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

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 quasiisotropic - QI), the influence of a kerosene flame exposure (116 kW/m2 and 1100 degrees C) on the composites structural integrity was examined as a function of exposure time (5-10-15min). The discussions on fire- and mechanicallyinduced 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 (micromeso-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 degrees 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.

Automated summary

This study investigated the influence of a kerosene flame on the residual mechanical tensile properties of composite laminates made from carbon fibers and PEKK thermoplastic. Longer exposure times to the flame resulted in increasing damages within the laminates, leading to a degradation in bearing capabilities. In quasiisotropic (QI) laminates, two distinct damaged regions were observed, highlighting the varying effects of the flame exposure.