# Size Effect on the Acoustic Emission Behavior of Textile-Reinforced Cement Composites
## 4 Discussion  
The trends from the curved plates described herein are in good correspondence with the straight plate AE behavior of the same material under three-point bending. Figure 10 shows the sliding average of the RT for straight elements with different widths, taken from a recent study [22]. The wider specimen (width of 400 mm), termed as the \"plate\", showed a much higher RT than the 23-mm wide beam. Gradually, with more time in the loading process, a shift to higher values is shown, as there is a reasonable shift from matrix cracks to delaminations. The results from the plane specimens confirmed that the curvature of the plate in the direction of propagation was not crucial and did not significantly influence the obtained trends [23]. However, a large increase in the lateral dimension imposed large discrepancies in the received waveform, increasing the RT by two to three times both in the curved (Figure 8) and straight plates (Figure 10). Considering the waveforms of Figure 5a, a measure of attenuation was obtained. Extra propagation of 100 mm reduced the absolute amplitude by 72% (first sensor: 8.135 V, second sensor: 2.286 V). A possible side reflection needed to propagate a total of 300 mm (150 mm from the center of the plate to the edge and back), so without any other effect, it corresponded to three times the aforementioned decrease, leading to an amplitude of only 2.2% of the original. This is too small to be pinpointed in the waveform of a resonant transducer and, as such, cannot influence the measurement of the RT. Nevertheless, the calculated expected time of these negligible reflections in the plates of 300 mm, considering a propagation velocity of approximately 3000 m/s, would be 100 \(\upmu\)s. In the waveform, any corresponding contribution at that time was not clearly observed. We can conclude that for the wide specimen (300 mm), the energy that propagated sideways was actually lost and practically was not expected to show again in the waveforms. For the less wide specimens (50 mm and 25 mm), the time of reflection arrival was less than 20 \(\upmu\)s. Therefore, the contribution of the reflections was included in the main body of the waveform. Due to the duration of the main pulse, the plate wave dispersion and the sensitive, resonant nature of the transducers, isolating the reflection contributions was not a straightforward process. To examine how the possible characterization would be influenced by the geometry, Figure 11 presents the AF vs. RT graph for the matrix cracking sources (first 100 hits) in the beam and plate. All individual points are visible, along with the average of the populations (larger symbols) and the standard deviations in the ellipses. The shift of the populations is obvious, with the plate data expanding to much higher RTs and lower frequencies, while there is no overlap for 65% of the populations, indicated by the ellipses. Figure 10: Sliding average of the rise time for plane TRC specimens with different widths under three-point loading. The practical implication of this shift in AE values can be understood by considering the dominant crack classification methodology that is described in the literature and the RILEM recommendations [2]. In general, a high frequency and low RT value are connected to the tensile mode of cracking. On the other hand, a low frequency and higher RT indicates the shear type of failure. Therefore, a relative shift between values would be interpreted as a change of the dominant fracture mode. However, when we compared different type of specimens, like in the case herein, this difference in AE parameters was only the result of the different geometry (in this case, the dimension lateral to propagation, termed as the width), despite the underlying basic fracture mechanism being the same (matrix cracking). As seen in Figure 11, there was a difference of more than 50 kHz in the average frequency value, while the RT was more than doubled for the case of the plate compared with the thin beam.