On the physical relevance of power law-based equations to describe the compaction behaviour of resin infused fibrous materials

Abstract The mechanical performance of structural parts (e.g. a 20 m+ aircraft wing) made from pre-impregnated (prepreg) fibre reinforced polymers (FRP) are intimately linked to the material's meso‑scale (sub-mm length) and micro-scale (μm length), which in turns depends upon the way the fibre mat impregnated with resin has been processed. Particularly important is the way the resin flows through the fibre bed, as that influences the level of porosity and the orientation of the fibres in the final part by controlling the stack's change of thickness during manufacture. In this paper, the applicability of a recently proposed phenomenological model, able to predict the thickness evolution of a wide class of prepreg stacks under processing conditions, is investigated. One of the model's main advantages is that, unlike other flow-compaction models, it can capture the transition between squeezing and bleeding flow. The model's significance for a variety of reinforcements (i.e. UD carbon, UD glass and carbon fibre plain weave) and resin systems (i.e. first and second generation of toughened thermoset and thermoplastic resins) is demonstrated. A new physics-based interpretation of the material parameters is proposed. This gives rise to an analytical expression relating the thickness evolution with time to the applied temperature and pressure cycles, the viscosity of the resin, and meso‑ and micro- geometrical characteristics of the reinforcements. The contribution paves the way towards a better understanding and control of the variability and increased digitalisation of the design and manufacturing processes of composite structures.