# Optimization of Composite Material Repair Patch Shape Based on Strength Analysis
## 5 Tensile Test of Composite Laminate Panels Repaired with Different Patch Shapes
### Static Tensile Experiment of Rounded Square Patch Repair Component  
The stress-strain curves for plates repaired with square patches are shown in Figure 33. It can be observed that the ultimate strength of R1 is the highest among the repaired specimens, while R4 exhibits the lowest ultimate strength. The average elastic modulus of circular patch repairs is greater than that of square patch repairs, but the elastic modulus of circular patch repairs shows greater dispersion. For the square patch repairs, a serrated pattern is observed after the tensile stress exceeds the ultimate strength. This is mainly due to the discontinuous load-bearing capacity caused by the square step on the base plate. Figure 33: Stress–displacement curve of rounded square patch specimens. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Parameters** & **Elastic Modulus** &
\begin{tabular}{c} **Ultimate Failure** \\ **Strength** \\ \end{tabular} \\ \hline
**C1** & & 13,260 & 388.653 \\
**C2** & & 11,922 & 379.611 \\
**C3** & & 14,095 & 436.874 \\
**C4** & & 11,244 & 386.238 \\
**C5** & & 11,500 & 381.897 \\
**C6** & & 10,185 & 452.596 \\ \hline
**Average** & & 12,034 & 404.311 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Material parameters of circular patches obtained from tensile test/MPa. Table 6 provides the elastic modulus and the tensile ultimate strength values of plates repaired with square patches based on experiments. In terms of elastic modulus, they are arranged from highest to lowest, as follows: R6, R2, R4, R1, R3, and R5. Unlike plates repaired with circular patches, the dispersion of the two parameters for plates repaired with rounded square patches is not significant. The ultimate strength of each plate, from highest to lowest, is as follows: R3, R1, R6, R2, R5, and R4. Based on the data processed through the DIC test, strain contour maps of the plates repaired with square patches are provided in the y-direction before and after the failure of specimen R1, as shown in Figures 34 and 35, and the others are similar. The patch and the whole plate region are shown in Figure 36.  
The comparison of the average displacement in three directions between the patch and the whole plate region of specimen R1 is listed in Figures 37-39. It can be observed that before Dev1/ai0 reaches 52 (stress at 312 MPa), specimen R1 exhibits a good correlation in average displacements in the x, y, and z-directions. However, after the tensile stress exceeds 312 MPa, significant displacement deviations are observed in all three directions, indicating a separation between the patch and the base plate. Considering the tensile ultimate strength, the repair quality of specimen R1 is relatively reliable. Figure 35: Y-direction strain after fracturing of specimen with a square patch. Figure 36: The patch area (Black Rectangle) of the rectangle repaired plate for DIC test. Figure 37: Average displacement of the entire board and the patch in the x-direction of sample R1. Figure 38: Average displacement of the entire board and the patch in the y-direction of sample R1. Figure 39: Average displacement of the entire board and the patch in the z-direction of sample R1. strength of the plate is determined to be 329 MPa. Preliminary assessment indicates that the bonding of the repair patch is firm and the reinforcement effect is reliable. Furthermore, Figures 43-45 provide the average displacement in the x, y, and z-directions for specimen R3. It can be observed that the openings in the x-direction for the two average displacement lines gradually increase after 180 MPa (Dev1/ai0 is 30), indicating the onset of separation between the patch and the base plate. In the y-direction, the two displacement lines coordinate well, and the two lines remain mostly parallel before  
Figure 41: Average displacement of the entire board and the patch in the y-direction of sample R2. Figure 42: Average displacement of the entire board and the patch in the z-direction of sample R2. material failure.