## 3 Results and Discussions
### Impact Damage Mechanism  
There are two types of impact damages in composite laminates: intra-ply damage and inter-ply damage. The intra-ply damage includes pull-out, matrix crack, fiber/matrix, debonding, and fiber  
Figure 11: Comparison of non-impacted side diagram for the case of 64-J impact energy. (**a**) Results of experimental tests; (**b**) Results of numerical calculations. Figure 10: Comparison of non-impacted side diagram for the case of 32-J impact energy. (**a**) Results of experimental tests; (**b**) Results of numerical calculations. breakage, and the inter-ply damage includes delamination [30; 31]. Pull-out occurs due to the weaker interface between the fiber and matrix. Matrix cracks initiate at the upper layers of laminate that are in contact with the impactor, and are usually oriented in planes that are parallel to the fiber direction in unidirectional fiber composites. Fiber/matrix debonding occurs when the fiber is pulled out from the matrix. Fiber breakage occurs later than matrix crack and delamination in the impact process, and it is a precursor to catastrophic penetration mode. Delamination is caused by transverse impact after reaching a certain energy, and it develops in the presence of a matrix crack. Figure 12 shows the cross-section contour plot of the Mises stress of composite laminates for the case of 32-J impact energy. The contour plot of Mises stress represents the stress distribution of composite laminates, and the von Mises stress is used to predict the yielding of materials under complex loading. From the contour plot of Mises stress, it can be seen that there is a write blank area, which refers to the failure elements that have been deleted. At the time of 4 ms, the laminate with stacking sequence [0/90]\({}_{8}\) (Figure 12a) and [0/SMA/90/(0/90)\({}_{7}\)] (Figure 12c) show the damages, including indentation, matrix crack, delamination, and fiber breakage. The laminate with stacking sequence [(0/90)\({}_{4}\)/SMA/(0/90)\({}_{4}\)] (Figure 12b) and [0/SMA/90/(0/90)\({}_{3}\)/SMA/(0/90)\({}_{4}\)] (Figure 12d) represents the damages, including indentation, matrix crack, and delamination, without fiber breakage. At the time of 8 ms, the laminate with stacking sequence [0/90]\({}_{8}\) (Figure 12a) and [0/SMA/90/(0/90)\({}_{7}\)] (Figure 12c) shows further, more serious damages compared that at 4 ms. However, the laminate with stacking sequence [(0/90)\({}_{4}\)/SMA/(0/90)\({}_{4}\)] (Figure 12b) and [0/SMA/90/(0/90)\({}_{3}\)/SMA/(0/90)\({}_{4}\)] (Figure 12d) represent smaller damages compared with those at the time of four ms. Furthermore, most of the Mises stress measurement data at the time of 8 ms in Figure 12b-d are smaller than the Mises stress values at the time of 4 ms. This is mainly due to the SMAs playing an active role in the impact process, which recovers part of the elastic strain of composite laminates in the non-perforation case. The other reason for different Mises stress values is because of the different stiffness of composite laminates in various directions. Figure 12: Cross-section contour plot of Mises stress (Pa) of composite laminates in the damage process for the case of 32-J impact energy. Figure 13 shows the cross-section contour plot of Mises stress of composite laminates for the case of 64-J impact energy. In contour plot of Mises stress, the elements that reached yield strength have been also deleted. At the time of 1 ms, the damages status of all of the composite laminates show almost no difference, which is the early elastic stage in the impact process. At the time of 2.5 ms, the Mises stress values exhibit a significant difference in different laminate types. At the time of 4.5 ms, all of the composite laminates show delamination and fiber breakages, whereas the laminate-stitched SMAs have much smaller damage than the laminate-unstitched SMAs. In addition, the laminate with stacking sequence \([(0/90)_{4}/\text{SMA}/(0/90)_{4}]\) exhibits SMAs bucking, which shows that part of the SMAs cannot recover in the impact perforation stage.