The damage tolerance of thick S-2 glass-fabric reinforced polyester, vinylester, and epoxy matrix composite panels subjected to ballistic impact loading has been examined. The damage shape and size was examined using ultrasonic NDE. After inspection of the damage, residual compression strength of the impacted panels was measured. The hand laid-up glass-polyester composite suffered from a large damage size, and low strength in compression after ballistic impact (CABI). The glass-vinylester and glass-epoxy panels manufactured using the SCRIMP process showed small damage sizes and high residual strengths. Through-thickness stitching of glass-vinylester and glass-epoxy panels was found to reduce the impact damage size, but the difference was quite small and did not translate into a substantial strength increase. The performance of panels with a ceramic tile bonded at the impact site was also examined. Model predictions of the CABI strength based on compression failure at the net section are compared to measured data, and implications for design of damage tolerant materials are discussed.

There are two predominate modes of failure in a compressively loaded laminate that has been subjected to ballistic damage. Analysis has shown that the stress concentration and the membrane failure are sensitive to the m-plane stiffness matrix of the damaged region, while delamination growth failure is dependent on the size and location of the damage as well as the fracture toughness of the material. An experimental study investigated means of improving damage tolerance ranging from resin toughness to through-thickness stitching. From the experimental study, it was found that improving fracture toughness by through-the-thickness stitching resulted in a reduction in damage size. Stitching may cause severe reductions in stiffness of the damage region as projectile energy is absorbed through the fiber breakage and pullout mechanisms. Thus, while stitching improves multi-impact performance, it may also lead to an increase in fiber damage, causing the compression after ballistic impact (CABI) failure mode to shift fi-om delamination growth to membrane failure. A design trade-off exists between maximum fracture toughness and stiffhess reduction induced within the ballistically damaged region. By examining the factors affecting residual strength (i.e., fracture toughness, inclusion stifmess, loading condition, and finite width effects) the methodology to develop design chart may be produced to optimize damage tolerance.