Mechanical Behavior of a Triaxially Braided Textile Composite at High Temperature

Polymer Matrix Composite Materials (PMCMs) have gained increasing attention from the aerospace industry over the past several years, as illustrated by their increased use in aircraft, like the Boeing Dreamliner or the Bombardier C-series. Their high strength to weight ratio, combined with the possibility to tailor their properties, provides great opportunities for the replacement of traditional materials, such as steel and aluminum. Textile composites can yield a broad range of mechanical properties, depending on matrix and fibres properties, fibres orientation and distribution within the composite, and the yarns volume fraction. In particular, carbon fibres provide high tensile modulus and strength, when compared to other reinforcing fibres. The yarns interlacing provides additional impact resistance. However, the use of these materials is usually restricted to low temperature applications due to the matrix properties degradation with temperature. The development of new temperature resilient polyimide matrices paved the way for new applications for PMCMs, especially in the gas turbine industry. In high temperature oxidative environments, the matrix properties degradation can significantly affect the composite's overall mechanical behaviour, even if the fibres can safely withstand much higher temperatures. Of particular interest are structural components exposed to temperatures of up to 300°C under permanent regimes. The literature has shown that PMCMs behaviour at elevated temperatures depends on the reinforcement specific architecture as well as the matrix' thermal stability. The work presented in this thesis aimed at understanding the influence of viscoelasticity, temperature and aging on the mechanical behaviour of a textile composite using experimental, analytical and numerical tools. The studied material was a triaxially braided composite with fibres in the 0°/±60° directions. The yarns were made of carbon fibres, embedded in an MVK10 temperature resistant polyimide matrix. The first step consisted in developing analytical and numerical frameworks to predict viscoelastic behaviour in textile composites. Simulations were performed for both braided and woven textile architectures, at different stiffness contrasts and yarns volume fractions. The analytical framework accuracy was verified with the help of the numerical simulations. An important finding of this study was that the analytical framework, combined with the Mori-Tanaka model, leads to relatively accurate predictions for both the permanent and transient parts. Therefore, the authors believe that the Mori-Tanaka model with an adjusted aspect ratio to take into account yarn curvature is reliable for predicting viscoelastic behaviour in textile composites. The textile composite that was studied in this project did not display viscoelastic behaviour, due to the high yarn volume fraction. However, the framework remains relevant for higher temperature applications or lower yarn volume fractions. The second step was to investigate the temperature effect on the tensile behavior of the carbon/MVK10 triaxially braided composite material studied in this project. To achieve this goal, a series of room and high temperature tensile tests on both matrix and composite samples were performed.