Dual Scale Porosity and Interlaminar Properties of Composite Materials

Abstract: In the strive towards reduced fuel consumption and lower emissions, low structural weight is becoming a key factor in the design of advanced vehicle and aerospace structures. Whereas most traditional construction materials are seemingly reaching their limitations, composite materials with their high specific properties offer possibilities to further reduce weight. In high demand structural applications, the quality of the composite material is of utmost importance, requiring the material to be void free and the matrix well distributed as a binder for the load carrying reinforcement. To achieve proper wetting of the fibres, knowledge of the flow resistance of the porous fibre reinforcement is required. It is normally expressed in terms of permeability.Fibre reinforcements in composite materials are normally regarded as a heterogeneous porous media since both fabric and tows are porous but at different length scales. In order to numerically compute the permeability of such media, one of following two approaches can be used. Either filaments are added one-by-one into the modelled geometry (resolved model) or the tows are considered as porous homogenised media. In the latter case expression for the intra-tow permeability is needed.In this thesis, a porous homogenised tow model is benchmarked with a resolved model to the level of refinement possible without being too expensive computationally. Based on this approach, the permeability of complex three- dimensional (3D) textiles is computed utilizing computational fluid dynamics (CFD) analysis. The effect of inter- and intra-tow porosity on the overall permeability of 2D and 3D structures is analysed and discussed in relation to contradictions found in past studies. A clearer picture of the problem is presented, which will be helpful in future modelling and understanding of the permeability of complex structures. In an experimental study, the overall fibre volume fraction as well as the tow compaction are varied and their influence on the permeability is measured. Experimental studies show good agreement with numerical simulations.The interlaminar shear strength of thermoplastic composite materials is studied and the influence of specimen size is examined. Using finite element (FE) analysis it is shown that size effects may be partly due to statistical effects and partly due to the higher number of composite layers in thicker specimens.The effect of processing on the interlaminar delamination toughness of car-bon/polyamide 12 (C/PA12) is studied. It is observed that processing conditions have vital effect on the interlaminar delamination of thermoplastic composites. The mode I crack energy release rate (GIc) of C/PA12 is found to be 15 times higher than for conventional thermoset based composites and 1.5 times higher than for a thermoset composite with stitched reinforcement through the thickness. The best performing C/PA12 composite is manufactured in a hydraulic press equipped with a cold tool, thereby showing potential for both cost and time efficient manufacturing.