## 3 Results and Discussion
### Low-Speed Impact Analysis of Designed Energy-Absorbing Box  
#### 3.2.1 Deformation Mechanism and Mechanical Response  
The high-resolution video camera recorded the whole experiment process in order to observe the deformation behavior of the energy-absorbing box in the low-speed impact experiments. The schematic diagram of the low-speed impact compression deformation characteristics of the test sample at different compression times and the von Mises stress cloud diagram of the collision impact simulation are shown in Figure 12.  
When the axial collision occurs, the failure of the traditional thin-walled energy-absorbing box is manifested as the strength failure limit at the middle position of the structure, and the macroscopic performance is the tearing of the drum structure generated  
Figure 12: Experimental characteristics of the automobile energy-absorbing box filled with porous triply periodic minimal surface (TPMS) structure under 4.4 m/s speed impact, and the von Mises stress cloud diagram of three energy-absorbing boxes under different collision moments of 0 ms, 3 ms, 6 ms, 9 ms, and 12 ms.  
by compression. The failed traditional thin-walled energy-absorbing box is cut with a wire electrical discharge machine, and then the L-shaped thin walls of the upper and bottom parts are generated. The shear stress generated by the designed energy-absorbing box structure is at an angle of 45\({}^{\circ}\) with the axial load along the direction generated by the curved surface structure. The complete failure of the designed energy absorption box structure is represented by shear fracture along the internal structure of the porous TPMS structure until the structure densification of the designed energy-absorbing box and the load-displacement curve transitions from the plateau section to the exponential growth stage (Figure 13).  
During the low-speed impact experiment, the load and displacement data are collected by sensors, and the mechanical properties of different TPMS compression boxes are evaluated by experimental testing and finite element simulation. The load-displacement curve of the designed energy-absorbing boxes obtained after finite element simulation post-processing is shown in Figure 14a. The load-displacement curves obtained from three sets of low-speed impact tests and finite element simulations for each type of energy absorbing box are summarized (Figure 14b). After calculation, the error values between the data obtained from three sets of low-speed impact tests on each energy absorbing box sample and the data obtained from finite element simulation are within 5%.  
The buckling deformation of the traditional thin-walled energy-absorbing box tympan expansion will bring local stress concentration, and unstable deformation mechanisms are prone to bending and torsion phenomena during collisions. In contrast, the deformation form of the energy-absorbing box filled with the porous TPMS structure is stable, showing that it collapses layer by layer to the compacted state. The relatively uniform distribution of stress changes after the impact process is conducive to improving the utilization efficiency of the energy-absorbing box structure, eliminating local stress concentration, and avoiding premature structural failure caused by uneven stress distribution on the shell. When the crash box is compressed by the distance of the first breakage judgment point, the traditional thin-walled crash box only bears a load of 103.6 kN. Surprisingly, the designed energy-absorbing boxes filled with the porous TPMS structure bear 18.46% and 32.73% more than it, 122.72 kN and 137.51 kN, respectively. The results of the dynamic impact simulation are used as a corroboration of this conclusion.  
Figure 13: Internal shape damage of three energy-absorbing boxes in the phase of collision plateau.