Modelling and experimental analysis of aerospace composites in fire
Creators: name: Nawaz, Naadia
Publisher: name: RMIT University
Year: 2024

description: The aim of this Master of Engineering (MEng) - Aerospace thesis is to study the structural response and failure of aerospace grade composites during exposure to fire. Key aspects of the research are to analyse the thermal and mechanical responses of laminates and sandwich composites in high temperature and fire environments, which result in softening and structural failure. The research is concentrated on carbon/epoxy laminates and sandwich composites, which are the most common composite materials used in aerospace structures. To achieve the general aim of this research project, the thermal response of aerospace composites will be validated using existing thermal models for fibreglass composites. In addition, the modification of existing models to ensure the applicability to aerospace composites is an aim of this research. Another key aim is the validation of a modelling approach through the generation of experimental data. By achieving these aims it is envisaged to have a validated modelling approach that can be used by the aerospace industry for fire assessment of structural composites.<br><br>The research work presented in this MEng thesis falls into three main categories: literature review; development of thermal and mechanical modelling approach for aerospace laminates and sandwich composites; and validation through experimental fire structural testing. A critical review of the scientific literature regarding the fire response of<br>composite materials is presented in the thesis. The existing thermal modelling approach applied to composites is reviewed with identification of the fundamental factors such as heat conduction, resin decomposition and gas flow. A review of structural models is presented that can analyse changes to the mechanical properties of composites in fire, such as strength reduction, buckling effects and creep resistance. The thermal and mechanical response of various composite systems in different heating conditions and fire environments is assessed in the literature review. Based on the literature review, it is apparent that there is a lack of analysis and data on laminates and sandwich composites used in aerospace structures, and this provides the rationale for the MEng work.<br>The thermal modelling presented in this thesis is based on existing models for composite laminates and sandwich materials that analyse the response of the composite in fire including heat flow, decomposition of the polymer matrix, flow of volatile gases into fire, reactions between fibres and char, pressure rise etc. The thermal modelling approach applied to fibreglass composites has been extensively validated, as published in literature, and can be adapted for carbon fibre reinforced composites. The accuracy of the models to predict the thermal response has been validated through experimental fire testing. Carbon/epoxy laminates, of varying thickness (4, 10 and 20 mm), and sandwich<br>composites have been exposed to various heat fluxes (temperatures) to generate data for the temperature-time response. This data is used to validate the approach undertaken to yield thermal predictions. The good agreement obtained between experimental measurements and model predictions is presented in this thesis.<br><br>The approach taken to modelling the structural response of aerospace composites is established by defining the mechanisms applied in existing models and the reasoning behind the analysis. As the failure mechanisms experienced by different composite systems (and in various loading conditions) will not be identical, the correct selection of structural effects is made. The high heat fluxes (&gt;10kW/m²) analysed in this research<br>eliminate any effects of creep behaviour, while strength and stiffness reduction are significant factors. The reduction of strength and stiffness is determined through experimental testing at increasing temperatures for composite laminates and sandwich structures. The work performed in this research forms part of a two-step modelling approach which comprises of a thermal analysis stage coupled to a mechanical analysis stage. The mechanical analysis combines with the thermal analysis to produce a thermomechanical model that has the ability to make predictions of the load-time response of carbon fibre composites. This is validated through fire structural testing performed on composites under one-sided heating and applied load. The extensive experimental validation is performed on composite laminates of varying thickness and heat flux, and sandwich composites at varying heat fluxes. The accurate prediction of failure times and comparison to theoretical models is presented in this thesis.