A large, full-scale (6 m tall, 2.5 m wide) full-composite ship hull section was subjected to three consecutive impacts with an impact energy level (∼20 kJ) mimicking a realistic heavy crash. The structure was continuously monitored with acoustic emission (AE) sensors during the impacts, which allowed the possible degradation of the composite hull to be assessed. Unsupervised learning was applied to AE data features to enable the categorization of the damage accumulated during the consecutive impacts. The implemented unsupervised learning routine was a combination of automatic Laplacian data feature selection followed by density-based spatial clustering of applications with noise (DBSCAN), for which the hyperparameters were automatically optimized with a silhouette-driven approach. Three predominant damage mechanisms (sandwich-core crushing/cracking, skin-core debonding, and matrix cracking) were identified through the clustering solution (with 0.9 mean silhouette and 6% outliers) and verified by AE feature bands found in the literature. The AE-based damage categorization was validated with impact slow-motion videos and postimpact digital laser shearography inspections. The study highlights the capability of the developed categorization methodology to be deployed for online monitoring, where SHM algorithms are retrained during ship operation to keep improving diagnosis accuracy. The results show that in the online training scenario, where Impacts 1 and 2 were used for training and Impact 3 was used for testing, the categorization performance was comparable to when data from all three impacts had been used for training, with a mean silhouette of 0.884 and only 2% outliers. Altogether, the damage categorization routine demonstrated reliability and stability for handling realistic AE data variability and different data availability scenarios. This study is an important step toward a complete condition diagnosis (comprising detection, localization, categorization, and quantification) of full-scale composite ship structures, which is crucial for estimating their remaining useful life in real time and thereby for enabling their condition-based maintenance.

This study investigates the multifunctional potential of metallized polyphenylene sulfide (PPS) nonwovens integrated as electrically conductive interlayers in glass fiber-reinforced polymer (GFRP) composites. The PPS nonwovens were coated with a nickel–phosphorus layer via electroless plating and embedded between the laminate plies. The system was evaluated both as an electrothermal heating element for de-icing and as a sensing layer for non-destructive testing. For de-icing applications, icing wind tunnel tests were conducted under glaze-ice and mixed-ice conditions. The integrated heating layer enabled complete ice removal within approximately 120 s for both icing regimes, while the distributed Joule-heating mechanism ensured stable and spatially uniform surface temperatures. Furthermore, the metallized nonwovens were successfully applied as an internal thermal excitation source in shearography, allowing clear identification of impact-induced damage, including delamination. The incorporation of the metallized PPS interlayer also enhanced the mechanical performance of the GFRP composite, with flexural strength increasing from 944 MPa (reference) to approximately 1164 MPa. Dynamic mechanical analysis indicated a slight increase in glass transition temperature from 132 °C to 141 °C. These findings demonstrate that metallized thermoplastic nonwovens provide an effective approach to designing multifunctional composites for advanced engineering applications.

Shearography and electronic speckle pattern interferometry (ESPI) have historically been developed in limited collaboration. Both techniques have a significant entry barrier for new researchers to get reliable results. The situation is even worse regarding data and code availability: only three documented and publicly available shearography datasets and very limited open software realisations exist. The data sharing aspect gets more critical. First, AI developments are well reported, while only two datasets were published. Second, developments in phase processing are reported without publicly available code. This limits reproducing and validating the results. Following an example from open data challenges in digital image correlation (DIC), this presentation highlights the Open Science issues and proposes three shearography datasets with inspection of composites. This presentation intends to initiate a discussion in the field that could lead to better practices on data and code sharing.

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.

Structural integrity of composite materials is vital for aerospace applications, requiring advanced non-destructive inspection (NDI). This study combines shearography and digital image correlation (DIC) to assess carbon-fiber reinforced polymer (CFRP) samples after the impact. Impact testing via a drop tower induced different damage types, detected by both techniques. Four-point bending test enhanced defect visibility. The measurements were performed by shearography and 3D DIC using the same cameras from the 3D shearography system. Shearography identified defects earlier than DIC, while DIC offered bigger range of displacements and strain measurements highlighting the complementary role of both methods and multimodal NDI test’s effectiveness in detecting impact damage and manufacturing defects.

Non-destructive inspection (NDI) of small-scale defects in fiber-reinforced composites is an urgent challenge to ensure the structural integrity of safety–critical components. Shearography is a non-contact and full-field optical NDI method that can be used to characterize surface strain components under loading. Thermal loading is widely used in shearography because of the advantages of being non-contact and convenient for in-service inspection. Shearography has received considerable industrial acceptance for the inspection of aerospace and marine composite structures, however its efficacies in detecting small mm and sub-mm defects have not been fully characterised. Besides, one major issue regarding shearography with thermal loading is fiber-related deformation or fiber noise, which can affect the efficacy of defect detection significantly, especially when detecting small and deep defects. In this study, the novel shearography pair method is proposed and developed to reduce fiber noise for reliable inspection of small mm and sub-mm defects in composites. The defect detection capabilities of the proposed method have been studied and compared with conventional shearography practice and with fast Fourier transform (FFT) and principal component analysis (PCA) based signal processing algorithms. The results demonstrate that the proposed shearography pair method has the advantages of less fiber noise, improved inspection results, and being faster with reduced number of datasets. It enables the detection of mm and sub-mm defects (down to 0.6–0.8 mm in diameter) in composites; these inspection results are one of the smallest defect sizes detected with shearography and reported in literature.

This study presents the implementation of two speckle-based optical measurement techniques-digital image correlation (DIC) and intensity-based dynamic speckle imaging (DSI)-integrated with high-speed camera acquisition for the dynamic characterization of composite materials under impact loading. The DIC method, relying on the correlation of printed or painted speckle patterns recorded before and after deformation, enables precise quantification of displacement fields. In parallel, the DSI approach captures temporal variations in speckle intensity, improving the detection of subsurface defects and the visualization of velocity distribution associated with internal processes. Our results demonstrate that DSI serves as a valuable complementary tool to the DIC technique.

The objective of this work is to study defect detection efficacy using embedded carbon nanotube (CNT) fibers as heaters for shearography and thermography. For that, GFRP laminates with various amounts of CNT-doped strips embedded at different layers have been manufactured. Impact tests are performed to create realistic damage in the GFRP specimens for inspection. Shearography and thermography non-destructive testing of the GFRP laminates are performed simultaneously with embedded CNT strips as heating sources before and after the impact test. This research was performed as part of the Horizon Europe COMP-ECO project (grant agreement 101079250). In the future, we aim at developing a novel demonstrator of a composite aerospace structure with integrated CNT-doped sensors that support NDT and enable self-sensing and in-situ SHM capabilities.

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.

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.

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.

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.

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.

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.

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.

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.

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.