A typical vehicle structural armor is comprised of a strike face supported by a thick-section composite backing plate. The composite backing plate provides support and confinement of the strike face under high-energy projectile impact and it must also be damage tolerant when subjected to low velocity impact during vehicle operations. In these applications, damage may occur within layers and between layers (as delamination), degrading structural and ballistic performance. In this study, mechanisms for retention of laminate stiffness after transverse impact are investigated by incorporating tough interlayers to reduce the initiation and propagation of delamination that can otherwise significantly reduce stiffness in thick section composites. Insertion of interlayers reduces transverse shear stress (responsible for initiation of delamination) through decoupling of the layers and increases toughness of the interface reducing the extent of delamination propagation. Plain-weave S-2 glass/SC15 epoxy thick sections (with and without interlayers) were fabricated (nominal dimensions: 28”x28” in-plane, 1.2” thick and areal weight of 10-11 lb/ft2) and tested in a large drop tower at an impact energy of 7.4 kJ. Digital Image Correlation (DIC) was used to measure dynamic deflection and strains on the back-face. Through-transmission C-Scans show significant delamination failure in the baseline panel (no interlayers), whereas no delamination was observed in the panel with interlayers (durability). Stiffness retention in the baseline panel is a low 44%, while the interlayer panel is able to retain 91% (damage tolerance) under the same impact conditions. A first-generation model using LS-Dyna (MAT 162) has confirmed the mechanisms leading to the suppression of delamination. The study has identified the need for new characterization methods that are required to improve model predictions leading to an experimentally validated design tool for optimal placement of interlayers.
2022-04-06 08:00:00
