Assessing the structural integrity of cultural heritage objects is of great importance for their structural conservation and long-term preservation. This paper focuses on the development of a non-destructive inspection (NDI) approach using 3D shearography to evaluate the structural integrity of wax-resin lined paintings, specifically for The Night Watch (1642), a large-format 17th-century canvas painting by Rembrandt van Rijn (1606–1669) that is on display in the Rijksmuseum, Amsterdam. The Night Watch has a complex treatment history that has many old repairs of structural defects and damages (holes, tears, etc.) and three wax-resin relinings. In 2021, before a new structural intervention involving retensioning of the canvas support, it was vital to evaluate the structural integrity of the painting, specifically the condition of the treatment carried out in 1975–76 when, among other actions, several long cuts in the area of Captain Frans Banninck Cocq's breeches were repaired and an old canvas insert in the drum was replaced. To assess the structural condition, we applied 3D shearography to quantitatively analyse the in- and out-of-plane surface strains with controlled thermal loading. First, a safe loading procedure was developed by inspecting a representative wax-resin lined test painting where reference delaminations and structural repairs to canvas supports were reliably identified with 3D shearography by raising the temperature with 1-2 °C. As part of Operation Night Watch, in November 2021 an in-situ investigation was carried out in the Rijksmuseum gallery. Two areas of interest in The Night Watch, the restored slashes in the Captain's breeches (0.5 × 1 m) and the canvas insert in the drum (0.2 × 0.5 m), were inspected from the reverse of the painting. Results revealed no critical structural problems associated with the repaired slashes, nor with adhesion of the lining. For the patched canvas in the drum, it showed higher in- and out-of-plane strain variations. Overall, 3D shearography provided valuable non-destructive inspection results for assurances regarding the structural integrity of the 1975 repairs and the adhesion of the lining canvas in The Night Watch.

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Composite materials, e.g., carbon fibre-reinforced polymers (CFRPs), have been increasingly adopted in safety-critical structures across different industries. However, various defects including delaminations and fibre breakage can occur in the composite structures that may endanger the whole structure severely. Therefore non-destructive testing (NDT) of composites is critical to ensure structural integrity and safety. It is important to advance the capabilities of NDT of composite materials towards early-stage damage, e.g., small defects of millimetre scale, to avoid future failure. The objective of this work is to study the detection of small defects in CFRP laminates using shearography with thermal loading. In this paper, a thermal-mechanical FEM model was established in Abaqus to assist shearography inspection of the composite laminate. This FEM assistance is capable of evaluating different thermal loading schemes for defect detection. A rational selection of the reference and signal interferograms from the heating/cooling sequence is determined for reliable defect detection. We will present both experimental and numerical results on the detection of small defects in CFRP.@en

Thick glass fiber-reinforced polymer (GFRP) composites, e.g. thickness of more than 50 mm, are increasingly used in a wide variety of industries, particularly in the marine and wind energy sectors. Defect detection and characterisation in these composites remain appealing challenges due to the material complexity and the presence of various manufacturing and in-service defects. In this study, we propose a novel shearography method with controlled surface temperature (CST) heating for deep defect detection (i.e., 15 mm depth and more) in thick GFRP laminates. The proposed CST heating has been developed based on analytical solutions to control the maximum surface temperature of a test object during shearography inspection. Numerical and experimental studies have been performed to analyse the defect behaviour and defect detection under various heating scenarios, a topic which is rarely reported for thick composites with shearography. Compared with conventional shearography, the CST shearography method maximises heating energy input with a controlled and stable maximum surface temperature for deep defect detection. Results indicate an enhancement of about 27% in defect signal for the defect at 15 mm depth in comparison to conventional heating. The results also provide insight for implementing an efficient inspection in terms of the inspection duration and the number of datasets. This study makes a step towards safe, quantitative and predictable inspection of deep defects in thick composites.

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The assessment of the structural condition of cultural heritage objects is important for conservation interventions and their long-term preservation. This investigation concerns The Night Watch (1642), a large-format 17^th-century canvas painting by Rembrandt van Rijn that is on display in the Rijksmuseum, Amsterdam. This painting, which has a complex treatment history, has various damaged areas and has undergone three wax-resin relinings. In 1975 the canvas was slashed twelve times with a serrated dinner knife, including several long slashes in the area of Captain Frans Banninck Cocq’s breeches. In 2021, prior to a proposed new structural intervention involving retensioning of the canvas, it was important to evaluate the structural condition of the repaired slashes and of another repair, specifically an old canvas insert in the drum. For this, an in-situ inspection was carried out in the Rijksmuseum as a part of Operation Nightwatch. 3D shearography instrument with thermal loading was used to inspect these two areas of interest on the reverse of The Night Watch. The results showed that the out-of-plane strain in the breeches does not show any large deviations, which alleviated conservators’ concerns about the adhesion of the lining canvas and stability of previous repairs in this region. The patch in the drum showed higher out-of-plane strain variations. This was explained by the lower quality of the patched canvas compared to the repaired slashes in the breeches of Banninck Cocq. Overall, 3D shearography provided valuable inspection results for assurances regarding the structural integrity of the 1975 repairs and the wax-resin lining in The Night Watch, reducing the risks and providing the confidence to proceed with the planned retensioning of the canvas.

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The performance of defect detection in composite materials using digital shearography is important for correct decision-making in non-destructive testing. In this work, we compared a high-resolution 24-megapixel digital still camera (DSLR) and a conventional medium-resolution 5-megapixel camera to determine the detectability of blind holes in an aerospace-graded carbon-fiber reinforced polymer (CFRP) sample. The hole diameters ranged from 0.2 to 3 mm with a material thickness of 4 mm and the test sample dimensions of 200×200 mm. The sample was heated and observed from the front (defect-free side) by three halogen lamps for 5 minutes in pulsed heating mode. Speckle interferograms were acquired during the heating and cooling phases from both cameras simultaneously using identical shearing interferometers and shearing distances. Phase maps were calculated using the 4+4 temporal phase step algorithm and then unwrapped. Further, defect-induced deformation (DID) phase maps were obtained by polynomial curve fitting. The DID phase maps obtained from the two cameras were compared. Blind holes with diameters up to 1 mm were detected, which are one of the smallest defects detected with shearography and reported in literature. In addition, the DLSR camera was able to detect holes of 0.8 mm in diameter. We observed that nearly comparable detection capabilities were obtained from both cameras, even though the spatial resolution of the second camera (DLSR) was 5 times higher. Possible reasons of this limitation include effects such as fiber-related deformation in CFRP and speckle noise.

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The colour of the ground layers of a painting has an influence on its visual appearance. In addition to the commonly used white ground layers, other colour ground layers have been used, for example, the grey ground layer used in Peter Paul Rubens’s painting Portrait of Clara Serena Rubens helps the colour transition of the skin tones. Understanding the effects caused by the colours of the ground layers is of significance for both technical art history and conservation. Optical non-destructive testing (NDT) techniques are useful tools for the investigation of paintings, for example, optical coherence tomography (OCT) can be used to study the surface and subsurface layers non-destructively. In this work, the interaction of light with paint and ground layers is modelled to supplement OCT measurements of paintings with ground layers. A previously described near-infrared light range OCT system provides high spatial and depth resolution measurements. A four-flux model has been developed for analysing the light interaction in the paint and ground layers. This model considers forwards-propagating collimated light, backwards-propagating collimated light, forwards-propagating diffuse light and backwards-propagating diffuse light. The model uses the optical material properties, including refractive index (RI), absorption and layer thickness, as input. This paper describes the construction of the model and an evaluation of its performance by comparison with OCT data.

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The use of thick composites and sandwich structures is increasing rapidly in marine, aerospace, and wind energy industries [1–3]. For example in the marine sector, sandwich structures consisting of glassfiber laminate skins bonded to a foam core are attractive because of the advantages of being light-weight, resistant to corrosion and underwater shocks, and cost-effective [4]. The thickness of these structures can be more than 50 mm. Nevertheless, various defects including delaminations and fiber breakage tend to occur in thick composites because of material complexity. These defects can arise from extreme loads such as impact and blast and can degrade material properties and structural integrity significantly. Hence, it is important to advance non-destructive testing (NDT) towards composite structures of significant thickness. The objective of this study is to perform shearography NDT of a large-scale thick composite structure, specifically a composite ship hull section in a shipyard environment. Shearography is a full-field and non-contact optical NDT method. It reveals defects by comparing two states of deformation of a test object. By applying a suitable loading, the defects can be revealed by looking for defect-induced anomalies in fringe maps or phase maps, which can be related to surface strain components. The composite ship hull section is a RAMSSES (www.ramsses-project.eu) demonstrator at Damen Shipyards. Before shearography inspection, multiple impact tests surpassing helicopter emergency landing loads (https://vimeo.com/522716506) have been performed on the hull shell and its composite helicopter deck for proving the resilience of composites to harsh marine environments. We will present our experimental results on shearography inspection of the impact damage in this large-scale composite structure. A total area of about 1×1.5 m2 was inspected by stitching six fields of view of 0.6×0.6 m2. Different heating scenarios including step heating as well as a mechanical loading were performed for shearography NDT. A brief comparison between thermal loading and mechanical loading on thick composite inspection with shearography will also be reported. Our previous work with a 51 mm thick marine laminate [5] showed that defects at 5 to 20 mm depth can be detected successfully using shearography with thermal loading. Here we aim at bringing the technique out of the laboratory and extending shearography to applications to composites with a thickness of more than 50 mm. @en

This study aims at improving shearography non-destructive testing (NDT) of deep defects in thick composites with thermal loading. Instead of conventional global heating (GH), the core idea is to apply novel spatially modulated heating (SMH) for shearography NDT. In this paper, the finite element method (FEM) has been used to advance shearography towards a quantitative inspection tool for thick composites. Both GH and SMH have been performed experimentally and modelled in Abaqus to evaluate the corresponding efficacies in the detection of deep defects. SMH was achieved by using a halogen lamp with a Fresnel lens. The heat flux distribution on the specimen surface was taken into consideration for defect detection, a factor which is rarely reported in shearography inspection. Besides, the influence of different reference states on shearography NDT of deep defects in thick composites has also been studied by looking into the defect-induced phase maps from shearography. The results indicate that the proposed SMH can improve deep defect detection with shearography in thick composites by 2 to 3 times that of GH. It should be addressed that a similar and defect-free reference sample is currently necessary to compare with a defective one.

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Thick composite materials are commonly used as load-bearing structures in marine applications. Developing a suitable and sophisticated non-destructive testing (NDT) method for thick composites is an urgent challenge to improve the safety, reliability and maintenance of these structures. Digital shearography has become an important NDT technique for detecting defects in thin composite materials because of the advantages of high sensitivity to deformation change, and whole-field measurement. So far, the efficacy of shearography for thick composite inspection (e.g. thickness as more than 50 mm) has not been fully characterised. This paper combines finite element methods (FEM) and experimental tests to investigate the defect detection capabilities of shearography for inspecting thick glass fiber-reinforced polymer laminates. A thermal–mechanical model was established by computing equivalent thermal and mechanical properties and was evaluated by experimental shearography testing. In order to reliably simulate major defects in thick composite, flat bottom holes were manufactured in the specimen. Both simulations and experiments show that shearography is a promising technique to inspect thick composites. The thresholds for defect-induced phase change and the corresponding defect-induced deformation are determined for shearography testing of thick composites in this paper. Afterwards, the effect of mechanical boundary conditions on defect-induced deformation is studied by FEM.

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With the increasing application of thick composites in marine, wind energy and aerospace industries, the inspection of thick composites becomes more and more challenging when considering the variety of thick structures (e.g., laminate, sandwich, honeycomb structures). Shearography is a full-field and non-contact optical non-destructive testing (NDT) method which is normally used to inspect composite laminates up to 10 mm while for the thick composite laminates (e.g., with the thickness of more than 50 mm), its performance is not clear yet. In shearography NDT, a defect-induced anomaly is revealed from fringe or phase maps obtained by comparing two states of deformation of the specimen to be inspected. Thermal loading is widely used to deform the specimen due to its advantages of convenience for on-site inspection and cost-effectiveness. The objective of this study is to improve the defect detection capabilities of shearography when used to inspect thick composites. For that, spatial modulated thermal excitations are investigated. A thick composite model has been built in Abaqus to assist the shearography inspection. Various kinds of spatially modulated heating including local heating and global heating are explored for thick composite inspection with shearography in order to evaluate the corresponding efficacies in defect detection. We will present both experimental and numerical results on spatial modulated thermal loading. Defect-induced shearographic responses subjected to local and global thermal excitations will be discussed in this paper, including the influence of short-time heating and long-time heating on thick composite inspection. Current results indicate that long-time heating is more favorable when inspecting deep defects in thick composites, and with local heating it is possible to increase the defect-induced signal when compared with global heating.

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Monitoring of extreme dynamic loadings on composite materials with high temporal and spatial resolution provides an important insight into the understanding of the material behaviour. Quantitative measurement of the surface strain at the first moments of the impact event may reveal the initiation of the failure mechanisms leading to damage. For this purpose, we developed a shearography instrument for strain measurements during a severe impact event at µs temporal resolution. This paper presents the design, development and experimental measurement of the surface strain during an impact on aluminium and composite samples. The final design realises measurements of the in- and out-of-plane surface strain components to improve coupling of experimental data with the numerical models. The experiments on aluminium and composite specimens revealed the main elastic material response to be in the first 1-2 µs after the impact followed by the initiation and propagation of flexural waves causing in- and out-of-plane deformation. Further analysis of the wavefronts will be used as input and validation data for new numerical and analytical models of the impact response of composites and for validation of other experimental techniques as acoustic emission and embedded piezo sensors. The set of technical parameters of the developed shearography instrument makes it one of the most extreme applications of shearography for material characterisation. The framework for this work is the “EXTREME Dynamic Loading – Pushing the Boundaries of Aerospace Composite Material Structures” Horizon 2020 project.

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Structural delamination in mural paintings is a complex phenomenon and is considered among the most frequent types of damage. In conservation practice, the most common technique to identify structural detachments is the percussion method. Full-field optical techniques based on interferometry, such as shearography, can provide a more scientifically substantiated evaluation of the condition of heterogeneous structures of wall paintings. The empirical nature of the percussion method was observed during the condition assessment of two medieval wall paintings in Maria Church, Nisse, the Netherlands. It can be argued that, to allow the formulation of specific treatment needs for structural delamination in wall paintings, accurate defect mapping and characterisation is needed. The application of shearography was believed to provide a holistic representation of the condition of the structure of the wall painting depicting St. Christopher in Maria Church. Preliminary comparison of the methods involved revealed a degree of matching between results obtained. Discrepancies, i.e. areas deemed extremely vulnerable during percussion testing that were not detected by shearography, are debatably caused by the misinterpretation of the acoustic response during percussion testing or the inability of shearography to detect in depth structural defects. Further research regarding shearography should focus on providing more information about the depth of structurally delaminated areas within the heterogeneous layered structure of wall paintings.

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In this study, we describe a method for measuring the spectral reflectance of a paint layer at both the surface and in the volume of the paint layers. We first present a fringes model which illustrates the possibilities for spectral reconstruction using a Short-Time-Fourier-Transform algorithm. We investigate the remaing percentage errors and identified that there is a strong fluctuation along the wavelength range of the spectrometer. Then, we demonstrate the validity of our approach experimentally by measuring the spectral reflectance of a paint layer using a custom-made visible light optical coherence tomography system. There, we reconstruct the spectral reflectance of a paint layer by probing the surface and a depth range below the surface. Finally, we show the importance to include a wavelength sensitive correction in the reconstruction for taking into account the spectral shape of the light in the reference path of the interferometer. This work is part of the Down To The Ground project, in which the results of the OCT inspection will be used directly by a consortium of technical art historians and conservators.

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In this study, we compare the hyperspectral imaging capabilities of a custom-built visible light OCT with those of a commercial grade hyperspectral camera. Using the Short-Time-Fourier-Transform algorithm on the OCT signal, we estimate the variation in the surface spectral response from two paint pigments. Our study aims at comparing the spectral measurements obtained from calibration samples and to estimate an optimal working point for OCT-based hyperspectral processing.

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PowerPoint presentation: Dutch initiative of innovative companies and knowledge institutes combine and develop knowledge and experience in inspection, production, repair and maintenance of composites.@en

In this study we investigate the possibility of spectra stitching in the context of Spectral Domain - Optical Coherence Tomography (SD-OCT). The aim is to reach a high axial resolution while keeping sampling issues to a low level (slow decay in depth) but still operating with the fastest camera line rate available. The paper focuses mainly on simulations of spectrometer signals and the stitching procedure. It briefly introduces the experimental system. The findings of this study are relevant to most of the SD-OCT systems and could also be transferred to Swept Source OCT (ß-OCT) where they will help to increase the axial resolution capabilities.@en

This work presents the design and the latest experimental results on the surface strain measurements during an impact event obtained with the EXTREME high-speed shearography instrument. The shearography technique is used in this project to provide a quantitative measurement of the surface strain development at the first moments of the impact event (μs time scale) which may reveal the initiation of the failure mechanisms in composite materials. Experimentally measured surface strain components over the field of view will be used as input and validation data for new numerical and analytical models of the impact response of composites. The new configuration of the shearography instrument realises measurements of the in- and out-of-plane surface strain components to improve coupling with the numerical models. Two viewing directions (shearing interferometers) with a double-frame approach are used to capture the interferograms during the impact. The interferometers realise a double-imaging Mach-Zehnder scheme for the spatial phase-shifting with independent control of the shearing amount and the carrier frequency. The set of technical parameters of the developed shearography instrument makes it one of the most extreme applications of shearography for material characterisation. The framework for this work is the "EXTREME Dynamic Loading - Pushing the Boundaries of Aerospace Composite Material Structures" Horizon 2020 project.

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Shearography is a non-destructive testing technique that provides full-field surface strain characterization. Previous inspection of flat objects or simple geometric shapes has been reported. However, real-life objects especially in aerospace, transport, or cultural heritage are not flat, but their inspection with shearography is of interest for both hidden defect detection and material characterization. Accurate strain measurement of a highly curved or free-form surface needs to be performed by combining in-line object shape measuring and processing of the shearography data in 3D. Previous research has not provided a general solution for that. This paper presents a new approach of 3D shape shearography which is based on the integration of a structured light projector for in-line shape measuring with 3D shearography. For the experimental part, a 3D shape shearography system prototype was developed and its performance was evaluated with a cylinder specimen loaded by internal pressure and compared with strain gauges.@en

This paper presents the full research and development cycle of a high-precision absolute linear encoder based on a standard calibrated scale. Already available and used in industry standard scales of invar alloy were employed in the study. The scales have incremental indexes with 1 mm spacing intervals without an absolute code. However, the existing technical task is to measure the absolute position in the range of 2 m with accuracy less than 5 μm. For that, the developed encoder rationally combines magnetic measuring channel for the index numbering and an optical channel for the precise estimation of the encoder position. First, for the development, a simulation was performed to synthesize and analyze an image of an index. This image was used to develop a real-time double-threshold image processing algorithm to estimate the index position. Later the developed image processing algorithm was verified by preliminary testing and supported by a presented three-stage calibration procedure. The final measurements proved that the designed and developed encoder has the accuracy of 1.65 μm (3 standard deviations) at the speed up to 3 m/s. The possibility of use of standard calibrated scales with the presented encoder to solve existing and new industrial tasks forms the value of this paper. A possible use of the existing scales also provides unification and compatibility with conventional metrology equipment.

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This work presents the design and preliminary results of a high-speed shearography instrument in development for surface strain components measurements during an impact event. Composite materials are vulnerable to extreme dynamic loadings such as blade off events or foreign object damage as their mechanical properties are strain rate dependent. The development of new instruments to reconstruct extreme dynamic events will provide important insight into the understanding of the behaviour of composites. Shearography provides a quantitative measurement of the surface strain components with a high sensitivity as it is a non-contact interferometric technique. The current configuration of the shearography instrument realises measurements of the out-of-plane surface strain components during an impact using a double frame approach. The first experimental results reveal phase maps registered during an impact event with μs temporal resolution. Later the experimentally measured surface strain components will be used as input and validation data for new numerical and analytical models of the impact response of composites. The overall set of technical parameters of the developing shearography instrument makes it one of the most extreme applications of shearography for material characterisation.@en