## 1 Introduction  
Adhesive-bonded structures have attracted considerable interest in the aerospace, marine, automotive, civil and other industries due to their cost-effectiveness and superior performance characteristics. The advantages of these structures include even load distribution, reduced weight, the ability to bond dissimilar materials, high strength/weight ratios, and improved fatigue life, impact resistance, and residual strength [1; 2; 3]. In the aerospace sector, for example, weight reduction is especially valuable, as it contributes to lower CO\({}_{2}\) emissions, enhanced aircraft performance, and greater cost efficiency. The use and popularity of lightweight materials compared to metal alloys has increased significantly in aviation, aerospace, and motorsports due to their efficiency [4]. In modern aircraft models, such as the Airbus A350 XWB, carbon fiber accounts for 50% of the total weight, an increase of 30% compared to the earlier A380 model [5]. Nevertheless, the integrity of adhesively bonded structures can be compromised by external loadings, aging, and environmental conditions, which can lead to the forming of various defects [1]. Common defects to which adhesive joints are prone include interface defects such as delaminations, debonding, porosity, and voids [6]. These defects are often invisible and can result in catastrophic failures if undetected. Therefore, regular application of reliable non-destructive testing (NDT) methods is crucial for monitoring the mechanical properties and structural integrity of adhesive joints [7; 8; 9]. Various NDT methods can be applied for the inspection of adhesive joints, including thermography [10], ultrasound [11; 12], guided waves [6], eddy current [13], and X-ray radiography. These methods do not affect the performance of the structure and can provide valuable information regarding the presence, quantity, location, size, and type of defects [11]. The selection of a particular NDT technique depends on the inspection requirements, as each technique has its advantages and limitations. For example, radiography is effective in detecting porosity, voids, and inclusions and providing accurate information on the location of defects, but is expensive and involves exposure to radiation. Thermography is non-contact and can quickly inspect large areas, making it suitable for detecting surface defects, but its effectiveness decreases with depth, making it less reliable for detecting subsurface defects. Eddy current testing is sensitive to small surface cracks and corrosion and works well with conductive materials, but has limited depth penetration. Ultrasound, on the other hand, can detect both surface and subsurface defects, making it versatile, but requires skilled operators and direct contact with the surface. Guided waves can inspect large areas efficiently, especially in complex structures, but are sensitive to environmental noise and may require signal interpretation [14]. Therefore, the selection of suitable techniques depends on the object structure and inspection requirements. Despite the wide range of NDT techniques available, the detection of interface defects in adhesive joints remains challenging [15]. This is particularly true in cases where surface curvature causes amplitude variations in ultrasonic testing, making defect detection less reliable [16]. Recent advances, such as the use of ultrasonic guided waves and post-processing algorithms, have attempted to improve the detectability of defects in multilayered structures. Techniques such as the generation of A0 mode ultrasonic guided waves, the weighted root mean square (WRMS) processing algorithm, and other optimization techniques have demonstrated improved accuracy by correlating wave properties with the presence of interface defects [17; 18; 19]. [PERSON] et al. [7] investigated zero-volume kissing bonds using both linear and non-linear ultrasonic testing. Their findings showed that high nonlinearity corresponded to weaker bond strength, while lower nonlinearity indicated stronger bonds.