# Damage Imaging in Lamb Wave-Based Inspection of Adhesive Joints
## 4 Conclusions  
In this paper, an experimental analysis of the Lamb wave interaction with damage in the adhesively bonded joints was conducted. The study consisted of a visual assessment and statistical analysis of the obtained results. The presented investigations support the following conclusions:  
* The analysis of maps that were obtained from the SLDV measurements provided the initial identification of defects with different efficiencies. The clearness of the SLDV maps varied depending on the shape and position of damage. * The actual defect shape and location were determinable based on the appropriate damage imaging, where recorded out-of-plane vibrations were processed by means of the WMRS. The effectiveness of the applied method was strongly dependent on the calculation parameters (weighting factor and time window), excitation frequency, and a type of damage. * The extension of the time window led to the increase in differences in the WRMS values between the damaged and undamaged areas of the adhesive joint. As a result, defects in the adhesive film could be identified more accurately. * By comparing different variants of weighting factors, the constant one did not provide efficient results due to the high influence of the incident wave at the excitation point. This effect was reduced by the weighting factor, which was represented by the linear or square function that achieved comparable results. For specimens with internal defects, moderately greater effectiveness of the linear variant was observed. * The shape and location of defects had a great impact on the effectiveness of their identification. Both the presence and the size of defects were possible to determine in all of the contested specimens based on the WRMS damage maps. The clearest results were obtained for stripe defects parallel or skewed to the primary direction of wave propagation. The disbonded areas in the shape of the strip perpendicular to the direction of wave propagation were also determinable, however, the identification was easier when such area was at the beginning or at the end of the lap. The most difficult to detect were the defects in the internal part of the adhesive film. Figure 20: Histograms of WRMS values for specimen #6 with different excitation frequencies (\(T=3.00\) ms, \(w_{r}=r\)). The differentiation of effectiveness of the method for different types of defects resulted from the occurrence of different wave phenomena at the boundaries of defects. Stripe defects, skewed, or parallel to the direction of propagation triggered a diffraction of wave fronts that provided a significant disturbance in wave propagation and resulted in a higher intensification of signals. Defects in the shape of a perpendicular strip were related to wave reflection that did not disrupt the propagation as diffraction did. Finally, internal defects were the most difficult to identify because of the local characterization of occurring wave phenomena. * The statistical analysis of the calculated WRMS values successfully supplemented the visual assessment of the damage images. The interpretation of the histograms helped to explain the effects that were observed on the WRMS maps. The use of the relative distance between peaks of bimodal distribution was proposed as the indicator for the optimal determination of WRMS parameters. Based on this indicator, it was proven that the largest value of the time window obtained damage maps with the highest clearness. Moreover, it was possible to determine the range of the optimal value of the weighting factor and to quantify the defect areas. The statistical analysis of the influence of the excitation frequency also enabled a comparison of the effectiveness of the three different frequencies. Two higher frequencies providing bimodal distribution were proven to be more effective than the lowest excitation frequency providing unimodal distribution.