Transverse Cracking in Glass Fibre Reinforced Plastic Laminates.

Unidirectional glass fibre reinforced epoxy and polyester laminae having a range of fibre contents, Vf, were tensile tested normal to the fibre direction. The transverse failure strain was found to drop with increasing Vf. At high Vf, the kies strain magnification theory accounted reasonably well for the measured transverse failure strain, but at low Vf the composites failed at strains smaller than those predicted by the theory. This was considered to be due to the nonuniform fibre distribution. Transverse cracking was then studied in 0°, 90°, 0° glass fibre/epoxy sandwich laminates. At comparatively large 90° ply thicknesses, systematic cracking of the 90° ply occurred at an onset strain of ~0.55%, but the crack spacing was greater in laminates with thicker transverse plies. At small transverse ply thicknesses, cracking was constrained; this constrained cracking is explained in terms of a model based on the energy available for cracking. Thermal strains generated during fabrication and poisson strains were also assessed; the thermal strains were found to be quite small but the poisson strains were significantly large and for some laminate geometries they were responsible for splitting of the longitudinal plies along the fibre direction. This longitudinal splitting was also constrained at small 0° ply thicknesses. Structural studies revealed that transverse failure initiated by fibre debonding. Crack propagation then occurred by the coalescence of the individual debonds leading to the formation of microscopic cracks. Fibre debonding can account for the deviation from linearity observed in the stress-strain curves of the laminates at ~ 0.3% strain and also a coincident visual whitening effect. The results of this work have important technological implications in that low strain damage can be minimised by dispersing the plies in a laminate on a fine scale