Simulation and Testing of Structural Composite Materials Exposed to Fire Degradation

Fiber-reinforced polymer composites are attractive for load-bearing aeronautical structures, but their use near fire zones is limited by matrix pyrolysis and the loss of through-thickness integrity. Validating thermo-structural models requires temperature data from inside the laminate rather than at its boundaries. This paper reports in situ through-thickness monitoring of cross-ply carbon/epoxy (AS4/8552) laminates under one-sided flame exposure at approximately 1000 °C. Fiber Bragg Grating sensors were embedded during lay-up at 10, 50, 90 and 100% of the thickness; temperature gratings were sheathed in 0.5 mm brass capillaries to decouple them from matrix strain, with bare gratings alongside to recover the mechanical component. Two configurations were tested: a 50-ply simply supported plate and a 30-ply plate clamped on four sides. Results are compared with a three-dimensional transient COMSOL model in which transverse conductivity and moduli switch at 673 K to represent degradation. The model reproduces the ordering and strongly non-uniform spacing of the measured plateaus (520, 375, 290 and 290 °C), and their magnitude to within 3% at the two stations closest to the flame, though it approaches steady state more slowly than the experiment and overpredicts the plateaus near the unexposed surface by about 20%. The simply supported plate degraded to roughly 70% of its thickness without delamination, whereas restrained expansion in the clamped plate produced interlaminar cracking through the full thickness.