## 4 Discussion  
The example described above demonstrates that multi-layered structures are rather complex when it comes to the description of guided waves travelling through them. To understand this, numerical analysis tools are of an essential need. In the case considered here, the SBFEM was used to calculate dispersion curves and mode shapes, as well as to analyse the propagation of guided waves in a plate consisting of an isotropic metal bonded to anisotropic carbon fibre reinforced layered material. The method allows appropriate modes to be identified and their interaction with different damage types to be analysed. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Mode** & **Frequency** & **Wavelength** & **Crack in** & **Delamination** & **Delamination** \\  & **kHz** & **mm** & **Aluminium** & **at Position (1)** & **at Position (2)** \\ \hline
1 & 100 & 15.6 & ✓ & ✓ & ✓ \\
2 & 475 & 5.3 & ✓ & ✓ & ✓ \\
3 & 950 & 2.9 & ✓ & ✓ & ✓ \\
4 & 400 & 14.6 & ✓ & ✓ & ✗ \\
5 & 475 & 15 & ✗ & ✓ & ✓ \\
6 & 700 & 7.8 & ✓ & ✗ & ✗ \\
7 & 860 & 11.6 & ✗ & ✓ & ✗ \\ \hline \end{tabular}
\end{table} 
Table 4: Comparison of the mode-damage interaction for the damage positioned in different parts of the structure. ✓, modes which reflect from the damage; �, modes which did not reflect from the damage. Figure 8: Frequency–wavenumber spectra of wave modes propagating in the aluminium-CFRP plate and reflecting from the 10 mm delamination between the third and the fourth by [Position (2)]: (**a**) for in-plane and (**b**) out-of-plane components while exciting Mode 2 at 475 kHz, and (**c**) for in-plane and (**d**) out-of-plane components while exciting Mode 5 at 475 kHz. Results show that there are wave modes, which are sensitive to the damage defined in the composite and metallic part of the structure, represented in Modes 1-3 with respect to the case described here. To be therefore able to separate the damage in the aluminium liner from the damage in the CFRP overwrap, the characteristic interaction of a mode sensitive to the damage only in one part is needed. This has been observed for Mode 4 in the case of the crack in the aluminium liner and for Mode 5 in the case of the delamination at both positions defined in CFRP. One has to be careful here, in the sense that the right modes are used and that a damage in the CFRP part will not be mistaken for a damage in the aluminium liner. This is why it is important to be able to excite modes in different constituent parts of the component, and that chosen modes show characteristic interaction only with the damage in one of the parts. As a remark, in the present work, a case of the CFRP being delaminated at numerous locations across the thickness has not been considered so far, which would represent an impact damage. Supposedly, more modes may interact with such an impact damage, and this may result in higher amplitudes of the reflected modes than in the case of a single delamination considered here. As for micro-cracking of the CFRP overwrap, a different approach is necessary where guided wave modes are analysed based on the change of their phase [10] or group velocity [13], and not the reflection from the damage. A solution in that regard for the reception as well as the excitation of the desired modes could be an interdigital transducer (IDT) being specifically \"tuned\" to the frequency and wavelength sensitive to the respective damage to be monitored. A 15 mm pitch between its electrodes may be suitable for the cases described here, which corresponds to Modes 4 and 5 when driven at 400 kHz and 475 kHz, respectively. Excitation of Mode 4 will allow the damage in the aluminium part and excitation of Mode 5 the damage in the CFRP part to be identified. A design of such an IDT based monitoring system could be to get it integrated at the aluminium-CFRP interface. The monitoring system configuration (pattern) should be adapted to as many modes as possible being sensitive to the damage to be monitored. Here, thin and flexible polymer materials such as PVDF can be used.