## 2 Research Method
### Shear Analysis  
To properly manufacture the shear steel, the stress distribution generated during shearing was first analyzed through a finite element (FE) analysis. The plate material used in the stress analysis was a 1.2 mm thick B steel (22 MnB5) with a tensile strength of 1.5 GPa. Additionally, the clearance of the upper and lower shear steels in the shearing process was 8% of the thickness of the material, because it is the most commonly applied clearance in the shearing of general thin-plate materials [19; 20]. To determine the mechanical properties of the material required for the analysis, a tensile test was performed at a constant speed of 2 mm/min using an Instron 5988 universal testing machine (Instron, Norwood, MA, USA). Table 8 lists the average values, excluding the minimum and maximum values, from five repeated tests. Because mechanical trimming is performed after press hardening, the flow stress of press-hardened 22 MnB5 used in the FE analysis of the mechanical trimming process was obtained via a tensile test. The mechanical properties of 22 MnB5 and the process conditions used in the FE analysis are summarized in Table 9. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Material**} & **Yield Strength** & **Tensile Strength** & **Elongation** \\  & **(MPa)** & **(MPa)** & **(\%)** \\ \hline
22 MnB5 & 1100 & 1598 & 8.91 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Mechanical properties of 22 MnB5. Figure 5: Hardness measurement results for each heat treatment condition. Figure 6 shows the shape of the shear surface that manifested during shearing. It is desirable to suppress burr generation because burring lowers workability and causes a gap with the mating material during bonding. These burrs are generally regulated to be less than 5-10% of the material thickness. The shear of a metal plate is considered to be a form of ductile failure due to defect growth, and several theoretical studies have been conducted to simulate this phenomenon. A general type of damage model can be expressed as Equation (4):  
\[C_{normalized\ CL}\ =\int F(S1,\ S2,\ldots)d\overline{e}, \tag{4}\]  
where \(d\overline{e}\) is the effective strain. Damage \(F\) is defined as a function of the process variables (Si), the values of which are integrated according to the deformation history. It was assumed that failure occurs when the accumulated damage value reaches the critical value C, which was applied accordingly. Although a variety of damage models have been proposed in a number of studies, the Cockcroft and Latham destructive models, which are known to be the most accurate through experimental verification, are the most widely used models for predicting ductile fracture during the shearing of shear steel with a simple shape [21]. The generalized Cockcroft and Latham fracture model is shown in Equation (5) [22]:  
\[C_{normalized\ CL}\ =\int_{0}^{\overline{e}_{f}}\Bigl{(}\frac{\sigma_{max}}{ \overline{\sigma}}\Bigr{)}\ d\overline{e}, \tag{5}\]  
where \(\overline{e}_{f}\) is the fracture strain, \(\sigma_{max}\) is the maximum principal stress, and \(\overline{\sigma}\) is the effective stress. That is, in the Cockcroft-Latham model, the growth of damage increases with respect to the maximum principal stress in the tensile direction, where failure occurs when the value reaches a certain limit. Figure 7 shows the shear analysis concept applied to the stress analysis. The ductile failure threshold of the material used in the fracture analysis was 0.8, which was derived from a finite element analysis of the shear test and using a correlation method between the test and analysis [23]. The physical property values obtained from the tensile test were used  
Figure 6: Shape of shear surface produced by shear processing.