Opposed-flow flame spread over carbon fiber reinforced plastic under variable flow velocity and oxygen concentration: The effect of in-plane thermal isotropy and anisotropy

Abstract

This work explores opposed-flow flame spread over carbon fiber reinforced plastics (CFRPs) with an emphasis on the in-plane thermal isotropy and anisotropy depending on carbon fiber (CF) orientation. CFRP sheets with different CF orientations were fabricated by laminating unidirectional prepregs in different directions and combusted via a chimney where flow velocity and oxygen concentration were variable. The limiting oxygen concentration (LOC) under buoyant flow significantly increased with CF orientation angle. At more than 45°, a flame did not spread at all but was extinguished immediately after ignition even at 60% O2. The flammability of CFRPs with CF orientation angles of 0°, 15°, and 30° was further investigated with varying opposed-flow velocities and oxygen concentrations. The LOC of CFRP was higher than that of polymethyl methacrylate (PMMA) over all tested opposed-flow velocities regardless of CF orientation. The response of LOC to opposed-flow velocity varied with CF orientation angle. At 0°, the LOC was not overly sensitive to opposed-flow velocity but almost constant at 31% O2. Meanwhile, the LOCs at 15° and 30° linearly decreased with increased opposed-flow velocity. Flame spread for CFRP was much faster than that for PMMA because of the high thermal conductivity of carbon fibers. The flame spread rate of CFRPs was affected by CF orientation and that at 0° was highest among the three tested CFRP sheets. IR images used to visualize the in-plane temperature distribution during flame spread showed that the preheat region was significantly varied according to CF orientation. Varying CF orientation, therefore, causes the in-plane thermal anisotropy, resulting in the differences in the LOC and flame spread rate. The findings from this work help us evaluate the fire risk and hazard of CFRP and serve to discuss flame spread behaviors of other anisotropic materials.