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

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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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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THICK composite materials, e.g., thickness of more than 50 mm, are increasingly being used across diverse industry sectors owing to their significant advantages of weightsavings, superiormaterial properties and load-carrying capability. These materials tend to be adopted in safety-critical applications such as large primary or secondary load-bearing structures, where mechanical failures would result in serious consequences. However, various defects and damage may occur in thick composites that endanger structural integrity and safety severely. Hence to improve the maintenance, safety and reliability of these structures, it is crucial to develop inspection methods capable of defect detection and characterisation for composite structures of significant thickness. To date, the nondestructive testing and evaluation (NDT&E) of thick composite structures still remain an urgent challenge due to their material and structural complexity, significant thicknesses, and the presence of various manufacturing and in-service defects....@en

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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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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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

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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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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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