Shearography System Research Articles

Simultaneous measurement of six displacement gradients using Digital Shearography

Optical methods show significant promise in accurately measuring all components of the displacement gradient tensor in three-dimensional space, excluding those related to depth measurement. However, dynamic measurements using Digital Shearography face challenges due to the complexity of acquiring sufficient phase information in one capture. This paper introduces a novel Digital Shearography system with triple beams and spatial phase shift capabilities. It allows simultaneous measurement of in-plane and out-of-plane phase distributions in a single image. Three laser beams, each with distinct wavelengths, are strategically positioned both horizontally and vertically to illuminate the object under test. Utilizing beam splitting and redirection, the system achieves dual shearing directions and captures shearograms using a 3CCD camera. Phase information extracted via the spatial phase-shift method enables determination of the six measurable displacement gradients and corresponding strain components. The manuscript provides a comprehensive explanation of the system’s theory, supported by experimental results, and explores potential applications.

General education and training of NDT personnel, including NDT Education at Universities. in South Africa

Following the 16th World Conference in Durban, South Africa, The South African Institute for NDT (SAINT) was mandated to invest the monies made from the WCNDT to establish a professional qualification for NDT personnel. In South Africa, a professional qualification must comply with the national qualification framework (NQF) requirements, which stipulate the rules to assure that qualifications for different occupations are at the same level. For NDT this meant, that the qualifications to ISO 9712 (ISO TC 25107 syllabus) would not suffice to serve as a qualification. There exists a requirement that classroom training, classroom practical training and work integrated practical training should each make up at least 30 % of the work. Anybody who had a look at ISO 9712 will realise that 160 hours of classroom training and 180 days (1080 hours) practical training would not cut it, as the training content for the 1080 hours is not defined in any standard. This state of affairs required the definition (curriculum development) of what has to be taught and how. The paper will elucidate this outcome. Concerning university education in South Africa one has to mention the NDT diploma and advanced diploma in NDT offered by the Vaal University of Technology (VUT). These courses focus on teaching NDT theory across a broad spectrum. Students have to undergo work integrated learning (WIL) and then successfully pass the practical examination at NDT Level II in order to be certificated to perform NDT in line with codes and standards. The Tshwane University of Technology (TUT) offer a subject in NDT as part of the applied physics diploma, where the emphasis is on Nuclear Technology and Photonics. At the University of Cape Town (UCT) in the department of mechanical engineering, digital shearography is offered as a research field for master degree studies. They have a well-equipped optical laboratory for this purpose and have manufactured a shearography system which they support and improve on an ongoing basis. At the University Pretoria (UP) the subject Non-destructive Testing was offered at the post graduate degree level, as part of the course work for Asset Integrity Management. Most practical NDT technology development in South Africa is performed in industry at the company’s own costs. Some examples will be mentioned.

Super-compact shearography based on a single diffractive optical element with 3-in-1 phase mask.

This Letter communicates a new, to the best of our knowledge, designing framework of shearography. The three elementary functional parts of quantitative shearography, namely imaging, shearing, and phase shifting, are integrated into a single diffractive optical element (DOE), named a 3-in-1 phase mask. The idea breaks through the conventional designing routine of shearography, and converts it from the combination of individual optical elements to the spatial manipulation of phase. The slicing, splicing, and alternating strategy is proposed to generate the 3-in-1 phase mask from a given number of sequenced Fresnel lenses and a modified echelle grating. The operating component is merely a DOE, which renders the optics naturally coaxial. The delivered shearography system enjoys a super-compact configuration, a high level of robustness and stability, and the potential for implementing outside optics laboratories. Crucial system parameters, e.g., shear amount, shear direction, working distance, can be readily shifted on call by re-making the 3-in-1 phase mask. The future of the present idea is in its shape and seems promising with lithography, micromachining, and metasurfaces.

Phase estimation via riesz transform in laser speckle interferometry for large-area damage imaging

A direct phase estimation (DPE) via Riesz transform from a speckle pattern (images) for visualizing barely visible impact damage (BVID) occurring in layered composites is proposed in this paper. Rather than complex optical hardware setup for phase extraction in commercial digital speckle pattern interferometry (DSPI) or shearography (SG) systems, the proposed numerical (software) DPE implementation via Riesz transform with a log-Gabor filter enables a compact, portable, and cost-effective system for large-area real-time nondestructive inspection (NDI) due to its simplicity, stability, and efficiency.Using speckles in laser speckle interferometry (LSI) speckle pattern is intrinsically noisy; the correlation-based phase signatures derived from Riesz transform followed by the log-Gabor filter is implemented to supplant the conventional phase-difference algorithm to localize damage regions, especially in the case where the high-power diode laser (low coherence) can cause serious phase drift. By enveloping the monogenic signal through Riesz transform for direct phase retrieval is more intuitive than TPS or SPS. This yields visualization of the damage feature with higher fidelity. Consequently, a phase-correlation algorithm becomes a more robust operation to cope with speckle images from the interferometry.This full-field optical SG system was demonstrated and verified in an impacted damaged honeycomb composite panel using the proposed phase-correlation algorithm under thermal stress loading for large-area inspection in near real-time. The visualized damage images enhanced by principal component analysis (PCA) agreed with images from pulse laser/LDV and X-ray CT scan. The DPE algorithm has a promising potential to apply on any optical metrology for instantaneous phase estimation and can combine with selected imaging conditions for multi-damage detection.

Simulation Dataset Preparation and Hybrid Training for Deep Learning in Defect Detection Using Digital Shearography

Since real experimental shearography images are usually few, the application of deep learning for defect detection in digital shearography is limited. A simulation dataset preparation method of shearography images is proposed in this paper. Firstly, deformation distributions are estimated by finite element analysis (FEA); secondly, phase maps are calculated according to the optical shearography system; finally, simulated shearography images are obtained after 2π modulus and gray transform. Various settings in the parameters of object, defect, load and shearing in those three steps could prepare a diverse simulation dataset for deep learning. Together with the real experimental images taken from a shearography setup, hybrid trainings of deep learning for defect detection are performed and discussed. The results show that a simulation dataset, generated without any real defective specimen, shearography system or manual experiment, can greatly improve the generalization of a deep learning network when the number of experimental training images is small.

Review of Shearography for Dual-Directional Measurement

Shearography is a coherent optical technique that allows the identification of the first derivative of deformation in the shearing direction. Due to direct measuring strain information, shearography is suited for non-destructive testing and evaluation (NDT/NDE). However, if there is a small defect parallel to the shearing direction, the first derivative of deformation in the direction has no noticeable change, and the defect is not visible. Therefore, the development of a shearography system with dual-directional simultaneous measurement of the first derivatives of deformation both in x- and y-directions is highly demanded in the field of NDT/NDE. It is suited to inspect complicated defects, such as long and narrow slots, microcracks, etc. This paper presents a review of shearography for different dual-directional systems developed in the last two decades. After a brief overview of shearography, the paper will display two dual-directional shearographic techniques—temporal phase-shift (TPS) and spatial phase-shift (SPS) methods. TPS dual-shearing systems are suited for static measurements, while the SPS dual-shearing systems are useful for dynamic measurements. The basic theories, optical layouts, and comparisons are presented. The advantages and disadvantages of practical applications are discussed.

Robust dynamic phase-shifting common-path shearography using LCPG and pixelated micropolarizer array

A concise and dependable dynamic phase-shifting shearography system based on common-path interferometer is proposed. Two lateral sheared images with common optical path are generated using a liquid crystal polarization grating (LCPG). The sheared images of the object under test interfere with each other, and a pixelated micropolarizer array is used to introduce pixelated carrier, enabling the measurement of dynamic deformation using dynamic phase-shifting. Compared with the ordinary imaging system, the shearography system only needs to insert LCPG between an imaging lens and a camera with pixelated micropolarizer array. The system is resilient to environmental disturbances benefitting from the compact common-path and dynamic phase-shifting method. Theoretical analysis and experiments have been completed, showing that the system can fully exploit the advantages of shearography and improve the reliability of shearography applications.

Sagnac Interferometer Based Digital Shearography for Simultaneous Dual Sensitivity Measurement

This paper presents a single camera digital shearography system with dual sensitivity to enable simultaneous measurement of minor and large shearograms in a single field measurement. Digital shearography has many benefits, such as its non-contact measurement, high sensitivity and resistance to turbulence. It has been successfully used for testing of minor deformation gradients in nondestructive testing (NDT). Most digital shearography systems can only work in a single measurement field. As a result, minor shearograms may be overlooked, resulting in incomplete detection in a full-field measurement. The application must allow for multiple measurements in different fields of view or overloading to reveal small defects. The former increases costs, while the latter can result in huge surface deformation that may mask important information. A novel dual sensitivity shearographic system that can simultaneously identify minor and large shearograms in a single field measurement has been developed. The setup is based on the modified Sagnac Interferometer, which uses a polarized Michelson interferometer as a substitute for one of its three mirrors to create additional shearing channels. Two pairs of orthogonally polarized waves carry the large and small field of view information individually without cross-interference on the target screen. The carrier frequency spatial phase shift method is adopted to separate the signal on the frequency domain. The system's performance for dynamic imaging of an object with defects was experimentally investigated. Theory derivation and description of the setup are shown in detail in this paper.

A Synchronous Measurement System for the First Derivative of Out-of-Plane Deformation of Double Ends

In current practical applications, synchronously obtaining the information about out-of-plane deformation of double ends is often necessary. However, traditional measurement systems cannot achieve the synchronous measurement of double ends, as two sets of measurement systems are often needed. This study solves the problem by designing a synchronous measurement system for the first derivative of the double-end deformation based on shearography. The first derivative distribution of the double-end deformation is measured synchronically by imaging the two end surfaces in different areas of the camera’s target plane through different shear structures. In addition, by placing apertures in each Michelson shear structure, the synchronous dynamic measurement of both ends and the independent adjustment of the shear and spectral distribution in the Michelson shearography system are realized. The principle of the measurement system is introduced in detail, and the feasibility and practicability of the proposed system are verified by experiments.

Dynamic Wind Turbine Blade Inspection Using Micro-Polarisation Spatial Phase Shift Digital Shearography

Shearography, as a novel non-destructive evaluation technique, has shown notable ability in the detection of composite materials. However, in current shearography practices, the phase shifting and loading methods applied are mainly static. For instance, vacuum hood or force loading facilities are often used in phase-shifting shearography, and these are hard to realise with robotic control, especially for on-board inspection. In this study, a dynamic process for detecting defects in the subsurface of a wind turbine blade (WTB) using spatial phase shift with dynamic thermal loading was proposed. The WTB sample underwent a dynamic thermal loading operation, and its status is captured by a Michelson interferometric-based spatial phase shift digital shearography system using a pixelated micro-polarisation array sensor. The captured images were analysed in a 2D frequency domain and low-pass filtered for phase map acquisition. The initial phase maps underwent a window Fourier filtering process and were integrated to produce a video sequence for realisation of visualising the first derivative of the displacement in the process of thermal loading. The approach was tested in experimental settings and the results obtained were presented and discussed. A comparative assessment of the approach with shearography fringe pattern analysis and temporal phase shift technique is also presented and discussed.

A Compact Laser Shearography System for On-Site Robotic Inspection of Wind Turbine Blades

Shearography is an optical technique in the field of non-destructive evaluation (NDE) of various materials. Its main advantages are that it is non-contact type and can cover a large area in a single inspection. As a result, although it has been widely acknowledged as an effective technique particularly for NDE of composite materials to detect subsurface defects such as delamination, disbond, cracks and impact damages, the use of shearography for on-site inspection of wind turbine blades (WTBs) has not been reported. This is due to wind causing structural vibration in the WTB. The solution in this paper is to make the shearography sit on the WTB during inspection when the WTB is parked, so that the relative motion between the shearography and the WTB is minimized within the tolerance of the shearography system. The ultimate goal of the solution is to enable a robot assisted shearography system to inspect the WTB on-site. This paper presents the research work on a new shearography design for integration with a robotic climber for on-site WTB inspection. The approach is tested and evaluated in experimental settings, and comparative assessment of the approach with other robotic NDE techniques is carried out. The results demonstrate the potential benefits and suitability of the approach for on-site robotic inspection of WTBs.

Dual-lens system in enlarging the viewing angle for the Michelson interferometer.

Shearography has been widely accepted as a non-destructive evaluation tool in engineering. However, the field of view is usually limited due to the use of the Michelson optical arrangement if the working distance is strictly constrained. In order to improve efficiency, we propose a dual-lens system to expand the view angle of the Michelson optical arrangement. Two imaging lenses, which are entitled as the objective lens and camera lens, are placed in front of and behind the Michelson interferometer, respectively. The optical arrangement has been set up and compared with the well-known 4f optical system. It is found that the proposed optical arrangement is simple to construct, and the brightness distribution is uniform over the whole image. The new setup has been applied to the inspection of a cylindrical steel shell with a rubber plate bonding on its inside surface. The debonding defects have been evaluated using the proposed shearography system at a working distance of 200 mm. By selecting an objective lens with a focal length of 2.8 mm, the viewing angle was enlarged in inspecting a field with 500 mm by 500 mm at one shot.

Shearography and its applications – a chronological review

This paper presents the activities in the field of shearography in chronological order and highlights the great potential of this holographic measurement technology. After a brief introduction, the basic theory of shearography is presented. Shear devices, phase-shift arrangements, and multiplexed shearography systems are described. Finally, the application areas where shearography has been accepted and successfully used as a tool are presented.

Spatial-light-modulator-based dual shearing direction shearography.

We propose a dual shearing shearography system based on a spatial light modulator (SLM). Compared to spatial phase shift shearography, the advantages of this system include its simple structure, relatively high light efficiency, and good phase map quality. Digital shearography is a fast, practical, non-contact, whole-field, and anti-turbulent optical approach to non-destructive testing (NDT) and strain measurement. Because the shearing direction determines the strain direction being measured, tests using multiple shearing directions are sometimes required to obtain strain in different directions and detect all defects. Various setups, based on the spatial phase shift method, have been proposed to solve the issue. While some of these setups perform well, they may also introduce new problems, such as poor phase map quality and low light efficiency. We present a sequential dual shearing shearographic system with good phase map quality and high light efficiency. Due to the SLM's high-speed response, capable of reaching hundreds of hertz, SLM-based dual shearing direction shearography allows for fast temporal phase shifting and shearing direction switching while providing very good phase map quality. Unlike the spatial phase shift method, which has low light efficiency due to its need for a small aperture to enable a relatively large speckle size to cover multiple pixels, the proposed method is based on a fast temporal phase shift and does not have this limitation. In addition, SLM can provide a programmable and adjustable shearing method in any direction and distance, which is beneficial for strain measurements and NDT requiring strain measurements in different directions using a small and precise shearing distance. We describe in detail the theory derivation and non-destructive testing application results for the SLM-based dual shearing direction shearography system.

Panoramic dual-directional shearography assisted by a bi-mirror.

A panoramic dual-directional shearography system is proposed to simultaneously determine out-of-plane deformation derivatives in two directions and globally inspect the object to be tested. A dichroic filter (DF), a 3CCD camera, and dual-wavelength light are used in the proposed shearography configuration. The dual-wavelength light coupled with the corresponding imaging sensors of the 3CCD camera provides independent color signals and shearograms. Through adjustment of the tilted stereo-angle of the DF, which offers a second wavelength-dependent measurement, an additional independent image-shearing can be introduced into the setup. The auxiliary bi-mirror surrounding the object helps to fully illuminate the object surface and capture it in a single shot. Theoretical analysis and experimental results demonstrated the utility of the system.

Multi-directional shearography based on multiplexed Mach–Zehnder interference system

ABSTRACTShearography is only sensitive to deformation in the shearing direction, and the deformation of object defects after loading may occur in multiple directions. This work reports a multi-direction shearography system that uses spatial phase-shift methods to detect the object from multi-shearing directions, effectively avoiding the missed detection of directional defects. A single laser is utilized to illuminate an object, and a single CCD camera records images in the multiplexed Mach–Zehnder interference system. First, the aperture stops in suitable size and location are set to produce different spatial carrier frequencies. Second, the shearing amount is independently adjusted by using different devices. Finally, the Fourier transform method is used to extract the phase information from the frequency domain. This system can be used for nondestructive testing of multi-directional defects and the feasibility of the method is verified by theoretical analysis and experiment.

A spatial-phase-shift-based defect detection shearography system with independent adjustment of shear amount and spatial carrier frequency

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.

Spatial phase-shift digital shearography for simultaneous measurements in three shearing directions based on adjustable aperture multiplexing

Because the sensitivity of shearography is determined by its shearing direction, aeolotropic defects could be difficult to detect in nondestructive testing (NDT) using a digital shearography system with a single shearing direction. We report an adjustable aperture multiplexing spatial phase-shift digital shearography system for simultaneous measurement of displacement derivatives in three directions. By setting the aperture parameters properly, three spatial carrier frequencies are produced within the speckle pattern, and the information of three interferograms can be separated in the frequency domain. Phase maps of the three shearograms can be obtained by applying a windowed inverse Fourier transform, which enables simultaneous measurement of the displacement derivative. The system is suitable for NDT with a dynamic load. The capability of the triple-directional spatial phase-shift digital shearography system is described by theoretical discussions as well as experiments.

Real-time dual-sensitive shearography for simultaneous in-plane and out-of-plane strain measurements.

A real-time, dual-sensitive shearography system using a single-wavelength laser was developed for simultaneous and dynamic in-plane and out-of-plane strain measurements. The shearography system is capable of measuring crack-tip deformation fields quantitatively. A spatial multiplexing technique based on Fourier transform is employed for simultaneous and dynamic multi-component phase retrieval. Two slit spatial filters and a common-path shearing interferometer are used to obtain an improved phase quality for crack-tip deformation measurements. Mode-I fracture experiments under three-point bending were conducted to validate the feasibility and the capability of this method.

Dual-directional shearography based on a modified common-path configuration using spatial phase shift.

This paper describes a dual-directional shearography system to address the issue of two-dimensional characterization of the surface strain. A common-path configuration coupled with an additional light path is used to provide the shearing in two directions. One of the three interfering beams is shared by both directional shearograms to improve the light efficiency and enhance the robustness of the system. The two directional shearograms are carried by different spatial carriers to distinguish one from the other. The spatial carrier is introduced by the single-aperture-lens Wollaston prism configuration. Rather than the conventional method in which the aperture is fixed at the front focal point of the imaging lens, a general case is considered by introducing a variable distance between the aperture and the imaging lens. The influence of the aperture-lens distance on the spatial carrier is then analyzed, which enables the separate control of the shearing amount and the spatial carrier. Two types of dual-directional shearography are presented to demonstrate the feasibility and the flexibility of the system. Type I is the simultaneous dual lateral shearography in orthogonal directions, and Type II is the simultaneous lateral and radial shearography. The spatial carrier introduced by the single-aperture-lens Wollaston prism configuration is discussed, and a configuration in which the Wollaston prism and the aperture are located at different sides of the lens is recommended for further shearography applications.