# A Numerical Investigation of Delamination Response of CNT/Epoxy Film Interleaved Composite
## 2 Materials and Experiments  
The mode II interlaminar fracture toughness test described in [23] is briefly presented here. In the experiments, the laminates are made by woven carbon fiber prepreg (WSN3K, SK Chemical, Seongnam-si, Korea) and unidirectional carbon fiber prepreg (USN200A, SK Chemical). For the fabrication of CNT/epoxy film, bisphenol-A liquid epoxy resin (YD-128, Kukdo Chemical, Seoul, Korea) and urethane pre-polymer modified liquid epoxy (UME-330, Kukdo Chemical) were used as the matrix and diethyltoluenediamine (DETDA, Sigma-Aldrich, St. Louis, MO, USA) was used as the curing agent. COOH-modified MWCNT (NC3101, Nanocyl, Sambreville, Belgium) were used as the nano-reinforcement. For high content CNT/epoxy film, without solution mixing, which rapidly increases the viscosity of CNT and resin mixture, or chemical vapor deposition (CVD), which requires complicated processes, equipment, and high temperature, the CNT/epoxy mixture was first dispersed with a horn type ultrasonic probe for 12 h. After that, using a three-roll milling machine, the CNT/epoxy mixture of high viscosity according to the high content of CNT was further dispersed through shear force. For a good dispersion of CNT, it was fabricated by film casting machine through three-roll milling 5 times. An epoxy film without CNT was also fabricated for comparison. To examine the effect of interleaving of CNT/epoxy film to prevent CFRP delamination, mode II interlaminar fracture toughness values of fourteen different CFRP laminate configurations were measured (Table 1).  
The prepregs were cut to 200 \(\times\) 200 mm and then laminated to 20 layers. After lamination of 10 th layer, a 70-mm long PTFE film is placed on the front of the specimen, and the remaining 10 layers are laminated. This is for crack initiation of delamination. For specimens interleaved with epoxy film or CNT/epoxy film, the film is placed from the end of the PTFE film to the end of the specimen as shown in Figure 1. It is cured by hot-press at 140 \({}^{\circ}\)C for 1.5 h. More detailed process conditions and materials are described in [23]. Subsequently, it is cut to a width of 20 mm to complete the specimen production. The mode II interlaminar fracture toughness can be evaluated as follows [24]:  
\[G_{IIC}=\frac{9a^{2}P_{c}\delta_{c}}{2B(2L^{3}+3a^{3})} \tag{1}\]  
where \(B\) is the specimen width, \(a\) is the crack length, \(P_{c}\) is the critical load at the crack growth initialization, \(\delta_{c}\) is the critical deflection value at the loading point, and \(L\) is half the length and half the thickness of the specimen.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Type** & **Interleave** & **Code** \\ \hline \multirow{8}{*}{Unidirectional CFRP} & Baseline & UB \\ \cline{2-3}  & Epoxy film interleaved & UE \\ \cline{2-3}  & 1 wt\% & U1 \\ \cline{2-3}  & 3 wt\% & U3 \\ \cline{2-3}  & 5 wt\% & U5 \\ \cline{2-3}  & 7 wt\% & U7 \\ \cline{2-3}  & 9 wt\% & U9 \\ \hline \multirow{8}{*}{Woven CFRP} & Baseline & WB \\ \cline{2-3}  & Epoxy film interleaved & WE \\ \cline{2-3}  & 1 wt\% & W1 \\ \cline{2-3}  & 3 wt\% & W3 \\ \cline{1-1} \cline{2-3}  & 5 wt\% & W5 \\ \cline{1-1} \cline{2-3}  & 7 wt\% & W7 \\ \cline{1-1} \cline{2-3}  & 9 wt\% & W9 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Specim Configurations.