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Fire stability of glass-fibre sandwich panels: The influence of core materials and flame retardants
(2017)
Fire resistance has become a key property for structural lightweight sandwich components in aviation, shipping, railway vehicles, and construction. The development of future composite materials and components demands adequate test procedures for simultaneous application of compression and fully developed fire. Therefore an intermediate-scale approach (specimen size = 500 mm x 500 mm) is applied with compressive loads (up to 1 MN) and direct application of a burner to one side of the specimens, as established in aviation for severe burn-through tests. The influence of different core structures (polyvinylchloride foam, polyisocyanorate foam reinforced by stitched glass bridges, and balsa wood) was investigated for glass-fibre-reinforced sandwich specimens with and without flame retardants applied on the fabrics, in the matrix, and on surface for each specimen at the same time. Times to failure were increased up to a factor of 4. The intumescent coating prolongs the time to failure significantly.
What is more, using the intrinsic potential of the front skin together with the core to protect a load bearing back skin in sandwich panels, the design of the core – here using the wood core – is the most promising approach.
A test set-up in intermediate scale was conceived to investigate the structural integrity of materials under fire. The task was to develop a realistic test scenario targeting component-like behaviour. Carbon-fibre-reinforced sandwich specimens (500 X 500 X 20 mm) were used to examine failure mechanisms, times to failure and critical failure loads under compression. Fire tests were performed with fully developed fire applied to one side of the specimen by an oil burner. In a first test series, the applied load was varied, but the fully developed fire remained unchanged. In general, times to failure were short. Decreased load levels resulted in prolonged times to failure and led to a different failure mechanism. Results obtained in the test series were compared with a bench-scale study (150 X 150 X 20 mm) investigating identical material. The comparison clearly revealed the influence of size on the time to failure and the load-bearing capacity.
Assessing the structural integrity of carbon-fibre sandwich panels in fire: Bench-scale approach
(2019)
The fire resistance of lightweight sandwich panels (SW) with carbon fibre/epoxy skins and a poly(methacryl imide) (PMI) foam core is investigated in compression under direct application of a severe flame (heat flux=200 kW m−2). A bench-scale test procedure was used, with the sample held vertically. The epoxy decomposition temperature was quickly exceeded, with rapid flash-over and progressive core softening and decomposition.
There is a change in failure mode depending on whether the load is greater or less than 50% of the unexposed failure load, or in other words if one or two skins carry the load. At high loads, failure involved both skins with a single clear linear separation across each face. There is an inflection in the failure time relationship in the ∼50% load region, corresponding to the time taken for heat to be transmitted to the rear face, along with a change in the rear skin failure mode from separation to the formation of a plastic hinge. The integrity of the carbon front face, even with the resin burnt out, and the low thermal diffusivity of the core, both play key roles in prolonging rear face integrity, something to be borne in mind for future panel design. Intumescent coatings prolong the period before failure occurs. The ratio of times to failure with and without protection is proposed as a measure of their effectiveness. Apart from insulation properties, their adhesion and stability under severe fire impact play a key role.
The fire behaviour of light-weight material used in structural applications is regarded as the main challenge to be solved for mass transportation. The task is to perform realistic experiments, including a mechanical test scenario under fully developed fires, to improve the material's reliability in structural applications. Our approach utilises an intermediate-scale test set-up (specimen size 500 × 500 mm) to apply realistic compressive loads and fully developed fires directly to one side of a carbon-fibre-reinforced sandwich composite. Three different intumescent coatings were applied to sandwich structures and compared to a bench-scale study. The results emphasise intumescent coatings as a promising method to sustain fire resistance, multiplying the time to failure. Nevertheless, the realistic intermediate-scale test using severe direct flame application underlines the extremely short failure times when the actual composite components are tested without any additional insulation.