## 4 Discussion
### Grading Thickness (GT) Design  
Table 9 and Figure 22 summarize the results on the performance metrics for the cellular structure with a SWT design configuration. To understand the meaning of these findings, Figures 23-25 will be used.  
From Table 9 and Figures 22-24, it is evident that the gradient has an insignificant impact on the performance metrics. The main contributor to the performance metrics is the thickness and length. Unlike the previous three designs, the length of the structure significantly affects gradient design. Figure 25 depicts the force-displacement diagram. It shows that the deformation modes are in a progressive and stable manner.  
Based on \(E_{SEA}\), the cellular structures with the highest specific energy absorption from each group (UT, SWT, AT, GT) were selected. Sample S3 and its corresponding samples S6 and S9 with 100 and 150 mm, respectively had the highest \(E_{SEA}\) among all specimen with 35 kJ/kg (S9), which is comparable to the results found by [29; 30]. Sample S15 was the highest among the SWT type with 28.8 kJ/kg and its corresponding samples of 100 and 150 mm length are S21 and S27, respectively. The best \(E_{SEA}\) for the AT type was sample S31 with 29 kJ/kg and the corresponding S37 and S48 of 100 and 150 mm, respectively. The best \(E_{SEA}\) for the GT type was sample S48 with 29.6 kJ/kg and a gradient of 0.004. It is apparent that the UT configuration design will be the most suitable candidate as the crash energy absorber to replace existing designs in EV. The \(E_{SEA}\) value obtained for the UT design (35 kJ/kg) is much higher than those reported for metallic aluminum forms (less than 10 kJ/kg) [31], however comparatively close with aluminum honeycomb structures (less than 45 kJ/kg) [31].  
Figure 23: Interaction plots between design parameters and performance metrics for the GT design.  
Figure 24: Mean effect plots for each performance metric with respect to the design factors.  
It should be noted here that in the simulations conducted in this study, the input kinetic energy was around 31 kJ as had been described in Section 3 of this paper. The maximum value of the energy absorbed by the best design reported (S9) is 8.2 kJ. Even though this is much lower than the input energy, what is important here is the specific energy absorption (\(E_{SEA}\)). For sample S9, this value is 35 kJ/kg. Which means in order to be able to absorb 31 kJ of the input energy, ideally we will require around 0.9 kg of S9 [43, 48]. In terms of densification, as seen in the force displacement diagrams depicted in Figures 13, 16, 21 and 25 it is obvious that the design structures collapsed beyond the densification point, leading to a high value of forces which practically will not be safe for vehicle occupants. However, this is avoidable when a sufficient structural mass (crash element) is designed. For example, in the case of sample S9, ideally if 0.9 kg of structural mass (series configurations of several S9 design) is used, the deformation will not reach the point of densification as the input energy will be absorbed by the multiple S9 design arranged in a series configuration. Such studies as presented here are primarily investigating potential structural designs for crashworthiness application to reduce time and cost. Potential candidate designs will then have to undergo a full-scale vehicle crash test (simulation) with some optimization to achieve the desired design parameters to absorb the crash energy [49].