Pipe Weld Investigation using Shearography

Abstract: In this paper the full‐field optical technique of shearography is used to fully characterise the surface strain of the region around a welded joint in a pipe. Shearography is an interferometric optical technique sensitive to displacement gradient, a parameter that is closely related to the surface strain. The surface of the material at a welded joint is uneven, but measurements may still be performed using full‐field non‐contact optical methods. In this paper the authors present a multi‐component shearography system, which measures the in‐ and out‐of‐plane displacement gradient components by determining three non‐orthogonal displacement gradient components and performing a coordinate transformation. To compensate for non‐planar object surfaces the coordinate transformation procedure incorporates a correction for the object shape, measured using the same apparatus. The instrument was used to determine the six components of displacement gradient, thereby fully characterising the surface strain. In comparison with resistance strain gauges, an accuracy of ±11 μɛ was achieved over a limited measurement range.