# A New Method for Detecting Weld Stability Based on Color Digital Holography
## 3 Experiments and Results
### Evaluation of Weld Stability
Figure 8d shows the center section line of the y-direction deformation field with the load from 293.67 N to 301.46 N. Using the method shown in Figure 7d, the position of the weld seam can be determined at \(\mathrm{y}\in[150,250]\) based on the significant changes in the slope of the three curves (red box) and the holographic reconstruction image (red box). The variance curve of the butt-jointed aluminum alloy plate is shown in Figure 8e. Near the weld (\(\mathrm{y}=159\) pixel), there is a peak in variance at \(\mathrm{y}=159\) pixel, which is \(1.509\times 10^{-4}\) mm\({}^{2}\). Next, \(\overline{\mathrm{x}}=0.0991\) mm can be obtained, and finally, the coefficient of variation \(\mathrm{c}_{\mathrm{v}}=0.1240\) can be obtained from Equation (6). Hence, it can be concluded that the weld stability of the lap-jointed aluminum alloy plate is good. Figure 9a-c show the three-direction deformation field of the defective butt-jointed alloy plate with the load from 293.67 N to 294.45 N. It can be seen that the deformation of the defective butt-jointed aluminum alloy plate mainly occurs in the y-direction, and the weld stability of the defective butt-jointed aluminum alloy plate would be evaluated only based on the deformation in the y-direction. Figure 9d shows the center section line of the y-direction deformation field with the load from 293.67 N to 294.45 N. Using the method shown in Figure 7d, the position of the weld seam can be determined at \(\mathrm{y}\in[210,290]\) based on the significant changes in the slope of the three curves (red box) and the holographic reconstruction image (red box). In addition, there are pits at the weld of the defective butt-jointed aluminum alloy plate, and the thickness of the weld is almost the same as that of the aluminum alloy plate, resulting in insignificant changes in the slope of the three curves. The variance curve of the defective butt-jointed aluminum alloy plate is shown in Figure 9e. When y changes from 210 pixels to 290 pixels (red box), the variance has a peak value of \(3.327\times 10^{-6}\) mm\({}^{2}\) at \(\mathrm{y}=246\) pixel. Next, \(\overline{\mathrm{x}}=0.0046\) mm can be obtained, and finally, the coefficient of variation \(\mathrm{c}_{\mathrm{v}}=0.3965\) can be obtained from Equation (6). Hence, it can be concluded that the weld stability of the defective butt-jointed aluminum alloy plate is poor. Figure 7: Stability evaluation of lap-jointed aluminum alloy plate. (**a**) x-direction deformation field; (**b**) y-direction deformation field; (**c**) z-direction deformation field; (**d**) the center section line; (**e**) the variance curve. Therefore, an optical system based on color digital holography technology was designed successfully for detecting the weld stability. This optical detection system has many advantages, such as non-contact, non-destructive, dynamic, and full field detection, as well as detecting the weld stability of the workpieces in service. Of course, more parameters are needed to further evaluate the stability of the weld seam.