## 4 Results
### Test Results According to GFRP Panel Thickness  
Flexural strength test results as a function of GFRP panel thickness are shown in Figure 7. The control specimens showed flexural strengths of 341.33, 321.26, and 287.11 MPa, respectively, as the GFRP panel increased in thickness from 9.2 to 12.2 to 15.2 mm. The flexural strengths of the sand-blasted specimens were 397.13, 470.21, and 491.11 MPa, respectively.  
Figure 6: Fracture deformations of test specimens.  
Figure 7: Flexural strength as a function of GFRP panel thickness in hybrid panels.  
In general, flexural strength increased with the thickness of the GFRP panel and the flexural strength of sand-blasted specimens was consistently higher than that of the control specimens. This shows that the sand-blasted specimens featured stronger adhesive forces than the control specimens. In addition, for sand-blasted specimens, the GFRP panel strength was greater than that of the steel panel, showing increased strength as a function of thickness. In contrast, the flexural strength of the control specimens decreased as a function of specimen thickness. This indicated that the control specimens were limited by weaker adhesive forces. As a result, the interfacial separation occurred before flexural failure.  
Specimen fracturing after the flexural tests is shown in Figure 8. The influence of GFRP panel thickness was not significant with the control specimens. The splitting fractures ultimately occurred at all interfaces regardless of GFRP panel thickness. With sand-blasted specimens, fracturing occurred at the interface between the steel and GFRP panels. Relative to the control, there was a significantly decreased tendency for interfacial delamination. Thus, fracture by interfacial delamination could be minimized or prevented by adjusting the thickness of the GFRP panel.  
In general, destruction of steel panels and FRP composites is due to delamination within the FRP composite, fiber delamination, the debonding of interfaces between steel panels and FRP composites, debonding between the steel panel and the adhesive, and the yielding of the steel panels [28; 29; 30; 31; 32]. In this study, the effects of steel panel surface modification and GFRP panel thickness on the overall strength of a hybrid panel were evaluated. The thickness of the steel panel was selected such that yielding of the steel panel would not cause failure. If the steel panel was too thin, it could fail before interfacial delamination occurred. In contrast, if the steel panel was too thick, it could lead to premature delamination at the GFRP/steel interface prior to the steel yielding [28]. Therefore, future studies should include an optimization of steel panel thickness. In addition, previous studies have shown that the arrangement of the fibers and matrix resin can have a significant effect on the mechanical properties and fracture behavior of FRP composites [28; 29; 30; 31; 32].  
Figure 8: Fracture deformation as a function of hybrid panel thickness.  
Generally, the strength decreased as the size of the FRP composite increased. However, this generality depended on the arrangement of fibers in the composite and the results were not always consistent [38; 39; 40; 41]. This result was also obtained with FRP composite materials consisting of steel and GFRP panels. It is unlikely that size effects were significant in the current research, since fracturing tended to occur at the interface between the steel panel and the GFRP panel prior to destruction of the GFRP panel [38; 39; 40; 41]. Further study considering the size effects in the adhesion failure of steel/GFRP hybrid panels is needed.