Interface interaction between cross laminated timber (CLT) and fiber composites under ballistic impact

Enhanced Cross Laminated Timber (ECLT) leverages the layered nature of CLT by introducing historically relevant ballistic resistant materials (e.g. steel, E-glass, etc.) between wood layers to increase overall ballistic resistance. Researchers have explored proof of concept ballistic experiments utilizing different wood species, enhancing materials, and manufacturing techniques since 2017. However, for integration in design standards and development of analytical models, we must predictably understand interactions at the interface between wood and enhancing materials. This article presents the results of 80 ballistic experiments using thermally modified coastal western hemlock and three different fiber-based enhancing materials. Tests were performed at the United States Army Engineer Research Development Center Fragmentation Simulation Facility under a novel protocol aimed to experimentally and physically examine interface interactions at the multi-material interface, considering strain rate dependency and inherent material variability in the wood. The experimental results showed the ballistic resistance increased on average between 34 and 59 Joules (5.1 % and 6.8 % gains), excepting one outlier with the addition of an adhered interface between thermally modified coastal western hemlock CLT and three different fiber enhancing materials. Additionally, post mortem computed tomography imaging showed an increased fracture zone projected area in the CLT ply adjacent to the enhancing material layer that replicated typical interface failure profiles in composite-ceramic armors.