# Behavior and Damage Characterization of Impulsively Loaded Cross-Laminated Timber Panels
## 3 Results
### Damage Characterization
#### 3.3.2 Net Force History as a Damage Characterization Tool  
The design of the impactor assembly, which featured dynamic load cells, allowed for the determination of the net impact force history applied to each specimen. The net impact force history provides data on the precise moment of contact between the impactor and specimen and the duration of the impact. The net impact force history made it possible to observe the development of new damage in the specimens based on fluctuations in the forces sensed by the impact load cells. The net impact force history data were paired with a review of the high-speed video footage to characterize failure modes. Figure 22 illustrates the net force history for specimen CLT5-10 with an impact that occurred at 36.72 ft/s (11.19 m/s). Significant fluctuations in the force throughout the history were observed to be correlated with the development of new damage, e.g., a shear crack, or the growth of damage that occurred earlier in the impact event. In Figure 22, several significant fluctuations in force are labeled, and and these fluctuations are discussed below. 1. t = 29.82 ms: Contact between the felt programmer and specimen is made. Load rapidly increases beyond this point in time. 2. t = 31.23 ms: Peak force occurs at this time. Rolling shear cracks located above and below the mid-span develop and widen during the load duration. 3. t = 31.23 to 32.51 ms: Crushing in the lower and upper support plywood bearing plates is observed. Additional rolling shear cracks and longitudinal cracks develop. The crushing of the bearing plates and the development of shear cracks promote the rapid decline of force. 4. t = 32.51 to 34.55 ms: Crushing of bearing plate plywood continues. Tensile rupturing commences in a single lamella in the extreme tensile lamina, and longitudinal flexural cracks develop in the extreme compressive lamina. 5. t = 34.55 ms to 37.44 ms: Progressive tensile rupturing of the lamellas in the extreme tensile lamina. Fall and rise of force is indicative of failure of individual CLT components or the occurrence of damage and the subsequent redistribution of load. 6. t = 37.44 ms to 43.35 ms: Complete failure of the extreme tensile lamina occurs. Rapid oscillations in the net force history are caused by bending of the wood fibers in the lamella in the extreme tensile lamina. 7. t = 43.35 ms to 46.65 ms: Peaks at and after this point occur due to the deceleration and retraction of the impactor and the wobbling of the aluminum pusher plate toward and away from the direction of impact. Figure 22: CLT5-10 Damage detection in net impact force history. Figure 23 shows frames from the high-speed video at the approximate time associated with the net impact force history times described above. Note, the numbering in Figure 23 references the numbering in Figure 22. Figure 24 shows the CLT5-10 post test. The damage progression for three-laminae CLT specimens developed in a similar fashion to the five-laminae specimen. Rolling shear cracks typically preceded longitudinal shear cracks, followed by the progressive rupturing of the tensile lamellas and the eventual failure of the specimen. Figure 25 illustrates the net impact force history for specimen CLT3-5 with an impact that occurred at 9.93 m/s (32.58 ft/s). Significant fluctuations in force are labeled and are discussed below. Figure 23: CLT5-10 Damage progression. Figure 24: CLT5-10 Post-dynamic test. t = 41.556 ms: Contact between the felt programmer and specimen is made. Load rapidly increases beyond this point in time. 2. t = 42.493 ms: Rolling shear cracks develop in the lower half of the specimen. A RS crack merges with a longitudinal shear crack that spreads the lower boundary in the beam, causing partial loss of composite action. 3. t = 42.600 ms: Peak force occurs at this time. 4. t = 43.431 ms: RS cracks that previously developed continue to widen. A longitudinal flexural crack develops in the compressive lamina and a lamella in the tensile lamina ruptures.