# A Novel Method for the Mechanical Testing of Sports Mouthguards
## 3 New Method for Testing Sports Mouthguards
### Methods
#### 3.1.2 Experimental Setup  
A drop tower test was the chosen method for the impact testing of the mouthguards for this work. A disadvantage of a drop tower test is that the speed of the impact is limited by the maximum height of the tower. To achieve impact speeds of 43.5 ms\({}^{-1}\), a ball would need to be dropped from a height of 96.4 m; however, by converting the speed of each impact to an impact energy, the impact energy can be reproduced in the laboratory by increasing the mass of the impact object. A distribution of impact energies was calculated from the distribution of ball speeds, shown in Figure 2(b). This distribution was used as a basis for setting up the impact tests using the drop tower apparatus. It is important to test the mouthguards at various impact energies to ascertain how they respond to impacts \begin{table}
\begin{tabular}{c c c} \hline \hline
**Material** & **Young’s Modulus (GPa)** & **Hardness (GPa)** \\ \hline Enamel & 95 & 3.5 \\ Aluminium & 72 & 1.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of hardness and Young’s modulus for enamel and aluminium [20]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Mouthguard** & **Test 20 J** & **Test 40 J** & **Test 60 J** & **Test 70 J** & **Test 100 J** \\ \hline Mean thickness (mm) & 4.97 & 4.88 & 4.88 & 4.84 & 4.80 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Measured mean thickness for each mouthguard. with shots of different speeds. As a result, a range of impact energies were selected as a reference for the impact testing, displayed in Table 4. Specimens were used only once for each impact energy experiment. \begin{table}
\begin{tabular}{l c c c c c} \hline \hline
**Parameter** & **Test 20 J** & **Test 40 J** & **Test 60 J** & **Test 80 J** & **Test 100 J** \\ \hline Mass (kg) & 1.94 & 1.94 & 1.94 & 1.94 & 1.94 \\ Drop height (m) & 1.05 & 2.10 & 3.15 & 4.21 & 5.26 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Overview of experiments specifications to obtain the different impact energies. Figure 3: Overview of the new experimental methodology for studying mouthguard impacts at high impact energies, detailing (**a**) the dimensions of the mouthguard including the manufactured mouthguard and mouthguard fixture, (**b**) demonstrating the experimental setup including camera position and high-speed imaging for analysis, and (**c**) showing the mechanical approach used to analyse the hockey ball mouthguard impact using a 2 DOF mass-spring-damper model based on high-speed imaging. The drop tower required the impactor, consisting of hockey ball and holder, to be attached to the drop rod via a screw at the end of the drop rod (Figure 3b). The ball could not be attached directly using a screw, so a ball fixture was created to connect it to the drop rod. The ball fixture also acted as a mass increase to the drop equipment to achieve the required impact energies. The ball fixture was made from an aluminium alloy cylinder. To attach the ball to its fixture, tape was applied around the circumference of the ball and fixture. The fixture had a threaded hole on the top face that the drop rod screws into, holding it in place during the test. The specimens were mounted on the mouthguard fixture and positioned so that the hockey ball was aligned to the middle of the specimen's width, as shown in Figure 3a,b. A high-speed camera was used to record the deformation of the ball and the mouthguard for the duration of the impact at 20,000 fps with an image resolution of 704 \(\times\) 520 pixels. (Supplementary Material). Figure 3b shows the calibration method by considering the known length of the black tape, which is 0.018 m. The use of the high-speed camera enables the observation of the deformation of the mouthguard and the hockey ball separately. This allows a better understanding of the interaction between hockey ball and mouthguard material performance during impact.