## 2 Materials and Methods
### Fixture Design
#### 2.1.2 Identification of the Thickness of the Top Plate  
The top plate was initially designed to be 10 mm thick. However, when impacted with an energy of 1850 J using an impact mass of 145 kg and impact velocity of 5 m/s, plastic strains were observed at the point of impact and around the edges of the inserts for the anti-buckling supports, while a \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Value** & **Parameter** & **Value** \\ \hline Density & 7.85 g/cm\({}^{3}\) & Hardening Parameter & 0.7 \\ Young’s Modulus & 202 GPa & Hardening Exponent & 0.4 \\ Poisson’s Ratio & 0.3 & Failure Plastic Strain & 0.16 \\ Yield Stress & 230 MPa & Maximum Stress & 560 MPa \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Material properties of C40 steel [27]. full elastic situation was desired. Although the drop tower used was only capable of delivering 1800 J of energy, the numerical analysis was conducted with 1850 J to allow a safety buffer of 50 J. The plate thickness was increased in increments of 5 mm until the deformations were completely within the elastic range. Plastic strains were still observed in two corners and in the middle of the plate when the thickness was increased to 20 mm. No plasticity was observed at 25 mm thickness; hence, the top plate was designed to be 30 mm thick, allowing 5 mm as a margin of safety justified by the plausible difference between the actual material properties and the ones used for the numerical model, which were obtained from an online database. Figure 3 shows the difference in the wideness of the plastic areas between the 10, 20, and 30 mm thick plates. The thickness of the bottom plate was fixed at 15 mm, and no plastic strains were observed as the specimen absorbed the impact energy. Hence, it was not necessary to optimize the thickness of the bottom plate. 1.3 Identification of Optimum Diameter, Spacing, and Unsupported Height of the Anti-buckling Columns  
Three different geometrical parameters were considered in sequence for the design of the anti-buckling supports: column diameter, column spacing, and unsupported height. The investigation was conducted with a 550 J impact with a 22 kg top plate impacting at 7 m/s. In order to assess the effect of the change in diameter of the anti-buckling columns on the deformation and impact behavior and arrive at the optimum value for the diameter, three different values were investigated: 5, 10, and 20 mm. As seen from Figure 4, a 20 mm diameter led to slight fronding on top and local buckling at the base with an extension of 30 mm along the length of the composite plate. The 5 mm diameter supports led to local buckling with an extension of 40 mm along the length of the composite plate, although no damage was observed at the top. The 10 mm diameter supports did not cause any damage on top and led to slight local buckling (as observed by the lighter shade of the elements) with an extension of 25 mm along the length of the specimen. Thicker 20 mm supports impeded fronding at the bottom and led to non-progressive damage at the bottom, which could have caused slight fronding to initiate at the top of the specimen. Thin 5 mm  
Figure 3: Top plate optimization with 1850 J impact. Images show the equivalent plastic strain field. From top to bottom: 10 mm thick plate, 20 mm thick plate, and 30 mm thick plate. diameter supports did not offer enough lateral support against vibrations caused by the impact, which led to an increase in buckling when compared to that observed using 10 mm supports. Hence, 10 mm thick supports provided the best solution. Moreover, a 10 mm diameter allowed space for threaded inserts to be added, which were used to change the unsupported height. Unsupported height is the length of the specimen that is not held in place by the anti-buckling supports, effectively causing the supports to not touch the base plate.