Kerosene flame behaviour of C/PEKK composite laminates: Influence of exposure time and laminates lay-up on residual mechanical properties

Abstract

This work examines the influence of a kerosene flame on the residual mechanical tensile properties of composite laminates manufactured by thermo-consolidation process from unidirectional (UD) carbon fibers and a Polyether Ketone Ketone (PEKK) thermoplastic. Considering laminates with two stacking sequences (orthotropic or quasi-isotropic - QI), the influence of a kerosene flame exposure (116 kW/m2 and 1100 °C) on the composites structural integrity was examined as a function of exposure time (5-10-15min). The discussions on fire- and mechanically-induced damage mechanisms are supported by fractographic analysis of specimens. Not surprisingly, longer exposures to kerosene flame lead to gradually increasing damages within the laminates at different scales (micro-meso-macro), ultimately resulting in degrading the bearing capabilities of the material. With respect to virgin specimens, the axial strength decreases by almost 60 and 40% (after a 15min exposure) in orthotropic and QI laminates, respectively. At the same time, the axial stiffness is about 30 and 20% lower (after a 15min exposure). Contrary to orthotropic laminates, two well-defined areas are observed through the thickness in QI laminates: a severely degraded one and a virtually undamaged one. The first one corresponding to the exposed surface side is significantly damaged (fibers oxidation and pyrolysis of the PEKK matrix leading to an extensive delamination). The second one corresponding to the back-surface side is characterized by two plies with very little damage. When loaded in tension, the “undamaged” plies are characterized by the breakage of 0° fibers. This failure mechanism shows that these “undamaged” plies are capable to bear significant portions of the tensile loading contrary to orthotropic specimens in which extensive delamination prevails through the whole thickness.