## 3 Experimental Results and Analysis
### Testing for Damage Detection Capabilities (O2)
These values were also cross-validated via FEM analysis: Eigenfrequencies at the region of 50 KHz were computed at 49,545 Hz and 49,946 Hz for -C1 and -C2 slabs, respectively. Again, as observed at the region of 40 KHz, eigenfrequencies equal to 39,936 Hz and 40,088 Hz were computed for -C1 and -C2 slabs, respectively. In all cases, the percent errors of the experimentally obtained (via power spectra) eigenfrequency values with respect to those computed via FEM analysis are extremely small. In both cases, the spike occurrences indicate alterations in structural condition, because at these frequencies no significant content was present in signals from pristine slabs (see also Figure 6). Note that in many other (mainly lower) frequency areas, one may find circumstantial evidence of spikes associated to signals from some but not all damaged slabs, in some cases somewhat undermined by noise. Hence, such frequency areas have been systematically disregarded, in favor of frequency bands where a selected number of the most prominent spikes may be clearly denoted. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**FEM Eigenfrequencies in Hz for** & \multicolumn{3}{c}{**Power Spectra Eigenfrequencies**} & \multicolumn{3}{c}{**Absolute Value of \% Error:**} \\  & **Indicated Slab: (I)** & \multicolumn{3}{c}{**in Hz for Indicated Slab: (II)**} & \multicolumn{3}{c}{**abs(I)-(II)/(I)**} \\ \hline
**-B** & **-C1** & **-C2** & **-B** & **-C1** & **-C2** & **-B** & **-C1** & **-C2** \\ \hline
4982.4 & 4986.9 & 5158.5 & 5235 & 5230 & 5233 & 5 & 5 & 1.4 \\
20,389 & 20,373 & 19,985 & 20,142 & 20,140 & 20,140 & 1.2 & 1.1 & 0.8 \\
34,804 & 34,798 & 35,257 & 34,868 & 34,865 & 34,868 & 0.2 & 0.2 & 1.1 \\
47,402 & 47,253 & 47,017 & 47,553 & 47,555 & 47,555 & 0.3 & 0.6 & 1.1 \\
65,038 & 65,007 & 65,061 & 64,898 & 64,900 & 64,895 & 0.2 & 0.2 & 0.3 \\
89,994 & 90,038 & 90,064 & 90,135 & 90,133 & 90,133 & 0.2 & 0.2 & 0.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Eigenfrequencies of type-B, -C1 and -C2 slabs as determined by the solution of the finite elements model compared with the ones picked up in signal power spectra. Figure 8: Power spectrum of response of type-C1 and -C2 slabs tested following P2 (type-B shown for comparison): Hidden strip acts as a sensor and damage causes spikes at 40 and 50 KHz (see insets). There is also evidence that the damage level (one or two notches) may lead to spikes of different amplitude, since at both frequency ranges of interest (around 40 and 50 kHz), -C1 slabs feature power spectra with larger spikes than those from -C2 slabs. The analysis in [28] (specifically in Ch. 10), proposes an estimation of the damping coefficient from such plots, by associating spikes of narrower angle (i.e., more abrupt) to lower damping for a given frequency. The fact that, at both frequencies, the spikes from -C1 slabs (in red and dark red) have a slightly narrower angle than those from -C2 slabs (in green and dark green) may indicate that damping slightly increases with notches, as should theoretically be the case [25]. Hence, damage is definitely detectable by considering a number of the most prominent spikes of the tested slab (mainly at higher frequencies) and comparing with the corresponding group of spikes from pristine slabs. Currently, frequencies around 40 and 50 KHz seem suitable for detecting damage, with occurrence of new spikes pointing to structural alteration (notches) of the tested slabs.