Damage Accumulation in Non-Crimp Fabric Based Composites.

Damage development in non-crimp fabric glass/epoxy cross-ply laminates has been studied under quasi-static loading. Two laminate lay-ups have been studied: NCFA (WARP/WEFT)s and NCFB (WEFT/WARP)s. The warp tows are slightly wavy, approximating to a misalignment that varies sinusoidally, whereas the weft tows are well aligned. A warp knit stitching was used to hold the fibres together in a fabric form. All laminates were fabricated using a wet lay-up process to impregnate two layers of fabric prior to curing. Uniaxial quasi-static tension tests were carried out on specimens of each laminate type, with some specimens prepared with polished edges and some without. Damage morphology and laminate microstructure were observed in detail and the development of damage quantified as a function of applied strain using both photography and microscopy. Transverse ply matrix cracking was the major type of damage observed. The manner in which damage accumulated and the crack morphology which developed in these laminates, were found to be similar to that observed in cross-ply laminates. As a result of this similarity, damage could be quantified by an area crack density measurement analogous to that used for cross-ply laminates. The NCFB type laminates were found to have a more complex crack morphology than the NCFA types, attributed to the wavy, misaligned nature of the warp 90° tows in the NCFB laminates. As an aid to quantifying the damage, a new concept has been introduced. The 'linear loop density' is defined as the number of loops per unit length of non-crimp fabric either in the warp or weft direction. The relationship between the laminate linear loop density and the saturation crack density was compared and good correlation occurred for the unpolished edge NCFA specimens, where matrix crack initiation sites are shown to be associated with the resin- rich regions caused by the loops. However, in the NCFB laminates, the much higher crack densities at failure were consistent with the lack of influence of the loops on crack accumulation (since the transverse tows are more dispersed in these specimens). The degradation in laminate modulus as a result of the accumulation in cracking was modelled using three slightly different shear-lag analyses. Good predictions for both laminate types are obtained when using a shear-lag analysis that assumes a parabolic variation of the shear in the longitudinal plies and of the longitudinal displacements in the transverse ply. In addition, the waviness of the warp 0° tows has been modelled by approximating this waviness to a sine wave, and the predicted effective 0° ply modulus using this method agrees well with the modulus derived from experimental results