PSI - Issue 77

João Queirós et al. / Procedia Structural Integrity 77 (2026) 475–483

476

Keywords: Damage identification; Laminate plate; Digital Shearography; Thermography

1. Introduction Structural composite materials have throughout the years widespread into a large variety of applications. Their combination of high load-bearing capacity and low weight makes them particularly attractive for the aerospace industry. Composite structures, despite their advantages, are often more susceptible to damage and defects than common metal structures. This is due to their inherent anisotropy and generally higher brittleness. Specifically, they are highly sensitive to low-energy impacts, which can cause internal delaminations while leaving little to no visible indication on the surface, thus resulting in Barely Visible Impact Damage (BVID), which according to Airbus, result from impacts below a 50 J threshold (Talreja and Phan, 2019). This type of damage is particularly insidious because it can significantly compromise structural integrity while being very difficult to detect through visual inspection. The detection of this and other types of damage is therefore crucial for enabling timely maintenance, emphasizing the need for reliable and robust inspection techniques. Furthermore, when inspecting a structure that is in active service, it is essential to ensure that the process does not compromise its integrity, thereby maintaining its suitability for continued use if deemed fit. This necessity leads to the field of Non-Destructive Testing (NDT), which encompasses a variety of techniques that utilize different physical principles to detect and evaluate damage or defects in structures without damaging them (Wang et al., 2020). Among the different NDT techniques, there is a particular interest in optical ones due to their ability to perform non-contact inspection over large areas with high sensitivity. This study focuses on two of these optical techniques, commonly used in the inspection of composite structures, namely, Active Thermography (AT) and Digital Shearography (DS). In AIT, a controlled thermal excitation is applied to the inspected structure using halogen or flash lamps. The thermal response at the inspection surface is observed by an infrared camera, displaying local thermal differences relative to the surrounding regions, and, therefore, revealing the presence of internal damage or defects (Ibarra Castanedo and Maldague, 2013). Two of the most widely used AT variants are Pulse Thermography (PT) and Lock in Thermography (LT). In PT a thermal pulse, with rectangular amplitude over time, is generated with duration ranging from microseconds to seconds (Talreja and Phan, 2019). Meanwhile, in LT, a sinusoidal wave form is employed. This type of excitation has the advantage of allowing the estimation of the damage depth through the analysis of the phase difference between the specimen thermal response and the thermal excitation (Ibarra-Castanedo and Maldague, 2013). Digital Speckle (DS) is a non-contact, high-resolution interferometric optical technique. It measures the surface displacement or strain field caused by an external load, such as mechanical stress or thermal changes. When internal damage is present, there is a local stiffness reduction in the region around it. This reduction is revealed as a variation in the measured strain field, allowing DS to detect any type of damage that induces stiffness changes. This NDT technique is highly effective at identifying subsurface damage in composite structures, such as delaminations, cracks, and other damage typologies (Hung et al., 2009; Wei et al., 2024; Tao et al., 2023; Zhang et al. 2022). Both techniques are non-contact and capable of detecting subsurface damage, both of which are key attributes for the inspection of composite structures. Since composites are more damage-sensitive, the non-contact nature of these methods offers a significant advantage over techniques which require physical contact and require care to avoid inadvertently harming the structure during inspection. As means to enhance damage detectability through image-based analysis, post-processing methods are frequently employed. For thermal image processing, different algorithms can be used, with one of the most common being Principal Component Analysis (PCA). PCA is a statistical data post-processing technique that is used to enhance damage/defect visibility by transforming a thermal image into a set of orthogonal features ordered by significance (Rajic, 2002). This decomposition reveals subtle flaw signatures that are otherwise masked by dominant components