# Behavior and Damage Characterization of Impulsively Loaded Cross-Laminated Timber Panels
## 2 Materials and Methods
### Impulsive Center-Point Testing System Design  
The behavior of impulsively loaded CLT was investigated experimentally using an impulsive center-point testing system at Georgia Tech's Structural Engineering and Materials Laboratory. The impulsive center-point testing system comprises two groupings of components, the reaction components and the impact components. The reaction components consist of a three-point flexural testing fixture, a steel mounting plate, and a modular reaction block wall to which all the reaction components are anchored (Figure 2). The impact components consist of an ultra-high-velocity actuator, or a _blast generator_ (BG), anchored to a reaction wall, a rail system and frame, and an impacting mass that is fastened to the actuator piston rod (Figure 3). Figure 1: CLT Panel Specim Processing: (**a**) cutting in progress, (**b**) partial cut with circular saw, (**c**) completed test specimen. Figure 2: Testing System: (**a**) Impact components—alternate view, (**b**) Reaction components—clamping assemblies not installed. Impulsive loading is applied using the BG, an ultra-high velocity, computer-controlled, hydraulic actuator (Figure 4). The BG delivers an impulse by impacting a specimen with a variable mass at a controlled rate and duration in a highly repeatable manner. The model BG25 is capable of accelerating a 110 lbs (50 kg) mass up to 112 ft/s (34 m/s) [26]. Pressurized oil, which is further compressed using nitrogen gas in a pressure accumulator, enables high actuator velocities. The piston rod is accelerated as the poppet valve opens and releases the pressurized hydraulic oil into the acceleration chamber. Similarly, the opening and closing of the poppet valves are controlled to release hydraulic oil from the acceleration chamber. The combination of the deceleration chamber nitrogen gas pressure and the depressurization of the acceleration chamber causes the piston rod to retract in a controlled manner. The impactor may be fired in one of two modes: _flyer_ or _impactor_. This research utilizes the _impactor_ mode, where the impactor is mechanically fastened to the actuator piston rod and remains attached throughout the test. The BG is programmed such that the target velocity is reached upon impact with the specimen. The impact with the specimen and programming of the BG will cause deceleration of the mass and retraction of the piston rod and impactor. The process described for the _impactor_ mode allows for the delivery of a singular pulse to a specimen and allows for the loading history to be tailored to be blast-like in duration and shape. There is evidence that the rolling shear failure mode may be sensitive to damage accumulation [8]; therefore, preventing the introduction of multiple pulses by the BG via the use of the impactor mode or specimen response is critical. Additional details regarding the blast generator at Georgia Tech are provided by [PERSON] et al. [27]. [PERSON] [28] provides additional details regarding the firing modes possible with the blast generator. The supporting reaction frame, shown in Figures 5 and 6, is a three-point flexural testing fixture that provides supports for the specimen and clamps it in a vertical orientation. The reaction frame is divided into an upper and a lower support frame. Two discrete frames were designed to prevent visual obstruction of a specimen's response to an impact. The reaction frame beams are designed with slotted holes to enable the adjustment of the specimen length by moving the cross beams. Four built-up beams act as guides and supports to the cross beams. Figure 3: Impact components. Two clamping assemblies, top and bottom, are affixed to the reaction frame. Each clamping assembly has four threaded rods that are tightened to a specified torque value. Figure 4: Schematic of the MTS blast generator. Figure 5: Supporting reaction frame design: (**a**) perspective view, (**b**) side view. Figure 6: Supporting reaction frame, top view.