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}

Accurate diagnosis of the level of consciousness in patients with prolonged disorders of consciousness (DOC) is critical for designing therapeutic plans, determining rehabilitative services, and predicting prognosis. The DOC^{1 – 4} mainly include coma (unwakefulness and reflex behavior only), unresponsive wakefulness syndrome (UWS), and minimally conscious state (MCS). Patients in UWS⁵ are clinically awake but unaware of themselves and the surrounding environment. Unlike UWS, patients in MCS⁶ present minimal, inconsistent, but reproducible purposeful evidence of awareness. The clinical evaluation of awareness and diagnosis of these two groups of patients is challenging. At present, subjective bedside behavioral assessments are still regarded as the clinical reference standard for evaluating the awareness of patients with DOC.⁷ However, it has been reported that the misdiagnosis rate was $\sim 40\%$.³ In fact, accurate evaluation of brain function and detection of residual awareness in patients with DOC is complicated, time-consuming, and challenging due to the cognitive, motor, and sensory impairments of the patients, as well as to other factors, such as the examiner and environmental conditions.

We propose a hybrid spatiotemporal three-dimensional phase unwrapping algorithm for use in digital speckle pattern interferometry (DSPI). The feature of the proposed algorithm is the integration of one-dimensional temporal and two-dimensional spatial phase unwrapping algorithms. By demodulating the phase on a single reference point or multiple reference points using temporal phase unwrapping and on each separated phase map region using spatial phase unwrapping, the DSPI with the spatiotemporal three-dimensional phase unwrapping algorithm can realize the measurement of dynamic absolute displacements and the determination of abrupt phase changes which are usually caused by object discontinuities. We demonstrate that the presented algorithm can overcome the drawbacks of the traditional spatial and temporal phase unwrapping algorithms.

An approach to precise measurement of small roll angle based on digital speckle pattern interferometry (DSPI) is introduced. A simple relationship between the roll angle and interferometric phase distribution is established by analyzing the mechanism of fringe formation in the process of roll detection using a dual-beam DSPI, and reasonable approximate processing. The presented method enjoys some important advantages, such as being free of a cooperative target and a very high measurement resolution. In our experiments, a measurement resolution of 1 µrad, which can compete with the best resolution of the existing methods, is achieved. Theoretically, the measurement resolution can even be improved to tens of nano-radians at the cost of increasing the size of the measuring device. The performance of the method is also demonstrated by a comparison experiment.

Phase fringe patterns in speckle pattern interferometry need to be denoised before they are processed further to ensure phase unwrapping reliability and phase determination accuracy. A fast phase denoising method combining the sine–cosine transform and the stationary wavelet transform, with a detailed, unified and adaptive parameter setting of the wavelet function, decomposition level, threshold selection and threshold function, is proposed for a speckle interference image. Owing to the ability of the sine–cosine transform to retain an abrupt phase change, and the stationary wavelet transform to quickly remove the phase noise, the proposed approach can be used to denoise the phase fringe pattern effectively and quickly. These characteristics are verified by simulation as well as practical experiments. Compared with the popular phase denoising method—the windowed Fourier transform with a window size of 20—the proposed method can save the running time by at least 16 times in a practical application test. This speed advantage will be more important when a high-resolution camera is used in dynamic measurement.