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The response of advanced composites to low‐velocity projectile loading was investigated. The impact failure mechanisms of composites containing various fibers with different strength and ductility were studied by a combination of static indentation testing, instrumented falling dart impact testing, acoustic emission monitoring, and scanning electron microscopy (SEM). The composites containing fibers with both high strength and high ductility (eg., polyethylene (PE) fibers) demonstrate a superior impact resistance as compared to those containing fibers with high strength (eg., graphite fibers) or high ductility (eg., nylon fibers) but not both. Upon impact loading, the composites containing PE fibers usually exhibited a great degree of plastic deformation and some level of delamination. These mechanisms acted to dissipate a significant amount of strain energy prior to the penetration phase of the impact process. No through penetration was observed in all the samples containing more than three layers of PE fabric except when loaded at relatively high rates and low temperatures. Although certain levels of delamination also took place in other composite systems, very little plastic deformation occurred, allowing ready penetration of the projectile. The stacking sequences in the hybrid laminates studied were found to play a critical role in triggering or inhibiting the processes of plastic deformation and delamination and, therefore, controlling their energy absorption capability. The penetration resistance of composites appeared to be dictated by the fiber toughness. The later property must be measured in a simulated high‐rate condition.

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Fiber‐reinforced composites used in the production, storage, and transport of energy are often exposed to extreme environments such as cryogenic temperatures and/or nuclear radiation. The fraction resistance of composites subjected to these conditions are of fundamental interest. A study was undertaken to examine the effects of Gammaradiation on the structure and properties of fibers, matrix and their interfacial bonding in terms of their influence on composite failure mechanisms. Also thermomechanical analysis was conducted to estimate the residual thermal stresses due to differential thermal expansion coefficients both between fiber and matrix and between two laminae. Results indicate that the impact energy of a composite laminate could be increased by 100 percent by immersing the specimen in liquid nitrogen for only five min. Samples impact‐tested at cryogenic temperatures were found to possess a great degree of delamination and crack bifurcation. Thermomechanical analysis and SEM investigation both reveal that microcracking and small‐scale delamination are promoted by differential thermal contraction. Such effects could be responsible for the observed crack branching and delamination phenomena during impact loading. Although under certain circumstances the Gammaradiation may yield a small increase in fracture resistance it generally degrades the cohesive strength of the matrix and reduces the interfacial bonding between fiber and matrix. Results of a mechanical and microscopic analysis are presented and discussed.