Laser Shearography Research Articles

Hybrid Inspection Method using Three Dimensional Scanning, Lock-in Thermography and Laser Shearography

The proportion of composite materials (such as CFRP) to the metal used on the modern aircrafts is rising, imposing different kind of failure modes. Since the composite structures are known to be sensitive to the impact loading, there is need for means to assess the sub-surface damage in the structure rapidly. Royal Netherlands Aerospace Centre NLR has an extensive track record on contactless non-destructive inspection (NDI) methods based on optical sensors, such as 3D surface scanning, lock-in thermography and laser shearography. The combination of these methods (multi-domain inspection) enables us to assess the structural integrity of an aircraft outer surface in a short time, reducing inspection costs and the “down time” of the aircraft. Recently, NLR is working towards a 3D oriented mesh environment of an object, enhanced with NDI data, providing sub-surface damage information. By automatically stitching the 3D object images, it is possible to expand this method for a complete scan of an aircraft surface. Furthermore, the thermographic and shearography imaging information has been integrated into the 3D surface accounting for the image distortion from the different measurement angles. In this paper, the results from the various studies will be presented involving integration of the 2D measurements with the 3D scan mesh. Keywords: Thermography, Shearography, 3D scanning, Data fusion

Viewpoint Generation Using Geodesics and Associated Semi-Automated Coverage Path Planning of Panels for Inspection

The coverage of a surface using multiple viewpoints is a topic of great interest for robotic path planning in inspection applications. Two approaches for coverage path planning are broadly addressed in the literature—geometric methods and optimization methods. While the optimization methods may be the most flexible, they are frequently difficult to implement in practical applications due to their NP hard nature. We present here a geometric algorithm for the coverage path planning of panels used for aerospace applications using a generic camera model that can represent area inspection techniques like thermography and laser shearography. This algorithm relies on drawing a 2D grid on the 3D surface of the panel using geodesic lines on the surface. The coverage of the surface is performed by propagating geodesic lines from a starting point until the patch thus covered diverges too much from a flat surface, and after that, the coverage is continued from another point. The propagation of the geodesic lines is stopped when they begin to converge or diverge, and we define two criteria for the stoppage. We show that the proposed algorithm has good results for 3D virtual models and emphasize its speed, simplicity, and reliability for such applications.

Infrared-Induced Laser Shearography: Enhanced Multimodal Features Recognition for Interfacial Defects in SIR/GFRP Composite Structures

Infrared-Induced Laser Shearography: Enhanced Multimodal Features Recognition for Interfacial Defects in SIR/GFRP Composite Structures

Review And Future of Non-Destructive Testing Methods for Composites in Aircraft

The application of carbon fiber in aerospace requires more advanced non-destructive testing methods due to its unique properties such as anisotropy. In this paper, mainstream non-destructive testing methods are introduced, including visual inspection, resonance test, acoustic emission, eddy current test, ultrasonic test, laser shearography, infra-red thermography, and X-ray. Their strengths and weaknesses are compared, and the common challenges are exposed. To solve those problems, some developing techniques such as hybrid methods and deep-learning-based automated inspection are proposed.

A simplified method for mapping samples with a fringes pattern in laser shearography

A simplified method for mapping samples with a fringes pattern in laser shearography

Viewpoint planning and 3D image stitching algorithms for inspection of panels

Non-Destructive Testing using optical methods such as Thermography and Laser Shearography are gaining importance in the inspection of aerospace composite structures due to their non-contact, area based and non-hazardous nature. Here we address the problem of fully covering a curved panel using a robotic system which holds the NDT system up against the panel, and stitching the acquired images together so that the images with defects can be visualized in a single model. A formal way of defining view-points in 3D based on the structure’s 3D model and stitching the gathered image data back onto the 3D model has been lacking in the literature. We present algorithms for defining a set of viewpoints for robotic systems using Area based NDT and for doing full 3D image stitching on the pointcloud generated from the 3D model of the structure. A virtual model or digital twin of the inspection process taking into account the details of the structural panel, robot and camera model is developed and used for getting inputs from the user for the viewpoint planning process as well as displaying the output after image stitching. We show excellent results for synthetic image capturing process as well as good results while using a prototype 5-axis cartesian robot with an IP camera to represent the NDT system.

A Past, Present, and Prospective Review on Microwave Nondestructive Evaluation of Composite Coatings

Recent years have witnessed an increase in the use of composite coatings for numerous applications, including aerospace, aircraft, and maritime vessels. These materials owe this popularity surge to the superior strength, weight, stiffness, and electrical insulation they exhibit over conventional substances, such as metals. The growing demand for such materials is accompanied by the inevitable need for fast, accurate, and affordable nondestructive testing techniques to reveal any possible defects within the coatings or any defects under coating. However, typical nondestructive testing (NDT) techniques such as ultrasonic testing (UT), infrared thermography (IRT), eddy current testing (ECT), and laser shearography (LS) have failed to provide successful results when inspecting composite coatings. Consequently, microwave NDT techniques have emerged to compensate for the shortcomings of traditional NDT approaches. Numerous microwave NDT methods have been reported for composite coatings inspection. Although existing microwave NDT methods have shown successful inspection of composite coatings, they often face several challenges, such as low spatial image quality and extensive data interpretation. Nevertheless, many of these limitations can be addressed by utilizing microwave NDT techniques with modern technologies such as soft computing. Artificially intelligent techniques have greatly enhanced the reliability and accuracy of microwave NDT techniques. This paper reviews various traditional NDT techniques and their limitations in inspecting composite coatings. In addition, the article includes a detailed review of several microwave NDT techniques and their benefits in evaluating composite coatings. The paper also highlights the advantages of using the recently reported microwave NDT approaches employing artificial intelligence approaches. This review demonstrates that microwave NDT techniques in conjunction with artificial intelligence approaches have excellent prospects for further enhancing composite coatings inspection and assessment efficiency. The review aimed to provide the reader with a comprehensive overview of most NDT techniques used for composite materials alongside their most salient features.

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.

Procedure for detecting the shape and size of defects on metallic substrates under composite repairs using shearography.

This paper presents a numerical-experimental procedure to characterize through-holes with arbitrary shapes present on metallic substrates under composite material sleeves using pulsed laser shearography and dynamic excitation combined to the finite element method. The so-called fitting process consists of matching experimental and numerical results in order to determine the shape and dimensions of the holes under the composite repair, or even quantify defects between layers of the composite laminate. The results show that the technique is capable of characterizing, in the worst case, the geometry of a hole with 83% accuracy and its respective area with a maximum error of approximately 20%. The advantageous results achieved in this research show that the fitting process can be very useful for real applications in the oil and petroleum industry.

Ultrasonic Welding of Glass Fiber Reinforced PP Thermoplastic Composites: An Investigation of the Outer Layer Orientation and the Fiber Volume Fraction

This paper presents an experimental study of the influence of the orientation of the outer layer of polypropeylene (PP) reinforced with E-glass fiber laminate (GF/PP) and the influence of the fiber volume fraction on the quality of the welded joint using an ultrasonic welding process. An orthogonal L 16 array (OA) design of experiment was conducted in this paper based on the Taguchi method to evaluate the effect of the orientation of the outer layer and the fiber volume fraction, on the welding process parameters; the welding energy, the amplitude of vibration, the welding pressure, the holding pressure and the holding time were considered in order to achieve a high weld quality. The experiments were carried out using a 15 kHz ultrasonic welding unit with a maximum supplied power of 4000-Watt. GF/PP laminates with fiber volume fraction of 36% and 46% were used in this paper, and the GF/PP laminates were either unidierctional or had a 90 degree outer layer orienation. A 0.127 mm thick polypropeylene film was used as a flat energy director (ED). The evaluation of the weld quality was measured by the apparent shear strength of the single lap welded joints, and by using laser shearography as a non-destructive inspection technique . The failure mechanism of the single lap joint was monitored, using a high speed digital imaging system. A combination of the highest selected level of welding energy, lowest level of amplitude, lowest level of welding pressure, and the lowest level of both hold time and hold pressure of a unidirectional GF/PP with the lowest fiber volume fraction, were found to achieve a higher apparent shear strength of the welded adherends, as compared with the apparent shear strength obtained with the presence of the flat energy director for the same level of factors. A confirmation experiment was conducted to measure the predicted apparent shear strength and compare it with the measured apparent shear strength from the test.

Imaging techniques for defect detection of fiber reinforced polymer‐bonded civil infrastructures

The present article provides a state-of-the-art review of imaging techniques used for defect detection of fiber reinforced polymer (FRP)-bonded civil engineering structures. Compared to the conventional techniques by equipping a structure with stress wave sensors, the contactless imaging techniques feature efficient instrumentation, convenient data acquisition, and rapid evaluation in the procedures of nondestructive testing (NDT). Recently, a substantial progress in utilizing electromagnetic waves for development of imaging NDT techniques (e.g., synthetic aperture radar, infrared thermography, laser shearography, and laser reflection technique) with the purpose to identify debonding or delamination of FRP-bonded structural systems has been made. As another electromagnetic wave-based imaging technique, X-ray computed tomography is promising for exploration of the damage evolution in this structural system, despite little application in structural health monitoring of real infrastructures. Apart from these imaging techniques, more recently, there have been computer-aided motion magnification techniques for defect identification. The motion magnification technique only requires a digital camera and a computer with vision algorithm, which can amplify the motion of defect region and render it visible in a video. This advanced imaging technique achieves high-resolution measurement, simultaneous full-field inspection, and straightforward defect identification in the scene. In this review, both Eulerian motion magnification and phase-based motion magnification techniques for structural visual inspection are presented and discussed. Furthermore, the present article recommends the combination of imaging NDT techniques with artificial intelligence approaches (e.g., deep learning algorithms) to realize the automated and efficient defect detection towards FRP-bonded civil infrastructures.

Nondestructive Visualization and Quantitative Characterization of Defects in Silicone Polymer Insulators Based on Laser Shearography

Effective defects detection method for silicone polymer insulators is desperately needed to eliminate defective insulators in factory examination and make early warnings for accidents of in-service insulators. This paper investigates a laser shearography method that reveals surface deformation anomalies induced by internal defects after applying thermal excitation. Samples with artificial defects of different sizes and depths in the silicone rubber sheath were tested. Both fringe patterns and phase distribution maps were generated after the experiments for defects identification by indicating the displacement derivative with respect to the shearing direction. The relationship between the defect parameters and the relative phase difference was discussed, and a parameter named detection sensitivity was defined to evaluate the system detection ability on different defects. Finally, Canny edge detection algorithm and proposed image gradient method were employed to quantitatively estimate the defect size, and the ability of identifying defects was also improved. With its noncontact testing, full-field measurement, real-time imaging, and visual results, laser speckle imaging would have quite promising applications in nondestructive testing of silicone polymer insulators and it can be extended for other composite materials insulating equipment.

Capability of non-destructive techniques in evaluating damage to composite sandwich structures

PurposeDefects can be caused by a number of factors, such as maintenance damage, ground handling and foreign objects thrown up from runways during an in-service use of composite aerospace structures. Sandwich structures are capable of absorbing large amounts of energy under impact loads, resulting in high structural crashworthiness. This situation is one of the many reasons why sandwich structures are extensively used in many aerospace applications nowadays. Their non-destructive inspection is often more complex. Hence, the choice of a suitable non-destructive testing (NDT) method can play a key role in successful damage detection. The paper aims to discuss these issues.Design/methodology/approachA comparison of detection capabilities of selected C-scan NDT methods applicable for inspections of sandwich structures was performed using water-squirt, air-coupled and pitch-catch (PC) ultrasonic techniques, supplemented by laser shearography (LS).FindingsTest results showed that the water-squirt and PC techniques are the most suitable methods for core damage evaluation. Meanwhile, the air-coupled method showed lower sensitivity for the detection of several artificial defects and impact damage in honeycomb sandwiches when unfocussed transducers were used. LS can detect most of the defects in the panels, but it has lower sensitivity and resolution for honeycomb core-type sandwiches.Originality/valueThis study quantitatively compared the damage size indication capabilities of sandwich structures by using various NDT techniques. Results of the realised tests can be used for successful selection of a suitable NDT method. Combinations of the presented methods revealed most defects.

Shearography Technique on Inspection of Advanced Aircraft Composite Material

The A320 aft fixed fairing is an advanced composite structure fabricated from laminate carbon woven and combines with honeycomb core which is externally mounted on the flap track fairing as a wing part of Airbus aircraft. These flaps are movable control surfaces to widen up the surface of the wing in order to increase the lifting. This paper discussed on the inspection of A320 aft fixed fairing by using shearography NDT technique. Shearography technique reveals the stress-affected zone due to additional loading that can be realized by the laser speckle correlation on the inspected composite panel. This A320 aft fixed fairing is identified as a scrap composite panel which having different sizes and orientations of defect. The results showed that the changes of shearing direction in laser shearography technique are an effective means to reveal the defects propagation in the composite component. The shearing at Y-axes direction is able to detect the cracks at horizontal orientation while shearing at X-axis has successfully detects the cracks at vertical orientation. Therefore, shearography could be utilized as an alternative inspection for other NDT method for inspection of composite material.

A novel method to produce kiss-bonds in composites components for NDI and characterisation purposes

Kiss-bonds (kissing bonds) are a defect type that feature a localised loss of structural continuity within the material, yet the material remains in intimate contact across the defect. Typically, shear and normal tensile stresses cannot be conducted across such defects (although, pure compressive stresses are possible). Kiss-bond defects are difficult to detect reliably – both within the bulk of the material (interlaminar) and within bond-lines of adhesively bonded joints or repairs – using conventional Non-Destructive Inspection (NDI) techniques. Compounding this issue is the lack of a reliable technique to create representative kiss-bond defects in a controlled fashion for the purpose of NDI equipment calibration or development, or scientific investigation.A novel method for manufacturing composite material test panels with kiss-bond defects (for research or NDI calibration, for example) in a controlled and repeatable fashion has been developed. Small areas of two adjacent pre-preg plies were pre-cured before being incorporated within a laminate. During consolidation, no bonding occurs between the pre-cured areas, thus creating a kiss-bond defect of known geometry. Test panels with 6 × 6 mm and 10 × 10 mm kiss-bond defects were manufactured. The robustness of the technique was verified using ultrasonic and laser shearography NDI methods; 7 of the 10 manufactured defects were classified as kiss-bonds, with the remaining 3 identified as dis-bonds.

The Effectiveness of Laser Shearography for the Inspection of Wall Thinning in a Large Aluminum Plate

Laser shearography is a powerful optical tool that is able to directly measure the displacement derivatives using interferometric effects. Stroboscopic shearography provides a full field, reliable and repeatable inspection process that facilitates defect visualization. The aim of this paper is to investigate the effectiveness of laser shearography for inspection of a large aluminum plate with a large wall thinning. A modified Michelson interferometer is introduced in order to inspect the entire aluminum plate simultaneously. The wall thinning is visualized as the region of the anomaly in the quasi-uniform and homologous shearography fringe pattern. The boundaries of the wall thinning cannot be indicated precisely due to the nature of the formation of shearography fringes. Therefore, the small shearing method is applied to determine the dimensions of the wall thinning. This approach allows for the detection of the wall thinning in the wider range of excitation frequencies. The wall thinning is successfully detected using the small shearing method as a light rectangle in the inconspicuous background. Finally, a novel averaging approach is proposed to eliminate the effects of noise and uncertainty in the calculation of the boundaries. Using this approach, the shearography interferometric effects are significantly reduced.

Interference Information Processing Technology for Defects of Aluminum Honeycomb Structure in Aircraft

In order to get the interference information extraction aircraft aluminum honeycomb structure defect extraction, reliability of testing. This paper uses image processing technology to find a method about image processing composite, with the help of laser shearography detection method detecting detection. The flaw speckle results show that, after Fourier filtering, unpacking of step by step, linear trend processing of the image, gray scale processing method can extract powder composite suited interference information, effectively improve the reliability of detection of the scattered laser displacement suited, provides a new image processing program to help speckle image processing.

Study on Laser Speckle Detection Characteristics of Aircraft Aluminum Honeycomb Structure

In order to detect the aircraft aluminum honeycomb structure defects quickly and accurately, improve the detection reliability, this paper uses the orthogonal optimization theory, using laser shearography detection method detectes the specialized design and manufacture of specimens through experimental data, combined with the theoretical analysis and calculation, find the main influencing factors of detection ability is the heating time, followed by the minimum phase shift modulation, and finally gain setting. In this paper, the optimization scheme of laser misplacement speckle interference detection is provided, which provides a theoretical basis for test parts detection.

Identification of an effective nondestructive technique for bond defect determination in laminate composites—A technical review

Laminate composites are commonly used for the production of critical mechanical structures and components such as wind turbine blades, helicopter rotors, unmanned aerial vehicle wings and honeycomb structures for aircraft wings. During the manufacturing process of these composite structures, zones or areas with weak bond strength are always issues, which may affect the strength and performance of components. The identification and quantification of these zones are always challenging and necessary for the mass production. Non-destructive testing methods available, including ultrasonic A, B, and C-Scan, laser shearography, X-ray tomography, and thermography can be useful for the mentioned purposes. A comparison of these techniques concerning their capacity of identification and quantification of bond defects; however, still needs a comprehensive review. In this paper, a detailed comparison of several non-destructive testing techniques is provided. Emphasis is placed to institute a guideline to select the most suitable technique for the identification of zones with bond defects in laminated composites. Experimental tests on different composite based machined components are also discussed in detail. The discussion provides practical evidence about the effectiveness of different non-destructive testing techniques.

Non-destructive Methods for Damage Assessment of Composite Sandwich Structures

Sandwich structures are capable of absorbing large amounts of energy under impact loads which results in high structural crashworthiness. Comparison of detection capabilities of selected C-scan NDT methods applicable for inspections of sandwich structures was performed using water-squirt, air-coupled and pitch-catch ultrasonic techniques, supplemented by laser shearography. Test results have shown that water-squirt and pitch-catch techniques are the most suitable methods for the core damage evaluation. Air-coupled method showed lower sensitive for detection of some artificial defects and impact damages in honeycomb sandwiches when unfocused transducers were used. The combination of the presented methods was able to reveal most of the defects.