## 4 Conclusions  
This paper introduced an unreplicated saturated factorial design as an effective factor screening method for the research of van rollover crashworthiness and weight optimization. The thickness values of 19 structural components of the vehicle's body were taken as the design variables, and the key structure specific energy absorption, the maximum angular velocity of the rollover body center of mass, and the displacement of the roof were investigated as response values. Three unreplicated saturated factorial design methods, including the semi-normal probability graph method, the Lenth method, and the Dong93 method, were used to screen six activity factors. The iterative optimization of the Dong93 method successfully reduced the value of Error I. By combining graphical and numerical analysis methods, the problem of factor screening in the optimization of vehicle rollover crashworthiness was solved more objectively and effectively. Screening out more comprehensive positive factors using only one design shortened the entire roll crashworthiness research cycle and reduced the cost of the test. Furthermore, a multi-objective optimization model for the roll crashworthiness and weight of vans was established, taking six activity factors as design variables. By using the optimal Latin hypercube design, the KRG model for _SEA_, _In_, \(M\), and \(\omega\) was constructed. A hybrid optimization algorithm combining particle swarm optimization and the artificial immune system method was applied to solve the multi-objective optimization problem established in this paper. This algorithm made full use of the population diversity of the artificial immune algorithm, added mutation and crossover operators to the PSO, and ultimately produced a more uniform Pareto solution set. The algorithm effectively improved the global search ability and optimization efficiency, and ultimately reduced the total weight of the selected parts by 34.84%. Meanwhile, the displacement of the top cover and the maximum angular velocity of the vehicle's center of mass decreased by 8.17% and 8.70%, respectively, with the specific energy absorption of the key structure increasing by 9.65%. That is, the van's rollover crashworthiness was enhanced significantly. These results indicate that the unreplicated saturated factorial design is effective in the design of vans in terms of roll crashworthiness and weight optimization, and it can be applied to the research of frontal crashes, side crashes, and roll crashes of other vehicles, breaking away from the dependence on traditional experience and the limitations on vehicle types. Although rollover crashworthiness and weight optimization based on changes in the thickness values of body parts is a direct and effective method, the range and degree of its optimization are limited by the sizes of the parts themselves. If topology optimization or composite materials, such as carbon fiber composites, were applied to the design of the key parts selected in this paper, the rollover crashworthiness and weight optimization of vehicles would be improved to an even greater degree. This topic is worthy of further research. Conceptualization, D.Z. ; methodology, D.Z. ; software, M.D. and Y.L. ; validation, Y.M. ; formal analysis, M.D. ; writing--original draft preparation, D.Z. ; writing--review and editing, Y.M. ; visualization, M.D. ; funding acquisition, Y.M. All authors have read and agreed to the published version of the manuscript. This research was funded by Wuhan University of Technology, grant 4501465542. Not applicable. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Terms** & **M (kg)** & **In (mm)** & **\(\omega\) (rad/s)** & **SEA (J/kg)** \\ \hline Before optimization & 61.917 & 124.9 & 4.6 & 637.21 \\ After optimization & 40.345 & 114.7 & 4.2 & 698.7 \\ Optimization range & 34.84\% & 8.17\% & 8.70\% & 9.65\% \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Rollover crashworthiness and weight optimization results for the van.