Abstract
Failure modes of Spectra polyethylene fiber-reinforced composites were examined under ballistic impact loading. Both woven fabric-reinforced laminates and angle-plied unidirectional fiber-reinforced laminates were found to exhibit sequential delamination, cut-out of a plug induced by through-the-thickness shear, and combined modes of shear and tensile failure offibers as obserxed in the cases of glass and graphite fiber composites. At low areal density, both laminates demonstrated similar ballistic limits. However, as areal density increased, differences in ballistic limit became more apparent, with angle-plied composite laminates showing higher values. When subjected to the repeated impact of a constant striking velocity below the ballistic limit, a progressive growth of local delamination was observed until gross failure of composites occurred. The use of lower striking velocity of the projectile led to the increase in cumulative numbers of impacts for full penetration defining an impact fatigue lifetime profile. The results of impact testing indicated that Spectra fiber-reinforced composites with vinyl ester resin matrix have a higher ballistic limit and longer impact fatigue life at a given striking velocity than the polyurethane matrix composites. Less effective absorption of impact energy by flexible polyurethane matrix composites was attributed to much more restrained pattern of delamination growth. Correlated with the results of dynamic mechanical analysis, these trends indicated that the stiffness of resin matrices plays an important role in controlling the ballistic impact resistance of Spectra fiber composites. Damping factor and resultant stress wave attenuation characteristics of materials seem to be less important parameters.
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