# Mechanical Characterisation and Modelling of Elastomeric Shockpads
## 5 Discussion
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{\begin{tabular}{c} **Shockpad and** \\ **Drop Height** \\ \end{tabular} } & \multirow{2}{*}{**Source**} & \multirow{2}{*}{\begin{tabular}{c} **Shock** \\ **Absorption** \\ **(\%)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Energy** \\ **Resolution** \\ **(\%)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Vertical** \\ **Deformation** \\ **(mm)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Contact** \\ **Time (ms)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **RMSE (\% of** \\ **Max Strain)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Mean** \\ **Error (\%)** \\ \end{tabular} } \\ \hline \multirow{2}{*}{\begin{tabular}{c} Berleburger \\ 55 mm \\ \end{tabular} } & Experimental & 41 & 55 & \(-4.1\) & 18.5 & - & - \\  & FE & 38 & 63 & \(-4.1\) & 17.4 & - & - \\  & Difference (\%) & 8 & 13 & 0 & 6 & 6 & 7 \\ \hline \multirow{2}{*}{\begin{tabular}{c} Berleburger \\ 35 mm \\ \end{tabular} } & Experimental & 48 & 52 & \(-3.4\) & 18.7 & - & - \\  & FE & 45 & 59 & \(-3.6\) & 18.3 & - & - \\  & Difference (\%) & 7 & 12 & 6 & 2 & 14 & 8 \\ \hline \multirow{2}{*}{\begin{tabular}{c} Berleburger \\ 15 mm \\ \end{tabular} } & Experimental & 54 & 53 & \(-2.5\) & 20.1 & - & - \\  & FE & 50 & 53 & \(-2.8\) & 19.9 & - & - \\  & Difference (\%) & 8 & 0 & 11 & 1 & 18 & 8 \\ \hline \multirow{2}{*}{\begin{tabular}{c} Holywell \\ 55 mm \\ \end{tabular} } & Experimental & 51 & 50 & \(-6.7\) & 23.8 & - & - \\  & FE & 52 & 56 & \(-6.6\) & 23.1 & - & - \\  & Difference (\%) & 2 & 11 & 2 & 3 & 14 & 7 \\ \hline \multirow{2}{*}{\begin{tabular}{c} Holywell \\ 35 mm \\ \end{tabular} } & Experimental & 57 & 48 & \(-5.8\) & 25.1 & - & - \\  & FE & 59 & 51 & \(-5.7\) & 24.5 & - & - \\  & Difference (\%) & 3 & 6 & 2 & 2 & 9 & 4 \\ \hline \multirow{2}{*}{\
\begin{tabular}{c} Holywell \\ 15 mm \\ \end{tabular} } & Experimental & 63 & 45 & \(-4.1\) & 27.1 & - & - \\  & FE & 62 & 44 & \(-4.3\) & 25.6 & - & - \\ \hline \multirow{2}{*}{
\begin{tabular}{c} Holywell \\ 15 mm \\ \end{tabular} } & Experimental & 2 & 2 & 5 & 6 & 6 & 4 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Experimental and FE simulation results from first drop of AAA for the Berleburger and Holywell shockpads. associated with all impacts at different heights when compared to the Holywell shockpad. This may be due to the reduced thickness of the Berleburger shockpad, resulting in the mass and impact foot having a shorter deceleration distance. The strains for each shockpad remained similar despite the difference in thickness, suggesting that thickening of the Berleburger shockpad may lead to better shock absorbing properties. Calibration of the material models to the experimental data was a multi-step process (Figure 7). The microfoam material model proved to be the more accurate as it allowed for the cellular characteristics of the shockpads to be captured. Relative density is an important characteristic for cellular materials as it expresses the density of the cellular material in relation to the dense solid [24]. The model was able to incorporate the relative density of the shockpads and thus make a distinction between the density of the solid compared to that of the cellular material. As this value could be estimated for both the shockpads, the variable could be fixed whilst optimizing for the remaining model coefficients. The FE simulation was deemed accurate if the mean error across all comparative measures was less than 10%, with no individual error greater than 15% (Table 3). These values were chosen for a number of reasons but principally due to the experimental values used in the error assessment also being subject to errors. There was an uncertainty associated with the shockpad thickness values due to their construction method and granular nature (standard deviation from 15 thickness measurements was 0.1 mm for the Berleburger and 0.9 mm for the Holywell; Table 1).