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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.
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.