# Damage Imaging in Lamb Wave-Based Inspection of Adhesive Joints
## 1 Introduction
Recently, the RMS has been successfully used in various damage detection applications [25; 26; 31; 32; 33; 34; 35; 36]. [PERSON] et al. [30] investigated aluminum and composite plates as well as composite elements of a helicopter, all with the damage being simulated by an additional mass. They concluded that better resolution in WRMS maps corresponded to higher values of the weighting factor (linear or square). Localization of damaged rivets in a stiffened plate structure based on RMS maps was conducted by [PERSON] et al. [31] where the authors used a five-cycle burst of different frequencies (5 kHz, 35 kHz, and 100 kHz). The best results were obtained in the case of an excitation signal of 100 kHz, where the wavelength was shorter than the distance between two rivets. [PERSON] and [PERSON] [32] considered an aluminum plate with notches of various orientation (tangential, perpendicular, and diagonal to the incident wave front). Based on RMS images, only tangential damage was successfully detected without a comparison to the intact images. [PERSON] et al. [33] used radially weighted and factored RMS for damage localization in an aluminum plate. Detection of delamination in a composite T-joint using RMS images was studied by [PERSON] et al. [34]. [PERSON] et al. [35] presented experimental investigations on the detection of disbonds in a stiffened panel based on the RMS plots. Wave packets with three different frequencies, namely 100 kHz, 250 kHz, and 300 kHz, were applied. The authors concluded that the excitation frequency is an important consideration for the inspection of disbonds. In the conducted works, only higher frequencies (250 kHz and 300 kHz) were able to detect damage. [PERSON] et al. [36] applied the WRMS to localize the notches and corrosion areas in aluminum plates and chose 2 as the weighting parameter. [PERSON] et al. [25] studied the detection of delamination in a composite plate using WRMS with the weighting factor set as 2.7. They also proposed a selective WRMS that enabled the elimination of edge reflections. Detection of disbond inserts in composite elements was performed by [PERSON] et al. [26] using RMS and three-dimensional (3D) scanning. In the conducted experimental investigations, it was noticed that in-plane RMS maps provided a better indication of damage than out-of-plane RMS maps. Summarizing all of the works that are reported above, the main problem that is connected with damage imaging using WRMS is the fact that the weighting factor is set arbitrary with no clear explanation for the choice of a specific value. This study presents an approach to the damage identification of adhesively bonded joints that is based on the Lamb wave propagation. The guided wave field was measured with the use of scanning laser Doppler vibrometry and then processed by means of the weighted root mean square. The object of the research was a single lap adhesive joint of metal plate-like structures. To verify the feasibility of the proposed approach, nine specimens with defects that were differing in size and position were tested. The influences of different WRMS parameters (i.e., the time window and weighting factor), as well as excitation frequencies, were analyzed. A novel element in this research was the application of the statistical analysis in determining the WRMS parameters. It has been proven that the obtained WRMS maps of damaged joints were characterized by bimodal distributions that created the opportunity to determine the optimal value of the time window, weighting factor, and excitation frequency.