Microscopy analysis of carbon fiber oxidation in composite coupons burned and smoldered at three different heat fluxes

Abstract

Carbon fiber composites consisting of woven carbon fiber layers embedded in a matrix of resin have replaced traditional materials used in modern aircraft. Composite materials respond uniquely from aluminum and titanium alloys in a fire in that they smolder after the flames are extinguished. Flame-out occurs spontaneously once the resin is vaporized or turns to carbonaceous residues or char during the burn. During smolder, resin char and fibers oxidize, producing additional heat and emitting partial combustion products. Previous work investigated oxidation properties and chemical emissions from two sets of carbon fiber composite coupons each with different resin systems when burned and allowed to smolder for 30 min at three heat fluxes (35, 50, 85 kW/m2) in a cone calorimeter. In that study, an increase in heat flux correlated with increased composite decomposition and emissions during smolder. Carbon fibers in the middle layers from the composite samples were analyzed via microscopy, and the results indicated that fiber diameter was similar across all samples, regardless of resin or heat flux. In the present study, carbon fibers in the upper layers of the previously burned and smoldered composites were analyzed via microscopy. The analysis indicated that in the upper delaminated composite layers closest to the heat source, fiber diameter was reduced at 50 and 85 kW/m2 versus 35 kW/m2 and compared to fiber diameter in the middle layers of composites at all heat fluxes. This phenomenon was limited to the upper layers of the composite, which provides evidence to support the concept that temperature and oxygen transport dictate smolder reaction kinetics and are limited in the in-depth direction between fiber layers.