Study the character of rock fracture mode using acoustic emission technique

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Study the character of rock fracture mode using acoustic emission technique

 

Author: Gao, Yuan
Title: Study the character of rock fracture mode using acoustic emission technique
Degree: M.Sc.
Year: 2011
Subject: Rocks -- Fracture.
Acoustic emission.
Rock mechanics.
Fracture mechanics.
Acoustic emission testing.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Civil and Structural Engineering
Pages: xi, 85, 8 leaves : ill. (some col.) ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2462909
URI: http://theses.lib.polyu.edu.hk/handle/200/6336
Abstract: Fracture propagation is a hot topic in rock mechanics. There are three types of cracks induced when a fracture propagates: mode I—tensile crack, mode II—shear crack and mode III—anti-plane shear crack. Most of the previous research used the acoustic emission (AE) to study the fracture propagation in rock, which is very useful technique to detect the initiation and propagation of the microcracks inside the rock. But the AE technique was mainly used on locating crack. It is believed that different modes of cracking would have different AE energy and waveforms. Therefore, some characteristics, such as duration and amplitude in waveform of acoustic emission event, are very useful to classify the modes of rock fracture. Thus, this report presents an experimental study on the characteristics of mode I—tensile crack, mode II—shear crack and mode III—anti-plane shear crack by using the acoustic emission (AE) technique. Three point bending test, Punch through shear test and Antiplane punch through shear test are used to study the mode I, mode II and mode III cracks respectively. The testing results show that the mode II has bigger fracture toughness (4.67 MPa√m) and energy release rate (409 J/m²) than mode III and mode I. For AE analysis, the directions of first motion of P wave of tensile crack are downward. But for the shear crack, each event has both downward and upward of P wave first motion direction due to different locations of sensors. For the anti-plane shear crack, the events also have upward/downward first motion direction, but more complicated than mode II. In this study, it is found that although the testing under tensile stress, but only 85% cracks are the tensile fractures. For the punch through shear test (mode II test), 55% crack are the shear fractures. The mode III testing result contains 45% tensile fracture, 20% shear fracture and 35% anti-plane shear fracture. In the full waveform analysis, the mode I—tensile fracture have long duration (19.77 μs), small maximum amplitudes of AE events (below 0.2 mv) and high dominant frequency (439.2 kHz); the mode II—shear fracture had very short duration of acoustic emission events (3.06 μs), large maximum amplitudes (below 0.8 mv), and lower average dominant frequency (374.2 kHz); the mode III—anti-plane shear fracture has relatively longer duration (25.915 μs), small maximum amplitudes (below 0.2 mv), and much lower dominant frequency (176.8 kHz). All above characteristics for three modes cracks are firstly summarized in this study, and hopefully it could help to classify different modes of rock fracture.

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