## 1 Introduction  
Recently, adhesive bonding technology has gained considerable attention in various industrial sectors, especially in the aviation, automobile, civil, and marine industries due to the weight reduction of the structure, uniform distribution of mechanical stresses, fairly high strength-to-weight ratio, and ability to join dissimilar materials [1, 2, 3]. Adhesive joints also provide improved performance of the structure and reduce fuel consumption, which benefits the economic sector. All the advantages and abilities of adhesive joints make this technology the most effective and practical for joining material components, in comparison to traditional bonding techniques, such as viewing, welding, etc. Adhesives are used to bond metal and composite materials to produce different structural components and advanced materials. In the aerospace industry, a substantial number of parts are fabricated using the adhesive bonding of metal and composite materials; however, the manufacturing processes are still unfavorably manual, resulting in various types of defects in the components [4, 5]. Factors that can affect the durability of adhesive joints and cause the occurrence of different defects are moisture, poor curing, contamination, a constant structural load, impact force, and others [2, 3]. Distinct types of defects, such as delamination, debonding, and weak or kissing bonds, can appear in adhesive joints. These internal defects are extremely critical for structural integrity due to their invisibility and the strong influence on bonding quality. As a result, it becomes difficult to predict the mechanical behavior of the joints in the case of the presence of internal defects [6]. The presence of flaws or discontinuities in the bonded joints should be avoided at all costs for implementing them in critical aerospace components to provide environmental safety, safety of human life and health, and economic benefits. Recently, many different investigations were carried out using different non-destructive testing (NDT) methods, such as ultrasonic [7], thermography [8; 9], eddy current stimulated thermography [10], and others for the inspection of adhesive joints [11; 12; 13; 14]. However, each non-destructive method has its own advantages and limitations upon which the selection of the appropriate method depends. Moreover, the choice of method depends on the structure to be inspected, since most of the methods become impractical after the structure is assembled due to the geometrical complexity of the structure, one-sided access, or even coverage with other layers of functional material [15]. The ultrasonic testing technique is widely and successfully used for the inspection of different structures. Ultrasonic non-destructive testing is advantageous and widely used in identifying and locating defects and can be utilized to evaluate the coherence of the adhesive bonded joints. This method was used by many researchers for the characterization of adhesion quality [16; 17]. Adhesive joints can have complex geometry, roughness of the surface, curved surface, or interface layers of the component, which in addition can be not parallel, with varying thicknesses of the materials and their types. All these factors influence the ability of the technique used to detect defects. [PERSON] et al. [18] investigated the detection of disbonds in multi-layered structures using the laser ultrasonic technique. The technique used was pulse-echo mode for defect detection, and quantitative evaluation was used for defect sizing. Numerical simulations and ultrasonic inspection were also used for the detection of fatigue debonding in composite lap joints in the work of [PERSON] et al. [19]. [PERSON] et al. [20] used numerical and experimental investigations of ultrasonic guided waves. Disbonds were detected because of the variation in the arrival time of the ultrasonic waves.