## 4 Discussion on Comparison of NDT Techniques  
Six different ultrasonic NDT techniques for the evaluation of adhesive bonding quality have been investigated. Aluminium-epoxy-aluminium bonded structures with interface debonding have been investigated with ultrasonic bulk waves as immersion through-transmission, immersion pulse-echo, scanning acoustic microscopy, and air-coupled ultrasonic through-transmission methods. Additionally, two guided wave inspections have been performed: air-coupled to contact transducer and two contact transducers. According to different ultrasonic NDT results, the defect detection performance in immersion techniques is better than of the air-coupled methods. Specifically, immersion techniques--with through-transmission (Figure 11), pulse-echo (Figure 12), and scanning acoustic microscopy (SAM, Figure 13), identified five different defects successfully. Immersion through-transmission C-scan results suggest that the defect number 1 has different properties than other defects. The same abnormality is observed in immersion pulse-echo and SAM results; however, the level of contrast difference is less than using the through-transmission technique. This abnormality might be caused a physical phenomenon such as the lack of air within the double-sided release film. On the other hand, air-coupled  
Figure 19: Contact-to-contact guided wave results: (**a**) A-scan; (**b**) B-scan; (**c**) slice at time = 54.24 \(\upmu\)s.  
through-transmission ultrasonic NDT was successful to visualize four defects (2-5) as seen in Figure 14b. In this case, the defect number 1 was not identified because the active part of the transducer is exposed outside the plate boundary. Since there is no barrier between ultrasonic transducers, the ultrasonic wave is directly transmitted to the receiver causing significantly high amplitude values. Moreover, the second defect at left shows the higher transmission of the ultrasonic bulk wave than in the case of a perfect bond; therefore, the identification of this defect is questionable. Guided wave inspection with air-coupled to contact transducer has shown that three defects with defect numbers 2-4, which are at the centre of the investigation, have been identified as defects (Figure 18). With contact-to-contact transducer guided wave inspection, four areas are identified as defects (Figure 19). The defect number 1 is difficult to detect due to the wave packet reflected from the structure boundary. Additionally, ultrasonic NDT techniques have been compared, according to defect sizing performances. For each NDT technique, the decibel drop method had been employed to determine the size of the defects (Figure 20) [30]. According to this technique, a slice with normalized amplitude from the observed C-scan or B-scan is crossed with a horizontal line at the desired decibel drop. For immersion ultrasonic NDT and scanning acoustic microscopy (SAM) results, the slice has been selected in the middle of the defects. The slice selections in air-coupled through-transmission C-scan and guided wave B-scans are performed according to the highest contrast observed. The defect size has been calculated with the \(-6\) dB drop method except for air-coupled through-transmission NDT technique where the drop line is selected at \(-12\) dB. The calculated defect sizes are reported in Table 3. Moreover, the relative errors have been estimated according to the known defect size (12.7 mm). It can be observed that the immersion techniques and SAM can size the defects with higher precision than the air-coupled and contact techniques. Compared to the guided wave inspections, the air-coupled through-transmission method shows higher accuracy of defect sizing. In-between guided wave techniques, the contact-to-contact method determines defect size better than the air-coupled to contact method.