Experimental and Numerical Studies of S2-Glass Fiber/Toughened Epoxy Composite Beams Subject to Drop-Weight or Ballistic Impact

A combined experimental and 3-D dynamic nonlinear finite element approach was adopted to study composite beams subject to drop-weight or ballistic impact. The composite specimens, made of S2 glass-reinforced toughened epoxy (44% fiber volume fraction, cured at 350°F), had 24 layers (approximately 6.35 mm) with various stacking sequences. They were damaged by impacts using either an Instron-Dynatup 8520 instrumented drop-weight impact tester (low-velocity impact) or an in-house high-speed gas gun (ballistic impact). For both types of tests, the time-histories of dynamic strains induced during impact were recorded using strain gages mounted on the front and back of the composite beam specimen. For drop-weight impact tests, the time history of impact force was also recorded; whereas for ballistic impact tests, only the impact velocity was calculated from the recorded change in voltage outputs, which resulted from the traversing of the impactor through two optical paths formed by two sets of diode laser-amplified photo diode pairs. The commercially available 3-D dynamic nonlinear finite element software, LS-DYNA, incorporated with a proposed nonlinear anisotropic damage model, was then used to simulate the experimental results. Good agreement between experimental and FEM results can be seen from comparisons of dynamic strain and impact force histories and damage patterns. Once the proposed nonlinear anisotropic damage model was verified by experimental results, further finite element simulations were conducted to predict the ballistic limit velocity (V₅₀) for penetration prevention.