## 2 Strength Evaluation of Seat Frame
As shown in Figure 4a, the front passenger seat of the multi-purpose vehicle was designed to mount two-point and three-point seat belts on the one seat frame. Therefore, both the shoulder block and the lap block were applied to the three-point seat belt, and only the lap block was applied to the two-point seat belt. The material and initial design thickness dimensions of the seat frame considered in the RDO were shown in Figure 4b. The seat frame assembly of the FEA model was generated with shell elements, and the shoulder block, the lap blocks and the seat cushion foams were generated as solid elements. The seat belts were generated with shell and 1-D seat belt elements. Rigid link elements were used to connect each seat structural part. The surface contact conditions were applied to all the parts where the contact behavior would occur. The numbers of elements and nodes of the FEA model were 21,591 and 22,601, respectively. The principal material properties of the FEA model are summarized in Table 2.  
As shown in Table 2, SAPH440-P had the highest yield strength and ultimate strength, and STKM had the highest plastic strain at rupture. The yield strength and the ultimate strength of SPCC and STKM were similar, and the plastic strain at rupture of SAPH440-P was the same as the SPCC. The constitutive model used in this analysis was an elastic-plastic model with strain rate effects. In this constitutive model, deviatoric stresses are updated, and yield function is checked to accept the deviatoric stresses. The load condition included the shoulder/lap loads and the inertial load shown in Figure 1 based on ECE R14 regulations. The shoulder/lab load of 1350 N was applied to the shoulder/lab blocks in contact with the seat belt, and at the same time the inertial load equal to 20 times the weight of the seat was applied to the entire center of gravity considering the application angle of 10\({}^{\circ}\). All the maximum loads were applied instantaneously at the same time, and simulation was performed for 0.08 s while the maximum load was maintained. Contact conditions were applied to all parts of the analysis model so that the initial loadings applied to the seat belt could be transmitted to the entire structure of the seat frame, as well as the seat belt anchorage points and the seat frame constraint points. The boundary condition was implemented by constraining all the degrees of freedom of each of the mount points, which are bolted on the front and rear brackets among the seat assembly parts, as shown in Figure 4b. In the numerical simulation of the seat belt anchorage strength, the maximum plastic strain and dynamic deformation were calculated using LS-DYNA/Explicit, a general-purpose FEA program. The partial rupture or breakage of any anchorage or surrounding area under the required force was investigated from the plastic strain contour results at the final simulation step. The plastic strain contour results are presented in Figure 5.  
As shown in Figure 5, the parts that were at risk of failure due to the concentration of high plastic strain were the rear seat panel (#1), the seat belt connection part of the center bracket (#2), and the front bracket (#3). The reason for the high strain distribution in #1 was that the rear bracket to which the boundary condition was applied and the center bracket to which the seat belt was mounted were connected. The strain concentration of #2 was because the seat belt was directly connected to the center bracket. In the front bracket of #3, the shoulder load and the lap load acted at the same time, and the boundary conditions were applied directly, resulting in a high concentration of strain. As shown in Figure 5, the plastic strain result for the initial design of the seat frame was confirmed to be less than 50% of the plastic strain at rupture of the material properties shown in Table 2. Therefore,