ACOUSTIC AND ELECTROMAGNETIC EMISSION FROM CRACK CREATED IN ROCK SAMPLE UNDER DEFORMATION

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1 ACOUSTIC AND ELECTROMAGNETIC EMISSION FROM CRACK CREATED IN ROCK SAMPLE UNDER DEFORMATION YASUHIKO MORI 1, YOSHIHIKO OBATA 1 and JOSEF SIKULA 2 1) College of Industrial Technology, Nihon University, Izumi 1-2-1, Narashino, Chiba , Japan; 2) Czech Noise Research Laboratory, Brno University of Technology Technicka 8, CZ Brno, Czech Republic Abstract In order to characterize the generations of electromagnetic emission (EME) and cracks created inside the materials in detail, we measured EME under monotonously increasing and repeated compressive loading of the granite sample. In the loading tests, AE signals were simultaneously measured with the EME signals, so that the generation of EME could be directly compared with the entire fracture process of the rock sample estimated by AE. In the uniaxial compressive tests, the sample was loaded at different displacement speeds from 0.02 to 0.5 mm/min. In comparison with AE, the number of EME events discriminated is lower, because of a lower signal-to-noise ratio of EME channel. The relationship between signal amplitudes of EME and AE suggests the existence of the correlation between AE and EME. The EME signals with the larger amplitude are associated with the AE signals of larger amplitude. Dispersion observed in the EME vs. AE signal amplitude plots is related to crack orientation with respect to EME electrodes. This paper also demonstrates that the simultaneous measurement of AE and EME would be useful for estimating the rock in-situ stress, as an example. Keywords: NDT, Electromagnetic emission (EME), Cracks, Granite Introduction It has been known that the changes in geo-electric potential and the anomalous radiation of geo-electromagnetic waves were observed before major earthquakes. These phenomena also have been observed in laboratory experiments on rock samples, and it was found that micro- and macro-cracking processes are often accompanied by acoustic and electromagnetic emission [1-7]. Generation of cracks in solids is accompanied by the generation of electric charges and the mechanical vibrations. Mechanical vibrations generate acoustic emission (AE) signal. The crack surfaces are electrically charged due to the loss of chemical bonds. Electric charges at the faces of the newly created cracks constitute an electric dipole or quadrupole system. They are sources of electromagnetic field and measurable quantity of this phenomenon is called electromagnetic emission (EME). EME signal can be detected by capacitive electrodes placed on the sample surfaces of low electrical conductivity. In this case the electric field vs. time is detected. A coil can detect the magnetic field of good electrical conductors. Experimentally we have observed two different type of EME signals like damped harmonic motion with: (i) exponentially time-dependent transient value (Fig. 1(a)) and (ii) constant average value (Fig. 1(b)). We suppose that the first one is generated by electrical dipole structure and second one is generated by an electrical quadrupole [8]. J. Acoustic Emission, 27 (2009) Acoustic Emission Group

2 (a) Signal generated by dipole. (b) Signal generated by quadrupole. Fig. 1 Two type of EME signal waveforms seen in the compression test of granite sample [8]. We also found that the voltage on the measuring capacitor is directly proportional to the electric charge distribution. The recorded electric signal is superposition of crack walls self -vibration given by the crack geometry and vibration due to an ultrasonic wave given by sample dimensions. The EME signal precedes the AE response and the time delay corresponds to the distance between AE sensor and crack position due to the difference of propagation velocities of sound and electromagnetic field in the sample. In order to characterize the generations of EME and cracks created inside the materials in detail, the measurements of EME were conducted under the monotonously increasing compressive loading test and the repeated compressive loading test of the granite sample. In the loading tests, AE signals were simultaneously measured with the EME signals, so that the generation of EME could be directly compared with the entire fracture process of the rock sample estimated by AE. This paper also demonstrates that the simultaneous measurement of AE and EME would be useful for estimating the rock in-situ stress, as an example. EME under Uniaxial Compression A rectangular block sample of Inada Granite with dimensions of 10 mm x 10 mm square and height of 30 mm was deformed under the uniaxial compression stress. Schematic sample assembly is shown in Fig. 2. Details of the loading set-up are described in [3]. AE transducer with a frequency range of 200 to 700 khz was mounted on the sample surface, as shown in Fig. 2(b), to detect the AE signals during the loading test. EME signals from the rock sample were detected as the electric potential change appearing between two electrodes A-A. The electrodes were formed by painting a conductive paste on the sample surfaces, as shown in Fig. 2(b). Both the EME and AE signals were amplified by 40-dB preamplifiers and led into the input channels of the AE system used. When an AE signal was detected at AE transducer, the AE and EME signals were digitized and stored on the memory in the AE system. The threshold level of 60 db was used for AE event detection. To eliminate interference arising from ambient noise, the sample and the preamplifiers were electromagnetically shielded by using a thick aluminum alloy chamber. In the uniaxial tests, the sample was loaded at different displacement speeds ranged from 0.02 to 0.5 mm/min, so that the effects of displacement speed on the generation behaviors of EME and AE were examined. 158

3 Fig. 2 Sample assembly: (a) setup for uniaxial loading and (b) arrangement of AE transducer, electrodes for EME detection, and strain gages. Fig. 3 A set of simultaneous measurement of EME and AE signals recorded in the compression test conducted on Inada Granite. The background noise of EME signal channel is larger than that of the AE channel, since the electrodes for EME detection forms a high-impedance circuit with more than 10-time higher equivalent noise resistance than that of corresponding AE transducer. This result in difficulty of discriminating the small amplitude EME signals expected. In Fig. 3, three strong AE signals are clearly recognized. As the onset of AE signal indicated by an arrow on the EME waveform matches the EME signal onset, it can be concluded that this EME signal is associated with AE. In this manner, even small amplitude EME signals, such as the signal generated near 100 µs in Fig. 3, are detectable. In the present experiments, most of EME signals recorded were classified into the type of signal generated by electrical quadrupole structure, of which waveform was shown in Fig. 1(b). 159

4 160 (b) displacement speed = 0.06 mm/min. (c) displacement speed = 0.5 mm/min. Fig. 4 Results of simultaneous measurements of AE and EME signals during monotonously increasing compression tests of the granite samples conducted at three different displacement speeds of 0.02, 0.06 and 0.5 mm/min. (a) displacement speed = 0.02 mm/min.

5 We have observed that the EME signal is generated several µs before AE generation. This delay in AE is expected as EME signal propagates with the velocity of electromagnetic waves while AE signal propagates with the velocity of mechanical waves. Origin of EME signal corresponds to the time of crack creation. This effect was used to improve crack localization [3]. The results of simultaneous measurements of AE and EME signals during monotonously increasing compression tests of the granite samples conducted at three different displacement speeds of 0.02, 0.06 and 0.5 mm/min. are shown in Figs. 4(a), (b) and (c), respectively. In Fig. 4, applied load (P) and dilatant strain of the rock sample (ε v ), cumulative AE event count (N AE ) and EME event count (N EME ), and amplitude of AE event signal (V AEp ) and EME event signal (V EMEp ) are plotted as a function of elapsed time of the loading (t). Generation of AE starts at a level of dilatant strain of the sample, where the dilatant strain deviates from the linear trend, and increases until the sample failure takes place. Generation of EME events starts after AE generation and increases with an increase of the applied stress. It is also observed that the generation of active EME events is peculiar to the stressing stage, at which the volume of sample changes from the contraction due to compression to the dilatancy developed in a direction vertical to the sample axis stressed. This result suggests that the EME signals were emitted from the micro-crack created in the tensile direction normal to the applied load. In comparison to AE, the number of EME events discriminated is lower, because of a lower signal-to-noise ratio of EME channel. It can be said that the displacement speed has no effect on the generation behavior of AE and EME during the stressing. However, there is a difference in the number of total AE event counts among three displacement speeds. Though the total event counts measured in the tests conducted at the displacement speeds of 0.02 and 0.06 mm/min show almost the same value of 20,000, while only one fifth of the value, i.e., 4000 counts were measured in the test at the displacement speed of 0.5 mm/min. The reason for the difference in event counts observed are explained as follows: The waveforms shown in Fig. 3 were the simultaneously recorded AE and EME signal waveforms detected in the test conducted at a displacement speed of 0.5 mm/min. By the visual observation three strong AE event signals are recognized in a period of 500 µs. In this case, a dead time for AE detection was set at 1 ms on the AE system used. Therefore, the AE system was triggered by the first hit signal generated at about 100 µs, and counted the signals as one event, even though three events were visually recognized. In general, it is expected that when the material is stressed, the time interval of micro-crack creation in the material becomes short with increasing deformation rate of the stressing. Thus, when the frequency of the occurrence of micro-cracks exceeds the AE dead time, as seen in Fig. 3, the AE event counts measured by the AE system show a smaller value than actual. Next, the plots of the signal amplitude of EME (V EMEp ) and AE (V AEp ) as a function of the elapsed time of loading, are shown in Fig. 4. These clearly demonstrate that the EME and AE events having the larger signal amplitude are generated with increasing time: i.e., with increase of the dilatant strain of the sample. This nature is irrespective to the displacement speeds tested. 161

6 Fig. 5 Relationship between EME (V EMEp ) and AE (V AEp ) signal amplitude. The EME signals detected in the loading tests, without exception, were accompanied with the generation of AE signals. Thus, the relationship between the signal amplitudes of EME and AE was analyzed for the experimental results shown in Fig. 4. The results, which are summarized in Fig. 5, strongly suggest the existence of the positive correlation between AE and EME in the signal amplitude. That is, the EME signals with the larger amplitude associate with the AE signals of larger amplitude. The signal amplitude of EME would correspond to the amount of the electric charges re-distributed on the newly created crack surfaces. It is expected that the amount of electric charges would depend on the size of the crack surfaces created. Dispersion observed on the relationship in Fig. 5 is related to crack orientation with respect to EME electrodes. 162

7 EME under Repeated Loading Rectangular block samples of Inada Granite, of which dimensions were 20 mm x 20 mm x 80 mm and 10 mm x 10 mm x 40 mm, were stressed in repeated uniaxial compression load. The sample was pre-loaded up to a certain value and then unloaded. It is then loaded to a larger value and then unloaded again. This loading-unloading procedure was repeated until the sample rupture took place. Each of repeated loading-unloading procedures was conducted at a constant displacement speed of 0.5 mm/min. The sample assembly for the repeated loading test was basically the same as that shown in Fig. 2. In the repeated loading test, the EME signals were amplified by 20 db using 3S Sedlak PA 21 ultra-low-nose preamplifier. The internal noise level of PA21 preamplifier was 3 nv/ Hz in the frequency range of 500 Hz 10 MHz. The output signals of PA21 were further amplified by 40 db with a PAC 1220A preamplifier. AE signals were amplified by 40 db with a PAC 1220A preamplifier. In addition, the sample assembly and the preamplifiers were electromagnetically shielded. The threshold levels were 70 db for EME signal and 60 db for AE signal. The result of simultaneous measurements of EME and AE signals during the repeated loading test conducted on the block sample of 20 mm x 20 mm x 80 mm is shown in Fig. 6. The loading history for the repeated loading test is shown as the stress curve, σ. After an initial stress of 12.5 MPa, which corresponds to approximately 10 % of compression strength of the rock tested, was applied, the maximum stress level of successive loading was increased by 12.5 MPa steps until the sample failure. In Fig. 6, EME and AE events are represented by the cumulative event counts measured for each stressing step. Fig. 6 History of the repeated stressing test and the result of simultaneous measurement of EME and AE signals conducted on Inada Granite sample of 20 mm x 20 mm x 80 mm. 163

8 Fig. 7 Expanded stressing steps #3 and #8 in the test procedure shown in Fig. 6. The EME signals are detected during every stressing step, with the exception of step #1. The EME signals are generated only in the stage where the applied stress increases and reaches at the maximum stress level for each step, whereas the generation of AE signals is observed not only in the stress increasing stage but also in the unloading stage. For example, the details of the stressing steps #3 and #8 are shown in Fig. 7. In the figures, the arrowhead indicates the stress corresponding to the maximum pre-stress for each step. In Fig. 7, open symbol plotted on the stress curve denotes the onset stress of active AE event generation during the stress increasing stage. Similarly, the solid symbol on the stress curve denotes the onset stress of EME signal. These onset stresses of AE and EME are referred as σ AE and σ EME, respectively. Fig. 8 Plots of estimated stresses σ EME and σ AE as a function of the pre-stress σ pre. The stresses plotted are normalized by the strength of the rock sample tested σ b. The values of the σ AE and σ EME for each repeated stressing step as shown in Fig. 6 and also for the experimental results obtained in the test conducted on the block sample of 10 mm x 10 mm x 40 mm were evaluated. In Fig. 8, the σ AE and σ EME obtained are plotted as a function of the pre-stress level σ pre. In the figure, these stresses are normalized by the compression strength of the rock tested, σ b. The Kaiser effect of AE is well recognized in the range of the pre-stress below 22 % of the σ b, giving the underestimation of pre-stress. Specifically, the amount of the underestimation is 10 % to 25 % of the actual pre-stress. This underestimation would be caused by 164

9 the fact that the AE due to the friction between the fracture surfaces induced during the previous loading process is measured in the early stage of each successive stressing step. However, it should be noted that in general, Kaiser effect shall be discussed within the region of elastic or elasto-plastic deformation. In the case of the sample tested, it is expected that the corresponding stress range is 30 % of the maximum strength. On the other hand, the onset stress level of EME signal, σ EME, estimates the pre-stress level within the deviation of 10 %, over a wide range of the pre-stress up to 90 % of the compression strength of the sample tested. This result clearly suggests that the emission of EME signals is associated with the creation and/or extension of micro-cracks, and is non-reversible phenomena, similar to the Kaiser effect of AE. Therefore, it can be concluded that the EME signal measurement can estimate more accurately the current stress level, to which the rock samples have been subjected, than that of AE activity. Summary Simultaneous measurement of EME and AE during the monotonously increasing compressive loading test and the repeated loading test has been conducted on granite samples to characterize the EME generation in detail. 1. EME signal is accompanied with the generation of AE signal. In comparison with AE, the number of EME events discriminated is lower, because of a lower signal-to-noise ratio of EME channel. 2. EME and AE signals are correlated and their amplitudes increase just before the sample rupture. 3. The relationship between signal amplitudes of EME and AE suggests the existence of the correlation between AE and EME. The EME signals with the larger amplitude are associated with the AE signals of larger amplitude. Dispersion is related to crack orientation with respect to EME electrodes. 4. The emission of EME signals is associated with the creation and/or extension of micro-cracks, and is non-reversible phenomena, similar to the Kaiser effect of AE. 5. EME signal is detected several µs before AE generation. This delay in AE is expected as EME signal propagates with the velocity of electromagnetic waves while AE signal propagates with the velocity of mechanical waves. Thus, the origin of EME signal corresponds to the time of crack creation, and this effect can be used to improve crack localization by AE. 6. EME signal measurement is applicable to estimate the current stress level, to which the rock samples have been subjected. The greatest advantage is that the current stress can be directly estimated without any post signal analysis. In addition, the detection of EME from rock sample under stressing is very simple and easy, and a usual AE system can be used to record and analyze the EME signals. Acknowledgement This work was partially supported by the Grant Agency of the Czech Republic under Grants 106/07/1393 and project VZ MSM

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