# Digital Shearography for NDT: Phase Measurement Technique and Recent Developments
## 1 Introduction  
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. Composite materials, however, have a higher possibility of having defects due to their complexity [1; 2; 3; 4; 5]. Composite materials, such as glass-fiber-reinforced materials, carbon-fiber-reinforced materials and honeycomb structures, usually have a multilayered structure with a likelihood of internal defects, particularly the separation between layers called delaminations. In addition, there are defects such as false adhesion, wrinkle, crack, impact damage, etc., which can reduce the mechanical properties of materials and even cause serious consequences. Therefore, nondestructive testing (NDT) plays an important role in ensuring that composite materials are free from defects and damage before putting into use.  
With the growing requirements for product performance and reliability, the need for better NDT techniques is urgent, especially for advantages such as real-time, whole-field, noncontact,and high-sensitivity [6]. Optical NDT techniques such as holography [7], electronic speckle pattern interferometry (ESPI) [8], shearography [9], and digital image correlation (DIC) [10] all have the characteristics mentioned above. Among them, shearography has been recognized as being one of the best NDT methods for inspecting delaminations of relatively thin panels made by composite materials and honeycomb structures due to its simple setup, direct strain measurement, and relative insensitivity to environmental interruptions [11; 12; 13].  
Shearography, also known as speckle pattern shearing interferometry, was first proposed on the basis of electronic speckle pattern interferometry (ESPI) in the 1970s. In comparison with other optical NDT techniques, shearography offers many advantages. It is similar to holography and ESPI, but it directly measures the deformation gradient and the strain information, while holography and ESPI measure just the deformation. This means shearography can reveal defects easier because defects in objects usually generate strain concentration. In addition, shearography is insensitive to small rigid body movement because a rigid-body motion does not produce strain. This means that shearography has a huge space for development in industrial operations.  
Regarding the development stage, shearography can be divided into photographic shearography [14], electronic shearography [15], and digital shearography [16]. Among them, digital shearography is the latest version and the mainstream of recent development. It uses image sensors (charge-coupled device (CCD) or complementary metal-oxide semiconductor (CMOS)) to capture information and analyze data. The development of the phase measurement technique was crucial for improving the measurement sensitivity and phase map quality of digital shearography [17; 18]. This paper will give a review of digital shearography for NDT. After a brief introduction of the principles of digital shearography, the paper presents the development process of the phase measurement technique and discusses the advantages and disadvantages of the temporal phase-shift algorithm and spatial phase-shift algorithm in detail. Then, its recent developments and applications for NDT are introduced. Finally, the potentials and limitations of digital shearography are shown and demonstrated.