## 10 Conclusions  
This paper has demonstrated how 3D scanning laser vibrometry can be used to conduct a thorough investigation of Lamb wave interaction with adhesively bonded stiffeners and adhesive disbonds. The work presented has demonstrated that the way in which Lamb waves interact with disbonds is dependent on the excitation frequencies.  
Analysis performed using a windowed RMS technique has revealed areas of high and low Lamb wave energy. This has revealed that, dependent on excitation frequency, conical fringes with high levels of Lamb wave energy can be found to be either reflected or transmitted through an adhesive disbond.  
RMS plots are also beneficial in revealing optimal structural areas for locating sensors to ensure a high probability of damage detection using Lamb waves. These plots demonstrate the complexity of sensor network design and show that considerations such as amplitude threshold are important for use in active sensing systems.  
The C-scan inspection revealed the suspected presence of PTFE tape within the disbonded region which had a significant influence on the interaction of the induced Lamb wave. By inducing a frequency swept Lamb wave and plotting the RMS of the velocity measurements taken using the 3D scanning vibrometer it was possible to clearly observe the Lamb wave interaction. This revealed the shape of the PTFE tape which correlated well with the results from the C-scan.  
This study has only considered the wave interaction when the actuator is in line with disbond. In reality, this may not be the case. Therefore, it would be beneficial to investigate the effect of the actuator being at different angles and distances relative to the disbond. Other considerations could also be taken into account including defect size in order to work towards achieving an optimisation platform for sensor network design. By using the 3D scanning laser vibrometer as a validation tool, it is possible to validate computational models of a Lamb wave interaction. Therefore it would be of significant benefit to be able to model several damage scenarios which would then allow a platform for sensor location optimisation to be produced.  
The work presented has shown that acousto-ultrasonically induced Lamb waves are a suitable method for the in-service monitoring of adhesively bonded structures for SHM applications. Careful consideration must be taken however when planning sensor location due to the complexity of the interaction with adhesive disbonds, in particular disbonds with irregular shapes.  
The authors would like to acknowledge the support of Engineering and Physical Sciences Research Council (EPSRC) Doctoral Training Grant (DTG) together with Airbus UK Ltd. for the funding of this project as well as [PERSON], [PERSON] and [PERSON] for their advice and assistance during this project. In addition, the authors wish to acknowledge the staff at Polytec Ltd. for their guidance of best practice for using the laser vibrometer.  
**Author Contributions:** [PERSON], [PERSON], [PERSON] and [PERSON] conceived the experiment; [PERSON] provided significant, industrially relevant design contribution to the project; [PERSON] was the principle researcher on this project, conducted the experiments, analysis and wrote the paper; [PERSON], [PERSON] and [PERSON] quality checked the written manuscript.  
**Conflicts of Interest:** The authors declare no conflict of interest.