# A Numerical Investigation of Delamination Response of CNT/Epoxy Film Interleaved Composite
## 4 Results and Discussion  
Graphs comparing experimental and numerical values using the mechanical properties of the prepreg laminate and single cohesive zone are shown in Figure 4. Among the various cases, the result of W3 is shown as representative. Both the experimental and the numerical load/displacement curves show a drop after the maximum force arrived in the ENF test. Delamination instantaneously overcomes the centerline of the specimen when it crack propagates. It was observed that the simulated results are in good agreement with the experimental results from origin to critical load. However, the numerical model was inconsistent with the experimental results after the critical load was reached. In the case of \"Simulation B\", peak load is consistent with the experimental value but the degree of load drop is calculated to be relatively small. Therefore, the curve after load drop does not match. Conversely, in \"Simulation A\", the curve after load drop is matched but the peak load is low.  
As another approach in this study, two different laws of traction-separation have been applied depending on the position of the cohesive zone. The length of the cohesive zone \(L\) is divided into \(L_{f}\) (the length of front cohesive zone) and \(L-L_{f}\). The first peak of the load-displacement curve, i.e., crack initiation phase, could be strongly affected by the edge shape of the nano-reinforcement and PTFE film, as well as the position, plastic dissipation, and crack initiating phenomena unlike the crack propagation phase. Thus, the cohesive zone is divided into two parts. To determine \(L_{f}\), the simulations with experimental results were conducted repeatedly. The cohesive zone parameters were set with the following steps. The first step is to determine the parameters to match the maximum load. Second, identify those parameters to predict the crack propagation phase. Finally, the parameters  
Figure 4: Experimental and simulation results with single cohesive zone (W3).  
Figure 3: ENF model of (**a**) initial stage, (**b**) just before crack propagation (W3, at 10 s).  
Figure 6: Experimental and simulation results with divided cohesive zone (WN).  
The associated cohesive parameters are presented in Table 4. In all cases, \(\Gamma_{II^{\prime\prime}}\) is less than \(\Gamma_{II^{\prime}}\) since once a crack starts, less energy is required for propagation. In the case of 3, 5 wt%, both \(\Gamma_{II^{\prime}}\) and \(\Gamma_{II^{\prime\prime}}\) are the highest as well as \(t^{\prime}{}_{max}\) and \(t^{\prime\prime}{}_{max}\). This is related to the fact that the CNT/epoxy film inhibits crack growth in both crack initiation and crack propagation phases when an appropriate amount of CNT is well dispersed in the epoxy. Conversely, if the amount of CNT is too large, agglomeration occurs, which leads to a decrease in stress transfer efficiency and crack propagation at the interface between resin and aggregated CNT. In addition, the \(\Gamma_{II^{\prime}}\) and \(t^{\prime}{}_{max}\) values of the UD laminates were higher than the \(\Gamma_{II^{\prime}}\) and \(t^{\prime}{}_{max}\) values of the WN laminates because of their higher moduli and strengths.  
By linking the geometrical features of CNT/epoxy film to the global mode II interlaminar fracture toughness properties of CFRP laminates, this represents a valuable tool for designers looking to enhance the interlaminar properties of laminates using interleaving. This finite element simulation analysis is powerful in that it can sufficiently predict the delamination behavior for non-carbon fiber or other interleaving types of reinforcement.