## 3 Experimental Results and Analysis
### Testing for Damage Detection Capabilities (O2)  
Section 2.2 described the second objective (O2) which should be achieved by following the testing procedure P2, namely that of obtaining signals useful for detecting and assessing (diagnosing) damage in the corresponding slabs, especially for vibrations of low frequency and high amplitude. Procedure P2 was applied to three type-B slabs (pristine condition), two type-C1 slabs (thus, involving one notch), and two more -C2 slabs (with two notches-see Figure 2). Testing involved vibrating the free slab end at 4 Hz, with mean value of 12.5 mm and amplitude of 25 mm, with the response signals (as generated by the reception coil) recorded at 200 KHz via a conventional oscilloscope. Even though the sampling frequency is larger than those used in traditional structural monitoring systems, it is by no means unobtainable, since no particular exotic equipment solutions are needed. Figure 6 shows the power spectra of the signal responses from the type-B slabs, with insets presenting frequency regions with the largest spikes from test data of at least two out of the three type-B slabs tested. From tests with type-A (see Section 3.1), it was deduced that an M-sensor slab provides meaningful signal (with respect to its frequency content) at high frequencies. At the same time, a convenient fact is that small damage may be quite clearly indicated at higher modes of vibration [25]. Hence, one should check whether the spikes noted in Figure 6 actually correspond to eigenfrequencies, so that they could be used for damage diagnosis purposes. Therefore, the FEM analysis described in Section 2.3 was carried out. Figure 7 presents indicative results of type-B, -C1 and -C2 slabs for two frequency ranges (around 47,000 and 90,000 Hz). These plots resulted from the finite elements modal vibration analysis and show the eigenfrequencies and eigenmodes of the combined structure (slab and ribbon) at two significant frequencies noted in signal power spectral analysis (Figure 6). As seen, type-B slabs show quite different behavior to that of type-C1, -C2 slabs for both frequency bands. The results from eigenfrequencies determination for a type-B (among others) slab are presented in Table 1 and compared with those resulting from Figure 6, whereas their percent error is also shown. Interestingly, the six largest spikes in Figure 6 are at frequency values extremely close to the eigenfrequencies of type-B slabs. The percent error shown in Table 1 is almost insignificant, except for the lowest eigenfrequency at 4982.4 Hz, where it reaches almost 5%. This is in agreement with findings of type-A slab testing (Section 3.1), where the M-sensor's output frequency content was significant mainly at higher frequencies. So, finite element  
Figure 5: Power spectrum of response of two type-A (in green and red) and two -A blanc (in dark grey and black) slabs tested following procedure P1. Evidence that the hidden strip acts like a sensor are given in insets. This result could be anticipated, even without the FEM analysis. In fact, every realistic implementation of a quasi-sinusoidal excitation would trigger harmonics in the response signal, with their magnitude in the signal power spectrum following a decreasing trend with growing frequency values. On the rare occasion of spikes actually becoming larger with growing frequencies, one may start thinking of such harmonics being close to eigenfrequencies of the tested structure. Figure 8 shows the power spectra of the signal responses from two type-C1 and another two type-C2 slabs. Power spectra of signals from type-B slabs are also presented for comparison. Considering the 6 largest spikes (as for the case of test data from type-B slabs), it results that type-C1 and -C2 slabs show spikes at approximately the same frequencies as type-B slabs (see Table 1) except for the spikes at 50,110 Hz and 40,090 Hz.