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ABSTRACT: Acoustic emission monitoring techniques are a powerful tool for analyzing deformation behavior and fracture characteristics of rock samples. Uniaxial compression tests have been carried out on three different types of sandstone for a better understanding of acoustic emission response with respect to rock deformation and damage development. All sandstone samples reveal congruent characteristic acoustic emission patterns which correlated directly with distinct stress-strain stages. However, the acoustic response and development within the individual stages prove to be entirely different due to variations in their microstructure. 1 INTRODUCTION Acoustic emission testing is used as a type of nondestructive testing technology in the ultrasonic regime. In contrast to conventional ultrasonic procedures AE tools do not actively produce waves but passively detect emissions from acoustic sources in materials which occur with sufficient suddenness to generate acoustic pulses. Mechanical loading like uniaxial compression tests on rock samples can serve as an external stimulus to generate acoustic emissions which result from local rapid stress-releasing events. Acoustic emission techniques became an important tool in rock mechanics where the effect is often referred to as "microseismic activity" (Hardy 2003). The fracture behavior of a rock is known to be governed by the formation, growth and interaction of microcracks. In numerous earlier works that studied the fracture process in connection with acoustic emission the basis for correlations was predominantly the count of the acoustic events. With this paper an attempt is made to characterize the acoustic emission activity more fully by taking the amplitudes of the events into account which particularly mirror the intensity of the acoustic response. Uniaxial compression tests have been carried out on various sandstone samples and AE monitoring was used to define several stages of failure development and the corresponding change in stress-induced strain with respect to variations in the petrography of the rocks.
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (1.00)
- Geology > Geological Subdiscipline (1.00)
ABSTRACT In this paper the results of a study on two main acoustic emission (AE) parameters – hit rate and energy- measured in laboratory on rock samples prior to failure are presented. Uniaxial test were carried out on Lavasan granite specimens, dimensions 45 mm diameter and 100 mm height, using a servo–controlled compression device with a constant rate of displacement. Four transducers were used for monitoring the acoustic emissions during loading. Acoustic emission event-hit data were recorded continuously until the failure of the specimens. Axial and radial deformations have also been measured by electric resistance gauges. Plotting of accumulated hit and the accumulated absolute energy versus time shows the absolute energy is a more appropriate parameter to monitor crack propagation and to predict catastrophic failure of specimens than the conventional AE's hit. By comparing the AE steps with volumetric deformation, we found that AE energy show definitively each steps of deformation that presented by Bieniawski (1967). 1 INTRODUCTION A number of techniques have been developed to detect crack growth and to study failure evolution in brittle materials. The most common of these involves the use of electric resistance strain gauges to measure slight changes in sample deformation that can be related to the closing and to the opening of cracks (Bieniawski, 1967). To a lesser extent, acoustic emission monitoring has been used to correlate the number of acoustic events to various strain gauge responses (Ohnaka and Mogi 1982; Khair, 1984; Lockner et al, 1991 and Eberhardt et al, 1998). Acoustic emissions (AE) are the stress waves produced by the sudden internal stress redistribution of the materials caused by the changes in the internal structure. Most of the sources of AEs are damage-related; thus, the detection and monitoring of these emissions are commonly used to predict material failure (Huang et al, 1998). Recording the number of AE events is the most conventional method for evaluation of damage procedure in rock. In addition, to recording the number of events and correlating this number to the measured deformation response in the rock, it is also possible to record certain properties of the AE waveforms. Generally, these waveforms are complex and using them to characterize the source can be difficult. Due to these complexities, AE waveform analysis can range from simple parameter measurements to more complex pattern recognition (Eberhardt, 1998). The characteristics of an acoustic event may also be used to approximate the release of kinetic energy through the AE event. The true energy is directly proportional to the area under the acoustic emission waveform which in turn can be measured by digitizing and integrating the waveform signal. The energy of event can be approximated as the square of the peak amplitude(Lockner et al., 1991) or the square of the peak amplitude multiplied by the event duration (Hardy, 2003). The resulting values are actually more representative of the event power (the units are given in dB), but are commonly referred to as energy calculations in the literature due to their approximately linear relationship with energy.
Acoustic Emissions During Deformation of Sandstone with Fluids
Muqtadir, A. (King Fahd University of Petroleum & Minerals) | Al-Dughaimi, S. (King Fahd University of Petroleum & Minerals) | Ali, A. Z. (King Fahd University of Petroleum & Minerals) | Kandil, M. E. (King Fahd University of Petroleum & Minerals) | Dvorkin, J. (King Fahd University of Petroleum & Minerals)
ABSTRACT: We experimentally investigate the effect of the pore fluid on AE in four sandstone plugs cored from a homogeneous slab. These plugs were fully saturated with brine, mineral oil, and hexane, while one of them remained dry. AE level was highest in the hexane-saturated sample, smaller in the oil-saturated sample, and even smaller in the brine-saturated sample. It was even smaller in the dry sample. It appears that in the liquid-saturated plugs, the AE level is monotonically related to the acoustic impedance of the pore fluid – the smaller the impedance the higher the AE. It does not appear to be related to the viscosity of the pore fluid.
- North America > United States (0.70)
- Asia > Middle East > Saudi Arabia (0.15)
- Geology > Mineral (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.64)
ABSTRACT The reliability of the more conventional non-destructive testing techniques, as applied to offshore steel jacket structures and semisubmersibles, to provide accurate assessment of defect severity or growth is questioned. The sources of error and inconsistency when applying these techniques to geometrically complex structures of relatively heavy section are discussed. The introduction of continuous monitoring techniques to establish structural integrity is reviewed and the promising acoustic emission analysis method is described in some detail. The use of acoustic emission analysis to monitor fatigue cracking or repaired cracks in the submerged part of offshore structures has been researched and applied to a number of platforms in the North Sea, together with laboratory and offshore exercises to assess the feasability of the technique. The extension of the method to topside applications, for which land based experience can be paralleled, is shown to offer a number of benefits when applied to pressurised components and systems, critical areas of the superstructure, slew ring cranes and general leak detection. INTRODUCTION When cracks are found in components or structures the factors which determine the servicability of that component or structure usually Centre around the size and/or history of the defect together with the material and loading parameters. In the case of fatigue normally all of the above must be considered. In order to obtain the information from which decisions can be made, non-destructive testing is applied. In theory, defect surface length and defect depth can be obtained at the time and perhaps compared with previous inspection reports. It is an unfortunate fact that rarely can any of these parameters be obtained with sufficient reliability or accuracy except perhaps for surface crack length. The situation progressively deteriorates once taken into environments which differ substantially from laboratory or normal field conditions.
Combination of Various Laboratory Tests to Investigate Rock Burst
Gottsbacher, L. (Institute of Rock Mechanics and Tunneling, Graz University of Technology) | Klammer, A. (Institute of Rock Mechanics and Tunneling, Graz University of Technology) | Schubert, W. (Institute of Rock Mechanics and Tunneling, Graz University of Technology) | Marschallinger, R. (University of Salzburg) | Hofmann, P. (University of Salzburg) | Zobl, F. (University of Salzburg) | Ketcham, R. (Jackson School of Geosciences, University of Texas at Austin) | Edey, D. (Jackson School of Geosciences, University of Texas at Austin)
Abstract The failure hazard rock burst is very dangerous as it can occur very suddenly and violently. Due to the fact, that tunnel and mining projects are in deeper areas than ever, the importance of investigating this hazard is more relevant than ever. The Graz University of Technology and the University of Salzburg are investigating rock burst with different laboratory tests to learn more about it. The test methods are uniaxial compression tests, acoustic emission tests, micro computed tomography and object based image analysis of thin sections of rock samples. To simulate the high stress state of rock that is prone to rock burst, stress close to the failure stress is applied on the rock samples. Simultaneously, the acoustic emissions in the sample are measured to investigate acoustic events in the sample, which are generated by micro cracks. After that, μCTscans of the loaded samples are performed to create 3D images of the micro-cracks in the rock sample. Subsequently thin sections are made from the samples for an object based image analysis. To evaluate the results various methods are used, from newly developed MATLAB codes to pattern recognition software to analyze the data. The findings allow a better understanding of the underlying mechanism of rock burst and indicate the usefulness of various testing methods to investigate the hazard. 1 Introduction Modern tunneling and mining projects are exploring deeper areas than ever before, because of this it is necessary to learn more about the hazard of rock burst. This failure is extremely dangerous, as it can occur very suddenly and is capable of releasing high amounts of energy. This is a high risk for the life of workers and the used equipment, it is very important to understand rock burst fully and find methods to predict it. As part of a research project financed by the Austrian Research Promotion Agency (FFG) the Graz University of Technology and the University of Salzburg are investigating this hazard, with the University of Salzburg focusing on the geological aspects and the Graz University of Technology on the rock mechanical aspects.
- Materials > Metals & Mining (0.95)
- Energy (0.70)
- Health & Medicine > Diagnostic Medicine > Lab Test (0.62)