Effects of thermal ageing and thermal cycles on the mechanical and thermomechanical behaviour of vacuum infused GFRP composites

The existing literature provides limited insight into the influence of thermal ageing (long term exposure to low or high temperatures) and thermal cycles on the properties of fibre reinforced polymer (FRP) composites, particularly those produced by vacuum infusion. This paper presents a study on the effects of thermal ageing and thermal cycles on the thermomechanical and mechanical behaviour of 6.5 mm thick glass-FRP (GFRP) laminates, produced by vacuum infusion. These laminates incorporated multi-axial reinforcement and used unsaturated polyester and vinyl ester resins. The laminates were exposed to (i) four different temperatures (-15 °C, 20 °C, 40 °C and 60 °C) for 180, 365 and 730 days, and (ii) 100, 200 and 300 thermal cycles, with temperatures ranging from - 15 °C to 60 °C. After each exposure period, different thermomechanical and mechanical tests were conducted to evaluate changes in the properties of the GFRP laminates, and Scanning Electron Microscopy analysis was performed to examine changes in their internal structure. The results indicate that the thermal ageing and thermal cycles applied in this study, which are representative of those typically encountered in civil engineering applications, generally resulted in slight to moderate changes in the above-mentioned properties of the laminates, and no clear differences were observed between the two resin systems in terms of property retention. Under both ageing protocols, the maximum reductions in tensile strength and modulus, compressive strength and modulus, flexural strength, in-plane shear strength and interlaminar shear strength were 8%, 5%, 8%, 16%, 9%, 5% and 1%, respectively. Additionally, the maximum decrease in glass transition temperature, amounting to 6.9 °C, was observed in vinyl ester based GFRP. Moreover, in most cases, these properties recovered significantly in later exposure periods. The performance of the GFRP laminates under thermal cycles was comparable to that under thermal ageing, indicating that, for most climates, the isolated effect of cyclic temperature variations is very limited.