## 4 Results and Discussion
### Acoustic Emission Signature Obtained with A Virtual Perfect Point Local Sensor  
Figure 16 shows the result obtained with a virtual perfect point-like local sensor situated at the position of the sensor C1. The velocity Vz calculated at a single node provides the signal recorded by a virtual perfect local point sensor. Based on these results, most of the energy of the wave is concentrated in the 400 to 800 kHz frequency band and a peak frequency appears around 700 kHz for fiber break located at the position P1. Without the sensor effect, the frequency content is higher, especially for fiber breaks near the sensor. Simulation also provides waveforms computed along a straight line on the surface in order to further analyze the characteristics of the wave propagation in the medium. In this case, the fiber break is located at the position P2 and the waveforms are calculated on a single node from the epicentre of the source to the boundaries of the gauge length on the top surface (out-of plane velocity Vz) and on the lateral surface (in plane velocity Vy). In both cases, the waveform amplitude significantly decreases in comparison to the one extracted at the epicentre of the AE source. These simulations suggest a shift in the main frequencies as the wave propagates from the source to the boundaries. One can observe changes in both the out-of-plane and the in-plane velocity amplitude and the frequency content (Figure 17). Finally, mainly the lower frequency components (around 500 kHz) appear far from the fiber break position. These are also information of \"primitive\" AE, which is defined as the waveform near the source (both in distance and in time). A high frequency content, around 900 kHz  
Figure 15: Descriptors vs. source/sensor distance for experimental signals and numerical signals detected by nano30 and PicoHF sensors: (**a**) amplitude in dB, (**b**) frequency centroid in kHz, e 2.8 mm and z = 0.9 mm. for the out-of-plane velocity and 700 kHz for the in-plane velocity, typifies the primitive AE signal, which corresponds to the signal detected at the epicenter of the source. For the in-plane velocity, the distance between the source and the epicenter point is equal to 2 mm. For the out-of-plane velocity, the distance is equal to 0.5 mm. Waveform analysis reveals a strong dependence of AE parameters on the distance between the source crack and the receiver point, stressing out that the influence of distance is crucial and should be taken into account for a successful identification of AE sources. Figure 16: SPWVD results and FFT obtained with a virtual perfect local point sensor for two different simulated fiber break (**a**) at position P1 and (**b**) at position P5 (e = 2.8 mm and z = 0.9 mm). Figure 17: _Cont._  
The comparison between the data recorded with the two kinds of sensors and the data recorded with a virtual perfect point-like local sensor shows that the data recorded by the picoHF sensor near the source are more representative of the frequency content than the data recorded by the nano30. These results may be very useful in order to choose the more suitable sensor. In this case, the modelling approach delivers very good information over experimentally inaccessible parameters. The fiber failure mechanism results in different amplitudes of symmetric and antisymmetric modes. Figure 18 shows the 2 DFFT for the simulated signal recorded on the faces of the specimen. It displays the dispersion curve of each mode through a colour map of its modulus as a function of the wave number \(k\) and the frequency \(f\). On the lateral edge of the sample, larger amplitudes of the measurable modes are observed than on the face. Of course, this finding must be qualified by the fact that the modes excited by a source placed outside the median plane of the sample do not necessarily have a strong component in the y direction. This is for example the case of the F\({}_{11}\) bending mode, which can be compared to the well-known A\({}_{0}\) antisymmetric mode for an infinite plate.