Investigation of thermal degradation and decomposition of both pristine and damaged carbon/epoxy samples with thermal history

Abstract

Although the investigation of thermal damage in composite materials has increased, the short-term thermal degradation and decomposition mechanisms in a non-oxidative environment have not been well established, as the reported thermal damage results were heavily influenced by the analyses of delicately collected volatiles and the presence of oxygen. The aim of this study, using scanning electron microscopy, Fourier transform infrared spectroscopy and thermogravimetry, along with kinetics, was to develop a post-mortem solid residual-based microscopic characterising methodology for thermal damage in carbon/epoxy composites induced by simulated lightning strikes. Fibre tufting, resin sublimation and discolouration were found to represent the severity of damage in three damage zones. Whilst fibre tufting was caused not by thermal sublimation but by returning shock waves, resin sublimation and discolouration were dominated by Joule heating. The carbon concentration in the damaged zones was found to radially decrease outwards and vice versa with oxygen. Decomposition started from epoxy dehydration, accompanied by discolouration and formation of water and carbonyl, and released chemical compounds like 2-propenal, acetylene and carbonyl-containing aromatic ether. The successful identification of these unique heating-formed functional groups provided the confirmation of the proposed methodology.