## 5 Conclusions  
This paper compares the performance of immersion, air-coupled, and contact ultrasonic non-destructive testing techniques in bonding quality evaluation. The aluminium-epoxy-aluminium single lap adhesive joints containing debonding have been investigated with both bulk wave and guided wave inspections. According to the investigation results, the following conclusions have been made:  
* For through-transmission techniques, the amplitude of the bulk wave decreases on the debonded area whereas in the case of guided waves and pulse-echo inspections, the amplitude increases when passing through the debonded area.
* The defect detectability and sizing performance of the immersion techniques and the scanning acoustic microscopy is higher than the performance of the air-coupled through-transmission and guided wave techniques.
* The relative comparison of ultrasonic NDT techniques in defect sizing has been performed by the ratio of average absolute defect sizing error to ultrasonic wavelength inside the adhesive bond. While the best performance is observed in the immersion through-transmission technique, the high performance for the air-coupled through-transmission technique cannot be disregarded.
* In this case study, ultrasonic techniques could be sorted from best to worst in defect sizing as: immersion through-transmission, SAM, immersion pulse-echo, air-coupled through-transmission, guided wave contact-to-contact, and guided wave air-coupled to contact.
* While immersion ultrasonic techniques and acoustic microscopy achieve high precision in defect detection and sizing, the structure under the test needs to be immersed in water. These NDT systems are bulky and expensive. Air coupled ultrasonic techniques reach lower precision than immersion techniques, but they allow non-contact inspection. Since the air-coupled through-transmission technique requires two-sided inspection, the guided wave inspections could be recommended for in-situ applications.
* In industrial applications, the structures can be inspected with less precise but more applicable techniques like guided wave ultrasonic methods, and if the abnormalities are detected, further  
Figure 21: Defect sizing normalized absolute errors per wavelength for each defect and ultrasonic NDT techniques.  
investigations with higher precision techniques, such as immersion and acoustic microscopy, could be used.  
Conceptualization, B.Y., A.A., E.J., and R.J.K.; methodology, B.Y., A.A., E.J., and R.J.K.; software, B.Y. and A.A.; validation, E.J., and R.J.K.; investigation, B.Y. and A.A.; formal analysis, B.Y. and A.A.; data curation, B.Y. and A.A.; writing--original draft preparation, B.Y., and A.A.; writing--review and editing, B.Y.; A.A.; E.J., and R.J.K.; visualization, B.Y. and A.A.; supervision, E.J., and R.J.K.; project administration, E.J.; funding acquisition, E.J. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the NDToAnIR project from the European Union's Horizon 2020 Research and Innovation program under the Marie Sklodowska-Curie, grant number 722134.  
The authors would like to acknowledge the contributions of [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] on the experiments performed; and the contribution of [PERSON] on the numerical investigations.  
The authors declare no conflict of interest.