Vision-based detection of bent circular tubes with ball joints

Circular tubes and ball joints are widely used in large-span grid structures. Bending of circular tubes is a common form of damage in grid structures, posing a potential risk of global safety. Traditional manual inspections are labor and time cost, and usually need high-altitude operation. To address this issue, an automatic bent tube detection method based on computer vision and deep learning is proposed in this article. In the method, ball joints in the image are identified and localized using YOLOv5s object-detection network, and then the tubes are identified by semantic segmentation conducted by a DeepLab v3+ network enhanced with the contextual transformer block. The axes of the bent tubes are extracted from the segmented masks by the skeletonization algorithms. Finally, bent-tube deflections are assessed as the perpendicular distance from the bent-tube axis to the joint anchor box centerline. The proposed framework is validated by eccentric compression tests on three circular tubes with ball joints. It is shown that the average error in deflection assessment by the proposed framework is less than 2 mm, with a maximum error rate of 3.7% which is deemed acceptable in engineering application.