Shear fracture evolution in rocks examined using a novel shear device associated with acoustic emissions

Accepted Manuscript Shear Fracture Evolution in Rocks Examined using a Novel Shear Device Associated with Acoustic Emissions Li-Hsien Chen, Wei-Chih Chen, Yao-Chung Chen PII: DOI: Reference:

S0013-7944(18)30303-5 https://doi.org/10.1016/j.engfracmech.2018.07.003 EFM 6069

To appear in:

Engineering Fracture Mechanics

Received Date: Revised Date: Accepted Date:

20 March 2018 2 July 2018 4 July 2018

Please cite this article as: Chen, L-H., Chen, W-C., Chen, Y-C., Shear Fracture Evolution in Rocks Examined using a Novel Shear Device Associated with Acoustic Emissions, Engineering Fracture Mechanics (2018), doi: https:// doi.org/10.1016/j.engfracmech.2018.07.003

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Shear Fracture Evolution in Rocks Examined using a Novel Shear Device Associated with Acoustic Emissions Li-Hsien Chen．Wei-Chih Chen．Yao-Chung Chen

Abstract This study constructs a new shear test method composed of an open-box-style shear device with a crack-opening-displacement (COD) extensometer associated with a non-destructive acoustic emission (AE) detection system for examining the complete load behavior and the shear fracture triggering position and subsequent fracture propagation of two rock types (Charcoal granite and Berea sandstone). The complete macroscopic view loading curve presents that the pre-peak shear stiffness and shear strength of granite are higher than those for sandstone. The post-peak load behavior of both rocks trend towards the unstable fracture type (Class II), whereas in the microscopic view, the localization of AE events occurred before the peak and is located near the middle of the specimen. The AE localization spatial distribution is in good agreement with the macro-crack position. This suggests that the AE localization L.-H. Chen Department of Civil Engineering, National Taipei University of Technology, Taipei 10608, Taiwan 

W.-C. Chen ( )  Y.-C. Chen

Department of Civil and Construction Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan e-mail: [email protected]

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distribution is related to the macro-crack initiation position. After localization, the AE events then propagate following the shear direction. The results obtained in this study provide a better understanding of the shear fracture mechanism of brittle rock material.

Keywords: Acoustic emission  AE localization  Shear fracture  Post-peak behavior  Granite  Sandstone

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Introduction Some important civil engineering mechanical properties such as cohesion (c) and friction angle () are usually obtained using the triaxial compression test or direct shear test. However, the microscopic behavior of material fractures is a key influence on these macroscopic engineering mechanical properties (Chen et al. 2013). In the traditional direct shear test, with low normal stress, the formed initiation cracks pinch at an angle with the shearing surface. New cracks form as the shear displacement increases. The initial crack represents a tension crack, whereas the later cracks are frequently shear cracks. Shear cracks link with tension cracks to form a continuous shearing surface. If a significant amount of normal stress exists, the shear stress continues to grow, eliminating irregular cracks and creating shear zones (Lajtai 1969). This fracture evolution including crack initiation and propagation during shear testing could be investigated using acoustic emission. A number of recent studies incorporated the AE technique into the direct shear testing of rock materials, rock-concrete joints, and coal-rock joints to elucidate the relationship between the AE energy rate and shear stress (Hong and Jeon 2004; Li et al. 2015; Moradian et al. 2010; Rim et al. 2005). However, a review of the above literature reveals no test results that describe the distribution and development of micro-cracks (AE events) in the materials during the shear tests. 3

To detect the location of occurring AE events, a minimum of five AE sensors should be employed using the arrival time difference method (Chen et al. 2015; Labuz et al. 2001). However, the closed-shear-box design of traditional direct shear test limits the installation space and number of AE sensors. Therefore, in this study, a novel shear device with an opened-shear-box design, was developed and used in conjunction with the AE technique to identify the location of micro-cracks within rock materials. Moreover, two classified post peak behavior fracture types for rock: snap through (Class I) and snap back (Class II), were defined using uniaxial compressive tests (Wawersik 1968), as shown in Fig. 1. In Class I (stable fracture type), the post-peak cumulative strain energy cannot sustain crack propagation. The carrying capacity of the specimen decreases with the increase in strain. In Class II (unstable fracture type), the post-peak strain or degree of specimen displacement decreases; however, the energy stored in the specimen can sustain crack propagation even without external pressure. Carpinteri et al. (2013) investigated the complete uniaxial compression test loading process and its corresponding AE activity for concrete and rocks and indicated that the AE activity was concentrated mainly in the post-peak regime. The number of AE events in the post-peak regime is proportional to the fracture released energy. In this study, a crack opening displacement (COD) control technique was used 4

to investigate the post-peak behavior of a shear test. Due to the opened-shear-box design a COD extensometer can be installed. The measured COD as a feedback signal was used to avoid an unstable specimen failure. The complete loading history, including pre- and post-peak stage with respect to AE evolution, can therefore be obtained.

stress

A

CLASS I

B

C

CLASS II D

E

strain

Fig. 1. Classification of rock failure behavior in uniaxial compression adapted from Wawersik (1968) The applicability of this new test method (inclined shear test) was discussed by Chen et al. (2013). They conducted the inclined shear test associated with the acousto-optic technique for gypsum and cement mortar material. They indicated that the cohesion (c) obtained by the inclined shear test was consistent with that obtained by the direct shear test, whereas the friction angle () was slightly higher. In this study, an inclined shear test coupled with acoustic emission was employed to investigate the 5

fracture evolution of rock materials (Charcoal granite and Berea sandstone).

Acoustic emission technique The AE technique is based upon the work of Kaiser (1953) who found that when materials are reloaded, the AE signals occur only if the reloading stress is greater than the maximum. According to ASTM E610-82, the AE phenomenon is defined as transient elastic waves generated by the rapid release of energy from localized sources within a material (ASTM 1999). Contrary to its use in the investigation of earthquake macroscopic-scale behavior, AE is one of the non-destructive techniques used for recording several micro seismic sources with minor energy releases (ASTM 2000). AE was also used to investigate micro-crack energy released in a study by Scholz (1968) that investigated the frequency–magnitude behavior of micro-cracks in rock under uniaxial and triaxial compression tests using AE data. Labuz et al. (1987) applied the AE technique to detect the fracture process zone in rock. Ohtsu (1987) used AE to assess the damage to concrete. Shah and Labuz (1995) classified the crack type in rock from the AE source characterization. In recent years the AE technique has been widely used to monitor masonry structural health (Han et al. 2015), detect the released energy during micro-crack propagation (Carpinteri et al. 2012; Carpinteri et al. 2013) and the location of micro-cracks (Carpinteri et al. 2007; Chen et al. 2015; Chen et al. 2009; Chen and Labuz 2006; ElBatanouny et al. 2012; Fortin et al. 2006; 6

Labuz et al. 2001; Lei et al. 2000; Liu et al. 2014; Tham et al. 2004; Xu et al. 2017; Yang et al. 2012; Zang et al. 2000) for cement mortar, concrete and rock brittle materials. The arrival time difference method (Chen 2001; Chen et al. 2009) was used in this study to process the difference in AE signal arrival time at each sensor. The AE signal travel distance for each sensor can be written as Eq. (1) using the arrival time difference method: (1) where ,

is the P-wave velocity; and

,

are the coordinates of the ith sensor;

and

are the coordinates of the source;

is the starting time from the source; and

is the corresponding arrival time;

is the statistical residual (error). Note

that the P-wave arrival time was determined in this study using the Akaike Information Criterion (AIC) (Akaike 1974). AIC was used to process the AE signal with an automatic procedure to eliminate false or doubtful onset times (Carpinteri et al. 2012; Niccolini et al. 2012). The AE locations were estimated using an iterative technique that varied the location until a minimum error was obtained. In Eq. (1) the source coordinates, and

and event time

,

are unknown. To solve these four unknown parameters,

four differences in arrival times are required. Therefore, a minimum of five AE 7

sensors should be used. Eight sensors were used in this study to increase the measurement accuracy. The AE positioning statistical analysis could thus be performed.

Experimental setup The experimental setup in this study consisted of (1) test specimen, (2) inclined shear device and (3) acoustic emission system, described as follows.

3.1 Test specimen A series of shear tests coupled with acoustic emission were conducted on the following natural rocks: Charcoal granite, a high strength rock, and Berea sandstone, a medium strength rock (Chen and Labuz 2006). Charcoal granite, quarried in Cold Spring, Minnesota, is a medium grained (0.2−6 mm), Precambrian granodiorite consisting primarily of plagioclase, microcline, quartz, hornblende and biotite (Zietlow and Labuz 1998). Less than 10-4 m/s of conductivity, 0.08% of porosity and 2.72g/cm3 density were reported by Krech et al. (1974). Berea sandstone is a fine to medium grained protoquartzite (0.1 −0.8 mm) cemented with silica and clay. It is early Mississippian and quarried in northern Ohio. The porosity was reported to be 20%, the conductivity 0.6 m/s and density 2.11 g/cm3 (Krech et al. 1974; Zietlow and Labuz 1998). The uniaxial compressive strength of Charcoal granite and Berea sandstone was obtained as 119 MPa and 48 MPa respectively by conducting uniaxial 8

compressive tests. Rock specimens were fabricated through cutting and grinding processes with the same dimensions: 70 mm long × 70 mm wide × 40 mm thick as shown in Fig. 2. In particular, grinding the four right edge angles was strictly required to ensure that the desirable boundary on lateral sides contacted with shear-box is uniform. Therefore, the shear force can be applied evenly on the test specimen surface.

（a）Charcoal granite

（b）Berea sandstone Fig. 2. Test specimens

3.2 Inclined shear device An open-box-style shear device (inclined shear device) was developed to perform a shear force test. It can be incorporated with nondestructive AE technology (Fig. 3) to investigate the macroscopic and microscopic fracture evolution of rock subjected to shear force. Symmetrical shear clamps were designed with an opened shear box. The shear clamps were combined with rollers capable of safely controlling for upper and 9

lower side slippage to prevent failure based on instability. Therefore, only one actuator was required to form two components: normal and shear forces. Different combinations of load components could be obtained by changing the shear angles. The device can also be installed in various load systems according to the test requirements. The multifunction, precision, high-stiffness servo-controlled hydraulic test system MTS 810 was adopted in this study as the load system.

Load

LVDT2

Opened shear box

AE sensor

Shear angle

LVDT1

Fig. 3. Inclined shear device and the installation of AE sensors and LVDT An inclined shear device can be incorporated with the AE technique, and also the crack-opening-displacement (COD) control technique during shear testing (Fig. 4).

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The MTS-produced model 632-02F-20 clip gage/extensometer was employed to measure the COD. The measured COD was used as the feedback signal to stabilize crack propagation. The complete loading history was then obtained. Additionally, two linear variable differential transformers (LVDT) were used to monitor the specimen displacement. These were installed on the bottom and top halves of the shear clamps (Fig. 3), and used to synchronously measure changes in the vertical and lateral displacement during the testing process.

Fig. 4. Installation of clip gauge/extensometer and two L type clips 3.3 Acoustic emission system This study applied the arrival time difference method in the three-dimensional AE positioning to obtain the positions of micro seismic sources. A piezoelectric transducer (Model S9225, Physical Acoustics Corporation, PAC) with frequency response over the 300−1800 kHz range was employed in this system as the AE sensor. As mentioned previously, eight sensors were used to increase the precision of locating

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the AE events. The sensors were attached to the specimen surface using adhesives (Instantbond 123, Hernon Manufacturing, Inc., Sanford, FL, USA) to receive signals that were processed using a preamplifier. The sampling rate and signal amplification of the data acquisition system were set to 8 MHz and 40 dB, respectively. A 7mv threshold was set to filter the signals to avoid background noise interference. Signals were selected greater than the 7mv threshold with a sonic rate between 100−1200 kHz. The setup threshold value was determined by trial and error. The minor voltage variations were then recorded in a binary file. The collected binary data were used to identify the wave shape characteristics, which determined the P-wave signal arrival time at each AE sensor. During the testing, when the single voltage reached the threshold value, the triggered channel could detect a valid micro seismic event source. Otherwise, it ignored the noise. If triggered, the other seven channels received and recorded the events synchronously. According to the differences in the arrival times between the eight AE sensors, the 3-D locations of micro-cracks were obtained. The control system converted the binary data into spatial coordinates. Matlab software was also used to assess and filter the waveform characteristics. A FORTRAN program was developed by the authors to locate the micro crack positions. The pencil lead fracture method suggested by ASTM (2000) was used to 12

calibrate the AE location. The statistical residual value of AE positioning less than 2 mm can be obtained within a measured distance of 70 mm. Therefore, cubic specimens with 70 mm length, 70 mm width and 40 mm thickness were designed for these tests.

Results and discussions By incorporating the constructed AE system into the developed inclined shear device with COD control technique, the shear fracture evolution of rocks including complete macro-view loading curve and the micro seismic fracture evolution were obtained. Two shear angles of 70⁰ and 80⁰ were adopted in the test. The test results are shown in Table 1. In Table 1, shear stiffness

was obtained by calculating the

average unload and reload curve slopes. The peak shear stress

and the shear

displacement at peak up were calculated based on trigonometric function geometric relationships, respectively, as shown in Fig. 5. The load level at AE localization was found according to the loading curve with respect to AE events accumulation. The details are discussed in the following section.

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Table 1. Inclined shear experiments on Charcoal granite (SxxG) and Berea sandstone (SxxS) at shear angle of 70⁰ (S70x) and 80⁰ (S80x) Microscopic view

Macroscopic view Test type

Peak shear stress

S70G

80.05

36.81

0.61

45

S80G

42.08

17.64

0.44

75

S70S

39.39

12.01

0.44

81

S80S

19.35

6.09

0.34

44

(MPa)

Shear displacement

Load level at localization

Stiffness before peak (MPa/mm)

at peak up (mm)

F LVDT2 D2 b

Normal force: N = F × cosb Shear force: T = F × sinb

T N

Normal stress: s= N / A Shear stress: ts = T / A Shear displacement: u = D1 × sinb – D2 × cosb

b

Normal displacement: v = D1 × cosb+ D2 × sinb

LVDT1 b D1

F Note: b is the shear angle, F is loading force provided by MTS, A is the contacted area between specimen and shear box, D 1 is the displacement measured by LVDT 1 and D 2 is the displacement measured by LVDT2

Fig. 5. Calculation parameters for the inclined shear test

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LL,L (%)

4.1 Complete macro-view loading behavior

The complete Charcoal granite and Berea sandstone loading processes at 70⁰ and 80⁰ shear angles are shown in Fig. 6 and 7, respectively. Prior to achieving peak strength, unload and reload scheduling was performed twice to obtain the hysteresis loop. The average unload and reload curve slopes before the peak were defined as the shear stiffness

. The obtained

of Charcoal granite is 80.05 and 42.08

MPa/mm at 70⁰ and 80⁰ shear angle; that of Berea sandstone is 39.39 and 19.35 MPa/mm at 70⁰ and 80⁰ shear angle.

is not constant under different shear

angle tests. This is because the normal component force increases continually rather than remaining at a fixed value during the loading process. The increase in normal stress altered the stress field at different shear angles, thereby affecting the stiffness.

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Fig. 6. Complete Charcoal granite loading process at different shear angles

Fig. 7. Complete Berea sandstone loading process at different shear angles The peak shear stresses

of Charcoal granite was found as 36.81 and 17.64

MPa with respect to shear angles of 70⁰ and 80⁰ respectively as shown in Fig. 6. 16

The

and corresponding shear displacement up (Table 1) decreased with the

increase in shear angle. This loading behavior can also be found in Berea sandstone as shown in Fig. 7. At higher shear angle testing, the smaller normal stress was performed to cause a decrease in the peak shear stress and the shear displacement at peak. This also affected the load behavior at the post-peak stage. The shear displacement at the post-peak stage tends to increase with the increasing shear angle. In addition, the post-peak failure type of both Charcoal granite and Berea sandstone under shear test tended towards Class II was observed. Note that, as expected, the peak shear stress and the pre-peak shear stiffness of Charcoal granite are higher than that of Berea sandstone.

4.2 Micro seismic Fracture evolution 4.2.1 AE evolution in time graph series An AE event indicates that a micro seismic crack occurred and that crack evolution is related to the loading process. A complete loading curve was obtained in this study. Therefore, the whole evolution of the AE activities could be investigated. Fig. 8 illustrates the relationship between the loading process and the AE events for Charcoal granite under the 70° shear angle test. The solid line in Fig. 8 is the loading curve corresponding to the load level, LL (defined as the shear stress divided by the peak shear stress), on the left vertical axis. The dotted line represents the correlation 17

between the accumulated number of AE events and the normalized shear displacement (defined as shear displacement divided by shear displacement at the peak shear stress). The red dots on the solid line mark the occurrence of AE events.

L

Fig. 8. Loading process and corresponding AE events accumulation for Charcoal granite at a 70⁰ shear angle In Fig. 8 the accumulated number of AE events increased with increasing shear displacement. The AE localization is defined as the rapid increase in micro-crack accumulation, as at point L where the slopes for the AE event curve accumulation change suddenly (Chen 2001). The AE localization phenomenon is explained by the fact that micro-cracks occur and are localized within an area under a certain load level due to a defect in that part of the original material or the stress concentration (Chen et 18

al. 2009). Therefore, the position at which these events occur is correlated to the initial macro-crack formation and propagation. Point L in Fig. 8 shows the localization where LL is approximately 45%. Following the same method, the localization of Berea sandstone at the 70° shear angle occurred at 81% LL was obtained, as shown in Fig. 9. The obtained localization LL (LL,L) of all samples tested in this study is shown in Table 1.

L

Fig. 9. Loading process and corresponding AE events accumulation for Berea sandstone at a 70⁰ shear angle Table 1 shows that the LL,L for Charcoal granite increased with increasing shear angle. As the shear angle increased, the smaller normal stress caused the growth of a smaller number of total AE events (Table 2), and a later localization occurrence. However, the LL,L of Berea sandstone decreased with the increasing shear angle. At 19

lower normal stress (80⁰ shear angle), sandstone was fractured rapidly, which led to detecting few AE events at the pre-peak loading stage. Only sixty-seven AE events were detected before the peak (Table 2). This may led to identifying the LL,L deviation for Berea sandstone at 80⁰ shear angle testing. Note that the Charcoal granite AE localization occurred earlier than that for Berea sandstone under 70⁰ shear angle testing. Table 2. Numbers of AE events at different test type Test type

Total AE events

Pre-peak AE events

S70G

16566

15371

S80G

8862

1114

S70S

8449

4803

S80S

4683

67

The detected numbers of AE events in Charcoal granite is larger than that in Berea sandstone, as shown in Table 2. The granite is a crystalline rock constructed of tightly interlocked particle arrangements (crystals), and the sandstone is a clastic rock composed of pieces of various rock types and assorted mineral grains (Goodman 1976; Goodman 1989). Therefore, the porosity of Charcoal granite (0.08%) is far smaller than that of Berea sandstone (20%). Meanwhile, the significant interlocking effect between the composed particles within Charcoal granite would cause more numbers of AE events.

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4.2.2 AE evolution in spatial graph series The spatial distribution of AE events before the peak in Charcoal granite at 70⁰ shear angle is shown in Fig. 10, where the hollow dots denote the positions of micro seismic sources (micro-cracks). Every graph is drawn according to the load levels. The spatial evolution of the AE events with time can then be observed. Observation of this series of spatial graphs indicates that when the load level LL was between 0% and 40%, during the early loading period, some AE events gathered and located near the middle of the specimen. At LL = 40−50%, the AE cluster located at the middle of the specimen is significant. After LL continued to increase, the greater AE events formed and continuously propagated following the shear direction.

Fig. 10 Development of AE events before the peak in Charcoal granite specimen (70⁰ shear angle) 21

From the findings in Fig. 8, the localization LL is defined as LL,L = 45%. AE distributions of 5% LL before and after the localization were investigated, as shown in Fig. 10 (the graph of LL = 45-5 (%) and LL = 45+5 (%)). The AE distribution at 5% LL prior to localization was relatively sparse. However, following localization, a dramatic increase in AE events was observed, with these AE events clearly concentrated near the shear surface. The AE localization occurred and located at the middle of the specimen. This may be related to the macro-crack initiation position. Fig. 11 shows the spatial evolution of AE events before the peak in Berea sandstone at 70⁰ shear angle. At early loading period, LL = 0−50%, contrary to the evolution of Charcoal granite, a few AE events occurred and distributed randomly. After LL exceeded 50%, greater AE events formed following the shear direction. The AE distribution graphs for 5% LL before and after the localization are also drawn in Fig. 11. Similar findings to Fig. 10, AE events increased rapidly as well as concentrated near the middle of the specimen at 5% LL after the localization (LL = 81+5 (%)).

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Fig. 11 Development of AE events before the peak in Berea sandstone specimen (70⁰ shear angle) Fig. 12 illustrates the final outline of macro-cracks in both rock materials used in this study after the test. The AE distribution of 5% LL prior and after the localization on Fig. 12 is shown in Fig. 13. In Fig. 13 the red dots are the AE events and the yellow line is a sketch of the macro-cracks. The distribution of AE events is in good agreement with the macroscopic scale crack positions. This implied that the AE localization is related to the initial macro-crack characteristics and the macro-crack propagation path. Therefore, the shear fracture behavior and mechanisms of rocks could be predicted and examined by applying the AE technique to the inclined shear test.

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Fig. 12 Final macro-cracks pattern

Fig. 13 AE distributions of 5% LL before and after the localization (red circles) and highlighted macro-cracks (yellow line) 24

Chen and Labuz (2006) conducted indentation tests on Charcoal granite and Berea sandstone at different indenter wedge angles. They found that the localization in these two rocks occurred at LL = 40−50%. However, at the shear test, localization occurred at LL = 45% and LL = 75% for Charcoal granite under 70⁰ and 80⁰ shear angle test, and LL = 81% for Berea sandstone under 70⁰ shear angle test, respectively, as shown in Table 1. The localization time for the shear test on average was later than that for the indentation test. The indentation test failure stress field belongs to the tensile type, whereas the inclined shear test is the shear type. Different stress paths may influence the localization occurrence time. More experiments are needed to clarify this issue.

Conclusion An opened-box-style inclined shear test was designed and used in conjunction with the nondestructive AE technique to track rock fracture development. Experiments were performed with two shear angles (70⁰ and 80⁰ ) using COD control for high and medium strength rock (Charcoal granite and Berea sandstone). The complete macroscopic view loading process and corresponding microscopic view crack evolution were obtained. In the macroscopic view, observing the complete loading curve indicated that the shear strength and the pre-peak stiffness of Charcoal granite are higher than that of 25

Berea sandstone. As the shear angle increased, the shear strength tended to decline since the normal stress decreased. Note that the post-peak load curve represented the Class II failure type for both test rocks. In the microscopic view, due to the interlocking effect, more AE events were detected in the Charcoal granite specimen than for Berea sandstone. The AE shear test localization occurred at LL = 45% and LL = 75% for Charcoal granite, and LL = 81% for Berea sandstone. When localization occurred, the AE events were clearly clustered near the center part of the specimen. A comparison of the spatial AE distribution of 5% LL prior and after the localization with the final macro-crack position shows good agreement. It is suggested that the AE localization distribution is related to the macro-crack initiation position. The macro-crack initiation actually formed before the peak shear stress occurred. Therefore, the macro-crack propagation behavior of rocks subjected to shear stress could be predicted using this newly-developed shear test method. It should be noted that as the shear angle increased, the normal stress and peak strength decreased, leading the AE localization of Charcoal granite to a later occurrence.

Acknowledgments The research funding provided by the Ministry of Science and Technology in Taiwan (98-2221-E-011-111-) is highly appreciated. 26

References Akaike, H. (1974). "A new look at the statistical model identification." IEEE transactions on automatic control, 19(6), 716-723. ASTM (1999). "Standard definitions of terms relating to acoustic emission." American Society for Testing and Materials. ASTM (2000). "Standard guide for determining the reproducibility of acoustic emission sensor response." American Society for Testing and Materials. Carpinteri, A., Corrado, M., and Lacidogna, G. (2012). "Three different approaches for damage domain characterization in disordered materials: Fractal energy density, b-value statistics, renormalization group theory." Mechanics of Materials, 53, 15-28. Carpinteri, A., Corrado, M., and Lacidogna, G. (2013). "Heterogeneous materials in compression: Correlations between absorbed, released and acoustic emission energies." Engineering Failure Analysis, 33, 236-250. Carpinteri, A., Lacidogna, G., Niccolini, G., and Puzzi, S. (2007). "Critical defect size distributions in concrete structures detected by the acoustic emission technique." Meccanica, 43(3), 349-363. Carpinteri, A., Xu, J., Lacidogna, G., and Manuello, A. (2012). "Reliable onset time determination and source location of acoustic emissions in concrete structures." Cement and concrete composites, 34(4), 529-537. Chen, L.-H. (2001). "Failure of rock under normal wedge indentation." Ph. D. Thesis, University of Minnesota, Minnesota. Chen, L.-H., Chen, W.-C., Chen, Y.-C., Benyamin, L., and Li, A.-J. (2015). "Investigation of hydraulic fracture propagation using a post-peak control system coupled with acoustic emission." Rock Mechanics and Rock Engineering, 48(3), 1233-1248. Chen, L.-H., Huang, K.-C., and Chen, Y.-C. (2009). "Acoustic emission at wedge indentation fracture in quasi-brittle materials." Journal of Mechanics, 25(02), 213-223. Chen, L-H., and Labuz, J. F. (2006). "Indentation of rock by wedge-shaped tools." International Journal of Rock Mechanics and Mining Sciences, 43(7), 1023-1033. 27

Chen, L.-H., Lin, G.-Z., Chen, Y.-C., and Yang, W.-X. (2013). "Using acousto-optic coupling technology to investigate the fracture evolution of rock-like materials during inclined shear tests." Applied Mechanics and Materials, 284, 1363-1367. ElBatanouny, M. K., Larosche, A., Mazzoleni, P., Ziehl, P. H., Matta, F., and Zappa, E. (2012). "Identification of cracking mechanisms in scaled FRP reinforced concrete beams using acoustic emission." Experimental Mechanics, 54(1), 69-82. Fortin, J., Stanchits, S., Dresen, G., and Guéguen, Y. (2006). "Acoustic emission and velocities associated with the formation of compaction bands in sandstone." Journal of Geophysical Research, 111(B10). Goodman, R. E. (1976). Methods of geological engineering in discontinuous rocks, West Publishing Company, United States of America. Goodman, R. E. (1989). "Introduction to rock mechanics." John Wiley & Sons, New York. Han, Q., Xu, J., Carpinteri, A., and Lacidogna, G. (2015). "Localization of acoustic emission sources in structural health monitoring of masonry bridge." Structural Control and Health Monitoring, 22(2), 314-329. Hong, C.-W., and Jeon, S.-K. (2004). "Influence of shear load on the characteristics of acoustic emission of rock-concrete interface." Key Engineering Materials, 270, 1598-1603. Kaiser, J. (1953). "Undersuchungen Uber Das Aufrterten Geraucchen Beim Zevgersuch." Ph. D Thesis. Technische Hochschule, Munich. Krech, W., Henderson, F., and Hjelmstad, K. (1974). "A standard rock suite for rapid excavation research Report: 16F. US. Bur. Mines, RI, 1974, 29P." Proc., International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, Pergamon, 198. Labuz, J. F., Shah, S. P., and Dowding, C. H. (1987). "The fracture process zone in granite: evidence and effect." Proc., International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, Elsevier, 235-246. Labuz, J. F., Cattaneo, S., and Chen, L.-H. (2001). "Acoustic emission at failure in quasi-brittle materials." Construction and building Materials, 15(5), 225-233. Lajtai, E. (1969). "Mechanics of second order faults and tension gashes." Geological Society of America Bulletin, 80(11), 2253-2272. 28

Lei, X., Kusunose, K., Rao, M. V. M. S., Nishizawa, O., and Satoh, T. (2000). "Quasi-static fault growth and cracking in homogeneous brittle rock under triaxial compression using acoustic emission monitoring." Journal of Geophysical Research, 105(B3), 6127. Li, W., Bai, J., Cheng, J., Peng, S., and Liu, H. (2015). "Determination of coal–rock interface strength by laboratory direct shear tests under constant normal load." International Journal of Rock Mechanics and Mining Sciences, 77, 60-67. Liu, H.-W., Chen, L.-H., Chen, Y.-C., and Chang, Y.-C. (2014). "Suggested continued heat-treatment method for investigating static and dynamic mechanical properties of cement-based materials." Construction and Building Materials, 69, 91-100. Moradian, Z., Ballivy, G., Rivard, P., Gravel, C., and Rousseau, B. (2010). "Evaluating damage during shear tests of rock joints using acoustic emissions." International Journal of Rock Mechanics and Mining Sciences, 47(4), 590-598. Niccolini, G., Xu, J., Manuello, A., Lacidogna, G., and Carpinteri, A. (2012). "Onset time determination of acoustic and electromagnetic emission during rock fracture." Progress In Electromagnetics Research, 35, 51-62. Ohtsu, M. (1987). "Acoustic emission characteristics in concrete and diagnostic applications." Journal of acoustic emission, 6(2), 99-108. Rim, H., Choi, H., Son, B.-K., Lee, C.-I., and Song, J.-J. (2005). "Experimental study for shear behaviour of pseudo rock joint under constant normal stiffness condition." Proc., Proceedings of the 31st ITA-AITES world tunnel congress on underground space use, Istanbul, 175-181. Scholz, C. H. (1968). "The frequency-magnitude relation of microfracturing in rock and its relation to earthquakes." Bulletin of the Seismological Society of America, 58(1), 399-415. Shah, K. R., and Labuz, J. F. (1995). "Damage mechanisms in stressed rock from acoustic emission." Journal of Geophysical Research: Solid Earth (1978–2012), 100(B8), 15527-15539. Tham, L. G., Liu, H., Tang, C. A., Lee, P. K. K., and Tsui, Y. (2004). "On tension failure of 2-D rock specimens and associated acoustic emission." Rock Mechanics and Rock Engineering, 38(1), 1-19. Wawersik, W. R. (1968). "Detailed analysis of rock failure in laboratory compression tests." Ph. D. Thesis, University of Minnesote. 29

Xu, J., Fu, Z., Han, Q., Lacidogna, G., and Carpinteri, A. (2017). "Micro-cracking monitoring and fracture evaluation for crumb rubber concrete based on acoustic emission techniques." Structural Health Monitoring, 1475921717730538. Yang, S.-Q., Jing, H.-W., and Wang, S.-Y. (2012). "Experimental Investigation on the Strength, Deformability, Failure Behavior and Acoustic Emission Locations of Red Sandstone Under Triaxial Compression." Rock Mechanics and Rock Engineering, 45(4), 583-606. Zang, A., Wagner, F. C., Stanchits, S., Janssen, C., and Dresen, G. (2000). "Fracture process zone in granite." Journal of Geophysical Research, 105(B10), 23651. Zietlow, W. K., and Labuz, J. (1998). "Measurement of the intrinsic process zone in rock using acoustic emission." International journal of rock mechanics and mining sciences, 35(3), 291-299.

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Highlights  A novel shear device with an opened-shear-box design was developed.  Acoustic emission was used to detect the micro-cracks position.  The complete load behavior was obtained using COD control technique.  Shear fracture initiation and propagation of granite and sandstone was observed.

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