## 9 Discussion  
It is apparent that the induced Lamb waves all interacted with the disbonded region of the stiffener. There was a quantifiable difference in the Lamb wave interaction when compared with the results for the healthy panel. This demonstrates that by using a statistical measure of the difference of the received Lamb wave such as the cross-correlation technique [42] it would be possible to use an acousto-ultrasonic system to routinely inspect the condition of bonded stiffeners. By combining this technique with a statistical measure of the received energy, such as calculating the RMS value as demonstrated in this study, the location and characterisation of the damage could also be potentially assessed.  
The results presented also demonstrate that the excitation frequency of an ultrasonic system is an important consideration for inspecting adhesively bonded stiffeners for disbonds. The 100 kHz excitation did not transmit through the disbonded region whereas the higher frequencies did. For damage detection systems, particularly those which are triggered on an amplitude threshold, this is particularly significant to fully ensure a high probability of detection.  
This study only considered a bond which had areas where the disbond spanned the whole width of the stiffener. In the case of real aircraft structure, this may not be the case. It would be beneficial to also be able to detect this type of disbond. The results from the 100 kHz excitation suggest that this could be possible by considering the response of the reflected signal. Therefore an acousto-ultrasonic system capable of pulsing and receiving at different frequencies would be beneficial.  
Figure 13: 100–500 kHz swept sine wave excitation out-of-plane RMS plot with an outline of the C-Scan over laid. Note the reduction in wave energy surrounding the PTFE tape region causing a reduction in high amplitude waves transmitting through the disbonded region.  
All three excitation frequencies used in this study demonstrated that a low amount of the induced Lamb wave energy was transmitted through the stiffener. This is clearly a consideration for acousto-ultrasonic SHM systems but should also be considered important for passive SHM systems such as acoustic emission (AE). AE induced Lamb wave propagation will be affected by the presence of adhesively bonded stiffeners on a structure. AE events occurring on one side of a stiffener may not be detected by sensors on the other side due to the decrease in Lamb wave amplitude. In addition to this, signals reflected from the stiffener may be interpreted incorrectly leading to added complication when locating and characterising damage using passive techniques. Therefore, the geometry of a structure is an important consideration for the design of a passive AE sensor network.  
The results from the RMS plots demonstrate that laser vibrometry is not only a powerful tool in increasing understanding of Lamb wave interaction but can also be used to indicate the size and shape of a defect. The RMS plots serve as a useful aid in considering optimal sensor locations for an active sensor network by highlighting areas of high Lamb wave energy.  
The transmission of the S\({}_{0}\) mode through the disbonded region seems mostly unaffected by the presence of the PTFE tape regardless of excitation frequency. The S\({}_{0}\) mode is mostly constructed of in-plane components which gives reason to suggest that in-plane modes maybe less influenced by the presence of kissing bonds. Further investigation using in-plane excitation would be beneficial to further investigate in-plane Lamb wave interaction with stiffeners.