The following are excerpts from “Overview of Digital Shearography for NDT” by Boyang Zhang, Xiaowan Zheng, Wan Xu, and Lianxiang Yang, which was published in Materials Evaluation, Volume 78, Issue 3 in March 2020. These excerpts have been lightly edited for ASNT Pulse. Access the full article in the NDT Library at ndtlibrary.asnt.org.
Digital shearography, an interferometric nondestructive testing (NDT) technique, has proven to be an invaluable tool for material inspections and evaluations by providing a full field and noncontact measurement of surface displacement derivatives, or strain information. Compared with holography and electronic speckle pattern interferometry (ESPI), environmental disturbances, such as a random rigid body movement, have less of an effect on digital shearography since rigid body movements do not generate strain information. This has enabled use of digital shearography for various applications outside of the laboratory. In the last decade, due to the rapid development of digital cameras and computer science, the measurement sensitivity and efficiency of shearographic technology has increased tenfold, thus increasing its utility. Here, we will present a brief review of shearography, including its basic principles, hardware configurations, and algorithms. Recent developments and applications are described and demonstrated. The overall review will be based on learning and understanding and will not provide a deep exploration into the principles and related derivative applications.
Introduction
Digital shearography was developed to address several limitations of holography/ESPI for practical applications. Its significant advantages include: (1) not requiring a reference light beam, which simplifies the optical setup and reduces the laser coherence length requirement and need for vibration isolation; and (2) direct measurement of surface strains (first-order derivatives of surface displacements). These distinct advantages have rendered shearography a practical NDT tool, and it has gained wide industrial acceptance for NDT of aircraft structures, particularly composite structures (Steinchen and Yang 2003). Conventional NDT methods such as X-ray and ultrasonic testing, which need to penetrate the object, perform poorly on some composite materials due to their complicated structure and uniform space inside. Digital shearography is based on the laser interference technique, which has an extremely high sensitivity to surface strain. Discontinuities inside the body generate strain concentrations on the surface under loading, and these strain concentrations can be detected by digital shearography.
Principle of Digital Shearography
The digital shearography schematic is shown in Figure 1. The test object is illuminated by an expanded laser, and the object image is captured by a digital camera connected to a computer. A required special shearing device is placed in front of the camera to introduce image shearing. The shearing device allows two nonparallel lights reflected from two different object points to interfere with each other on the camera sensor. If these two points are oriented in the X-direction, this is referred to as the X-shearing direction; otherwise, it is called the Y-shearing direction. The usage of the shearing device enables shearography to be a self-referenced interferometric system, which is a distinguishing feature when compared to other interferometric systems. Different optical elements and setups can be utilized as the shearing device (Steinchen and Yang 2003).
The most popular shearing device is the Michelson shearing interferometer, shown in Figure 2. In this device, shearing is created by tilting a mirror (shown as mirror 2) at a very slight angle, which brings laser rays from two different points on the object’s surface to one point on the sensor plane. These laser rays interfere with each other and produce a random interference pattern due to the rough surface.
This pattern is called a speckle pattern; an example of a speckle pattern is shown in Figure 3. Use of the modified Michelson interferometer as a shearing device has many advantages, including adjustability of the shearing direction and shearing amount, as well as its simplicity in introducing the temporal and spatial phase shift techniques.
The intensity of the speckle pattern recorded on the CCD video camera can be expressed as
where
I0 is the background,
γ is the contrast, and
θ is the phase difference between two points (P1 and P2).
After the specimen is loaded, the intensity becomes
where
Δ is the relative phase difference between light waves from points P1 and P2 before and after loading.
A fringe pattern, or shearogram, is generated by digital subtraction between loaded and unloaded speckle patterns. Because intensity cannot be negative, the absolute value of the subtraction is displayed as
The dark fringes, which result when Is = 0, occur at locations where Δ = 2nπ, where n is the fringe order. Through a digital real-time subtraction function provided by most software programs, a real-time shearogram can be observed. Figures 4a and 4b show typical shearographic fringe patterns for a center-loaded square plate with X and Y shearing directions, respectively. The shearographic fringe pattern is butterfly-shaped.
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Authors
Boyang Zhang: Laboratory of Optical Measurement and Quality Inspection, Department of Mechanical Engineering, Oakland University, Rochester, MI 48309
Xiaowan Zheng: Laboratory of Optical Measurement and Quality Inspection, Department of Mechanical Engineering, Oakland University, Rochester, MI 48309; Zhengzhou University of Light Industry, No. 5 Dongfeng Rd., Zhengzhou, Henan Province, PRC 450002
Wan Xu: Laboratory of Optical Measurement and Quality Inspection, Department of Mechanical Engineering, Oakland University, Rochester, MI 48309
Lianxiang Yang: Laboratory of Optical Measurement and Quality Inspection, Department of Mechanical Engineering, Oakland University, Rochester, MI 48309; 1-248-370-2283; email: yang2@oakland.edu
References
Steinchen, W., and L. Yang, 2003, Digital Shearography: Theory and Application of Digital Speckle Pattern Shearing Interferometry, SPIE Press, Bellingham, Washington.
End note:
Excerpted from, “Overview of Digital Shearography for NDT” by Boyang Zhang, Xiaowan Zheng, Wan Xu, and Lianxiang Yang, Materials Evaluation, Volume 78, Issue 3 in March 2020. DOI: doi.org/10.32548/2020.me-04
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Photo courtesy of Laser Technology Inc (LTI).