Synergy between pyrolysis and interface degradation on the flexural behavior of hybrid PEEK thermoplastic laminates under kerosene flame exposure

Abstract

The present work investigates the synergy between pyrolysis and interface degradation (induced by flame exposure) on the residual flexural behavior (three-point bending) of hybrid quasi-isotropic laminates consisting of carbon/glass fibers and a PEEK thermoplastic matrix. The effect of a kerosene flame exposure (116 kW/m² and 1100°C), on the composites structural integrity is examined as a function of exposure time (300 – 600 – 900 s) and distance to flame. The discussions on fire- and mechanically-induced damage mechanisms are supported by fractographic analysis of specimens (tomography and microscopy). The kerosene flame exposure involves in-plane and through-thickness temperature gradients within the plate specimen. Significant reductions in the flexural stiffness (−65%) and strength (−85%) occur after a 300 s flame exposure compared to virgin state, as the structural integrity of the fire-exposed central zone is severely affected. The effect of prolonged exposure time (ranging from 600 s to 900 s) is moderate on flexural behavior as the barrier formed by an extensive thermally-induced delamination contributes to relatively preserve the plies near the back surface. Regardless the fire exposure time, the specimens in the center seem much degraded with an elastic domain interrupted at 0.7% strain, as the fiber-matrix interface strength has been significantly reduced. The overall residual bending properties of CG/PEEK laminates follow master curves representing the correlations between the amount of char/porosity ratio and the changes in the flexural properties. These master curves provide a relevant design rule for composite parts to be used under critical service conditions (flame exposure).