Knitted glass fiber reinforced polypropylene laminates were fabricated using a combination of co-knitted fabrics and compression molding. The knitted fabrics with different stitch densities were produced using a weft knitting machine. Laminates of 4 and 16 plies were fabricated. Initially, static indentation tests were conducted to determine the approximate energies associated with puncturing of the laminates. Drop tower impact tests were conducted using the same constraint and tup used in the static tests. The trends in peak load and puncture energy observed during static compression testing were similar to those observed in dynamic tests. Parameters that define the impact damage resistance of knitted fabric composites were identified. Both peak load and puncture energy were sensitive to the stitch density, number of plies and imposed strain rate. Efforts were made to compare the impact damage resistance of knitted fabric composites with other conventional reinforcing architectures, including continuous strand mat (CSM), plain weave and orthogonal non-woven fabric. A sensitivity index was proposed to compare the impact damage resistance of these different material systems.

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The influence of a polyamide coating on the energy absorption behavior of an E-glass fabric-Vinylester composite was investigated using low velocity impact events to assess composite response. The evaluation included three different plate thicknesses and two concentrations of polyamide to achieve a thin and thick coating of the fabric reinforcement. Inelastic Energy Curves (IEC) were used to describe and compare the dominant failure modes. Thin ductile coatings were seen to enhance energy absorption primarily through crack blunting and enhanced frictional sliding, while thicker coatings appear to facilitate delamination in addition to matrix-coating slip due to the weak chemical bond between the vinylester matrix and polyamide. These mechanisms combine to allow greater plate deflections which in turn induce transitions between the dominant plate reactions and ultimately translate into greater energy absorption.

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Many structural advantages offered by composites due to their inherent tailorability await realization for lack of a clear understanding of their response under transient loading. One tool currently available for evaluation of impact response is the drop weight impact tower. In this paper, we address the influence of impact velocity in drop tower testing using woven and non-woven E-glass fabric-reinforced vinyl-ester composites fabricated by resin transfer molding (RTM). For the range of impact velocities attainable in a typical drop weight impact tower (< 7 m/s), impact energy and contact force determine the extent and type of damage. Impact velocity determines the initial loading slope and time taken to reach maximum load; however, it only marginally influences inelastic damage accumulation. Peak contact force increases linearly with impact energy and projected damage area up to the linear inelastic limit (LIL), which is described as the point where top surface penetration commences. While independent of impact velocity, LIL marks the upper limit of applicability for the well-known linear relationship between projected damage area and impact energy. Beyond LIL, these two composite systems offered no increased resistance to load application, suggesting that ultimate contact force may be an important impact metric.