PSI - Issue 77

H. Lopes et al. / Procedia Structural Integrity 77 (2026) 673–680 H. Lopes/ Structural Integrity Procedia 00 (2026) 000 – 000

674

2

1. Introduction The fundamental purpose of Non-Destructive Testing (NDT) is to assure structural integrity through the evaluation of components without compromising their future usability. This non-invasive assessment is crucial for ensuring the safety of critical assets, such as aircraft, pressure vessels, and bridges (Paul and Balamurugan, 2022). Still, widespread adoption of laminated composite structures (such as carbon and glass fiber-reinforced polymers), driven by the composites' superior strength-to-weight and stiffness-to-weight ratios, in industries like aerospace, automotive, and wind energy, presents new NDT challenges. Their complex, layered architecture and unique failure modes required the development of new specialized NDT techniques to reliably evaluate their structural integrity (Ali et al., 2024). Image-based NDT techniques are vital for detecting internal and external damage in laminate composite structures. These methods rely on capturing and analyzing visual data, often generated by a response to an external stimulus, to reveal internal damage, such as delamination, matrix cracking, and fiber breakage. Image-based inspection techniques are broadly classified according to their energy source and the resulting data: active thermography, radiography, ultrasonic methods, and surface deformation methods. The most prominent techniques in this last category are Digital Image Correlation (DIC), active Thermography, and Digital Shearography (DS). DS is a non-contact, interferometric optical technique, particularly renowned for its high resolution and exceptional sensitivity. It is officially certified by leading bodies, including NASA, the Federal Aviation Administration (FAA), and the European Union Aviation Safety Agency (EASA), for non-destructive inspection (NDI) of aerospace components (Saeedifar and Saleh, 2024). The method utilizes laser light to inspect structures, especially composites, for internal defects. Importantly, unlike Electronic Speckle Pattern Interferometry (ESPI) or Digital Holography, which measure simple surface displacement, DS measures the first-order derivative of the displacement, which can be assumed as a good approximation of strain or the rotation field on the structural surface (Kreis, 2006; Yang and Xie, 2016). DS is highly effective at revealing internal damage by analyzing the localized strain concentration or deformation produced when defects cause a reduction in stiffness. This capability has made DS a dominant optical NDT method, particularly for inspecting advanced composite structures, like laminated and sandwich plates. DS offers high sensitivity, capable of detecting strain variation in the sub-micrometer range. This precision makes it ideal for finding critical subsurface damage, such as delaminations or the lack of adhesion (disbonds). The technique uses a self-referencing optical path to create the interference pattern, making it inherently more robust than methods like ESPI (Yang and Xie, 2016). Because the interference is created between two wave fronts that share the same optical path (a common-path configuration), DS is inherently insensitive to rigid body motion and more tolerant of external perturbations, simplifying its use outside of a laboratory environment. DS provides full-field and non-contact measurements over large areas. Because a camera captures the interference pattern, the spatial resolution is determined by the camera's resolution, while the inspected area is defined by the system's field of view. Its ability to rapidly inspect large areas is a crucial advantage for quality control during manufacturing and for efficient in-service maintenance. DS is a versatile NDT technique employed across critical industrial sectors. In aerospace and aviation, DS is essential for inspecting high-performance CFRP components, sandwich panels, and bonded joints, such as wings, fuselages, and rotor blades. Key defects identified include delaminations, disbonds, crushed cores, and fluid ingress in honeycomb/foam structures, as well as checking the integrity of adhesion seams and metal-to-metal bonds (ASTM E2581-07; Krupka et al., 2005). The wind energy sector uses DS extensively for quality control and in-service inspection of large wind turbine blades, which are frequently susceptible to impact and delamination damage (Yang et al., 2016). Within the automotive industry, DS is used to perform quality control on tires, detecting bubbles and delaminations (Hung and Ho, 2005). Additionally, the technique is applied in marine and defense for composite component integrity checking (Vyas and Kazys, 2018), and in research, it is a key tool for measuring strain and vibration analysis (Yang and Xie, 2016; Hung, 1999). DS is an optical NDT technique that does not directly measure damage. Instead, it captures the gradient of out-of plane displacement (i.e., the rotation field) or the surface strain field induced by an external load. Damage is primarily identified by interpreting the resulting fringe pattern, known as a shearogram, which forms due to the applied load. Analysis can be performed either by directly examining the fringe pattern or by analyzing the corresponding strain

Information classification: Internal