Hot/wet properties of high temperature composites

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The replacement of metals with advanced composite materials within a range of industries has become commonplace. This is due to their excellent specific stiffness and strength, wide range of available constituents and array of manufacturing processes, making them suitable for a multitude of applications, including within the aerospace and energy sectors. As composites are employed in more extreme environments, there has been considerable interest in organic matrices that are able to withstand high temperatures and humidities. Thus, research has been conducted to develop novel thermoset systems with enhanced thermal and moisture durability properties. There has been specific interest in cyanate esters due to their desirable viscosity properties for resin infusion and high glass transition temperatures, making them suitable for a range of applications. Although there is some understanding of the behaviour of the neat resin, there is little knowledge on its performance when integrated within composite laminates. Delamination, the separation of layers, is the primary failure mechanism within composites. Fracture toughness is a measure of a material's resistance to delamination. Most tentative applications for these novel materials involve cycling components in the presence of heat and moisture which accelerates degradation mechanisms. Therefore, it is imperative to understand the interlaminar fracture toughness properties of these thermoset systems, with a particular focus on how they react in hot-wet conditions, before they can become routinely used within composites. This project will focus on the experimental testing of a leading aerospace-grade epoxy-based composite material, IM7-8552, in conjunction with composites containing the novel thermoset resins. This will allow a baseline set of properties to be acquired for performance comparison with the newly developed materials. Although regulations for moisture conditioning of test specimens exist, there is no universally accepted testing standard for acquiring fracture properties of composites at elevated temperatures and humidities. For this reason, a key objective of this project is to develop a more robust, reliable method for testing within extreme environments, along with obtaining the fracture properties themselves. Testing will initially be conducted in static and quasi-static conditions, followed by fatigue testing performed to mimic the conditions experienced by materials in-service under continuous cycling and vibration exposure whilst in high temperature environments. Fractographic analyses will be conducted to create qualitative measures of fractured surfaces, developing easily identifiable visual indicators of how the material failed in each set of testing conditions for future comparison. Once the behaviour of the novel thermoset systems within composites is more thoroughly understood, supported through rigorous testing within this project, the envisaged application for their use is within the turbofan engines developed by Rolls Royce. However, if these materials prove to have enhanced durability within environments involving repeated cycling at elevated temperatures, this will open them up for use within a broad spectrum of applications. These could include conventional gas turbines, advanced hybrid electric propulsion systems, motors, generators, and hydrogen storage tanks. The development of a vigorous protocol for static, quasi-static and fatigue testing in elevated temperatures and humidities will be a significant contribution to the fracture mechanics research field. The ability to confidently produce a data set of properties tested within these conditions is essential for the progression of high temperature composites as the boundaries for their use continue to be broadened.