## 3 Evaluation of Adhesive Joints by Ultrasonic Method  
Monitoring the condition of adhesive joints (including bonding technology) is important for their operation and durability. Destructive methods are not applicable in monitoring the condition of adhesive joints because the effect of such testing is damage to the joint. Therefore, it is important to control the condition of adhesive joints by nondestructive testing methods, including ultrasound.  
The use of different manufacturing technologies in the industry requires the adaptation of inspection methods, including nondestructive methods, to the structure and properties of the materials or connections. Therefore, the authors of the article determined the possibility of using ultrasonic testing in the evaluation of the bonding connection between the Ti6 Al4V as well as Ti6 Al4V-AM material with 3M 490 Scotch-Weld structural adhesive. Ultrasonic testing was conducted using seven heads and two types of specimens (with three different thicknesses), as described in Section 2.  
A 1 mm thick adhesive layer was applied to each sample. Before applying the adhesive, the surface of the sample was sanded with an abrasive disc and cleaned. The adhesive was applied at 20 \({}^{\circ}\)C and 50% humidity. Equal atmospheric conditions were maintained during the bonding of the adhesive to the sample surface. The setting time was about 24 h. Ultrasonic testing of the decibel pulse drop was then performed, as shown in Figure 6.  
Figure 6: Ultrasonic testing of adhesive joint—scheme; h = 10, 25 and 40 mm, d = 25 mm, a = 1 mm.  
Twenty ultrasonic measurements of the amplitude of the first two return echoes from the adhesive joint boundary were made for each head and each sample thickness of Ti6 Al4V and Ti6 Al4V-AM material. The decibel drop between the first and second pulses of the ultrasonic wave from the adhesive-sample boundary was determined by:  
\[R=20\times\log\left(\frac{H_{1}}{H_{2}}\right) \tag{3}\]  
The aggregate results of the decibel drop in the amplitude of the \(H_{1}\) and \(H_{2}\) pulses for the additively and conventionally manufactured adhesive-sample joint are shown in Figures 7 and 8.  
Figure 8: Decibel drop for samples with and without adhesive of Ti6 Al4V-AM material.  
Figure 7: Decibel drop for samples with and without adhesive of Ti6 Al4V material.  
Based on the test results, it should be concluded that the application of an adhesive layer causes an increase in decibel drops regardless of the material and the ultrasonic heads used during the experiment. The higher the frequency of the ultrasonic transducer, the decibel drop is also higher. The highest difference in decibel drop was obtained for the MB2S ultrasonic transducer at 2 MHz. The adhesive applied to the sample made with additive manufacturing technology caused a more than 60% increase in attenuation in such a joint in comparison with Ti6 Al4V material produced by conventional methods. For the other heads, the increases in the discussed ultrasonic parameter ranged from 3 to about 20%. Nevertheless, each time, higher attenuation was obtained for the printed material than for the samples made with conventional technology. This is mainly due to the more fine-grained structure of the printed material, which results in greater energy loss of the ultrasonic wave beam propagating through the sample material [33].