Acoustic emission monitoring of mode I fracture toughness tests on sandstone rocks

Experimental study on cracking process and fracture properties of granite under mode I loading by acoustic emission and digital image correlation

“…Guo et al 24 studied the mode I fracture behavior of granites with different particle sizes, and found that granite with coarse grain size (average grain size = 1.26 mm) exhibited a certain plastic deformation when approaching the peak, and ductile failure occurred after the peak, while the samples with other particle sizes suffered brittle failure. Similar postpeak brittle fracture behaviors also appeared in the studies of Kang et al, 39 Shao et al 40 and Muñoz‐Ibáñez al 25 . Compared with the existing research results, it can be also seen that if the sample does not show any plasticity in the prestage of the peak, that is, the whole fracture process increases linearly, the possibility of postpeak ductile failure is very small.…”

“…To investigate the cracking behaviors of granite under pure mode I loading, three‐point bending tests on SCB granite specimens with an average grain size of 1.67 mm and a / H ≈ 0.4 were carried out in this study. The mechanical test results revealed that all the three specimens exhibit a ductile fracture characteristic rather than the complete brittle behavior as reported by Guo et al 5,18,24 The rock type and the physical and mechanical properties of the rock itself are one of the influencing factors leading to the postpeak ductile fracture 25 . Guo et al 24 studied the mode I fracture behavior of granites with different particle sizes, and found that granite with coarse grain size (average grain size = 1.26 mm) exhibited a certain plastic deformation when approaching the peak, and ductile failure occurred after the peak, while the samples with other particle sizes suffered brittle failure.…”

“…For the location accuracy of AE events, there is a certain amount of errors in the obtained event source coordinates, whether using the results given by the AE system or a secondary processing of the waveform of the original data with Matlab and other computational software. The results reported by Muñoz‐Ibáñez et al 25 indicated that there was a location error of 3 mm when comparing the known position of pencil‐lead break with the source coordinates calculated by AE software. In addition, under the three‐point bending loading, the crack initiation and propagation inside the sample, resulting in a change in the wave velocity of the rock, is another source of the error in the AE event location 43 .…”

“…In this case, the probability of ductile failure is greater. For instance, Lu et al 36 successfully obtained the postpeak strain softening curve of red sandstone by using beam specimens with a / H ≈ 0.27, whereas, in the experimental study conducted by Muñoz‐Ibáñez et al, 25 the notch length of the SCB sandstone samples was set to be overly high, which is directly related to whether the rock has sufficient buffer space for tension/compression. In other words, the FPZ has almost no enough space for growth under these conditions, the likelihood of plastic deformation before the peak is considerably reduced, and the probability of ductility failure curves after the peak is approximately zero.…”

“…Li and Einstein, 16 Wong et al 23 and Guo et al 5,24 further investigated the microcracking mechanisms and the FPZ size by analyzing the spatial-temporal evolution, the moment magnitude and the source mechanism of AE events. Muñoz-Ib añez et al 25 assessed crack modes and cracking processes by comparison of the average frequency, rise angle and b value. AE can characterize microcracks at a microscopic scale, but is limited by the influence of noise signals and positioning accuracy, because it is rough to judge the deformation characteristics of crack tips.…”