Evaluation of the defect size in the curved surfaces using digital shearography NDT method

Digital shearography is an optical, full-field and non-destructive method that can be used in identification of the defects by using image processing techniques. In this method, the surface displacement gradients are directly extracted from the fringe patterns, which requires processing of the phase information in the fringe images obtained in the experimental tests. One of the most important parameters of the shearography is the shear distance, playing an important role in the detection of the defects and quantitative measurements. Any mistake in the selection of the appropriate value for this parameter, makes it difficult to identify the right defect size. This parameter needs to be corrected because the changing in the shear size on the curved surfaces severely reduces the accuracy of the estimating of the defect size. In this paper, a method for estimating the size of the defects in curved surfaces by processing the shearography images is introduced. Geometric analysis is used to calculate the size of the accurate shear distance along the curvature, which can be used to improve the results of the defect estimation. Improvement of the defect size estimation for defects of 10, 16 and 20 mm has been measured as 15, 9.6 and 15%, respectively. It concluded that the proposed method can be used for better estimation of the defect size in the curved surfaces.