## 1 Introduction  
Adhesive bonding is an attractive alternative to other joining technologies due to its high performance to weight ratio, ability to join dissimilar materials, and homogenous load distribution. Although the advantages of the adhesive bonding are very attractive to highly developed industries, such as aerospace, their usage is limited due to the lack of the knowledge in the non-destructive testing (NDT) of bonding quality [1]. Hence, research in non-destructive evaluation of bonding quality is of great interest for decades. The ultrasonic NDT techniques have been one of the most commonly used NDT techniques to evaluate bonding quality [2]. The ultrasonic NDT methods have been used not only to evaluate adheren integrity, but also for interface quality determination, debonding detection, and weak bond investigations [3]. Through-transmission immersion, ultrasonic NDT is considered the reference method within all ultrasonic NDT methods. The experiment itself requires two-sided access to the structure and the inspected component needs to be immersed in water [4]. Immersion pulse-echo ultrasonic inspection, on the other hand, requires only single-side access to the structure; however, it still requires water immersion of the sample [5; 6]. In particular, pulse-echo immersion ultrasonic NDT techniques have been utilized for bonding interface visualization [7], debonding detection [8], adhesive quality evaluation [9], kissing bond evaluation [10], and for the defect positioning in dissimilar joints [11]. Moreover, scanning acoustic microscopy (SAM) allows structures to be inspected with high frequency-high resolution. While SAM provides higher spatial clarity, the size of the structure is limited, the experimental set-up is bulky and investigations are performed in water. SAM has been employed for the evaluation of weak bonds in composite joints [12], weak bond imaging with holography [13], and porosity identification in dissimilar joints [14]. Nowadays, air-coupled ultrasonic NDT has also gained interest for in-situ applications because, as a rapid ultrasonic testing, it can be performed without any contact medium and it applies to the industrial test line [15]. Compared to immersion ultrasonic, the air-coupled ultrasonic NDT has several advantages. However, it still lacks the high spatial resolution and fast inspections that are achieved with the immersion ultrasonic NDT. In a recent study, the interfacial stiffness coefficients of weak bonds have been investigated with air-coupled ultrasound [16]. Moreover, interface visualization of the tri-layer bonded aluminium plate has been performed with the air-coupled through-transmission ultrasonic testing [15]. While air-coupled through-transmission needs two-sided access, it is possible to inspect structures with pitch and catch by the propagation of Lamb waves [17]. The guided wave (GW) ultrasonic NDT is another promising technique to evaluate bonding quality that allows large structures to be inspected in a short time. GW ultrasound can be employed with non-contact systems, such as air-coupled transducers and laser interferometer [18; 19; 20]. Moreover, it is possible to excite and receive guided waves in specific modes with contact transducers, electromagnetic acoustic transducer (EMATs) [21], and macro fibre composite transducers (MCF) [22]. However, it should be noted that the evaluation with GW inspections requires an understanding of the wave propagation within the sample, which, in most cases, is achieved only by highly complex numerical investigations [23]. Several studies used guided waves to investigate bonding quality. While some works focused on the debonding and delamination detection with Lamb waves and vibrometry systems [24; 25]; others discussed the scattering of the guided waves as a result of the change in interface quality [26; 27]. In addition, weak bonds had been investigated by using Lamb wave inspection [28].