## 2 Materials and Methods
### Construction of Discrete Element Model of FTB  
#### 2.4.1 FTB Model  
Due to the irregular external shape of FTB, it is challenging to accurately reproduce its external morphological features by traditional modeling methods. To obtain a more realistic three-dimensional representation and improve the fidelity of simulation experiments, a sample--whose length, width, and height closely approximate the average dimensions--was selected for modeling, as shown in Figure 7. A K500 automatic turntable 3D scanner (The manufacturer of the K500 automatic turntable 3D scanner is Nankai 3D Technology Co., Ltd., which is located in Shenzhen, China) was employed to perform a full 3D scan of the selected sample. The K500 scanner offers a reconstruction accuracy of less than 0.001 mm and can acquire up to \(1\times 10^{6}\) precise laser points per second, allowing for the generation of high-density 3D point cloud data. The scan data were processed using Techego 3D scanning software (Version: V3.2.1, 64-bit) to remove outliers and fill any missing regions, resulting in an editable 3D model file. The finalized model was exported in IGS format and then imported into the EDEM software (Version: 2025.1, BETA, Developed by CAE Systems). The particle generation function in EDEM was used to fill  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Intrinsic Parameters** & **Values** \\ \hline Poisson’s ratio (\(\mu\)) & 0.373 \\ Elastic modulus/Mpa & 7.93 \\ Static friction coefficients between FTB and the stainless steel plate \(f_{1}\) & 0.63 \\ Static friction coefficients between FTB and industrial rubber \(f_{2}\) & 0.66 \\ Coefficient of rolling friction for the interaction between FTB and the stainless steel plate \(\mu_{f}\) & 0.14 \\ Coefficient of restitution between FTB and the stainless steel plate \(e_{1}\) & 0.606 \\ Coefficient of restitution between FTB themselves, \(e_{2}\) & 0.366 \\ \hline \hline \end{tabular}
\end{table}
Table 1: The intrinsic parameters of FTB.  
Figure 6: Collision test between the FTB samples themselves. (**a**) Initial state; (**b**) maximum swing after collision. 1. High-speed camera. 2. Coordinate paper. 3. FTB samples. 4. Suspension thread.  
the scanned geometry with particles; the number of filling particles generated was 2532, thereby constructing the discrete element model of FTB, as presented in Figure 7b [37].  
The discrete element model consists of multiple particles, and the force between these particles is determined by the contact model. Among the available models, the Hertz-Mindlin with bonding contact model was adopted to construct the discrete element model for its capacity to form the bonding interactions between particles, enabling the simulation of biomechanical properties of FTB through the breakage of these bonds. After the model was imported into GOM Inspect software (Version: 2018.0.1, 64-bit, Developed by ZEISS Group), sharp edges and noise points were refined to generate a high-quality 3D model. This model was then imported into EDEM, where particles were generated and 14,444 bonding links were added, as illustrated in Figure 7c, resulting in the bonded-particle model of FTB. In the bonding model, cohesive forces were introduced between adjacent elements, causing the individual units to behave as a unified structure under loading. When external forces are applied, internal stress is generated within the bonding network. If the stress exceeds a critical threshold, some bond links between units are broken, eliminating their cohesive effect and leading to the separation of individual elements [38].