Composite materials have seen widespread use in the aerospace industry and are becoming increasingly popular in the automotive industry due to their high strength and low weight characteristics. The increasing usage of composite materials has resulted in the need for more effective techniques for nondestructive testing (NDT) of composite structures. Of these techniques, digital shearography is one the most sensitive and accurate methods for NDT. Digital shearography can directly measure strain with high sensitivity when combined with different optical setups, phase-shift techniques, and algorithms. Its simple setup and less sensitivity to environmental disturbances make it particularly well suited for practical NDT applications. This paper provides a review of the phase measurement technique and recent developments in digital shearographic NDT. The introduction of new techniques has expanded the range of digital shearography applications and made it possible to measure larger fields and detect more directional or deeper defects. At the same time, shearography for different materials is also under research, including specular surface materials, metallic materials, etc. Through the discussion of recent developments, the future development trend of digital shearography is analyzed, and the potentials and limitations are demonstrated.

Various full-field optical techniques such as digital shearography, digital holography, electronic speckle pattern interferometry (ESPI), thermography, and digital image correlation have been applied in recent years for full-field nondestructive testing (NDT).¹ 2. 3. 4. 5. ^{– 6} Of these techniques, digital shearography has demonstrated many advantages for real-world applications.⁷ 8. 9. 10. ^{– 11} Unlike other techniques, digital shearography uses a special shearing device to directly measure a displacement gradient rather than the displacement itself. This significant feature is particularly important for field NDT applications. Displacement gradients contain strain information and defects in materials generate strain concentration. As a result, digital shearography displays defects more directly. Rigid body motion also does not produce a displacement derivative. The relative insensitivity of digital shearography to environmental noises makes it better suited for field applications. Currently, digital shearography has become an industrial tool for NDT, especially, for NDT of delaminations in composite materials and honeycomb structures.¹² 13. 14. 15. 16. ^{– 17}

Digital speckle pattern interferometry and digital shearography are widely used in nondestructive testing due to their advantages of high speed, full field, and high sensitivity. However, traditional speckle pattern interferometry and shearography can only measure a single variable of deformation or strain. This study presents a modified common measurement setup that can simultaneously measure deformation and its first derivative. In the optical setup, a reference beam is introduced behind the shearing device to interfere with the object beams, thereby simplifying the spectrum and improving the quality of the phase maps. Then the spatial carrier technology is used to extract phase and achieve dynamic measurements. The proposed system also expands the measurement range using a ${4}f$ system. This study also presents a 3D optical setup based on the 1D system, which is more suitable for practical measurement applications in industrial areas. Theoretical derivation and experimental results are described and presented.

The flagship monthly journal of SPIE, Optical Engineering (OE) publishes peer-reviewed papers reporting on research and development in all areas of optics, photonics, and imaging science and engineering.