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Acoustic emission based mechanical behaviors of Beishan granite under conventional triaxial compression and hydro-mechanical coupling tests
International Journal of Rock Mechanics & Mining Sciences 123 (2019) 104125
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International Journal of Rock Mechanics and Mining Sciences journal homepage: http://www.elsevier.com/locate/ijrmms
Acoustic emission based mechanical behaviors of Beishan granite under conventional triaxial compression and hydro-mechanical coupling tests H.W. Zhou a, c, *, Z.H. Wang b, W.G. Ren b, Z.L. Liu b, J.F. Liu d a
School of Energy and Mining Engineering, China University of Mining and Technology, Beijing, 100083, PR China School of Mechanics and Civil Engineering, China University of Mining and Technology, Beijing, 100083, PR China c State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Beijing, 100083, PR China d College of Water Resources and Hydropower, Sichuan University, Chengdu, Sichuan, 610065, PR China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Beishan granite Strength Deformability Acoustic emission CTC test HM coupling test
Beishan area is now considered as a potential site of high-level radioactive waste (HLW) repository. In order to understand the mechanical behavior of Beishan granite, conventional triaxial compression (CTC) and hydromechanical (HM) coupling tests are conducted on Beishan granite. An acoustic emission (AE) monitoring sys tem is employed to estimate the rock damage evolution during the tests. The testing results show that the compressibility of the rock specimens increases and expansibility decreases with increasing confining pressure under CTC tests, resulting in failure pattern transforms from splitting failure to shearing failure. Under HM coupling, the compressibility decreases and expansibility increases with increasing pore pressure, and the micro cracks can keep relatively equilibrium state due to the existence of fluid. Based on AE hit counts, the crack closure stress σcc , crack initiation stress σ ci and crack damage stress σcd are 12.3%, 48.7% and 81.1% of the peak stress σ c under CTC tests, respectively. Under HM coupling, the crack closure stress σ cc is vanished and the crack initiation stress σ ci is higher than that of CTC tests. In this sense, pore pressure can facilitate crack development once the crack damage stress σ cd is reached. The fluctuation intensities of AE amplitude and frequency centroid (FC) increase during the loading process. The amplitude fluctuation range increases and FC decreases after peak during CTC tests. However, the amplitude fluctuation range reaches its maximum at peak stress under HM coupling tests, and FC decreases. Moreover, simplified models of failure process for crystalline rocks under compression and HM coupling are proposed.
1. Introduction Disposal of high-level radioactive waste (HLW) is one of the most challenging subjects across the world. The most important task is to isolate the HLW from the biosphere. At present, deep underground geological disposal is regarded as a feasible way. Granite, a potential host rock for HLW, is basically characterized by high strength and low permeability. It is reported that China plans to build a HLW repository in granite strata in Beishan, Gansu Province.1 Therefore, understanding of the mechanical behaviors of the host rock under conventional compression and hydro-mechanical (HM) coupling is of significant importance for the safety and long-term stability of the repository. It is well recognized that growth of micro cracks associated with crack coalescence, and propagation can eventually lead to failure of crystalline rock.2–9 Bieniawski conducted the compression experiments
to observe crack closure, crack initiation and crack propagation.2 Cracks in specimens subjected to uniform loading propagate relatively uni formly throughout the granite specimens, resulting in longitudinal splitting, parallel to the direction of axial loading.4 Tapponnier and Brace revealed that new transgranular cracks formed at around 75% of the peak stress, and crack density approximately doubled up to peak stress.6 In addition, under the condition of granite creep, crack inter action with other cavities seems to increase in time as the number of individual cracks increases, and crack-crack interaction seems to in crease near the onset of tertiary creep.7 On the other hand, development of micro cracks will also cause the change in rock volume,10–13 which is strongly related to the compaction and dilation of porous rock materials. Dilatancy is traced in the granite to open cracks which form parallel with the direction of maximum compression,10 and is directly proportional to cracking.11 Holcomb
* Corresponding author. School of Energy and Mining Engineering, China University of Mining and Technology, Beijing, 100083, PR China. E-mail address: [email protected] (H.W. Zhou). https://doi.org/10.1016/j.ijrmms.2019.104125 Received 16 April 2019; Received in revised form 6 September 2019; Accepted 25 September 2019 Available online 30 September 2019 1365-1609/© 2019 Elsevier Ltd. All rights reserved.
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International Journal of Rock Mechanics and Mining Sciences 123 (2019) 104125
observed decrease in sample volume, which is consistent with the healing of crack.13 In this instance, micro crack can affect the failure pattern of rocks. In most cases, failure patterns of rock specimens, tested in laboratory, usually changes from brittle failure to ductile failure.14–16 Therefore, rock damage at different stress levels should be investigated. Fluid flow through rock is of fundamental importance in HLW isolation. In crystalline rocks, fluid flows through a network of cracks under the flow pressure. For this, fluid in cracks could have a major influence on the propagation of crack networks, resulting in perme ability change.17–21 As a result, rock strength and deformation charac teristics will be changed.22–26 Generally, the ultimate strength and ductility of porous rocks are found to depend on effective confining pressure—the difference between external and internal pressures.22,23 Rice revealed that rock is dilatantly hardened in undrained deformation by the effective stress principle.24 And the transition from dynamic to stable failure occurred at lower ratio of pore pressure to confining pressure with decreasing strain rate. In this case, coupling between fluids and crack propagation needs to be studied for the safety of HLW repository. Scanning Electron Microscope (SEM) is widely used to observe propagation and coalescence of micro cracks in rocks.27–29 Now AE monitoring techniques provide detailed insight into fracture nucleation and growth.30–38 Lockner et al. observed quasi-static fault growth, slip and fracture nucleation of Westerly granite, using AE monitoring.30–32 Three stages of facture propagation are defined under constant strain loading. Cai et al. discussed the excavation damage zone based on AE and microseismic monitoring, and proposed σ cc , σ ci and σcd threshold during the excavation in hard rock strata.33–35 Furthermore, a coupled numerical method is used to study AE.36 Since crack initiation and propagation can be observed with AE with respect to the damage or rock fracturing, rock degradation can be well characterized. In the present study, CTC tests and HM coupling tests are conducted under different confining pressures and pore pressures, respectively. An AE monitoring system is employed to reveal the fracturing process of Beishan granite during the tests. Based on testing results, development of micro crack in the process of failure is studied in order to characterize the overall mechanical behaviors of granite.
2. Laboratory tests settings 2.1. Preparation of rock specimens The granite samples were taken from the Beishan area, Gansu Province, China. The specimens, with diameter of 50 mm and length of 100 mm, were prepared according to ISRM recommendations.39 Five specimens were prepared for CTC test, and four specimens were pre pared for HM coupling test. The basic properties and components of Beishan granite were investigated by Zhao et al. (2013).40 2.2. Test facilities The tests were carried out with a MTS815 Flex Test GT rock me chanics machine. The MTS815 tester has a maximum load capacity of 4,700 kN and can supply a maximum confining pressure of 140 MPa. The axial and lateral strains were measured by a pair of extensometers. The measurement ranges for the axial and lateral extensometers (LVDT) are 5 mm and 8 mm, respectively. A 12-channel portable AE system, which could work simultaneously with the loading system, was utilized to monitor AE events during the CTC and HM coupling tests. The triggering threshold of AE system was set to 28 dB. The sensor frequency was 200 kHz. As shown in Fig. 1, eight AE sensors were used in the tests. Four of them were a group and distributed evenly in a plane 5 mm away from the boundary of the specimen outside on the pressure cell. It should be pointed out that the elastic wave of the AE events monitoring during the experiment need first pass through a Teflon heat-shrink jacket enwrapped seamlessly around the sample, then the hydraulic oil, and finally the steel wall of the triaxial cell before reaching the sensors.40,41 Fortunately, the recorded AE data demonstrated that the monitoring method utilized in the tests is feasible. 2.3. Testing procedure For CTC tests, the confining pressures were set to 5, 10, 15, 20 and 30 MPa, respectively. In the first step, a vertical load of about 2 kN was applied in order to fix the position of the specimen. Then, the desired confining pressure was reached with a constant loading rate of 0.05 MPa/s to ensure that the specimen was under uniform hydrostatic stresses.42 Afterwards, the AE system was started and the axial stress was increased with a constant loading rate of 30 kN/min. Lateral
Fig. 1. Specimen with LVDT (a) and layout of AE sensors (b) for CTC and HM coupling tests. 2
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International Journal of Rock Mechanics and Mining Sciences 123 (2019) 104125
deformation control was used when the axial stress approached the failure strength. For the HM coupling tests, the specimens were immersed in water for 72 h in the vacuum suction device to reach the saturated state.43 The confining pressure was set to 20 MPa in all HM coupling tests. Pore pressures were set to 2, 4, 6 and 8 MPa, respectively. The desired pore pressure was reached with a constant rate of 1 MPa/min. The following loading process is the same as for the CTC tests.
regard as an elastic body. Therefore, only very few AE hits are generated, which can be defined as quiet period. The quiet period of AE hits ends when the axial stress reaches σ ci ,34,40 which is about 49% of the peak stress. Micro cracks are reactivated gradually by gradual increase of AE hits. The AE hits increase significantly when the axial stress exceeds σcd (dilatancy stress point) about 81% of the peak stress. The micro cracks interact with each other, which leads to crack coalescence and formation of fracture zone. There are no AE hits in the stage of stress drop. It demonstrates that the internal structure of the specimen has undergone significant change, and the fracture zone prevents the propagation of AE waves. In the residual stage, the number of AE hit increased dramatically because of unstable fracture zone.
3. Testing results 3.1. Strength and deformation behaviors under CTC tests
4.1.2. AE events characteristics In order to better understand the failure process, AE events at different stress levels are shown in Fig. 5. At point a, the axial stress is larger than σci . AE events are distributed randomly inside the specimen, and the number of AE events is very small. Therefore, the damage is merely limited to micro cracks closure. At point b, the axial stress almost reaches σ cd . However, AE events increase slightly and remain scattered inside the specimen. This indicates that micro cracks propagate slowly between the stress σci and σcd . Then, AE events increase significantly and its intensity increases because of coalescence of micro cracks and for mation of macro cracks. A large number of macro cracks are connected around the peak stress, which leads to the formation of splitting or shearing fracture zone. After the peak stress, AE events increase dramatically, which indicates that the fracture zone develops continu ously. As a result, there is a visible macro fracture cross the specimen. The accumulated numbers of AE events at different stress points (marked in Fig. 5) are listed in Table 4. At point a, the AE activity under different confining pressures is 8.23%, 2.99%, 3.05%, 4.22% and 3.29% of the total AE events, respectively. It shows that propagation of micro cracks is easier under low confining pressure. AE events generated from point b to point d account for 62.54%, 63.97%, 45.01%, 52.64% and 38.79% of the total AE events, respectively. The proportion shows decrease trend with increasing confining pressure, which indicates that the activity of micro cracks is marked under lower confining pressure. This matches the large expansibility under relatively low confining pressure, as mentioned in section 3.1. After the peak stress, AE events account for 15.75%, 17.80%, 45.92%, 34.92% and 47.53%, respec tively. The proportion increases with increasing confining pressure. The complete failure of specimens still lasts for a time period under relatively high confining pressure. In other words, the higher the confining pres sure, the more obvious the ductility.
The testing results of CTC tests are tabulated in Table 1, and the obtained stress-strain curves and failure patterns are shown in Fig. 2. With increasing confining pressure, the peak stress increases linearly. During the initial loading process, the specimens are in state of compression. Dilatancy appears when the stress reaches about 81% of the peak stress. Compared to the initial volume, the specimen is still in state of compression in the following loading process before the peak stress. In the process of stress drop, the volume of specimen expands rapidly. At the confining pressure of 5 MPa, axial splitting failure is observed. With increasing confining pressure, shear failure is dominant. A macro shear crack runs diagonally through the specimen when the confining pressure reaches 30 MPa. It can be obtained from Fig. 2 that the spec imen exhibits low compressibility and high expansibility under low confining pressure. As a result, the specimen is more likely to show splitting failure under low confining pressure. With the increase of confining pressure, the compressibility of specimen increases and expansibility decreases. Therefore, its ductility is characterized by shearing failure. 3.2. Strength and deformation behaviors under HM coupling tests The stress-strain curves of Beishan granite under different pore pressures are shown in Fig. 3. The testing results are given in Table 2. With pore pressure increasing, the maximum axial strain, peak stress and compressibility decrease. The dilatancy appears when the axial stress reaches about 82% of the peak stress. It can be obtained from Fig. 3 that the higher the pore pressure, the larger the expansibility. 4. AE characteristics of Beishan granite 4.1. CTC tests
4.1.3. Amplitude and frequency centroid of AE The frequency centroid (FC) and amplitude of AE were analyzed by choosing one AE channel, as shown in Fig. 6. Fluctuation of amplitudes under different confining pressures are basically the same. In the stress adjustment and quiet period, amplitude shows a single curve fluctuation due to few AE. The curve begins to fluctuate continuously after the axial stress exceeds σ ci , and the vibration intensity increases gradually. The fluctuation intensity of AE amplitude increases rapidly between σcd and peak stress. In the residual stage, the fluctuation intensity is still very high, and the amplitude is higher than that in the pre-failure stage. FC is a calculated frequency feature reported in kilohertz, which
4.1.1. AE hit counts characteristics The characteristics of AE hit counts observed during the CTC tests are shown in Fig. 4. According to observed AE hit counts, the damage stresses of Beishan granite under different confining pressures are esti mated (Table 3). At the beginning of the tests, the AE hits rate and in tensity are low. Only a few registered AE hits are generated because of sample setting and interface adjustment. In the meantime, micro cracks are gradually closed, which can contribute to the increase of AE hits. Almost all micro cracks are closed when the stress reaches σ cc .34,40 And σ cc is about 13% of the peak stress. Since then the specimen can be Table 1 CTC tests results of Beishan granite. Sample no
Confining pressure/MPa
Peak stress/MPa
Poisson ratio
Young modulus/GPa
Cohesion/MPa
Friction angle/�
Maximum compressibility/%
CTC-1 CTC-2 CTC-3 CTC-4 CTC-5
5 10 15 20 30
207.1 245.9 274.1 326.6 405.3
0.122
28.6
35.4
52.4
0.58 0.66 0.70 0.81 0.99
3
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International Journal of Rock Mechanics and Mining Sciences 123 (2019) 104125
Fig. 2. Stress strain curves and failure patterns under different confining pressures (CTC tests, Beishan granite).
350 300
2 MPa 4 MPa 6 MPa 8 MPa
Axial stress/MPa
250 200 150 100 50 0 -2.0
Axial stressAxial strain
Axial stressVolumetric strain -1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
Strain(%)
Fig. 3. Stress strain curves of Beishan granite under different pore pressures (HM coupling tests, Beishan granite). Table 2 HM coupling tests results of Beishan granite. Sample no
Confining pressure/MPa
Pore pressure/ MPa
Peak stress/ MPa
Maximum compressibility/%
HM-1 HM-2 HM-3 HM-4
20
2 4 6 8
311.7 316.1 307.4 245.1
1.07 1.06 1.05 0.91
results from a sum of magnitude time frequency divided by a sum of magnitude time, as equivalent to the first moment of inertia. In the prefailure stage, fluctuation of FC is basically the same as the trend of amplitude. However, FC reaches lower values in the post-failure stage. 4.2. HM coupling tests Fig. 4. Characteristics of AE hit counts under different confining pressures (CTC tests), confining pressure (a) 5 MPa (b) 20 MPa.
4.2.1. AE hit counts characteristics AE hits counts characteristics during HM coupling tests are shown in 4
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International Journal of Rock Mechanics and Mining Sciences 123 (2019) 104125
Table 3 Characteristic damage stresses under different confining pressures (CTC tests). Confining pressure/ MPa
σc
σcc
σcc =σc
σci
σci =σc
σcd
σcd =σc
5 10 15 20 30 Average
207.1 245.9 274.1 326.6 405.3 –
28 28 28 33 66 –
13.5% 11.4% 10.2% 10.1% 16.3% 12.3%
104 126 130 154 193 –
50.2% 51.2% 47.4% 47.2% 47.6% 48.7%
180.1 198.4 221.3 261.4 313.6 –
87.0% 80.7% 80.7% 80.0% 77.3% 81.1%
/MPa
/MPa
/MPa
/MPa
Fig. 6. Amplitude and frequency centroid of AE for confining pressure 20 MPa (CTC tests), (a) Amplitude (b) Frequency centroid.
of pore pressure. As shown in Table 5, σ ci is about 66% of the peak stress, which is greater than that of the CTC tests. This shows that micro cracks are more likely to be stable under the combined action of eternal force and pore pressure. Once the axial stress reaches σci , the AE hit rate in creases gradually in the following loading process, and is greater than that of CTC tests. AE hit rate is relatively low between σ ci and σ cd , and the accumulated AE hit count increases slightly. σ cd is slightly higher than that of the CTC tests (approximately 82% of the peak stress). AE hits increase rapidly after σ cd is exceeded. There are also AE hits around the peak stress, because fluid provides medium for AE wave propagation. In the post-peak stage, accumulated AE hit count increases steadily at a higher rate.
Fig. 5. Accumulative AE events at different stress levels incl. final failure state (CTC tests), confining pressure (a) 10 MPa (b) 15 MPa. Table 4 Accumulated number of AE events at different stress points under different confining pressures. Confining pressure/MPa
a
b
c
d
e
f
5 10 15 20 30
127 41 74 132 177
335 250 221 405 737
887 593 1122 1535 2088
1300 1127 1313 2052 2827
1510 1229 1838 2785 4603
1543 1371 2426 3129 5388
4.2.2. AE events characteristics Fig. 8 shows accumulated number of AE events during HM coupling tests (see different stress points in Table 6). At point a, the axial stress is larger than σ cd . However, the accumulated AE events are few and randomly scattered in the specimen. It can be concluded that the fluid inhibits the activation of micro cracks and increases the damage threshold. From point b to point c, it can be obtained that once the
Fig. 7, and the corresponding damage stresses are listed in Table 5. Unlike the CTC tests, there are no AE hits at the beginning of the tests. Therefore, σcc cannot be determined for the HM coupling tests. In other words, the closure of micro cracks is prevented because of the existence 5
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International Journal of Rock Mechanics and Mining Sciences 123 (2019) 104125
a 350
b Axial stress/MPa
c
300
Axial stress/MPa
c
b a
250
d
200 150
d
100
e
50
e
0 0.0
0.5
1.0
1.5
2.0
2.5
Axial strain (%)
(a)
b
a 350
Axial stress/MPa
300
Axial stress/MPa
Table 5 Damage stresses under different pore pressures (HM coupling tests). Pore pressure/ MPa
σc
σci
σci =σc
σcd
σcd =σc
20
2 4 6 8 –
311.7 316.1 307.4 245.1 –
203 202 202 168 –
65.1% 63.9% 65.7% 68.5% 65.8%
256.4 266.3 248.1 202.3 –
82.3% 84.2% 80.7% 82.5% 82.4%
Average
/MPa
/MPa
/MPa
c
a
Fig. 7. Characteristics of AE hits under different pore pressures (HM coupling tests), pore pressure (a) 4 MPa (b) 8 MPa.
Confining pressure/MPa
b
c
250
d
200 150 100
0 0.0
e
d
50
e 0.5
1.0
1.5
2.0
2.5
Axial strain (%)
(b)
damage threshold is reached, AE events increase dramatically in the following loading process. This indicates that the fluid facilitates prop agation of cracks. Under confining pressure 20 MPa, the difference in the number of accumulated AE events at peak stress between HM coupling tests and CTC tests is small. It indicates that the degree of the damage of the specimens at peak stress is approximately the same. Higher stress is needed for CTC tests to ensure cracks development, and for HM coupling tests, the fluid accelerates micro cracks development. During the postpeak period, AE events increase dramatically, and the AE number is larger than that of CTC tests. Therefore, the cracking degree of the specimen is elevated.
Fig. 8. Accumulative of AE events at different stress levels (HM coupling tests), pore pressure (a) 2 MPa (b) 6 MPa. Table 6 Accumulated number of AE events at different stress points under different pore pressures.
4.2.3. Amplitude and frequency centroid of AE The fluctuations of amplitude and FC under different pore pressures are basically the same, as shown in Fig. 9. The characteristics are similar to those of CTC tests. Amplitude vibrations are low when axial stress is 6
Pore pressure/MPa
a
b
c
d
e
2 4 6 8
200 162 226 120
704 3926 1839 735
2052 9803 2734 2062
4376 14131 5033 4690
5577 15657 8366 6828
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International Journal of Rock Mechanics and Mining Sciences 123 (2019) 104125
Fig. 9. Amplitude and frequency centroid of AE for pore pressure of 8 MPa (HM coupling tests), (a) Amplitude (b) Frequency centroid. Fig. 10. Simplified illustration of overall specimen failure process under different confining pressures (CTC tests), (a) Overall failure process, (b) Crack propagation.
less than σci . Continuous fluctuations occurs when the axial stress ex ceeds σci . The fluctuation intensity increases significantly when the axial stress exceeds σcd . The major difference is that the amplitude increases during the loading stage and reaches its maximum at the peak stress. FC does not change significantly, and an overall decline trend after peak is observed.
stress. The specimens show elastic characteristics when the axial stress reaches σcc , about 12.3% of the peak stress. The elastic range ends when the axial stress reaches σci , about 48.7% of the peak stress. In this elastic range, the specimen can be seen as an elastomer. As a result, AE activ ities are quite low. As the axial stress further increases and exceeds the elastic limit, micro cracks are reactivated and grains show intensive frictional contact. The AE activities increase slowly and uniformly distributed inside the specimen. When σ cd is reached, the volume of the specimen reaches its minimum value. Afterwards, the AE activity in creases rapidly. This indicates the propagation of micro cracks. AE amplitude and FC start to fluctuate intensively, which can serve as the precursor information of impending fracturing. The volume of the specimen increases to its initial volume when the peak stress is approached. In the stress drop stage, propagation of AE waves is inhibited by the fracture zone. Therefore, no AE signals are observed during the stress drop phase. In the residual stage, AE activity increases
5. Discussion Based on the testing results, simplified failure processes for crystal line rocks under compression and HM coupling are illustrated in Figs. 10 and 11, respectively. Compaction, dilation, failure pattern, crack prop agation and damage stress are taken into account in order to have a fullscale understanding of failure process. 5.1. AE-based overall failure process under CTC tests As shown in Fig. 10, the specimen is initially compressed at begin of the CTC tests, accompanied by the closure of micro cracks and grain friction, which results in a very small number of AEs under low axial 7
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International Journal of Rock Mechanics and Mining Sciences 123 (2019) 104125
increasing pore pressure. Few AE activities are monitored before σ ci . It indicates that the micro cracks are in relatively stable state due to the presence of fluid. The volume of specimen also decreases with increasing axial stress. When the axial stress reaches about 65.8% of the peak stress, σ ci is reached. This suggests a significant increase compared with that of the CTC tests. Afterwards, AE activities increase gradually, and AE amplitude and FC fluctuate slightly, indicating the friction of grains. When the axial stress reaches σ cd , the volume compression reaches its maximum. And with the pore pressure increasing, the compression tends to decrease. σcd is about 82.4% of the peak stress, which is approxi mately the same as that of CTC tests. With axial stress further increasing, AE activities increase significantly, and AE amplitude and FC fluctuate strongly. The volume of the specimen increases gradually, indicating crack propagation inside the specimen. Specimen reaches again its original volume when the peak stress is reached. In the residual stage, AE activity is more intense than that of CTC tests, suggesting that fluid promotes the propagation of cracks. As a result, the higher the pore pressure, the larger the expansibility. In addition, amplitude fluctuation reaches the maximum range and there is an overall decline in FC. 6. Conclusion The strength, deformation and AE characteristics of Beishan granite are obtained by CTC and HM coupling tests. Under different stress levels, the development of AE is in good agreement with the propagation of micro cracks. The failure pattern of crystalline rocks is well calibrated by AE activities. Based on the testing results, the following conclusions can be drawn. For the CTC tests, the compressibility of Beishan granite increases linearly with increasing confining pressure. The specimen begins to expand when the axial stress reaches 81.1% of the peak stress. Splitting failure is dominant under low confining pressure. The higher the confining pressure, the more pronounced the shear failure. For HM coupling tests, the compressibility decreases with increasing pore pres sure, as well as peak stress. The dilitancy point appears at 82.4% of the peak stress. AE activity shows different phenomenon under different stress levels, which can be used to determine the damage stresses. For CTC tests, σ cc , σ ci and σ cd are 12.3%, 48.7% and 81.1% of the peak stress, respectively. For HM coupling tests, σ ci , σcd are 65.8% and 82.4% of the peak stress, respectively. There is no σcc in HM coupling tests, and σci is much higher than that of the CTC tests. AE activity shows that the induced damage before σ cd is small and crack propagation accelerates after σcd under CTC tests. AE events pro portion at the same loading period changes under different confining pressures, which illustrates that failure pattern develops from splitting to ductile shearing failure with the increasing confining pressure. The propagation of AE events in HM coupling tests indicates that the damage threshold increases due to the presence of fluid. In addition, fluid con tributes to the activation of cracks once the stress exceeds σcd . AE amplitude and FC have same fluctuation trend in the CTC tests. The fluctuation intensity increases with the increasing axial stress. In the residual stage, the amplitude fluctuation increases, and FC decreases. For HM coupling tests, amplitude fluctuation range increases during the loading stage and reaches its maximum at the peak stress, the same as fluctuation intensity. However, the FC decreases after the peak stress. It should be noted that pore pressure affecting the rock strength is not thoroughly investigated in this research. In the future, more tests are needed to be conducted for further investigating the AE and fracturing characteristics of the rock subjected to different confining stresses and pore pressures. In addition, the permeability of Beishan granite also needs to be studied thoroughly.
Fig. 11. Simplified illustration of overall specimen failure process under different pore pressures (HM coupling tests), (a) Overall failure process, (b) Crack propagation.
rapidly. In addition, the amplitude is higher compared with that of the pre-peak stage, and the FC decreases. It also shows that lower confining pressure causes smaller compressibility of the specimen, and vice versa. Moreover, stress con centration is not uniform in the specimen due to the interaction between low peak stress and low confining pressure. As a result, the post-peak expansion is marked, which can be easily to observed in a form of splitting failure. On the other hand, interaction between high peak stress and high confining pressure makes the stress concentrated in the spec imen uniformly, therefore the post-peak expansion is relatively small, and shearing failure dominates. 5.2. AE-based overall failure process under HM tests
Acknowledgments
An illustration of the overall specimen failure process under different pore pressures with respect to HM coupling tests is shown in Fig. 11. Under the same confining pressure, the peak stress decreases with
This work was supported by the State Key Research Development 8
H.W. Zhou et al.
International Journal of Rock Mechanics and Mining Sciences 123 (2019) 104125
Program of China (2016YFC0600704), the National Natural Science Foundation of China (51674266) and the Yueqi Outstanding Scholar Program of CUMTB. Special thanks are due to Professor H. Konietzky for his help in improving the article.
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1 Wang J. On area-specific underground research laboratory for geological disposal of high-level radioactive waste in China. J Rock Mech Geotech Eng. 2014;6(2):99–104. https://doi.org/10.1016/j.jrmge.2014.01.002. 2 Bieniawski ZT. Mechanism of brittle fracture of rock: part II-experimental studies. Int J Rock Mech Min Sci Geomech Abstr. 1967;4(4):407–423. https://doi. org/10.1016/0148-9062(67)90031-9. 3 Wawersik WR, Brace WF. Post-failure behavior of a granite and diabase. Rock Mech Rock Eng. 1971;3:62–85. https://doi.org/10.1007/BF01239627. 4 Peng S, Johnson AM. Crack growth and faulting in cylindrical specimens of Chelmsford granite. Int J Rock Mech Min Sci Geomech Abstr. 1972;9(1):37–86. https://doi.org/10.1016/0148-9062(72)90050-2. 5 Hadley K. Comparison of calculated and observed crack densities and seismic velocities in Westerly granite. J Geophys Res. 1976;81(20):3484–3494. https://doi. org/10.1029/JB081i020p03484. 6 Tapponnier P, Brace WF. Development of stress-induced microcracks in Westerly granite. Int J Rock Mech Min Sci Geomech Abstr. 1976;13(4):103–112. https://doi. org/10.1016/0148-9062(76)91937-9. 7 Kranz RL. Crack growth and development during creep of Barre granite. Int J Rock Mech Min Sci Geomech Abstr. 1979;16(1):23–35. https://doi.org/10.1016/0148-9062 (79)90772-1. 8 Wong TF. Geometric probability approach to the characterization and analysis of microcracking in rocks. Mech Mater. 1985;4(3-4):261–276. https://doi. org/10.1016/0167-6636(85)90023-7. 9 Oda M, Katsube T, Takemura T. Microcrack evolution and brittle failure of Inada granite in triaxial compression tests at 140 MPa. J Geophys Res-Sol Ea. 2002;107 (B10). https://doi.org/10.1029/2001JB000272. 10 Brace WF, Paulding Jr BW, Scholz CH. Dilatancy in the fracture of crystalline rocks. J Geophys Res. 1966;71(16):3939–3953. https://doi.org/10.1029/jz071i016p03939. 11 Scholz CH. Microfracturing and the inelastic deformation of rock in compression. J Geophys Res. 1968;73(4):1417–1432. http://doi.org/10.1029/jb073i004p01417. 12 Brace WF. Volume changes during fracture and frictional sliding: a review. Pure Appl Geophys. 1978;116(4-5):603–614. https://doi.org/10.1007/BF00876527. 13 Holcomb DJ. Memory, relaxation, and microfracturing in dilatant rock. J Geophys Res-Sol Ea. 1981;86(B7):6235–6248. https://doi.org/10.1029/JB086iB07p06235. 14 Haeard H. Transition from brittle fracture to ductile flow in Solenhofen limestone as a function of temper ature, confining pressure, and interstitial fluid pressure. Mem Geol Soc Am. 1960;79:193–226. https://doi.org/10.1130/mem79-p193. 15 Horii H, Nemat-Nasser S. Brittle failure in compression: splitting faulting and brittleductile transition. Philos Trans R Soc Lond A. 1986;319(1549):337–374. https://doi. org/10.1098/rsta.1986.0101. 16 Wong TF, David C, Zhu W. The transition from brittle faulting to cataclastic flow in porous sandstones: mechanical deformation. J Geophys Res-Sol Ea. 1997;102(B2): 3009–3025. http://doi.org/10.1029/96jb03281. 17 Brace WF. Permeability from resistivity and pore shape. J Geophys Res. 1977;82(23): 3343–3349. https://doi.org/10.1029/JB082i023p03343. 18 Walsh JB, Brace WF. The effect of pressure on porosity and the transport properties of rock. J Geophys Res-Sol Ea. 1984;89(B11):9425–9431. https://doi.org/10.1 029/JB089iB11p09425. 19 Oda M. Permeability tensor for discontinuous rock masses. Geotechnique. 1985;35(4): 483–495. https://doi.org/10.1680/geot.1985.35.4.483. 20 Oda M, Hatsuyama Y, Ohnishi Y. Numerical experiments on permeability tensor and its application to jointed granite at Stripa Min Scie, Sweden. J Geophys Res-Sol Ea. 1987;92(B8):8037–8048. https://doi.org/10.1029/JB092iB08p08037. 21 Suzuki K, Oda M, Yamazaki M, Kuwahara T. Permeability changes in granite with crack growth during immersion in hot water. Int J Rock Mech Min Sci. 1998;35(7): 907–921. https://doi.org/10.1016/S0148-9062(98)00016-3. 22 Handin J, Hager JRV, Friedman M, Ferther JN. Experimental deformation of sedimentary rocks under confining pressure: pore pressure tests. AAPG (Am Assoc Pet
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Contents lists available at ScienceDirect
International Journal of Rock Mechanics and Mining Sciences journal homepage: http://www.elsevier.com/locate/ijrmms
Acoustic emission based mechanical behaviors of Beishan granite under conventional triaxial compression and hydro-mechanical coupling tests H.W. Zhou a, c, *, Z.H. Wang b, W.G. Ren b, Z.L. Liu b, J.F. Liu d a
School of Energy and Mining Engineering, China University of Mining and Technology, Beijing, 100083, PR China School of Mechanics and Civil Engineering, China University of Mining and Technology, Beijing, 100083, PR China c State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Beijing, 100083, PR China d College of Water Resources and Hydropower, Sichuan University, Chengdu, Sichuan, 610065, PR China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Beishan granite Strength Deformability Acoustic emission CTC test HM coupling test
Beishan area is now considered as a potential site of high-level radioactive waste (HLW) repository. In order to understand the mechanical behavior of Beishan granite, conventional triaxial compression (CTC) and hydromechanical (HM) coupling tests are conducted on Beishan granite. An acoustic emission (AE) monitoring sys tem is employed to estimate the rock damage evolution during the tests. The testing results show that the compressibility of the rock specimens increases and expansibility decreases with increasing confining pressure under CTC tests, resulting in failure pattern transforms from splitting failure to shearing failure. Under HM coupling, the compressibility decreases and expansibility increases with increasing pore pressure, and the micro cracks can keep relatively equilibrium state due to the existence of fluid. Based on AE hit counts, the crack closure stress σcc , crack initiation stress σ ci and crack damage stress σcd are 12.3%, 48.7% and 81.1% of the peak stress σ c under CTC tests, respectively. Under HM coupling, the crack closure stress σ cc is vanished and the crack initiation stress σ ci is higher than that of CTC tests. In this sense, pore pressure can facilitate crack development once the crack damage stress σ cd is reached. The fluctuation intensities of AE amplitude and frequency centroid (FC) increase during the loading process. The amplitude fluctuation range increases and FC decreases after peak during CTC tests. However, the amplitude fluctuation range reaches its maximum at peak stress under HM coupling tests, and FC decreases. Moreover, simplified models of failure process for crystalline rocks under compression and HM coupling are proposed.
1. Introduction Disposal of high-level radioactive waste (HLW) is one of the most challenging subjects across the world. The most important task is to isolate the HLW from the biosphere. At present, deep underground geological disposal is regarded as a feasible way. Granite, a potential host rock for HLW, is basically characterized by high strength and low permeability. It is reported that China plans to build a HLW repository in granite strata in Beishan, Gansu Province.1 Therefore, understanding of the mechanical behaviors of the host rock under conventional compression and hydro-mechanical (HM) coupling is of significant importance for the safety and long-term stability of the repository. It is well recognized that growth of micro cracks associated with crack coalescence, and propagation can eventually lead to failure of crystalline rock.2–9 Bieniawski conducted the compression experiments
to observe crack closure, crack initiation and crack propagation.2 Cracks in specimens subjected to uniform loading propagate relatively uni formly throughout the granite specimens, resulting in longitudinal splitting, parallel to the direction of axial loading.4 Tapponnier and Brace revealed that new transgranular cracks formed at around 75% of the peak stress, and crack density approximately doubled up to peak stress.6 In addition, under the condition of granite creep, crack inter action with other cavities seems to increase in time as the number of individual cracks increases, and crack-crack interaction seems to in crease near the onset of tertiary creep.7 On the other hand, development of micro cracks will also cause the change in rock volume,10–13 which is strongly related to the compaction and dilation of porous rock materials. Dilatancy is traced in the granite to open cracks which form parallel with the direction of maximum compression,10 and is directly proportional to cracking.11 Holcomb
* Corresponding author. School of Energy and Mining Engineering, China University of Mining and Technology, Beijing, 100083, PR China. E-mail address: [email protected] (H.W. Zhou). https://doi.org/10.1016/j.ijrmms.2019.104125 Received 16 April 2019; Received in revised form 6 September 2019; Accepted 25 September 2019 Available online 30 September 2019 1365-1609/© 2019 Elsevier Ltd. All rights reserved.
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observed decrease in sample volume, which is consistent with the healing of crack.13 In this instance, micro crack can affect the failure pattern of rocks. In most cases, failure patterns of rock specimens, tested in laboratory, usually changes from brittle failure to ductile failure.14–16 Therefore, rock damage at different stress levels should be investigated. Fluid flow through rock is of fundamental importance in HLW isolation. In crystalline rocks, fluid flows through a network of cracks under the flow pressure. For this, fluid in cracks could have a major influence on the propagation of crack networks, resulting in perme ability change.17–21 As a result, rock strength and deformation charac teristics will be changed.22–26 Generally, the ultimate strength and ductility of porous rocks are found to depend on effective confining pressure—the difference between external and internal pressures.22,23 Rice revealed that rock is dilatantly hardened in undrained deformation by the effective stress principle.24 And the transition from dynamic to stable failure occurred at lower ratio of pore pressure to confining pressure with decreasing strain rate. In this case, coupling between fluids and crack propagation needs to be studied for the safety of HLW repository. Scanning Electron Microscope (SEM) is widely used to observe propagation and coalescence of micro cracks in rocks.27–29 Now AE monitoring techniques provide detailed insight into fracture nucleation and growth.30–38 Lockner et al. observed quasi-static fault growth, slip and fracture nucleation of Westerly granite, using AE monitoring.30–32 Three stages of facture propagation are defined under constant strain loading. Cai et al. discussed the excavation damage zone based on AE and microseismic monitoring, and proposed σ cc , σ ci and σcd threshold during the excavation in hard rock strata.33–35 Furthermore, a coupled numerical method is used to study AE.36 Since crack initiation and propagation can be observed with AE with respect to the damage or rock fracturing, rock degradation can be well characterized. In the present study, CTC tests and HM coupling tests are conducted under different confining pressures and pore pressures, respectively. An AE monitoring system is employed to reveal the fracturing process of Beishan granite during the tests. Based on testing results, development of micro crack in the process of failure is studied in order to characterize the overall mechanical behaviors of granite.
2. Laboratory tests settings 2.1. Preparation of rock specimens The granite samples were taken from the Beishan area, Gansu Province, China. The specimens, with diameter of 50 mm and length of 100 mm, were prepared according to ISRM recommendations.39 Five specimens were prepared for CTC test, and four specimens were pre pared for HM coupling test. The basic properties and components of Beishan granite were investigated by Zhao et al. (2013).40 2.2. Test facilities The tests were carried out with a MTS815 Flex Test GT rock me chanics machine. The MTS815 tester has a maximum load capacity of 4,700 kN and can supply a maximum confining pressure of 140 MPa. The axial and lateral strains were measured by a pair of extensometers. The measurement ranges for the axial and lateral extensometers (LVDT) are 5 mm and 8 mm, respectively. A 12-channel portable AE system, which could work simultaneously with the loading system, was utilized to monitor AE events during the CTC and HM coupling tests. The triggering threshold of AE system was set to 28 dB. The sensor frequency was 200 kHz. As shown in Fig. 1, eight AE sensors were used in the tests. Four of them were a group and distributed evenly in a plane 5 mm away from the boundary of the specimen outside on the pressure cell. It should be pointed out that the elastic wave of the AE events monitoring during the experiment need first pass through a Teflon heat-shrink jacket enwrapped seamlessly around the sample, then the hydraulic oil, and finally the steel wall of the triaxial cell before reaching the sensors.40,41 Fortunately, the recorded AE data demonstrated that the monitoring method utilized in the tests is feasible. 2.3. Testing procedure For CTC tests, the confining pressures were set to 5, 10, 15, 20 and 30 MPa, respectively. In the first step, a vertical load of about 2 kN was applied in order to fix the position of the specimen. Then, the desired confining pressure was reached with a constant loading rate of 0.05 MPa/s to ensure that the specimen was under uniform hydrostatic stresses.42 Afterwards, the AE system was started and the axial stress was increased with a constant loading rate of 30 kN/min. Lateral
Fig. 1. Specimen with LVDT (a) and layout of AE sensors (b) for CTC and HM coupling tests. 2
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deformation control was used when the axial stress approached the failure strength. For the HM coupling tests, the specimens were immersed in water for 72 h in the vacuum suction device to reach the saturated state.43 The confining pressure was set to 20 MPa in all HM coupling tests. Pore pressures were set to 2, 4, 6 and 8 MPa, respectively. The desired pore pressure was reached with a constant rate of 1 MPa/min. The following loading process is the same as for the CTC tests.
regard as an elastic body. Therefore, only very few AE hits are generated, which can be defined as quiet period. The quiet period of AE hits ends when the axial stress reaches σ ci ,34,40 which is about 49% of the peak stress. Micro cracks are reactivated gradually by gradual increase of AE hits. The AE hits increase significantly when the axial stress exceeds σcd (dilatancy stress point) about 81% of the peak stress. The micro cracks interact with each other, which leads to crack coalescence and formation of fracture zone. There are no AE hits in the stage of stress drop. It demonstrates that the internal structure of the specimen has undergone significant change, and the fracture zone prevents the propagation of AE waves. In the residual stage, the number of AE hit increased dramatically because of unstable fracture zone.
3. Testing results 3.1. Strength and deformation behaviors under CTC tests
4.1.2. AE events characteristics In order to better understand the failure process, AE events at different stress levels are shown in Fig. 5. At point a, the axial stress is larger than σci . AE events are distributed randomly inside the specimen, and the number of AE events is very small. Therefore, the damage is merely limited to micro cracks closure. At point b, the axial stress almost reaches σ cd . However, AE events increase slightly and remain scattered inside the specimen. This indicates that micro cracks propagate slowly between the stress σci and σcd . Then, AE events increase significantly and its intensity increases because of coalescence of micro cracks and for mation of macro cracks. A large number of macro cracks are connected around the peak stress, which leads to the formation of splitting or shearing fracture zone. After the peak stress, AE events increase dramatically, which indicates that the fracture zone develops continu ously. As a result, there is a visible macro fracture cross the specimen. The accumulated numbers of AE events at different stress points (marked in Fig. 5) are listed in Table 4. At point a, the AE activity under different confining pressures is 8.23%, 2.99%, 3.05%, 4.22% and 3.29% of the total AE events, respectively. It shows that propagation of micro cracks is easier under low confining pressure. AE events generated from point b to point d account for 62.54%, 63.97%, 45.01%, 52.64% and 38.79% of the total AE events, respectively. The proportion shows decrease trend with increasing confining pressure, which indicates that the activity of micro cracks is marked under lower confining pressure. This matches the large expansibility under relatively low confining pressure, as mentioned in section 3.1. After the peak stress, AE events account for 15.75%, 17.80%, 45.92%, 34.92% and 47.53%, respec tively. The proportion increases with increasing confining pressure. The complete failure of specimens still lasts for a time period under relatively high confining pressure. In other words, the higher the confining pres sure, the more obvious the ductility.
The testing results of CTC tests are tabulated in Table 1, and the obtained stress-strain curves and failure patterns are shown in Fig. 2. With increasing confining pressure, the peak stress increases linearly. During the initial loading process, the specimens are in state of compression. Dilatancy appears when the stress reaches about 81% of the peak stress. Compared to the initial volume, the specimen is still in state of compression in the following loading process before the peak stress. In the process of stress drop, the volume of specimen expands rapidly. At the confining pressure of 5 MPa, axial splitting failure is observed. With increasing confining pressure, shear failure is dominant. A macro shear crack runs diagonally through the specimen when the confining pressure reaches 30 MPa. It can be obtained from Fig. 2 that the spec imen exhibits low compressibility and high expansibility under low confining pressure. As a result, the specimen is more likely to show splitting failure under low confining pressure. With the increase of confining pressure, the compressibility of specimen increases and expansibility decreases. Therefore, its ductility is characterized by shearing failure. 3.2. Strength and deformation behaviors under HM coupling tests The stress-strain curves of Beishan granite under different pore pressures are shown in Fig. 3. The testing results are given in Table 2. With pore pressure increasing, the maximum axial strain, peak stress and compressibility decrease. The dilatancy appears when the axial stress reaches about 82% of the peak stress. It can be obtained from Fig. 3 that the higher the pore pressure, the larger the expansibility. 4. AE characteristics of Beishan granite 4.1. CTC tests
4.1.3. Amplitude and frequency centroid of AE The frequency centroid (FC) and amplitude of AE were analyzed by choosing one AE channel, as shown in Fig. 6. Fluctuation of amplitudes under different confining pressures are basically the same. In the stress adjustment and quiet period, amplitude shows a single curve fluctuation due to few AE. The curve begins to fluctuate continuously after the axial stress exceeds σ ci , and the vibration intensity increases gradually. The fluctuation intensity of AE amplitude increases rapidly between σcd and peak stress. In the residual stage, the fluctuation intensity is still very high, and the amplitude is higher than that in the pre-failure stage. FC is a calculated frequency feature reported in kilohertz, which
4.1.1. AE hit counts characteristics The characteristics of AE hit counts observed during the CTC tests are shown in Fig. 4. According to observed AE hit counts, the damage stresses of Beishan granite under different confining pressures are esti mated (Table 3). At the beginning of the tests, the AE hits rate and in tensity are low. Only a few registered AE hits are generated because of sample setting and interface adjustment. In the meantime, micro cracks are gradually closed, which can contribute to the increase of AE hits. Almost all micro cracks are closed when the stress reaches σ cc .34,40 And σ cc is about 13% of the peak stress. Since then the specimen can be Table 1 CTC tests results of Beishan granite. Sample no
Confining pressure/MPa
Peak stress/MPa
Poisson ratio
Young modulus/GPa
Cohesion/MPa
Friction angle/�
Maximum compressibility/%
CTC-1 CTC-2 CTC-3 CTC-4 CTC-5
5 10 15 20 30
207.1 245.9 274.1 326.6 405.3
0.122
28.6
35.4
52.4
0.58 0.66 0.70 0.81 0.99
3
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Fig. 2. Stress strain curves and failure patterns under different confining pressures (CTC tests, Beishan granite).
350 300
2 MPa 4 MPa 6 MPa 8 MPa
Axial stress/MPa
250 200 150 100 50 0 -2.0
Axial stressAxial strain
Axial stressVolumetric strain -1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
Strain(%)
Fig. 3. Stress strain curves of Beishan granite under different pore pressures (HM coupling tests, Beishan granite). Table 2 HM coupling tests results of Beishan granite. Sample no
Confining pressure/MPa
Pore pressure/ MPa
Peak stress/ MPa
Maximum compressibility/%
HM-1 HM-2 HM-3 HM-4
20
2 4 6 8
311.7 316.1 307.4 245.1
1.07 1.06 1.05 0.91
results from a sum of magnitude time frequency divided by a sum of magnitude time, as equivalent to the first moment of inertia. In the prefailure stage, fluctuation of FC is basically the same as the trend of amplitude. However, FC reaches lower values in the post-failure stage. 4.2. HM coupling tests Fig. 4. Characteristics of AE hit counts under different confining pressures (CTC tests), confining pressure (a) 5 MPa (b) 20 MPa.
4.2.1. AE hit counts characteristics AE hits counts characteristics during HM coupling tests are shown in 4
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International Journal of Rock Mechanics and Mining Sciences 123 (2019) 104125
Table 3 Characteristic damage stresses under different confining pressures (CTC tests). Confining pressure/ MPa
σc
σcc
σcc =σc
σci
σci =σc
σcd
σcd =σc
5 10 15 20 30 Average
207.1 245.9 274.1 326.6 405.3 –
28 28 28 33 66 –
13.5% 11.4% 10.2% 10.1% 16.3% 12.3%
104 126 130 154 193 –
50.2% 51.2% 47.4% 47.2% 47.6% 48.7%
180.1 198.4 221.3 261.4 313.6 –
87.0% 80.7% 80.7% 80.0% 77.3% 81.1%
/MPa
/MPa
/MPa
/MPa
Fig. 6. Amplitude and frequency centroid of AE for confining pressure 20 MPa (CTC tests), (a) Amplitude (b) Frequency centroid.
of pore pressure. As shown in Table 5, σ ci is about 66% of the peak stress, which is greater than that of the CTC tests. This shows that micro cracks are more likely to be stable under the combined action of eternal force and pore pressure. Once the axial stress reaches σci , the AE hit rate in creases gradually in the following loading process, and is greater than that of CTC tests. AE hit rate is relatively low between σ ci and σ cd , and the accumulated AE hit count increases slightly. σ cd is slightly higher than that of the CTC tests (approximately 82% of the peak stress). AE hits increase rapidly after σ cd is exceeded. There are also AE hits around the peak stress, because fluid provides medium for AE wave propagation. In the post-peak stage, accumulated AE hit count increases steadily at a higher rate.
Fig. 5. Accumulative AE events at different stress levels incl. final failure state (CTC tests), confining pressure (a) 10 MPa (b) 15 MPa. Table 4 Accumulated number of AE events at different stress points under different confining pressures. Confining pressure/MPa
a
b
c
d
e
f
5 10 15 20 30
127 41 74 132 177
335 250 221 405 737
887 593 1122 1535 2088
1300 1127 1313 2052 2827
1510 1229 1838 2785 4603
1543 1371 2426 3129 5388
4.2.2. AE events characteristics Fig. 8 shows accumulated number of AE events during HM coupling tests (see different stress points in Table 6). At point a, the axial stress is larger than σ cd . However, the accumulated AE events are few and randomly scattered in the specimen. It can be concluded that the fluid inhibits the activation of micro cracks and increases the damage threshold. From point b to point c, it can be obtained that once the
Fig. 7, and the corresponding damage stresses are listed in Table 5. Unlike the CTC tests, there are no AE hits at the beginning of the tests. Therefore, σcc cannot be determined for the HM coupling tests. In other words, the closure of micro cracks is prevented because of the existence 5
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a 350
b Axial stress/MPa
c
300
Axial stress/MPa
c
b a
250
d
200 150
d
100
e
50
e
0 0.0
0.5
1.0
1.5
2.0
2.5
Axial strain (%)
(a)
b
a 350
Axial stress/MPa
300
Axial stress/MPa
Table 5 Damage stresses under different pore pressures (HM coupling tests). Pore pressure/ MPa
σc
σci
σci =σc
σcd
σcd =σc
20
2 4 6 8 –
311.7 316.1 307.4 245.1 –
203 202 202 168 –
65.1% 63.9% 65.7% 68.5% 65.8%
256.4 266.3 248.1 202.3 –
82.3% 84.2% 80.7% 82.5% 82.4%
Average
/MPa
/MPa
/MPa
c
a
Fig. 7. Characteristics of AE hits under different pore pressures (HM coupling tests), pore pressure (a) 4 MPa (b) 8 MPa.
Confining pressure/MPa
b
c
250
d
200 150 100
0 0.0
e
d
50
e 0.5
1.0
1.5
2.0
2.5
Axial strain (%)
(b)
damage threshold is reached, AE events increase dramatically in the following loading process. This indicates that the fluid facilitates prop agation of cracks. Under confining pressure 20 MPa, the difference in the number of accumulated AE events at peak stress between HM coupling tests and CTC tests is small. It indicates that the degree of the damage of the specimens at peak stress is approximately the same. Higher stress is needed for CTC tests to ensure cracks development, and for HM coupling tests, the fluid accelerates micro cracks development. During the postpeak period, AE events increase dramatically, and the AE number is larger than that of CTC tests. Therefore, the cracking degree of the specimen is elevated.
Fig. 8. Accumulative of AE events at different stress levels (HM coupling tests), pore pressure (a) 2 MPa (b) 6 MPa. Table 6 Accumulated number of AE events at different stress points under different pore pressures.
4.2.3. Amplitude and frequency centroid of AE The fluctuations of amplitude and FC under different pore pressures are basically the same, as shown in Fig. 9. The characteristics are similar to those of CTC tests. Amplitude vibrations are low when axial stress is 6
Pore pressure/MPa
a
b
c
d
e
2 4 6 8
200 162 226 120
704 3926 1839 735
2052 9803 2734 2062
4376 14131 5033 4690
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International Journal of Rock Mechanics and Mining Sciences 123 (2019) 104125
Fig. 9. Amplitude and frequency centroid of AE for pore pressure of 8 MPa (HM coupling tests), (a) Amplitude (b) Frequency centroid. Fig. 10. Simplified illustration of overall specimen failure process under different confining pressures (CTC tests), (a) Overall failure process, (b) Crack propagation.
less than σci . Continuous fluctuations occurs when the axial stress ex ceeds σci . The fluctuation intensity increases significantly when the axial stress exceeds σcd . The major difference is that the amplitude increases during the loading stage and reaches its maximum at the peak stress. FC does not change significantly, and an overall decline trend after peak is observed.
stress. The specimens show elastic characteristics when the axial stress reaches σcc , about 12.3% of the peak stress. The elastic range ends when the axial stress reaches σci , about 48.7% of the peak stress. In this elastic range, the specimen can be seen as an elastomer. As a result, AE activ ities are quite low. As the axial stress further increases and exceeds the elastic limit, micro cracks are reactivated and grains show intensive frictional contact. The AE activities increase slowly and uniformly distributed inside the specimen. When σ cd is reached, the volume of the specimen reaches its minimum value. Afterwards, the AE activity in creases rapidly. This indicates the propagation of micro cracks. AE amplitude and FC start to fluctuate intensively, which can serve as the precursor information of impending fracturing. The volume of the specimen increases to its initial volume when the peak stress is approached. In the stress drop stage, propagation of AE waves is inhibited by the fracture zone. Therefore, no AE signals are observed during the stress drop phase. In the residual stage, AE activity increases
5. Discussion Based on the testing results, simplified failure processes for crystal line rocks under compression and HM coupling are illustrated in Figs. 10 and 11, respectively. Compaction, dilation, failure pattern, crack prop agation and damage stress are taken into account in order to have a fullscale understanding of failure process. 5.1. AE-based overall failure process under CTC tests As shown in Fig. 10, the specimen is initially compressed at begin of the CTC tests, accompanied by the closure of micro cracks and grain friction, which results in a very small number of AEs under low axial 7
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International Journal of Rock Mechanics and Mining Sciences 123 (2019) 104125
increasing pore pressure. Few AE activities are monitored before σ ci . It indicates that the micro cracks are in relatively stable state due to the presence of fluid. The volume of specimen also decreases with increasing axial stress. When the axial stress reaches about 65.8% of the peak stress, σ ci is reached. This suggests a significant increase compared with that of the CTC tests. Afterwards, AE activities increase gradually, and AE amplitude and FC fluctuate slightly, indicating the friction of grains. When the axial stress reaches σ cd , the volume compression reaches its maximum. And with the pore pressure increasing, the compression tends to decrease. σcd is about 82.4% of the peak stress, which is approxi mately the same as that of CTC tests. With axial stress further increasing, AE activities increase significantly, and AE amplitude and FC fluctuate strongly. The volume of the specimen increases gradually, indicating crack propagation inside the specimen. Specimen reaches again its original volume when the peak stress is reached. In the residual stage, AE activity is more intense than that of CTC tests, suggesting that fluid promotes the propagation of cracks. As a result, the higher the pore pressure, the larger the expansibility. In addition, amplitude fluctuation reaches the maximum range and there is an overall decline in FC. 6. Conclusion The strength, deformation and AE characteristics of Beishan granite are obtained by CTC and HM coupling tests. Under different stress levels, the development of AE is in good agreement with the propagation of micro cracks. The failure pattern of crystalline rocks is well calibrated by AE activities. Based on the testing results, the following conclusions can be drawn. For the CTC tests, the compressibility of Beishan granite increases linearly with increasing confining pressure. The specimen begins to expand when the axial stress reaches 81.1% of the peak stress. Splitting failure is dominant under low confining pressure. The higher the confining pressure, the more pronounced the shear failure. For HM coupling tests, the compressibility decreases with increasing pore pres sure, as well as peak stress. The dilitancy point appears at 82.4% of the peak stress. AE activity shows different phenomenon under different stress levels, which can be used to determine the damage stresses. For CTC tests, σ cc , σ ci and σ cd are 12.3%, 48.7% and 81.1% of the peak stress, respectively. For HM coupling tests, σ ci , σcd are 65.8% and 82.4% of the peak stress, respectively. There is no σcc in HM coupling tests, and σci is much higher than that of the CTC tests. AE activity shows that the induced damage before σ cd is small and crack propagation accelerates after σcd under CTC tests. AE events pro portion at the same loading period changes under different confining pressures, which illustrates that failure pattern develops from splitting to ductile shearing failure with the increasing confining pressure. The propagation of AE events in HM coupling tests indicates that the damage threshold increases due to the presence of fluid. In addition, fluid con tributes to the activation of cracks once the stress exceeds σcd . AE amplitude and FC have same fluctuation trend in the CTC tests. The fluctuation intensity increases with the increasing axial stress. In the residual stage, the amplitude fluctuation increases, and FC decreases. For HM coupling tests, amplitude fluctuation range increases during the loading stage and reaches its maximum at the peak stress, the same as fluctuation intensity. However, the FC decreases after the peak stress. It should be noted that pore pressure affecting the rock strength is not thoroughly investigated in this research. In the future, more tests are needed to be conducted for further investigating the AE and fracturing characteristics of the rock subjected to different confining stresses and pore pressures. In addition, the permeability of Beishan granite also needs to be studied thoroughly.
Fig. 11. Simplified illustration of overall specimen failure process under different pore pressures (HM coupling tests), (a) Overall failure process, (b) Crack propagation.
rapidly. In addition, the amplitude is higher compared with that of the pre-peak stage, and the FC decreases. It also shows that lower confining pressure causes smaller compressibility of the specimen, and vice versa. Moreover, stress con centration is not uniform in the specimen due to the interaction between low peak stress and low confining pressure. As a result, the post-peak expansion is marked, which can be easily to observed in a form of splitting failure. On the other hand, interaction between high peak stress and high confining pressure makes the stress concentrated in the spec imen uniformly, therefore the post-peak expansion is relatively small, and shearing failure dominates. 5.2. AE-based overall failure process under HM tests
Acknowledgments
An illustration of the overall specimen failure process under different pore pressures with respect to HM coupling tests is shown in Fig. 11. Under the same confining pressure, the peak stress decreases with
This work was supported by the State Key Research Development 8
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International Journal of Rock Mechanics and Mining Sciences 123 (2019) 104125
Program of China (2016YFC0600704), the National Natural Science Foundation of China (51674266) and the Yueqi Outstanding Scholar Program of CUMTB. Special thanks are due to Professor H. Konietzky for his help in improving the article.
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