## 6 Conclusions  
This paper investigated tool failure modes and causes of the cutting-type energy-absorbing structure using experiments and numerical approaches. A new energy-absorbing tube structure is proposed and optimized for better crashworthiness and reliability. The tool failure modes were identified in the experiments, and the effects of tool failure on the collision responses were analyzed accordingly. The developed numerical model effectively verified the experimentally obtained collision response. Finally, a new energy-absorbing tube structure was proposed by eliminating the tearing-type chip separation mode. The average cutting force and steady-state cutting temperature are optimized using the Kriging surrogate model and NSGA-II algorithm. The main conclusions are as follows:  
* Different areas of the tool had different failure modes and causes. In the middle of the cutting edge, the temperature was lower than the heat-resistant temperature, so mechanical wear containing abrasive and adhesion wear occurred primarily. In contrast, the temperature in the tool's tip and surrounding area exceeded the heat-resistant temperature of the material, so the thermal wear, including transformation and oxidation wear, was mainly: the closer to the tip, the more significant the effect of thermal wear. Thermal wear plays a dominant role in tool failure.
* Tool failure yielded a significant impact on the performance of energy-absorbing structures. The difference in force changing between experimental and numerical results indicated that tool failure led to increasing and oscillating crash forces, which was not conducive to achieving a smooth cutting energy-absorption process.
* The new structure reduced the likelihood of tool failure due to mechanical and thermal wear. By eliminating the tearing mode, it had a significantly lower tip temperature, a more uniform temperature distribution in the cutting edge, and a flatter temperature rising. The cutting force was slightly lower but more stable, so the amount of energy absorption would not be affected. These improvements could better prevent tool failure due to high temperature, thermal shock, thermal unevenness, etc.
* The Pareto front of multi-objective optimization indicates that SST and MCF competed with each other. \(STT\) and \(MCF\) could be well balanced by the minimum distance method. The obtained optimal solution was \(STT\) = 514 K, \(MCF\) = 131 kN. The corresponding design variables were \(d=3.8 mm\), \(w=26.5 mm\), \(\alpha=7.1^{\circ}\).  
Figure 14: Pareto front of the multi-object optimization problem. (**a**) Original Pareto front; (**b**) normalized Pareto front.  
* This paper indirectly verified the different tool failure modes by studying the tool's temperature distribution via thermal-structural coupling finite element analysis. Future research will focus on a realistic simulation of tool failure and further study of the effect of the failed tool on cutting-type energy-absorbing structures.  
Conceptualization, X.W.; methodology and software, Q.G.; validation, X.W.; formal analysis and investigation, Q.G.; resources, M.W.; data curation, S.X.; writing--original draft preparation, Q.G.; writing--review and editing, X.W. and S.X.; visualization, Q.G.; supervision, S.X.; project administration, M.W.; funding acquisition, S.X. and T.Z. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China (grant no. 52175123 and no. 52172409) and the Sichuan Outstanding Youth Fund (grant no. 2022 JDJQ0025).  
Not applicable.  
Data Availability Statement: Data available on request due to restrictions.  
The authors declare no conflict of interest.