## 2 Materials and Methods
### Experimental Refinement of the Design  
The preliminary results proved the functionality of the fixture in accordance with the design and simulation results. As discussed in the introduction, two of the common drawbacks of the previously developed fixtures are tearing of the specimen and jamming of the fixture. However, it was observed that the circular anti-buckling columns with 10 mm diameters and 65 mm distance from each other were able to support the specimen without causing tearing. The 10 mm gap between the support and bottom plates provided the necessary space for the crushed parts to curve up freely and avoided jamming the fixture. Moreover, the high-speed camera from the side angle proved the success of the trigger in initiating the steady crushing of the composite element, and no buckling was recorded, which verified the functionality of the new anti-buckling fixture. Finally, no permanent plastic deformation was observed on the 30 mm thick steel top plate, which was aligned with the simulation results. Figure 8: Image of the anti-buckling fixture designed for crashworthiness evaluation of flat composite plates under axial impact load. Figure 7: Damage progression in a 24 ply thick carbon-fiber-reinforced polymer (CFRP) laminate. After gaining confidence in the applicability of the fixture, two kinds of materials were used for this initial study: glass-fiber-reinforced epoxy laminate made of 0\({}^{\circ}\)/90\({}^{\circ}\) fabrics according to the NEMA LI-1 standard [32] with a total thickness of 3 mm and GG630T-37 [33] carbon-fiber-reinforced epoxy laminate with a thickness of 2.5 mm made of high strength \(2\times 2\) carbon fabric twill. Table 4 reports the mechanical properties of the two composite materials, according to the supplier datasheets. Effects of unsupported height, impact mass, velocity, and energy were studied on these materials. Specimens were \(150\times 100\) mm flat plates with sawtooth triggers of 5 mm depth (Figure 9). First, the effects of the unsupported height were studied to see the different responses of the specimens at 35, 20, 10, and 5 mm. Due to the relative scarcity of the CFRP specimens, only glass fiber ones were used in this stage. Then, the effect of the impact mass and impact velocity were studied on both glass and carbon fiber specimens. In the literature, energy-absorbing capabilities are reported by various methods such as peak load and the mean crush force after that, crush force efficiency (which is the relation between these two forces), and SEA. Since in this fixture the metallic plate on top of the specimen is impacted by the metallic impactor of the drop tower, all tests show a huge initial peak and then a force drop to zero. This is because the contact between the impactor load cell and the upper plate is lost for some milliseconds due to the inertia. Therefore, peak forces and crush force efficiency are not used as energy \begin{table}
\begin{tabular}{c c c} \hline \hline  & **NEMA FR4 Glass Fiber Composite [32]** & **GG630T-37 Carbon Fiber Laminate [33]** \\ \hline Density & 2.07 kg/dm\({}^{3}\) & 1.56 kg/dm\({}^{3}\) \\ Elastic modulus & 24 GPa (ISO 178) & 60 \(\pm\) 2.21 GPa (ASTM D3039) \\ Tensile strength & 300 MPa (ISO 527) & 946 \(\pm\) 37.36 MPa (ASTM D3039) \\ Flexural strength & 500 MPa (ISO 178) & 624 \(\pm\) 48.05 MPa (ASTM D0790) \\ Compressive strength & 350 MPa (ISO 604) & 325 \(\pm\) 13.03 MPa (ASTM D3410) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Mechanical characteristics of the specimens according to the supplier data sheets. Figure 9: Images of the two triggered specimens used for experimental evaluations: (**a**) NEMA FR4 glass fiber composite [32]; (**b**) GG630T-37 carbon fiber laminate [33]. absorption indicators in this paper and only SEA values are reported. Figure 10 illustrates these areas on the typical force-displacement curve obtained from experiments using this anti-buckling fixture and also shows the crushed specimen from two visual angles.