## 4 Discussion  
Two tables are reported to synthesize the attained results, which are clustered for the left and the right stringers. In Tables 1 and 2, the detected damage extension is provided for panel PNL1 and panel PNL2, respectively. In the first case, the fault zone is revealed with a max difference of 12.5%; therefore, it can be assumed that extrapolated data and actual measures (NDI) are in good agreement. In the second case, predicted performance is still good, even it should be considered in that case, only a single stringer (the left one) is affected by debonding. In that case, the error estimated is 3.2% as algorithm is compared with C-scan output in the region of interest.  
These results are extremely encouraging, even if the damaged region is very extensive, and then easily detectable in principle by visual inspection. On the other side, it is evident that the precision shown by the proposed SHM system is relevant, from both the point of view of damage extension and damage position (start/end) detection. Furthermore, selected damage indexes assume remarkable value along the whole range of interest. The system is able to catch the strain variation correlated to the elastic energy that is released as debonding occurs, and consequent to the impact. Such energy corresponds to deformations of about 100 microstrains. The system, on the other hand is able to discern strains of about 10 microstrains that could be set as the \"observation\" threshold. If we consider that displacement may be represented as a fourth degree polynomial (free-free element), it follows that deformation has a quadratic dependence on the damage extension. Therefore, standing the observed results, it could be extrapolated that faults can be detected, whose size is up to a third of the currently investigated size. Such a value brings the SHM system capability to a bottom margin of 30 to 40 mm, wide enough for real applications where limits are often imposed at 80 mm (typical step of fasteners).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**Skin/Stringer Debonding on PNL1**} \\ \hline  & **Left Cap on** & **Right Cap on** & **Left Cap on** & **Right Cap on** \\  & **Left stringer** & **Left stringer** & **Right stringer** & **Right stringer** \\ \hline \begin{tabular}{c} _X-Cor Algorithm_ \\ _Position [mm]_ \\ \end{tabular} & from 890 to 980 & from 690 to 780 & from 400 to 470 & from 200 to 290 \\ \hline \begin{tabular}{c} _X-Cor Algorithm_ \\ _Extension [mm]_ \\ \end{tabular} & 90 & 90 & 70 & 90 \\ \hline \
\begin{tabular}{c} _C-scan_ \\ _Extension [mm]_ \\ \end{tabular} & 80 & 90 & 80 & 90 \\ \hline
\begin{tabular}{c} _Error [\%]:_ \\ _(XCor–Cscan) \_ **100** \\ \end{tabular} & +12.5 & 0 & \(-\)12.5 & 0 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Damage position estimation for PNL1.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**Skin/Stringer Debonding on PNL2**} \\ \hline  & \multicolumn{4}{c}{**FO on the Cap of Left stringer**} & \multicolumn{4}{c}{**FO on the Skin of Left stringer**} \\ \hline \begin{tabular}{c} _X-Cor Algorithm_ \\ _Position [mm]_ \\ \end{tabular} & from 360 to 480 & from 780 to 810 & from 880 to 930 & from 1040 to 1045 \\ \hline \begin{tabular}{c} _X-Cor Algorithm_ \\ _Extension [mm]_ \\ \end{tabular} & 120 & 30 & 50 & 5 \\ \hline \
\begin{tabular}{c} _C-scan_ \\ _Extension [mm]_ \\ \end{tabular} & 124 & - & - & - \\ \hline
\begin{tabular}{c} _Error [\%]:_ \\ _(XCor–Cscan) \_ **100** \\ \end{tabular} & \(-\)3.2 & - & - & - \\ \hline \hline \end{tabular}
\end{table}
Table 2: Damage position estimation for PNL2.