the use of Fick’s law in order to obtain comparative diffusion coefficient values. UD material showed higher diffusion coefficients by up to 21.56% as compared with their 2D woven equivalents. The mechanical properties dominated by the condition of the matrix and interfacial regions such as compression, shear, and impact resistance were assessed for un-aged and aged specimens. A reduction in strength was observed for all aged specimens. However, the observed reduction in strength varies depending on both the fibre architecture used and the specific load case. The largest effect on properties was observed in unidirectional materials, which leads to the recommendation that less conservative safety factors could be used for woven fibre architectures. Further work was undertaken to understand the effect of gradually increasing water absorption on interlaminar strength of UD materials, as this is the material that experiences greatest reduction in short beam strength (SBS). This work used Acoustic Emission (AE), optical strain measurements and microscopy to provide an assessment of the gradual change in failure mechanisms due to the degradation of a polymer’s structure with increasing water absorption. The SBS values decreased as immersion times were increased; the decrease was significant at longer immersion times (up to 24.47%). Failures evolved from a single shear delamination at low immersion times to more complex crushing, intralaminar cracking and multiple delaminations at long exposure times, suggesting that the true SBS was not determined at long exposure times. AE data supports this and was able to distinguish different material behaviours prior to final fracture for un-aged and aged specimens, and this was further validated using optical measurements and microscop