Finite-element investigation of the compressive strength of non-crimp-fabric-based composites

The compressive strength of composites based on non-crimped fabric (NCF) under axial loading has been investigated. The objectives are firstly to give insight into the mechanisms leading to failure, land secondly to provide guidelines for the fabric structure to optimise the NCF properties. The modelling approach is at the blanket scale where the main microstructural parameters can be accounted for based on experimental characterisation of the NCF composites (A.J. Miller, School of Industrial and Manufacturing Science, 1996). Using a two-dimensional repeating finite-element model through the thickness of a biaxial fabric, we relate the NCF compressive properties to the geometrical and mechanical characteristics of the constituents. It is established that mesobuckling of the 0 degrees-oriented tows leads to an overall shear instability of the model that can be associated with the experimental occurrence of failure. The coupling of tow crimp and material properties, especially the resin mechanical characteristics, is shown to very much modify the NCF compressive strength. The 90 degrees tow shape is also relevant as it defines the support provided against the 0 degrees tow plastic mesobu!ckling. Finally, it is seen that in order to improve the NCF compressive strength, the stitching tension during the manufacturing stage should be kept low so that the tows can spread. Thus, mesobuckling is delayed owing to the improvement in 90 degrees tow support and the reduction in 0 degrees tow crimp. These guidelines are consistent with compressive strength measurements carried out on NCF made during manufacturing trials (R. Backhouse, Eng D. Thesis, 1998). Improvement of the resin characteristics is also shown to bring about substantial improvement in predicted NCF compressive strength. (C) 1999 Elsevier Science Ltd. All rights reserved.