# Behavior and Damage Characterization of Impulsively Loaded Cross-Laminated Timber Panels
## 1 Introduction  
Cross-laminated timber (CLT) is an innovative multi-layered engineered wood product with proven performance in extreme events, such as earthquake, wind, and fire [1, 2, 3]. In recent years, CLT's great potential for application in the force protection of structures has been explored and demonstrated. [PERSON] [4, 5] demonstrated CLT's resistance to ballistic threats and introduced the concept of enhancing CLT's ballistic resistance further via the addition of polymer fabrics. [PERSON] et al. [6] performed live-blast testing on full-scale, multi-story CLT buildings and provided evidence for CLT's ability to resist varying levels of explosive loading in a real-world application. Additionally, several researchers have utilized shock tubes to subject CLT flexural specimens to simulated far-field blast loading and provide methods to model CLT's response to blast loading, obtain data on the behavior of commonly used connections in CLT to blast, and obtain data on the performance of retrofits in CLT subjected to blast loading [7, 8, 9, 10, 11]. While the experimental testing of CLT subjected to blast loading is a maturing field, the performance of CLT subjected to impulsive loading is not well understood. Shear failure modes, e.g., rolling shear (RS), can have deleterious effects on the overall structural integrity of CLT, and yet they remain elusive in impulsively loaded CLT. [PERSON] [4] stated that RS failures have been observed inconsistently in shock tube testing and not at all in live blast testing. Rolling shear strength and stiffness are tied to a CLT panel's out-of-plane bending performance. The global out-of-plane bending response for CLT panels is significantlyaffected by cross-laminae strength and stiffness. [PERSON] et al. [2] stated that RS is an important factor in the consideration of CLT ultimate limit state and serviceability limit state design. For example, premature failure of a CLT panel may be caused if the cross laminations were to fail due to low RS strength. Premature failure in this mode would cause the panel to lose its ability to transfer shear flow and thus its capability for composite action. Dynamic testing to date has primarily focused on the flexural response of CLT specimens subjected to shock, despite the dramatic effects of RS failures. Specifically, blast testing testing has focused on specimens with large span-to-depth ratios (approximately 12 to 35) [6; 7; 11], specimens with complex boundary conditions [6; 8; 9], and low strain rates on the order of \(10^{-1}\) s\({}^{-1}\)[7; 8; 9; 10; 11]. A few hypotheses that may explain the absence of shear modes of failure in dynamically loaded CLT include the following: experimental shock tests on CLT have avoided applying sufficient energy to produce a post-yield response in CLT; large span-to-depth ratios (i.e., \(\geq\)12) elicit a largely flexural response, whereas lower ratios would elicit greater shear deformation and a mix of flexural and shear failure modes; complex and/or stiff boundary conditions redistribute the stresses induced by shock loading, thereby precluding the initiation of shear failure modes; and material scale shock testing of wood indicates that strain-rate enhancement occurs at intermediate to very high strain rates (\(10^{1}\) s\({}^{-1}\) to \(10^{5}\) s\({}^{-1}\)) [12; 13; 14; 15; 16], whereas most experiments on CLT to date have focused on far-field blasts and thus low strain rates (\(\leq\)\(10^{-1}\) s\({}^{-1}\)). The goal of this research was to develop an experiment to study shear modes of failure in impulsively loaded CLT. A testing system and methodology were developed to investigate the behavior of low-span-to-depth ratio Spruce-Pine-Fir-South (SPFS) CLT panels subjected to impulsive loading in a center-point bending loading condition. A test series was designed to subject CLT panels to a variety of impulses in a loading condition that would promote shear modes of failure.