# 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 Circular Patch Repair Component  
The stress-strain curve, elastic modulus, initial failure strength, and ultimate failure strength of a composite laminate were measured through tensile testing, as shown in Figure 11.  
Table 4 provides the elastic modulus, initial failure strength, and ultimate failure strength values for specimens C1-C6, along with the mean values of these three parameters for comparison. According to Figure 11, in the plate repaired with circular patches, C6, C3, and C1 exhibit higher values of ultimate failure strength and better toughness. Based on this single criterion, it is considered that the repair effects of these three are better. According to the elastic modulus of the material, it can be observed that the higher the ultimate strength, the bigger the elastic modulus. It is believed that the C6 test specimen is relatively unique and its elastic modulus may be smaller due to insufficient clamping force applied by the fixture. According to the strength testing experiment, the ranking of repair quality is C6, C3, C1, C4, C5, and C2. The coupon C1 in the y-direction (vertical direction in Figures 12 and 13) strain maps of the side with a patch on the board before and after fracture were obtained using the DIC method (refer to Figures 12 and 13). The direction of the fracture opening in the cross-section is approximately parallel to the line connecting the two maximum strain areas in the y-direction before fracture. The other five coupons are similar. Figure 11: Stress–displacement curve of circular patch specimens. Figure 12: Y–direction strain before fracture of the specimen with a circular patch. The DIC analysis method provides a comparison chart of the average displacements in the x (vertical stretching direction), y (parallel stretching direction), and z (perpendicular to the board) directions for the circular region, with C0 as the center (Figure 14). In Equation (10), \(\sigma\) represents the stress value during the experimental process, \(F\) is the experimental tensile force, and S is the cross-sectional area of the specimens. Figure 14: The patch area (Black circle) of the circular repaired plate for DIC test. Figure 13: Y\(-\)direction strain after fracturing of specimen with a circular patch. The \(x\)-axis \"Dev1/ai0\" represents the voltage values corresponding to the tensile stress, which is proportionate to the experimental force, as shown in Formula (10). \[\sigma=\frac{F}{S}=6\times Dev1/ai0 \tag{10}\]  
In Equation (10), \"_Dev1_\" translates to \"the first board of the signal acquisition device,\" and \"ai0\" refers to \"the analog input signal acquired from channel 0\". As Dev1/ai0 increases, in the x-direction of specimen C1, the separation between the two average displacement lines gradually increases in Figure 15. As Figures 16 and 17 show, the average displacement lines of the patch and the entire board in the y and z-directions do not separate significantly. This indicates that the average displacement of the patch in the x-direction is not consistent with that of the entire board. The possible reasons for this are (1) inadequate adhesion of the patch, and (2) the patch is oriented at a 45\({}^{\circ}\) angle to the surface, with external loads applied along the y-direction while the x-direction remains free, resulting in inconsistent deformation under the influence of shear stress and the Poisson effect. Next, the average displacement cloud maps are shown in Figures 18-20 for specimen C2. It can be observed that the average displacement of the patch in the x-direction aligns well with that of the entire board, while in the y-direction it is poor compared to other specimens. The average displacement in the z-direction indicates that the patched region is approximately 0.6 mm higher than the motherboard, possibly due to excess adhesive in the patch area, although the two curves run parallel without significant separation. Figure 17: Average displacement of the entire board and the patch in the z-direction of C1.