As described in Chapter 2, the measurement results of simple shearography (one-beam illumination) contain both in-plane and out-of-plane strain components. At least two shearograms created from two illumination directions should be measured to obtain the pure in-plane strain components. For TPS-DS, due to the limitation of the phase-stepping procedure, the two shearograms need to be measured separately. As a result, TPS-DS can only measure in-plane strain under static loading situations. However, as mentioned in Chapter 6, with the introduction of the spatial phase shift technique, digital shearography can now measure the phase distribution from a single speckle pattern image, allowing for the application of phase shift digital shearography in nonstatic loading situations. Furthermore, as illustrated in Chapter 8, with multiple carrier frequencies, multiple phase maps can now be measured from a single pair of speckle pattern images. This capability of measuring the phase map of two shearograms in a single step allows for the possibility of achieving a pure in-plane strain measurement using digital shearography. This chapter introduces two approaches for achieving a pure in-plane strain measurement with multiple carrier frequencies. The first approach uses two measurement channels separated by proper filters to measure two shearograms from two measurement directions. The second approach uses a single measurement channel to record two shearograms from two measurement directions in a single speckle pattern image. Through the introduction of two different carrier frequencies, two mixed shearograms (in the time domain) can be separated in the frequency domain by Fourier transform. The optical setup, theory, and efforts to improve phase map quality are illustrated in detail.
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