Damage Tolerance of Thick-Section Composites Subjected to Ballistic Impact

This is a ARMY RESEARCH LAB ABERDEEN PROVING GROUND MD report procured by the Pentagon and made available for public release. It has been reproduced in the best form available to the Pentagon. It is not spiral-bound, but rather assembled with Velobinding in a soft, white linen cover. The Storming Media report number is A885093. The abstract provided by the Pentagon follows: 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 in-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 from delamination growth to membrane failure. A design trade-off exists between maximum fracture toughness and stiffness reduction induced within the ballistically damaged region. By examining the factors affecting residual strength (i.e., fracture toughness, inclusion stiffness, loading condition, and finite width effects) the methodology to develop design chart may be produced to optimize damage tolerance.