Modelling the gradual through thickness porosity formation and swelling during the thermal aggression of thermoplastic based laminates

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Impacting thermoplastic-based laminates by a high thermal energy -e.g. a flame—essentially causes the progressive deterioration of the matrix, involving solid-state transformations and dramatic variations of the thermomechanical properties. Throughout this process and because it is associated with significant through thickness gradients, the laminates retains a substantial capacity to sustain a mechanical load, even after matrix has melted. For temperatures higher than the melting temperature, the dominant mechanism of the matrix thermal decomposition is the formation of voids. Whereas they constitute a weakness from the mechanical point of view, they act as thermal insulators and contribute to the protection of the matrix on the side opposite to thermal aggression. Thus, describing accurately the kinetics of their formation is the key to a reliable control of the laminates thermomechanical properties evolution under fire conditions. As a prerequisite to this objective, the formation process was experimentally investigated. Results have evidenced the strong dependence of the porosity content and of the related swelling phenomenon to the time and temperature of thermal exposure. A mesoscopic Finite Element model representing porosities at a structural level was developed based on these observations. The porosity nucleation and the induced swelling were reproduced using a probabilistic approach to drive the progressive transformation of elements into porosities according to their thermal state.