Quantitative assessment of the fire performance of aerospace structural materials
Creators: name: Grigoriou, Katherine
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400101
subject: Aerospace materials
classificationCode: 400102
subject: Aerospace structures

description: Three structural materials commonly used in modern aircraft are aluminium alloys, carbon fibre-epoxy laminates and fibre metal laminates. The aim of this PhD project is to determine the softening behaviour of these materials when exposed to combined mechanical loading and fire. The specific materials studied are aluminium alloys (AA2024 and AA7075), quasi-isotropic carbon-epoxy composites, and a fibre metal laminate (GLARE). The thermal, physical and mechanical processes controlling the softening and failure of the materials under structural loading and one-sided heating by fire are determined. The two important structural loading cases of tension and compression are studied for different radiant heat flux conditions representative of fires having different flame temperatures. The term ‘fire’ is used in this PhD to describe the radiant heat which is emitted by fire and then absorbed by materials. The term ‘fire’ does not describe the flaming process itself nor direct flame impingement on the test materials. Due to the test methods used in this PhD project, the materials were not exposed to an actual fire, but were exposed to heat fluxes representative of the heat radiated by a controlled flame/fire. <br><br>This approach is similar to other testing methods used to measure the fire properties of composites and other combustible materials, such as the cone calorimeter which is used to measure the fire reaction properties (e.g. time-to-ignition, heat release rate) but does not use an actual fire in the test.  Presented in the PhD thesis is a comprehensive and critical review of published research on the thermal-mechanical properties of the aerospace structural materials. A review of research into the fire structural and fire reaction properties of polymer matrix composites is also presented. Based on the existing research, gaps are identified in the knowledge of the thermal response and structural survivability of aerospace-grade materials when exposed to combined loading and fire exposure as well as the residual strength properties of the materials after fire exposure. This PhD aims to address these research gaps.<br><br>The mechanical properties including creep as well as the thermal and physical responses of aircraft-grade aluminium alloys under fire conditions are experimentally investigated. The structural performance of AA2024 and AA7075 alloys under tension and compression loads is analysed using a thermal-mechanical finite element model and experimentally evaluated using fire-under-load tests. When exposed to the same radiant heat flux, both alloys reach the same front surface temperature, however the heat transfer through-the-thickness is more rapid for the 7xxx alloy due to its higher thermal conductivity. The structural survivability of the 7xxx alloy under tension or compression loading is inferior to the 2xxx alloy due to its faster heat transfer rate which accelerates softening. The effects of elastic, plastic, creep and other factors on the softening and weakening of aluminium alloys when exposed to fire is determined.<br><br>An experimental and analytical study into the structural performance of four aerospace-grade quasi-isotropic laminates subjected to combined fire and tension or compression loading is undertaken. Fire-under-load tests are performed on the laminates with different stacking sequences of the 0o, 90o, +45o and -45o plies. Laminates with 0o mid-plane plies have lower back-surface temperatures, slower tension softening rates and longer tensile stress rupture times compared to laminates with 45o mid-plane plies. Changes to the tensile properties of the laminates after exposure to fire are also investigated.<br>Experimental testing revealed that laminates with 0o mid-plane plies retain more tensile strength than laminates with 45o mid-plane plies for longer times during and after fire exposure. Laminates with 0o middle plies experience more extensive heat-induced delamination cracking which lowers their effective through-thickness thermal conductivity and traps heat near the fire-exposed surface. This slows the thermal softening rate of the load-bearing (0o) plies located towards the back surface, and this extends the fire structural survivability under tensile loading. The structural survivability of quasi-isotropic carbon-epoxy laminates under combined fire exposure and compression loading is less sensitive to the ply stacking sequence, and the reasons are explored. A model is presented to compute the softening and failure of carbon fibre-epoxy laminates under both tension or compression loading and exposure to fire.<br><br>The thermal and structural responses of fibre metal laminates (FML) under combined fire exposure and loading is investigated experimentally. The fire-under-load response of the FML is compared to its constituent materials of glass fibre composite and aluminium alloy. The monolithic glass fibre-epoxy composite reaches a much higher front surface temperature due to its lower thermal conductivity. Despite its higher temperature, the tensile failure stress of the glass fibre composite during fire-under-load testing is superior to the FML and aluminium at higher applied loads. At higher heat fluxes and lower loads the FML is structurally superior due to the lower back surface temperature and the capacity of the glass fibres to retain stiffness and strength. Under compression loading, the FML has a similar low failure stress as the glass fibre composite due to thermal softening and decomposition of the polymer matrix.<br><br><br>An experimental investigation is presented comparing the structural survivability of the aluminium alloys, carbon-epoxy laminates and FML under identified fire conditions. The thermal response, softening rate, deformation behaviour and structural survivability for the materials are compared. When exposed to the same radiant thermal flux, the surface temperatures of the carbon fibre-epoxy composites are much higher than the aluminium and FML due to differences in their thermal conductivity and fire-induced damage (i.e. delamination cracks, fire-matrix debonding) as well as decomposition processes. However, heat flow through the carbon fibre-epoxy laminates is much slower due to the lower through-thickness thermal conductivity and heat-induced damage. Under tensile loading, for the experimental test conditions used in this PhD project, the tensile load-bearing capacity of the carbon fibre-epoxy laminates are superior to the aluminium alloys and the FML when exposed to the same thermal flux. This is due to the capacity of the load-bearing carbon fibres to retain stiffness and strength to much higher temperatures. <br><br>The structural response of the materials under combined fire exposure and compressive loading is also compared. The softening rate and failure stress of the carbon fibre-epoxy laminate and FML is inferior to the aluminium alloys due to thermal softening of the matrix phase which significantly reduces the buckling stability. The research presented in the PhD thesis presents a deeper understanding of the structural response of aircraft materials when exposed to fire. The work not only gives a deeper fundamental scientific understanding of the phenomena controlling the softening, damage and structural response of aircraft materials, but has practical use in evaluating the relative fire structural performance of the materials when used in modern military, civilian or commercial aircraft.