# Behavior and Damage Characterization of Impulsively Loaded Cross-Laminated Timber Panels
## 3 Results
### Effect of Impulsive Loading on CLT Panel Dynamic Apparent Flexural Stiffiness  
The effect of impulsive loading on CLT panels was studied by evaluating changes to their mechanical properties and characterizing damage caused in relation to variations in rotational rigidity at the boundary conditions and impact energy. The dynamic apparent flexural stiffness, \(EI_{app\_dyn}\), was determined for each specimen by taking the slope of the linear, pre-peak region of the resistance function. Additionally, a high-speed video was reviewed to ensure that no visible damage had initiated in each specimen prior to the development of the peak impact force. A ratio, \(EI_{app\_dyn}/EI_{app\_QS}\) was used to compare the dynamic apparent flexural stiffness to the average apparent flexural stiffness for quasi-statically loaded specimens, \(EI_{app\_QS}\). The experiments showed a significant increase in dynamic apparent flexural stiffness in both three- and five-laminae specimens (Figures 16 and 17).  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Test Number**} & \multirow{2}{*}{**Span Length (in.)**} & \multirow{2}{*}{**L:h 1**} & \multicolumn{2}{c}{**Clamping**} & \multirow{2}{*}{**Impact Velocity**} & \multirow{2}{*}{**Impact Energy**} & \multirow{2}{*}{\(\mathbf{\ell}\) (\(\mathbf{s^{-1}}\))} \\  & & & **Torque (in-lbf)** & & **(m/s)** & & **(kJ)** \\ \hline CLT3-1 & 27 & 6.55 & 65 & 9.86 & 4.80 & 7.54 \\ CLT3-2 & 27 & 6.55 & 65 & 12.54 & 7.77 & 11.77 \\ CLT3-3 & 27 & 6.5  
At energies ranging from 1000 J to 2000 J, the five-laminae specimens exhibited a higher amplification of their dynamic apparent flexural stiffness than the three-laminae  
Figure 16: Dynamic apparent stiffness ratio for three-laminae specimens.  
Figure 17: Dynamic Apparent stiffness ratio for five-laminae specimens.  
specimens. The higher amplification in dynamic apparent flexural stiffness in the five-laminae specimens may be due to a combination of the following factors: redundancy inherent in the presence of additional logitudinal laminae in five-laminae specimens; increased stiffness due to the geometry of the five-laminae specimens; and amplification of the shear modulus in the cross laminae (i.e., rolling shear modulus, \(G_{RT}\)) due to high-rate impulsive loading and the presence of multiple cross layers in five-laminae CLT.  
At higher impact energies, there was a marked decrease in the stiffness ratio for the five-laminae CLT. The three-laminae specimens, however, displayed a consistent stiffness ratio between approximately 3.0 and 4.5 for most impact energies tested. In both the three- and five-laminae specimens, stiffness ratios are higher at lower energy levels and drop dramatically once a certain level of impact energy is reached, thereby creating distinct impact energy zones. A similar phenomenon is also noted in the three-laminae specimens with L:h = 14.55, although additional tests are required to determine a trend for these specimens in particular. In the three-laminae CLT specimens, the transition between zones occurs at a higher level of impact energy, \(E_{impact}\) (6500 J \(\leq E_{impact}\leq\) 7500 J), as compared to the five-laminae specimens (2000 J \(\leq E_{impact}\leq\) 4000 J).  
At higher levels of boundary-condition rotational rigidity, the three-laminae CLT exhibited a greater increase in dynamic stiffness (6% to 47%) compared to specimens tested at the baseline torque (i.e., T = 65 in-lbs) and similar impact energies. Increased boundary condition rotational rigidity in five-laminae CLT did not appear to affect the dynamic stiffness and no clear relationship was noted.