Carbon fiber-reinforced plastic (CFRP), which is a light and composite material, has a higher specific strength and stiffness than metal materials. However, owing to its low elongation, it is vulnerable to local impacts such as collision. Therefore, hybrid composite materials that can overcome the disadvantages of homogeneous materials by bonding CFRP and metal materials are increasingly popular. In this study, a physical surface treatment sandblast process was applied on a high tensile steel plate (CR980) manufactured by cold rolling to form another surface condition, and the bonding strength with CFRP was measured. In addition, spring-back due to the manufacturing process of the CFRP and CR980 hybrid composite material bonded with different surface roughness was observed. The bonding strength and the spring-back angle of the CFRP/CR980 hybrid composite material tended to increase with the increase in the surface roughness.

The variables of die temperature, pressure, time, lamination method, and forming count are applied to fabricate thermosoftening CFRP (CFRTP) with compression molding method. The mechanical properties of CFRTP are compared with those of thermosetting CFRP (CFRTS). The first lamination method is that one hotmelt is inserted into carbon fiber (five carbon fibers and four hotmelts). The optimal lamination conditions are the die temperature of 220°C, pressure of 6 MPa, and pressurization time for 10 min. The tensile strength of CFRTP of this lamination method is 400 MPa. For higher tensile strength value, the second lamination method was applied. CFRTP prepreg was prepared with one hotmelt and one carbon fiber by applying die temperature at optimal lamination conditions. Five CFRTP prepregs were laminated under the same conditions. When the CFRTP sheet was three formings, the tensile strength of 494 MPa could be obtained. CFRTS are prepared by laminating the epoxy prepregs at 140°C with the compressive pressure of 0.5 MPa for 30 min. CFRTP sheet has the lower tensile strength than CFRTS sheet by 223 MPa, but the flexural strength was higher by 61 MPa and by 1.0 J/cm² for Charpy impact test.

Many car components have been fabricated with different materials to improve collision toughness and safety during a car crash. However, as yet, no evaluation standard has been established that evaluates the replacement of auto parts. The standard must take into consideration mechanical properties of tensile and bending. A car crash test was conducted by the IIHS (Insurance Institute for Highway Safety) standard to evaluate safety, but it is expensive and hard to evaluate collision toughness for each car component. In this study, the side crash test was conducted by simulation and the collision characteristic of the center-pillar was evaluated. The center-pillar with CR420 was changed to HPF material. The absorbed energy and intrusion displacement during a side crash were compared to measure collision characteristic between CR420 and HPF. The result showed that the HPF was a feasible alternative material to enhance the collision characteristics and lighten weight of car component.

Finite element (FE) analysis of the tensile test of TWB-HPF 22MnB5 steel was performed and compared with the experimental results. To improve the accuracy of the simulation, the damage theory of FLD and ductile damage theory were used in 2D and 3D simulations. The tensile strength of 22MnB5 steel was determined under various welding heat inputs for FE simulation. Crack propagation of the welded region indicated that the fracture was observed in the base metal under normal welding conditions. Also, the crack propagated along the HAZ region due to higher heat input of the welding, and lead fractures have been highlighted as a potential complication.