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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.
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.
Intermediate-scale testing is indispensable when investigating the fire resistance under simultaneous compressive load of components made of glass- and carbon-fibre-reinforced composites (GFRP and CFRP). BAM is successfully operating an intermediate-scale test stand, developed for a specimen size of 500 mm x 500 mm (1000 mm). The fire resistance in terms of fire stability of CFRP and GFRP sandwiches are investigated, e.g. at 20 % of their compressive failure load at room temperature. Times to failure increase by up to a factor of 4 due to intumescent coatings. For GFRP sandwiches, different core structures with and without additional flame retardants show an astonishing impact on time to failure. CFRP shell structures are investigated on the intermediate scale with and without stringer reinforcements, resulting in completely different mechanical failure behaviour in the ultimate load test as opposed to the fire resistance test. The stringers become the only load-carrying part, while the shell acts as a protective layer. Thus the design exploiting this self-protection potential, i.e. the residue of the front skin protecting the load-bearing structure, is highlighted as a most promising route to enhance the fire resistance of lightweight materials.
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.
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.
An intermediate-scale fire testing approach on the structural integrity of lightweight materials
(2013)
Carbon or glass über composites and Sandwich structures, the lightweight materials of choice for aviation, naval, offshore and construction show an enormous energy saving potential. Their combination of excellent specific mechanical properties, high corrosive resistance and thermal insulation properties in combination with various adoptable fabrication techniques leading to mass and fuel cost reduction. The most limiting single factor for a wider use of fibre reinforced plastics (FRP) in particular as elements for structural application is believed to be their fire behaviour (Mouritz and Gibson, 2006). FRPs promote burning by themselves consuming the stabilizing polymeric matrix while embedded fibers (glass, carbon) persisting the flame (Mouritz et ah, 2006). Already at elevated temperatures (100 - 200 °C) the matrix softens with a loss in mechanical properties (Perret et al., 2011, Mouritz and Gibson, 2006). For this reason the stability of the structural component is decreased severely. Fire behavior becomes the major hazard to worry about, increasingly demanding targetoriented investigation, suitable testing and tailored development.
Experimental approaches in the bench-scale have been proposed to investigate the structural integrity in the past (La Delfa et al., 2009, Gibson et al., 2010, Seggewiß, 2011, Mouritz and Gardiner, 2002, Schartel et al.). Ascribed to the small-scale neither the mechanical properties nor the effects of fire may be represented satisfactorily. Flence, the task is to perform more realistic investigations under adequate compressive loads in fully developed fires, based on suitable specimen sizes. Also (La Delfa et al., 2009)) have announced that it is evident that larger scale test of composites are needed. The aim of this study is to present a developed intermediate-scale test setup to perform more realistic investigations (Hörold et al.). Mechanical loading is generated by a column furnace in terms of compression due to a more severe response of specimens in fire tests (Seggewiß, 2011, Gibson et al., 2012, Feih et al., 2008, Feih et al., 2007). An oil burner used to determine the burnthrough resistance of thermal/acoustic insulation materials provides fire directly onto one side of the specimen (Federal Aviation Administration, 2003). Generating a fully developed fire the NexGen burner offers a homogenous heat flux of ~ 180 kW/m2. The intermediate-scale is addressed by specimen sizes either 500 x 500 mm or 1000 x 500 mm with a maximum thickness of 50 mm. The specimen attachment is realized by a compression device that was designed to apply the compressive loads, figure 1. The test setup for specimens with component like dimensions allows realistic investigations up to structural failure in absence and presence of fire load. A first test series was carried out with different levels of loading while the fire remained unchanged. Failure mechanisms, temperature distributions, diversity of FRPs regarding fiber, matrix, lay-up and core as well as flame retardant Systems are in the scope of investigation.