Issue 40

K. Kaklis et alii, Frattura ed Integrità Strutturale, 40 (2017) 1-17; DOI: 10.3221/IGF-ESIS.40.01 12 Figure 13: Time evolution of acoustic emission parameters during the CCNBD test of the specimen 2.3 (mode I fracture). (a) The distribution of amplitudes of the 4 sensors located to the front face of the specimen, in conjunction with the applied load. (b) The mean hit rate of all sensors. (c) The cumulative distribution of hits and (d) the rise time of hits recorded in channel 4. Two-dimensional AE source localization was also conducted in order to gain additional information for the formation of the first macroscopic crack in specimens with mode I fracture. We have to note here that the small size of the specimens as compared to the size of the sensors (5mm diameter) and the size effect to the acoustic wave velocities result to inevitable large uncertainties to the estimation of the events location, leading to the scattering of the estimated positions. However, the rejection by filtering, of the events with low amplitudes close to the threshold (Amp<44 dB) may improve the overall pattern although it reduces their total number. A 2D location pattern derived from the signals recorded from the 4 sensors positioned at the front side of specimen 2.3 is illustrated in Fig. 14. From the first snapshot (t = 26 s) we may conclude that the AE activity initiates at the bottom tip of the chevron notch. Afterwards, events with higher amplitudes are observed to the upper tip suggesting that the stress concentration is higher in this region. However, during the formation of the macroscopic crack (t = 34 s) the events are randomly distributed around the two tips of the notch without any preferential orientation or position. Similar patterns were also estimated for the other specimens subjected to mode I fracture. In the case of the CCNBD tests of the specimens fractured under pure mode II loading, the AE activity exhibits some similar characteristics with the mode I loading, regarding the correlation of the AE parameters with the fracture process. The AE parameters during the test of a representative specimen (specimen 3.23) are shown in Fig. 15a-d. Considerable AE activity starts at around 57% of the fracture load (5.9 kN) as it is indicated from the uniform increase of the mean hit rate in Fig. 15b. However, a small number of hits with relative high amplitudes are also recorded from the beginning of