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Highlights • Specular surface is illuminated by scattered light to generate speckle patten. • The sensitivity factor is influenced by surface normal of the specular surface. • The specular surface is conjugated to the interferogram in the optical setup. • Defects below specular surface is detected by the proposed shearography. Abstract Shearography has been widely used in non-destructive testing due to its advantages of fast, full-field, and high sensitivity for nondestructive test and strain measurement. Shearography has been applied successfully to various industry applications. However, traditional shearography requires the surface under test to be sufficiently rough. This study presents a modified shearography setup that can be applied to workpieces with specular surface which is illuminated by light scattered from a rough plane to generate speckle patten. In the optical setup, the specular surface is conjugated to the phase map to simplify the positional correspondence between the interferogram and the internal defects. The sensitivity factor is analyzed through ray tracing method based on geometric optics which is influenced by the surface normal. The experimental results are described and presented as well. [ABSTRACT FROM AUTHOR]

Highlights • The SLM can change the phase of the light and the amount of shear. • The nonlinear random error in phase shift method can be eliminated. • The efficiency of detection can be greatly improved. Abstract The paper presents a new shearography system which applies a spatial light modulator (SLM). The system can control the shearing amount and direction as well as introduce the phase shift accurately and automatically, making shearographic testing easier and more practical. Shearography has the known advantages for nondestructive test and strain measurements of being fast, full-field, and having high sensitivity. Traditionally, the shearing amount is controlled by adjusting a reflector mechanically. Mechanical adjustment of the reflector is slow, inconvenient and not well suited for in-field NDT practice. For application of phase shift techniques, a piezoelectric transducer (PZT), which is usually used for application of phase shift techniques, introduces random error due to its tendency to be nonlinear. This paper introduces a new shearographic detection system with a spatial light modulator. The spatial light modulator controls the liquid crystal by using a birefringence effect and twisted nematic effect. The spatial light modulator can change the phase of the light and shearing amount accurately and automatically, thus eliminating the nonlinear random error in the phase shift method. Furthermore, this system also improves testing efficiency. In this paper, the fundamental of SLM shearography is presented. The capability of this optical configuration is demonstrated theoretically as well as through experiments. [ABSTRACT FROM AUTHOR]

This paper presents a Double Imaging Mach-Zehnder Spatial Carrier Digital Shearography (DIM-SCDS) system. Compared to traditional Spatial Carrier Mach-Zehnder shearography, DIM-SCDS has two advantages: one is that it can adjust shearing amount and spatial carrier frequency independently; the other one is that it can measure a larger area with a shorter working distance. Two pairs of imaging lenses and apertures are placed in the two arms of Mach-Zehnder interferometry, respectively. Shearing amount is adjusted by the mirrors in the two arms, and spatial carrier frequency is determined by the angle between the two apertures when they are mapped to the same optical axis. Shearograms of the object under test are recorded using a CCD. Shearogram phases are calculated as the measurement results. Finally, the performance of DIM-SCDS is proved by experiments. [ABSTRACT FROM AUTHOR]

This paper presents a simple spatial phase shift shearography based on the Michelson interferometer. A novel digital shearography set-up with a large angle of view, which is based on a 4 f system is demonstrated. In the system, the Michelson interferometer is used as a shearing device to generate a shearing distance by tilting a small angle in one of the two mirrors. In fact spherical wave fronts become plane after going through the first lens in 4 f system. Tilting the mirror in the Michelson interferometer also generates spatial carrier frequency. Sinusoid fitting method is applied to evaluate the phase. This system can generate a phase map of shearography by using only a single image. The effects of shearing angle, the choice of algorithm and comparison result are discussed in detail. The theory and the application are presented. [ABSTRACT FROM AUTHOR]

This paper presents a Dual Directional Sheared Spatial Phase-Shift Digital Shearography (DDS-SPS-DS) system for simultaneous measurement of strains/displacement derivative in two directions. Two Michelson Interferometers are used as the shearing device to create two shearograms, one in the x -shearing direction and one in the y -shearing direction, which are recorded by a single CCD camera. Two lasers with different wavelengths are used for illumination, and corresponding band pass filters are applied in front of each Michelson Interferometer to avoid cross-interference between the two shearing direction channels. Two perpendicular shearing directions in the two measurement channels introduce two different spatial frequency carriers whose spectrums are orientated in different directions after Fourier Transform. Phase maps of the recorded two shearograms can be obtained by applying a windowed inverse Fourier transform, which enables simultaneous measurement of dual directional strains/displacement derivatives. The new system is well suited for nondestructive testing and strain measurement with a continuous or dynamic load. The capability of the dual directional spatial phase-shift digital shearography system is described by theoretical discussions as well as experiments. [ABSTRACT FROM AUTHOR]

In the traditional microfield speckle pattern interference measurement system, due to the use of high-magnification objective lens, the working distance is small and the shearing device cannot be introduced to implement shearography measurement. Thus the distribution of the deformation gradient on a microstructure surface cannot be measured. A system for the synchronous measurement of the surface deformation and gradient distribution of a microstructure was designed. Through the combination of focusing lens, convex lens, and low-magnification objective lens, the microstructure surface was clearly magnified and the internal distance of the system was enlarged. Thus the shearing device can be introduced to realize the shearography measurement under the microfield view. The synchronous full-field measurement of the structural surface deformation and its gradient distribution was further realized by the introduction of a reference light and designing of an optical path switch. In addition, the designed system had a zoom feature, which can facilitate the measurement in variable field of view in a certain range. The feasibility of the system was verified by theoretical analysis and experiment, and the practicability was validated by testing the chip and circuit board. [ABSTRACT FROM AUTHOR]

Shearography is a very effective non-destructive testing method for detecting the internal defects of composite materials. How to improve the detection efficiency of this technique to help achieve the rapid and automatic recognition of multiple defects has always been a hot research topic for researchers. Based on the results of the shearography, we propose a method to identify multiple defects in the phase map using the spatial characteristics of Freeman chain code. First of all, we use Canny operator to binarize the phase map, then refine the fringe by morphological means, and finally identify the defects by the spatial characteristics chain code. In addition, Otsu is used to assist in judging the fracture or incomplete defects. For the fringes under vacuum loading or thermal loading, this method can detect the defects quickly and successfully. The theoretical description is given and the effectiveness and practicability of the method are verified by experiments. [ABSTRACT FROM AUTHOR]

• A solid method for generating shearography defect detection dataset is proposed. • Improved semantic segmentation network for unfiltered phase maps. • Real NDT experiments conducted to validate the method proposed in this study. Shearography combined with deep learning is increasingly becoming a commonly used approach in non-destructive testing (NDT). However, the validity of deep learning depends on the reliability of the training dataset, and the acquisition of real datasets is costly. In this study, the use of a shearography dataset that contains real noise-generating method is proposed. This dataset can generate random shearing direction and shearing amount defects only after one electronic speckle pattern interference experiment and annotation. Moreover, this study improves the existing semantic segmentation network and identify the defect location for the defect phase map containing noise accurately. Real NDT experiments using composite materials validate the effectiveness of the dataset generation method and semantic segmentation network proposed in this study. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a spatial phase-shift digital shearography system based on a modified Michelson Interferometer, which is capable of measuring out-of-plane deformation and its first derivative simultaneously. A reference beam is introduced into the Michelson Interferometer using an optical fiber. Two sheared images from the Michelson Interferometer are combined with the reference beam to generate two holograms and one shearogram. The Fourier Transform method is utilized to calculate the phase difference. To separate the spectrum in the Fourier Domain, the reference beam is tilted a small angle to create a spatial phase shift. An out-of-plane deformation and its first derivative can then be determined simultaneously from a single speckle interferogram by properly selecting filters for the spectrum image. The theory and the experimental results are presented. [Copyright &y& Elsevier]

• The shear amount and spatial carrier frequency are controlled independently. • The adjustment of shear amount is accurate. • The spatial phase technology improves the detection efficiency. Digital shearography has a wide application in defect detection. Phase-shift-technique-based shearography has a higher phase measurement sensitivity than the real-time shearography. This phase shift technique can be divided into temporal phase shift shearography and spatial phase shift shearography. The latter has the advantage of fast detection and can be used in dynamic detection. In a traditional spatial carrier frequency shearography system, shear amount adjustments and spatial carrier frequency are not completely independent. In some shearography systems, shear amount and spatial carrier frequency are adjusted by rotating mirrors. The amounts of rotation required to obtain the appropriate shear amount and spatial carrier frequency are different, thereby reducing the accuracy of detection results. In other shearography systems, although the two adjustments are divided into two parts, using a rotating mirror to obtain the appropriate shear amount will also introduce additional spatial carrier frequency which will also have some influence on the result. This paper introduces a detection method with independently adjusting shear amount and spatial carrier frequency. Shear amount adjustment is achieved by changing the written image of a spatial light modulator (SLM). No additional spatial carrier frequency is introduced. Spatial carrier frequency is only controlled by the relative position between the dual apertures. Experimental results of undamaged and damaged specimens indicate that this system is suitable for detecting the phase distribution of the deformation and internal defects. [ABSTRACT FROM AUTHOR]

Shearography has been widely used in non-destructive testing due to its advantages in providing full-field, high precision, real-time measurement. The study presents a pixelated carrier phase-shifting shearography using a pixelated micropolarizer array. Based on the shearography, a series of shearograms are captured and phase maps corresponding to deformation are measured dynamically and continuously. Using the proposed spatiotemporal filtering algorithm in the complex domain, the set of phase maps are simultaneously low-pass filtered in the spatial and temporal domains, resulting in better phase quality than spatial low-pass filtering. By accumulating the temporally adjacent phase, the phase corresponding to large deformation can be evaluated; thus, large deformations can be accurately measured and protected from speckle noise, allowing internal defects to be easily identified. The capability of the proposed shearography is described by theoretical discussions and experiments. [ABSTRACT FROM AUTHOR]