## 3 Results and Discussion  
Tensile strength was investigated to evaluate the effect of HNTs on the tensile strength of carbon fiber-reinforced composite material. The respective mean value of the eight sets was calculated excluding the minimum and maximum values. These results are shown in Figure 2a. The tensile strength of the test specimens increased in accordance with the added amount of HNT content up until 1 wt %. However, the tensile strength tended to decrease after 1 wt % HNT content. In particular, the specimen with an HNT content of >5 wt % had a smaller tensile strength than the specimens with no added HNT. In the case where the content of the HNT increased beyond a certain level, it is believed that by increasing the cohesion of the HNT, the tensile strength decreased according to the stress concentration in the agglomerated HNT. The interlaminar shear test was carried out to evaluate the interfacial properties of carbon fiber/epoxy composite with added HNTs. The ILSS with varying amount of HNTs is shown in Figure 2b. The ILSS of carbon fiber-reinforced epoxy composite without any added HNT was -26.29 MPa, which was even lower than the test specimen with 10 wt % HNT showing the smallest \begin{table}
\begin{tabular}{c|c|c|c|c} \hline \hline \multicolumn{5}{c}{**(a) Carbon Fiber Used as the Reinforcement**} \\ \hline Value Properties & Tensile Strength & Density & Elastic Modulus & Diameter \\ \hline Carbon fiber & 4694 MPa & 300 g/m\({}^{2}\) & 26.2 GPa & 11–15 \(\mu\)m \\ \hline \multicolumn{5}{c}{**(b) Epoxy Resin Used as the Matrix**} \\ \hline Value Properties & Equivalent Weight & Density (g/mL) & Viscosity (cps) & Total Amine Value \\  & (g/eq, EW) & DIN EN ISO 1675 & DIN EN ISO 2555 & (mgK OH/g) \\ \hline KFR-120 & 170–180 & 1.0–1.2 & 1000–1500 & - \\ \hline KFH-160 & - & 0.8–1.0 & 5–50 & 400–600 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mechanical properties of main materials used in this study. strengthening effect with the addition of HNTs. The ILSS improved by adding HNTs. In addition, when the added HNT content was up to 3 wt %, the ILSS increased with added amount, and starting at 3 wt %, as the content was increased, the ILSS decreased. When more than a certain amount of HNTs was added, the effect of the addition was found to be small. In the case where a comparatively high amount was added to the epoxy resin, even when dispersed, the HNTs clumped together and were not uniformly dispersed by visual observation. It was surmised that stress concentration due to such agglomeration decreased the strength. The left side in Figure 3 shows the SEM image of a test specimen with a fracture cut from the front side after the interlaminar shear test of the carbon fiber-reinforced composite with added HNTs. A test specimen with an HNT content of 10 wt %, compared to the test specimen with 3 wt % HNT content, had the maximum shear strength value, forming a significant interlaminar shear fracture, and the damaged side of the fractured surface was seen to be smooth. Figure 3: Internal image of the damaged zone (**left**) and a fracture (**right**) of HNT carbon fiber-reinforced composites taken using SEM equipment. Figure 2: Comparison of tensile strength (**a**) and interlaminar shear strength (**b**) of carbon fiber-reinforced composites according to HNT content. An image of the fractured surface, which was taken to evaluate the fracture's shape is shown in the right side of Figure 3. With an HNT content of 3 wt %, the pull-out phenomenon was relatively small in the entire fractured surface, and the epoxy resin was also observed to be adhering well to the surface of the carbon fiber. In addition, at an HNT content of 10 wt %, the pull-out phenomenon such as the white part in the SEM image of the test specimen appeared in every aspect of the fiber, and in particular, delamination appeared near or at the fiber-resin interface. It is believed that the reason delamination appears locally is because of the stress concentration from the agglomeration of the HNTs.