Effects of biaxial deformation of the knitted glass preform on the in-plane mechanical properties of the composite

The effect of biaxially deforming a weft-knit Milano rib fabric on the overall composite tensile and compressive properties has been studied for a glass/vinyl-ester knitted composite. A range of combinations of wale and course stretch ratios was considered. It was found that the tensile modulus, strength and strain-to-failure were all affected to varying degrees by fabric deformation, whereas the compression properties of these structures, on the whole, appeared to be closer to isotropic and relatively insensitive to fabric deformation. This latter observation is believed to be due to the dominance of the matrix properties. Despite not detecting any gross changes in the knit structure, it is believed that some re-distribution and re-orientation of the fibres did occur during fabric deformation, which in turn altered the relative contents and/or directionality of the fibres in the composites. Consequently, the tensile properties were affected by the stretching of the knitted fabric. Post failure microscopy revealed that tensile fracture typically occurred in the vicinity of the highly stressed crossover points of the knit structure. A feature of the compressive failure was kinking of the highly bent yarns, particularly in resin rich regions having reduced lateral support for yarns.