Environmental Effects of Moisture and Elevated Temperatures on the Mode I and Mode II Interlaminar Fracture Toughness of a Toughened Epoxy Carbon Fibre Reinforced Polymer

The use of composite materials within extreme environments is an exciting frontier in which a wealth of cutting-edge developments have taken place recently. Although there is vast knowledge of composites' behaviour in standard room temperature and humidity, there is a great need to understand their performance in 'hot/wet' conditions, as these are the conditions of their envisaged applications. One of the key failure mechanisms within composites is interlaminar fracture, commonly referred to as delamination. The environmental effects of moisture and elevated temperatures on interlaminar fracture toughness are therefore essential design considerations for laminated aerospace-grade composite materials. IM7/8552, a toughened epoxy/carbon fibre reinforced polymer, was experimentally characterised in both 'Dry' and 'Wet' conditions at 23 °C and 90 °C. A moisture uptake study was conducted during the 'Wet' conditioning of the material in a 70 °C/85% relative humidity environment. Dynamic mechanical thermal analysis was carried out to determine the effect of moisture on the glass transition temperature of the material. Mode I initiation and propagation fracture properties were determined using double cantilevered beam specimens and Mode II initiation fracture properties were deduced using end-notched flexure specimens. The effects of precracking and the methodology of high-temperature testing are discussed in this report. Mode I interlaminar fracture toughness, GIC, was found to increase with elevated temperatures and moisture content, with GIC=0.205kJ/m2 in 'Dry 23 °C' conditions increasing by 26% to GIC=0.259kJ/m2 in 'Wet 90 °C' conditions, demonstrating that the material exhibited its toughest behaviour in 'hot/wet' conditions. Increased ductility due to matrix softening and fibre bridging caused by temperature and moisture were key contributors to the elevated GIC values. Mode II interlaminar fracture toughness, GIIC, was observed to decrease most significantly when moisture or elevated temperature was applied individually, with the combination of 'hot/wet' conditions resulting in an 8% drop in GIIC, with GIIC=0.586kJ/m2 in 'Dry 23 °C' conditions and GIIC=0.541kJ/m2 in 'Wet 90 °C' conditions. The coupled effect of fibre-matrix interface degradation and increased plasticity due to moisture resulted in a relatively small knockdown on GIIC compared to GIC in 'hot/wet' conditions. Fractographic studies of the tested specimens were conducted using scanning electron microscopy. Noteworthy surface topography features were observed on specimens of different fracture modes, moisture saturation levels, and test temperature conditions, including scarps, cusps, broken fibres and river markings. The qualitative features identified during microscopy are critically examined to extrapolate the differences in quantitative results in the various environmental conditions.