# Damage Imaging in Lamb Wave-Based Inspection of Adhesive Joints
## 1 Introduction  
Adhesive joints of metal elements are a kind of non-separable connections with numerous applications, e.g., in the aviation, machine, or automotive industry [1]. The strength and durability of adhesive joints are essential for the safety of engineering structures. They are highly susceptible to any defects resulting from mistakes that were committed during the preparation and joining of elements, e.g., poor condition of the adherend surface or a lack of glue on a part of the overlay. Additionally, a gradual deterioration in the quality of the adhesive material cannot be neglected. Typical adhesive joints are not capable of being inspected based on visual techniques as the place of damage occurrence (the adhesive film) is hidden between the joined parts. Destructive testing provides very accurate results [1]. This approach allows for the determination of the influence of certain parameters (e.g., damage existence [2; 3] or adhesion condition [4]) on the strength of representative samples, which were usually prepared in laboratory conditions and to predict the state of existing joints. Destructive testing is not free of disadvantages, it is, in general, a high-cost solution and cannot usually be applied to existing structures. Therefore, different non-destructive testing (NDT) methods have been adopted for the inspection of adhesive joints [5] including ultrasonics [6; 7], thermography [8], radiography [7], digital image correlation [9; 10], or electromechanical impedance spectroscopy [11]. In recent years, one of the most promising techniques for damage assessment is the use of guided wave propagation (e.g., [12; 13; 14]). Lamb waves are a specific type of guided wave that occur in a media that is restrained by two parallel surfaces, e.g., thin plate-like structures. The ability of Lamb waves tointeract with any kind of defect makes them particularly useful in the diagnostics of various elements of engineering structures, including adhesive joints. In previous studies, guided waves have been proven to be an effective technique for the inspection of adhesive lap joints [15; 16; 17; 18], adhesive bonds between composites [19; 20; 21], adhesive butt joins of plates [22], or the detectability of the adhesion level [23]. In the guided wave propagation technique, the wave is usually excited at one point of a structure and is collected at a number of points that are spread over the whole structure. In general, two measurement strategies can be used. The first method depends on using a small number of ultrasonic or piezoelectric transducers, followed by an analysis of registered time histories. This strategy is mainly used for structural health monitoring (SHM). Another approach is directed towards non-contact techniques, such as scanning laser Doppler vibrometry (SLDV) and the sensing of the full guided wave field (e.g., [24; 25; 26]). This strategy is more appropriate for quality assurance testing and periodic nondestructive inspection. Particular signals that are registered at numerous points distributed over a monitored area provide a two-dimensional representation of propagating waves at specific time instances. If any damage exists in an analyzed structure, a disturbance in the wavefront shape can be observed. The perturbation is more or less clearly visible depending on the type, size, and position of the defect. This visual assessment of SLDV maps allows for one to determine the presence of damage, but does not give a clear image due to the multiple reflections of waves from discontinuities and element edges [25]. The determination of the actual shape and size of the existing defect requires additional processing and damage imaging [27; 28; 29]. One of the simplest but most efficient techniques is based on vibration energy distribution and utilizes the calculation of a root mean square (RMS). This method was improved to be more applicable for damage detection by defining the weighted root mean square (WRMS) method [30].