Shearografic detection of thermal waves for non-destructive testing

Phase-shifting shearography is a well-known speckle-interferometric method for remote non-destructive testing. Conventionally, a short, static loading is applied to the test object, and the shearography sensor monitors the displacement field of the object in order to find flaws. However, this method has some limitations: The depth of defects cannot be determined, and in some cases, the signal of a flaw is superposed by a large deformation of the sample itself which makes defect detection difficult. To overcome these drawbacks, the excitation can be performed modulatedly. This generates a thermal wave at the object surface, going along with a modulated object displacement, which can be monitored by a shearography sensor. After the measurement, the local phase and amplitude of the periodical object displacement can be retrieved by a pixelwise discrete Fourier transformation of the recorded stack of fringe images. Since all images are used for evaluation, the signal-to-noise ratio is substantially increased. The displacement of the test object itself is reduced since only the sine-coded object response is extracted by the Fourier transformation. Depth range is adjustable via the modulation frequency. This paper discusses the performance of this technique on model samples and demonstrates the advantages of this approach on modern automotive and aerospace structures.