# Behavior and Damage Characterization of Impulsively Loaded Cross-Laminated Timber Panels
## 4 Conclusions  
Impulsive loading was applied to low span-to-depth ratio (6.40 \(\leq\) Lh \(\leq\) 6.55) CLT panels in a center-point loading condition to promote shear modes of failure. A novel testing fixture was developed and used to mount and restrain CLT specimens subjected to impulsive loading via an ultra-high-velocity actuator. Three levels of boundary-condition rotational rigidity were tested to study its effect on CLT panel behavior. The test series was successful at eliciting rolling shear and longitudinal interlaminar shear failures in all CLT panels tested. Analysis of the resistance history verified that resistance dropped dramatically following the development of shear cracking. No dependency was noted on geometrical parameters, such as CLT layup, or test parameters, such as boundary condition rotational rigidity, span-to-depth ratio, impact energy, peak load, load duration, or impact impulse. Shear modes of failure have been largely absent from CLT panels that undergo live blast testing. The results and observations from these experiments suggest that the occurrence of shear modes of failure in impulsively loaded CLT panels may be due to the load condition and span-to-depth ratio rather than the boundary conditions, CLT panel layup and geometry, or intensity and duration of loading. The literature further supports this hypothesis as shear modes of failure have been observed in shock-tube testing of CLT where a four-point bending loading condition was employed [7]. A uniform loading condition, as may be applied by live blast loading, should be explored. Strain rate effects were noted in the apparent flexural stiffness of all specimens tested. The dynamic apparent flexural stiffness of CLT was 1.3 to 7.2 times greater than the quasi-static apparent flexural stiffness. For the three-laminae specimens, stiffness ratios were consistent across most kinetic energies tested with the stiffness ratio dropping at the higher end of the energies tested. For the five-laminae specimens, stiffness ratios were consistent at the lower range of kinetic energies tested, i.e., 1.0 kJ to 2.0 kJ, with stiffness ratios exhibiting a decreasing trend with increasing kinetic energies above a kinetic energy of approximately 4.0 kJ. There appears to be a transition zone in the stiffness ratios for kinetic energies ranging from 2.0 kJ to 4.0 kJ; testing was not conducted in this energy range, and as a result, additional testing is required to capture the change in stiffness in this range. The presence of shear modes of failure resulted in the dramatic loss of resistance in all specimens tested. The resistance histories provided a method for correlating loss of resistance to the development of damage. Additionally, the Direct Force Method may be used to aid in the characterization of damage in CLT. The development of a resistance function that captures CLT post-peak behavior is required as experimental data and analytical tools are limited. Lower levels of damage were observed in specimens with higher levels of boundary condition rotational rigidity. This observation implies that the provision of stiffer boundary conditions can reduce damage in impulsively loaded CLT panels. Additionally, relationships that correlate impact kinetic energy with maximum inbound displacement were developed for both CLT layups tested (Equation (5) (three-laminae) and Equation (6) (five-laminae)). Conceptualization, N.R.F. and L.K.S. ; methodology, N.R.F. ; software, N.R.F. ; validation, N.R.F. ; terminal analysis, N.R.F. ; investigation, N.R.F. ; resources, L.K.S. and R.G. ; writing--original draft preparation, N.R.F. ; writing--review and editing, L.K.S. and R.G. ; visualization, N.R.F. ; supervision, L.K.S. ; project administration, L.K.S. ; funding acquisition, L.K.S. and R.G. All authors have read and agreed to the published version of the manuscript. This work was supported by the United States Forest Service (USFS) through the Wood Innovations Grant under USFS Domestic Grant 17-DG-11083150-008.