Phase shifting shearography monitors the mechanical behaviour of an object under load, which makes it a valuable tool
for non-destructive testing. However, it cannot determine the depth of defects, and sometimes, the gradient of the
displacement of the whole object is so large that it hides small deviations caused by flaws. Our approach to overcome
these limitations is based on shearographic imaging of the gradient of the displacement field of an object that is
periodically loaded by a modulated excitation. After unwrapping the stack of fringe images, the local phase and
amplitude of the periodical object displacement can be retrieved by a pixelwise discrete Fourier transformation. The
displacement of the test object itself is mathematically reduced since only the sine-coded object response is extracted by
the Fourier transformation. Depth range can be adjusted since the thermal diffusion length of the thermal waves depends
on their frequency. Since all images are used for evaluation (and not only one fringe image like in conventional speckle-interferometry),
the signal-to-noise ratio is substantially increased. This paper discusses the performance of this
technique on model samples and demonstrates the advantages of this approach on modern automotive and aerospace
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.
Interferometrical methods like Shearography or Electronic-Speckle-Pattern-Interferometry (ESPI) are being used for
remote deformation measurements. For non-destructive testing, usually not the deformation of the whole inspected
object is of interest, but only the changes in the deformation field that are caused by hidden defects. By applying the
lockin technique, small local discontinuities can be monitored even on a large background deformation. Dynamic
excitation is performed by modulation of absorbed light intensity while object deformation is continuously recorded to
give a stack of fringe images. Instead of using only the information contained in the image with the best contrast, our
technique evaluates the whole image stack with respect to the local response to the coded input. The periodical
component of the deformation is extracted by Fourier transformation for the time dependent signal at each pixel. This
way the relevant information contained in the image stack is compressed to an amplitude- and a phase angle image. As
only defects contribute to a signal change in the phase image, the method is defect selective. Furthermore, the phase
change depends on depth where the defect is located since thermal waves are involved. One more advantage is the
substantial improvement of the signal-to-noise ratio.