Through thickness gradients in porosity content of a fire exposed thermoplastic matrix composite and its effects on mechanical load redistribution: Experiments and modeling

The exposure of a thermoplastic-based laminates to a kerosene flame leads to a highly heterogeneous and rapidly evolving temperature field within the laminates. This is due to the complex combination of conditions at play: a severe thermal aggression, a heterogeneous solid material with highly contrasted thermal and mechanical properties, and a variety of physical mechanisms involved during state transitions of the polymer, up to its decomposition into pyrolysis gas and char. Addressing this physical problem and its consequences on the degradation of mechanical properties is of major importance to the aeronautical industry in its development of lightweight solutions for structural parts subject to fire resistance specifications. This work deals with a quasi-isotropic C/PPS laminates subjected simultaneously to a kerosene flame exposure -in conditions inspired by certification tests- and to a mechanical loading. X-ray tomography analyses have allowed to characterize the porosity spatial distribution, highlighting the presence of different categories of pores and their connectivity. The proposed thermal modeling has proved its potential for accurately reproducing the through thickness evolution of the porosity content, both from ply to ply and inside plies. Notably, it captures the high intraply gradients observed experimentally. Extending the modeling to the thermomechanical behavior has also been shown to produce results consistent with experiments regarding stiffness degradation under flame exposure. This modeling could also demonstrate its potential to represent load redistribution from ply to ply over time within the first minute of flame exposure.