Dual-biprism-based digital image correlation for defect detection of pipelines

A dual-biprism-based stereo camera system, with an imaging model derived from geometrical optics analysis, is proposed for detection of defects on the external and internal surfaces of pipelines. The proposed system, which comprises two biprisms, a lens, a single camera, and common light sources, overcomes the difference in the camera specifications and synchronization mechanism of conventional two-camera systems. Moreover, different fields of views can be obtained by adjusting the angles of the two prisms and the distance between the dual-biprism and the camera. Measurements can be made even for regions in narrow and small areas using a coaxial illumination and speckle-based three-dimensional digital image correlation method. After a typical experiment performed to verify the efficacy of the proposed system, three different types of defects were accurately detected: (1) a local indentation on the external pipe wall; (2) a weld defect in the pipeline that was measured, and the shape and location retrieved after image processing; and (3) a crack on the internal surface of the pipe determined based on strain calculation. These results validate the proposed system as an efficient and convenient tool for defect detection.