Extraction of the outer edge of spot welds from the acoustic image with the aid of image processing

Resistance spot welding has been widely used in the industry to join steel sheets. The area of the spot weld is one of the important parameters because it directly relates to the strength of the weld. The purpose of this study is to estimate the area of the spot weld in a nondestructive way, and for this, ultrasonic imaging is used to extract the outer edge of the spot weld. The acoustic image of the welded interface was found to be affected by the surface roughness of the sample. To extract the outer edge of the spot weld from the acoustic image, the image was first binarized by adaptive mean thresholding, and then, the Sobel filter was adapted to the image for edge detection. It was found that the nugget diameters determined from the processed images were in good agreement with those that were measured directly from the cross section obtained through the destructive way. Finally, the change in the area of the spot weld against the number of time welding was monitored, and after welding 1500 times, the area was found to be decreased. This took place because the Cu electrodes were worn out.