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Triaxial compression system for rock testing under high temperature and high pressure
International Journal of Rock Mechanics & Mining Sciences 52 (2012) 132–138
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Technical Note
Triaxial compression system for rock testing under high temperature and high pressure Yangsheng Zhao a,n, Zhijun Wan b, Zijun Feng a, Dong Yang c, Yuan Zhang d, Fang Qu e a
College of Mining Engineering, Taiyuan University of Technology, Taiyuan, Shanxi 030024, China School of Mining and Safety Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221008, China c Mining Technology Institute, Taiyuan University of Technology, 030024, China d Huaiyin Institute of Technology, Huaiyin, Jiangsu 223003, China e China Jiliang University, Jinan, Hangzhou, Zhejiang 310018, China b
a r t i c l e i n f o Article history: Received 9 April 2010 Received in revised form 5 February 2012 Accepted 26 February 2012 Available online 5 April 2012
1. Introduction Rock mechanics always develops with improvement of rock testing technique, especially the improvement of rock testing machines. The latter has been gradually separated from material mechanics and has evolved to an independent subject. In early stages of its development, rock testing was carried out by ordinary material testing machines. Then, the discovery of the principles of stiff testing machine [1], the developments of rock triaxial testing machine with confining pressure [2,3], true triaxial tests machine [4], high temperature and high pressure triaxial testing machine [5–7,21], large-scale true triaxial testing machine, as well as other subsequent experimental improvement, have revealed many features of rock samples that were little known, and have greatly promoted the development of rock mechanics. The rapid growth of deep resources and energy exploitation is greatly relying on the knowledge of the mechanical properties of deep rock [8,9]. For example, the depth of oil drilling has reached 8000 m; the exploitation depth of the geothermal resources for high-temperature rock is greater than 5000 m [18–20]; the exploitation depth of the metal mining reached 3000 m; the temperature of underground coal gasification and underground fire is up to 1000 1C. However, most of the current existing high temperature and high pressure test machines in the world, due to their limited sample size (often 20 mm 40 mm) and normal experiment condition, can only test for the rock properties such as deformation and elastic waves, and hardly provide further data
n
Corresponding author. E-mail address: [email protected] (Y. Zhao).
1365-1609/$ - see front matter & 2012 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.ijrmms.2012.02.011
regarding rock engineering behavior. Therefore, our research group developed XPS-20 MN, a servo-controlled triaxial rock testing system of high temperature and high pressure, for high temperature and high pressure rock testing. This paper intends to describe the structure, functions, and technical considerations of the test machine. Examples of measured deformation properties and thermal expansion coefficients of coal and granite samples at high temperature and high pressure are also presented.
2. Main functions and parameters 2.1. Main functions The testing machine XPS-20 MN can be used to study rock mechanical behavior in a wide range of engineering fields, such as deep mining, underground coal liquefaction and gasification, geothermal exploitation, coal-bed methane recovery, deep oil and gas extraction, waste disposal, etc. The testing machine is designed to study rock properties including deformation, strength, solid-fluid-thermal coupling, rheology, permeability and thermal conductivity at high temperature and high pressure as well as at normal temperature; to study rock properties and laws in phase transitions, melting, heat and mass transfer, liquefaction, gasification, chemical reactions of solid minerals (coal, oil shale, etc.) at the mutual action of heat and stress; and to study the interaction between the deformation of rock mass and the drilling rig and to study hydraulic fracturing at high temperature and high pressure.
Y. Zhao et al. / International Journal of Rock Mechanics & Mining Sciences 52 (2012) 132–138
3. Structures of the test machine and key technologies of each component The testing machine consists of four parts: host loading system, high-temperature triaxial pressure chamber with temperature controlling system, auxiliary installing system, and testing system. The host loading system is the force source of the testing machine. The high-temperature triaxial pressure chamber with temperature controlling system is used for holding pressuring and heating the specimen. The auxiliary installing system is specifically designed for installing coal (rock) specimens in the high-temperature triaxial pressure chamber. The testing system can measure the temperature, pressure, deformation, permeability and acoustic emission, etc. of coal (rock) specimens. 3.1. Host loading system The host loading system consists of the following three parts: (1) The host framework consists of four-vertical columns with ‘‘bottom-push style’’, i.e., the piston moves upwards to press the specimen. Its vertical and lateral forces can both reach 10,000 kN. The vertical and lateral pressers are designed differently to ensure the stiffness of the testing machine. Inside the pressure chamber, the diameter of the vertical presser cylinder is 200 mm; while the pressers for lateral pressure are hollow cylinders with an inner diameter of 210 mm and outer diameter of 300 mm. Outside the pressure chamber, the pressers with larger diameter is transited to a smaller diameter gradually in order to ensure the overall stiffness of the testing machine satisfying the requirements for stiff testing machine. (2) The hydraulic pressure system is the power source of the host machine. The pressure is generated by three hydraulic pumps. The working pressure of the pump station is 25 MPa. For the purpose of servo control, we used the precisely proportioned servo valve produced by MOGO, which can accurately control the movements of the cylinders for axial pressure and lateral pressure. (3) The main console controls the movements of the host system. Its operation is fulfilled through a computer combined with a control panel. Incremental loading process is controlled using a computer program; while the adjustment of the presser and the movements of the material holding cylinder are controlled by the control panel. Axial and lateral loads are applied separately. The loading methods include constant load, constant load rate, constant displacement, constant displacement rate, saw tooth wave load, etc.
The high-temperature triaxial pressure chamber is not only a core component of the testing machine but also an important component producing the required temperature and pressure in the entire experiment. It is a thick-walled cylinder made of thermoplastic die steel (H13). The phase transition temperature of the cylinder is 800 1C. Its structure is like an inner–outer double-layer bucket. The two layers in the shrink sleeve structure are nested together by tight fit after a special treatment. This ensures that the cylinder will not damage at a high-pressure up to 250 MPa. The cylinder height is 960 mm, with outer diameter of 1060 mm and inner diameter of 300 mm. The heating, temperature adjustment and temperature measurements in the high-temperature triaxial pressure chamber are implemented by the temperature-controlled cabinet. The operation diagram of the high-temperature triaxial pressure chamber is shown in Fig. 1. The surface of the coal (rock) specimen is wrapped by heat-resistant alloy films. The heatresistant alloy film is connected with the H13 presser placed on the top and bottom of the coal (rock) specimen. The heat-resistant alloy films are charged with direct current from the temperaturecontrolled cabinet and produce heat to heat up the coal (rock) specimen and its surrounding salt ring (pyrophyllite ring) to the required temperature. The high temperature triaxial pressure chamber is heated using direct current that is generated by 380 V three-phase electricity, transformed and rectified in the temperature-controlled cabinet. The designed maximum output voltage is 30 V and the maximum output current is 1700 A with a maximum output power of 51 kW. 3.3. Auxiliary installing systems The auxiliary installing system is designed for setting up coal (rock) specimens. The designed maximum force is 2000 kN. The maximum pressure of the hydraulic pump is 25 MPa. There are
Pyrophyllite powder Press head Thermocouple Resistance chip Copper sleeve Sample H13 Tank
Salt
mica plate
H13 Tank
Thermocouple
Lead of thermocouple
The maximum axial load is 10,000 kN, the maximum lateral load is 10,000 kN, the maximum axial pressure on rock specimen is 318 MPa, the maximum lateral pressure on the rock specimen is 250 MPa (pseudo-triaxial confining pressure), the maximum pore pressure is 250 MPa, the specimen size is f200 mm 400 mm. The parameters of the drill are as follows: maximum travel is 450 mm, maximum loading is 200 kN, maximum torque is 500 Nm, tha maximum temperature is 600 1C, the holding time of pressures is over 360 h, and the offset of axial pressure and lateral pressure is less than 70.3%. The high-temperature triaxial pressure chamber has a highprecision control, of which the temperature sensitivity is less than 0.3%. Stress, deformation, pore pressure of inlet and outlet, temperature, torque and other parameters can be recorded automatically. The stiffness of the whole machine is greater than 9 1010 N/m.
3.2. High-temperature triaxial pressure chamber with temperaturecontrolled heating system
Lead of thermocouple
2.2. Parameters of the equipment
133
Press head Pyrophyllite powder Lateral press head Axial press head
Lateral strain/Axial stress/Lateral strain Fig. 1. Schematic diagram of high temperature triaxial compression chamber.
134
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three hydraulic cylinders, namely center cylinder, side-pressure cylinder and lower cylinder, working together to set up the specimen, the loose salt with ring resonator, and the pyrophyllite ring.
industries. Its main technical parameters include maximum temperature of 1100 1C, density of 7.9 g/cm3, resistivity 1.0470.05 O mm2/m, melting point of 1390 1C, tensile strength of 637–784 MPa, and elongation greater than 20%.
3.4. Testing system 4.2. Insulation and sealing The deformation of a specimen can be calculated by measuring the positions of the axial and lateral presser. The measuring instrument is a grating ruler. The measuring precision is 70.005 mm. The specimen load is calculated by the pressures converted from hydraulic pressures measured at the upper and lower chambers of the hydraulic cylinders with pressure gauges. The surface temperature of a specimen is measured by three thermocouples deployed with 1201 between each other at the upper, middle and lower parts of the circumference near the specimen. In order to increase measurement accuracy and reliability, an additional thermocouple is placed at the middle part of the specimen galvanic symmetrically. The real-time data of the thermometers can be transferred to a computer through the communication port. Based on the temperature value set in one of the four thermometers, the temperature in the heat stress chamber can be controlled automatically by the temperaturecontrolled cabinet. The permeability-measuring instrument consists of inflatable tools, high-pressure gas cylinders and flow meters, etc. The medium used for testing is nitrogen (N2). The flow-meters are soap-bubble meter and rota-meter, which are used for the small and large gas flow rate cases respectively. The drainage gas sampling method can also be used to measure the amount of exhaust gas (mainly used in measuring the gas emitted from coal samples). An AE-04 four-channel acoustic emission detection system is used. Its preamplifier gain is 40 dB; bandwidth is 0.005–1 MHz; the main gain is 9–60 dB; the sensor sensitivity is no less than 60 dB; and its standard resonant frequency is 140 kHz. The system is designed to handle multiple parameters, such as the located functions of linear, regional, and square. It can collect and process the AE parameters of each event, including ring counting, energy counts, event duration, and amplitude, etc.
4. Technical considerations 4.1. Heating elements The appropriate heating element and heating mode can generate temperature environmental in the high-temperature triaxial pressure chamber. The carbon fiber tape used at early heating stage is found to be easy to burn and break, so it is substituted by the alloy resistance film. The alloy resistance film has the following advantages: (1) a wide range of elongation and tensile resistance capacity; (2) longer usage duration; and (3) high temperature durability. The alloy resistance films can produce nearly uniform temperature distributions around the specimen. We changed the distribution of the heating power of the varistor by thickening the varistor in appropriate part of the sample and changing the resistance distribution of the varistor winding around the specimen to produce a uniform temperature field [10–13]. The alloy resistance used is nickel-chromium alloy film named Ni80Cr20. It has good deformation characteristics, thermal stability and mechanical strength. It also has advantages such as better seismic and antioxidant capacity in thermal states. It has been widely used for electric heating and heat treatment in chemical industry, machinery, metallurgy, national defense and other
In the development of this machine, the insulation and sealing between the upper part of the high-temperature pressure chamber and the upper presser of the test machine, the insulation and sealing between the axial presser at the lower part of pressure chamber and the axial presser of testing machine and those between H13 pressers above the specimen in the pressure chamber are important not only for the safe operation of the apparatus, but also for determining the accuracy and reliability of the experimental results. The materials for sealing and insulating are required to have highly stable performance at the conditions of high pressure and temperature. A layer of high temperature insulation pads is placed in the gap between the high-temperature pressure chamber and the platform underneath it. A layer of asbestos or mica plate is placed between the upper part of the high temperature and pressure chamber and the upper presser of the testing machine. Insulating plastic cast for high temperature is pasted around the ventilation (water) hole on the upper presser of the host machine play a role in sealing and insulating. To prevent the leakage of N2, seal is required between the axial presser of the host machine and the lower axial presser in hightemperature pressure chamber. A seal groove is manufactured on the surface of the axial presser of the host machine and a copper pad placed in it to make good sealing and insulation. Similarly, a seal groove plus copper pad are used to seal between two H13 pressers above the specimen in the pressure chamber.
5. Deformation and failure characteristics of coal samples at high temperature 5.1. Specimen preparation The coal specimens tested are taken from three gas coal seams in Xinglongzhuang Coal Mine in Yanzhou, China. Large lump coal samples were cut from mining face of 500 m deep. Each lump sample was cut into a cylindrical blank and then grinded carefully to meet the size requirements. The final dimensions of the samples were f200 mm 400 mm. The coal samples were in good integrity without obvious cracks. Potholes and broken corner were repaired by filling with blended pyrophyllite powder and silicate. 5.2. Deformation of coal samples [14,15] Fig. 2 shows the stress–strain relationship of a coal specimen at 600 1C. When g1–ga was lower than 20 MPa, both the axial and the volumetric strain increased linearly and the sample showed good linear-elastic characteristics. When g1–ga exceeded 20 MPa, the deformation of the coal sample went into plastic strengthening phase. Though the axial stress increased with the increasing axial and lateral deformation, the resistance for deformation is significantly reduced and the slope of the stress–strain curve is greatly reduced compared with the elastic stage. When g1–ga passed 75 MPa, the sample’s capacity to resist deformation still remained. Fig. 3 shows the failed coal specimen. The coal specimen was severely compressed from 400 mm to 280 mm in axial direction with an axial strain of about 30%, which is a unique
Y. Zhao et al. / International Journal of Rock Mechanics & Mining Sciences 52 (2012) 132–138
90
15 Gas coal from Shandong during thermal decomposition brown coal from Neimeng Xinao after thermal decomposition
80
Gas coal after pyrolysis 300-600°C
12
60
Young's modulus / GPa
stress (σ1-σa)/MPa
70
50 40 30 axial strain lateral strain volume strain
20 10 -0.01
135
0
9
6
3 0
0.01
0.02
0.03 0.04 strain
0.05
0.06
0.07
0.08
Fig. 2. Stress–strain curve of gas coal at 600 1C (sa ¼ 15 MPa).
0 0
100
200
300 400 Temperature/°C
500
600
Fig. 4. Young’s modulus of coal at variable temperature.
gas was expelled from the coal sample and left great amount of newly created voids. The voids were closed gradually with increased pressure and resulted in the contraction of the sample. With the increase of axial pressure to 120 MPa and a lateral confining pressure remaining at 15 MPa, the coal sample experienced significant axial deformation compared to only little lateral deformation. However, the deformation characteristics, due to thermal decomposition reaction, of the gas coal sample under high temperature and high pressure should be distinguished from the plastic deformation characteristics of other rocks. It should also be pointed out that the compression of the H13 presser at 600 1C, with Young’s modulus of 110 GPa at that temperature, was calculated only 0.27 mm, about 1.1% of that of the coal sample. The deformation of the coal sample was adjusted with consideration of the presser deformation.
5.3. Young’s modulus of coal samples at high temperature and high stress
Fig. 3. Coal sample after failure.
feature not found in other rock. At high temperature and high pressure, the coal sample showed high plasticity and its particle composition also showed the capacity of re-crystallization. Fig. 4 shows that the volume strain of the coal specimen is always positive in the entire loading process, that is, it is always in a state of volume contraction even though the axial strain of the sample reached about 30%. As a comparison, a drastic volume expansion was found for other rocks, such as granite [16], and quartzite [17]. The coal specimen deformed with sharp axial contraction and very small lateral expansion at 600 1C is very different from the deformation properties at normal temperatures. This is due to the thermal decomposition reaction bursted when the gas coal sample was heated from 300 1C to 600 1C. Approximately 1500 l
Fig. 4 shows Young’s modulus of the coal samples at different temperatures. At low temperature and the confining pressure less than 15 MPa, Young’s modulus of the coal samples was high (14.3 GPa for 50 1C). At higher temperatures, Young’s modulus dropped drastically. The data can be fitted by exponential functions with the correlation coefficient of 0.94. The fitted equation is Ec ¼ 19:6 expðT=199:2Þ1:55 GPa
ð1Þ
Fig. 4 shows the variation of Young’s modulus when the temperature of gas coal and brown coal samples was increased from room temperature to 600 1C. Young’s modulus of gas coal sample was 14.3 GPa at 50 1C and gradually decreased when sample temperature rose. The reduction of Young’s modulus slowed down at about 250 1C and accelerated sharply when the sample was heated from 300 1C to 400 1C. Due to the thermal decomposition reaction occurred during heating of the gas coal sample, great amount of gas was released and left numerous new voids and fissures. Young’s modulus of the gas coal sample dropped from 3.0 GPa at 300 1C to 0.4 GPa at 400 1C and then slowly dropped to 0.2 GPa at 600 1C.
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14
stress (σ1-σa)/MPa
12 10 8 6 20°C 100°C 200°C 300°C 400°C 500°C
4 2 0
0
0.002 0.004 0.006 0.008 strain
0.01
0.012 0.014 0.016
Fig. 5. The stress–strain curve of brown coal upon completion of thermal decomposition.
For each temperature, upon completion of the thermal decomposition reaction, additional axial load was applied to the sample and the corresponding deformation was measured. Young’s modulus at that temperature is then calculated based on the observed stress– strain curve. When the sample temperature is higher than 300 1C, Young’s modulus calculated using this method is high than the one measured when the thermal decomposition reaction was still ongoing. Fig. 2 shows the stress–strain curve of gas coal at 600 1C. Fig. 5 shows the stress–strain curves of brown coal at different temperatures upon completion of the thermal decomposition reaction. The calculation of Young’s modulus is described as follows. In cylindrical coordinate system, the stresses are expressed as:
sz ¼ s1 , sr ¼ sy ¼ s2
stage from room temperature to 120 1C, the thermal expansion of granite is small and the thermal expansion coefficient is 0.5 10 5/1C (Fig. 6). Middle temperature stage in the range of 120–460 1C: the thermal expansion rate of the granite continues to increase. Fig. 7 shows the thermal deformation characteristics of the 2 granite specimen at this stage. High-temperature section above 460 1C, the coefficient of thermal expansion of the granite is rapidly reduced and the deformation of its thermal expansion also decreases (Fig. 7). This is due to changing of some mineral crystals in granite at high temperature and melting or coming into phase transition of some minerals. In comparison with the linear expansion coefficient and temperature of the granite presented by Xu and Yu in [41], we found that at temperatures ranging 120–460 1C, the thermal expansion coefficient of granite increases non-linearly, and its rate of increase gets higher with temperature. Xu and Yu indicated that the thermal expansion coefficient of granite increases non-linearly with a slightly different temperature range of 150–560 1C. The maximum of the thermal expansion coefficient of the Luhui granite occurred at 460 1C compared with 560 1C presented in [41]. Both results show a peak point, i.e., a critical temperature, exceeding which the thermal expansion coefficient drops rapidly and the heat deformation becomes very gentle. (3) In general, the thermal deformation characteristics of granite presented in the present work and in [41] are very similar. The difference of temperature range and threshold temperature may be due to the differences of the structure and components of granite samples. Our test results show that under stresses corresponding to about 1000 m deep below ground surface, the thermal expansion
ð2Þ 0
where s1, s2 are the principal stresses, sz is the axial stress, and sr and sy are the radial and tangential stresses. According to Hooke’s law, ð3Þ
where E is Young’s modulus, n is Poisson’s ratio, and e1, e2 are the principal strains. Comparing the two equations from Hooke’s law, Young’s modulus E is expressed as E¼
ðs1 s2 Þðs1 þ 2s2 Þ ðs1 þ s2 Þe2 2s2 e2
Thermal strain
1 E
6.1. Specimen preparation The granite sample, Luhui Granite, was collected from Pingyi in Shandong Province in China. The collected lump granite sample was cut into a cylindrical blank. Then it was refined to meet the experimental requirements. The processed granite specimen was a gray, dense and crack-free cylinder with the diameter of 200.3 mm and height of 414.5 mm. It was in a state of natural water-bearing with the density of 2.71 g/cm3. 6.2. Thermal expansion of the granite sample at hydrostatic pressure of 25 MPa At all-around hydrostatic pressure of 25 MPa, the thermal deformation of granite can be divided into three stages: Low-temperature
Temperature /°C 300 400
500
600
700
-0.004 -0.006 -0.008 axial strain lataral strain volume strain
-0.01 -0.012
ð4Þ
6. The deformation properties of granite at high-temperature and high pressure
200
-0.002
-0.014 Fig. 6. Thermal strain of granite at all-around hydrostatic pressure of s1 ¼ sa ¼25 MPa.
Linear expansion coefficient /°C-1
1 E
e1 ¼ ðs1 2ns2 Þ, e2 ¼ ðs2 nðs1 þ s2 ÞÞ
100
0
1.8E-05 1.5E-05 1.2E-05 9.0E-06 6.0E-06 3.0E-06 0.0E+00 0
100
200
400 300 Temperature /°C
500
600
700
Fig. 7. Linear thermal expansion coefficient of granite at different temperatures.
Y. Zhao et al. / International Journal of Rock Mechanics & Mining Sciences 52 (2012) 132–138
coefficient of granite changes between 0.2 10 5/1C and 1.25 10 5/1C. However, Xu and Yu indicated that the thermal expansion coefficient of the granite is between 0.5 1C and 12 10 5/1C, which is about ten times higher. This is attributed to different confining pressures applied to the sample. When there is little to no confining pressure, thermal expansion coefficient is larger because thermal cracking may occur and micro-cracks may develop. However, when high confining pressure is applied, as the 200 400°C 300°C 500°C 200°C
stress (σ1-σa)/MPa
160 120 80 40 0 0
0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 0.0045 strain
137
tests presented in this paper, thermal cracking is restricted and resulted in a smaller thermal expansion coefficient [33–36]. 6.3. Deformation characteristics of granite at high temperature [22– 32,37–40] Fig. 8 shows the curve of axial stress vs. axial strain derived by increasing the differential stress (s1–s2) of granite at 200–500 1C, and at a constant confining pressure (sa ¼25 MPa). When the axial pressure is increased, axial compression became larger. When the differential stress (s1–s2) reached 128 MPa, the rock gets into a yield/flowing state. The rock showed greater lateral expansion deformation which increases rock total volume. Fig. 7 shows that the granite sample gradually turned to plastic after temperature reached 200 1C. The sample behaved apparent plastic when the temperature reached 400 1C and the deviate stress reached 110 MPa. The transition can also be seen on Fig. 9, at which stage Young’s modulus reduced significantly. In comparison, the samples showed linear elastic behavior at temperature 200 1C, even with a high deviatoric stress of 170 MPa. When the temperature reached 300 1C and the deviate stress reached 100 MPa, the sample entered a non-linear deformation stage, which is a plastic strengthen stage and features with a significant decrease of secant modulus.
Fig. 8. Axial strain–stress curve of granite at different temperatures (sa ¼ 25 MPa).
6.4. Failure mode of granite 70 60 Young's modulus/GP
Fig. 10 is a photograph of granite after failure at temperature 300 1C. After the granite failed at high temperatures, the main rupture appeared ‘‘X’’ shape and the two ends showed regular ‘‘top cones’’, which is a typical mode of shear failure.
2# sample zhu et al test results
50
6.5. Young’s modulus at different temperatures
Low temperature
Medium high temperature
40
high temperature
Fig. 9 shows that Young’s modulus of granite decreases with increasing temperature [24]. Their relationship can be expressed by the following exponential function:
30
E ¼ 60 expð0:0006TÞ 20 0
100
200
300 400 temperature/°C
500
600
Fig. 9. Young’s modulus of granite at different temperatures.
700
ð5Þ
where T is the temperature of granite, in 1C. In our test, when the temperature increases from room temperature to 600 1C, Young’s modulus of granite dropped by 25.1%. This is also shown in Fig. 11.
Fig. 10. Failure of granite specimen at 300 1C; (a) shear plane (b) full view.
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35
stress (σ1-σa)/MPa
30 25 20 15 20°C 100°C 200°C 300°C 400°C 500°C 600°C
10 5 0
0
0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 0.0007 axial strain
Fig. 11. Stress–strain curve of granite at different temperatures.
7. Conclusions The performance and technological innovations of a self-developed 20 MN rock triaxial test machine XPS-20 MN for high temperature and high pressure are presented in this paper. Compared with the common testing machine for high temperature and high pressure in geophysical exploration, the test machine has the following features: (1) Specimen of larger dimensions can be tested. The diameter of the specimen is up to 200 mm and height is up to 400 mm. Compared to ordinary rock testing machine, this system can test rock specimen with volume 80–170 times larger, which may improve the representativeness of the sample. (2) The test machine uses servo-control loading; therefore, the deformation of rock samples at a variety of temperatures can be easily examined. (3) The test machine can be used to study rock permeability and deformation characteristics in the thermal decomposition process. (4) Holes can be drilled in the specimen at high temperature and the creep characteristics can be tested. This paper also presents the test results of coal specimens at high temperature and high pressure. The tests revealed the stress–strain characteristics of the coal specimens at high temperature, particularly the enhanced plastic features. Young’s modulus decreases in a negative exponential function in regarding to the temperature. Test results revealing the thermal deformation and failure mode of the large-size granite specimens at high temperature and high pressure, and the changing of the thermodynamic parameters of the specimen, such as Young’s modulus, with temperature, are also presented as examples of tests carried out using the testing machine. The following findings are drawn from the test results: (1) The ‘‘X’’ shape fractures during failure of the granite specimen at high temperature and high pressure indicate a shear failure; (2) The linear thermal expansion coefficient of granite increases with temperature while its Young’s modulus decreases with the temperature. The elasticity modulus decreases with the temperature in an exponential function with the temperature. References [1] Vutukuri VS, Lama RD, Saluja SS. Handbook of Mechanical Properties of Rocks. Clausthal. Trans Tech Publications; 1974. [2] Tao ZY. The Theory and Practice of Rock Mechanics. Beijing: China Water Conservancy Press; 1981 (in Chinese). [3] Metal Mine 1976;6:1–7 . (in Chinese). [4] Zhang JZ, Lin TJ. Stress conditions and the variation of rupture characteristics of a rock as shown by triaxial tests. Acta Mech Sin 1979;15(2):3–10 (in Chinese).
[5] Ren AH. 800 t high temperature and high pressure servo-controlled triaxial rock rheology testing system. Chin J Geophys 1988;31(2):9 (in Chinese). [6] Shi ZQ, Zhou MQ. Designing of 800 MPa high temperature and high pressure triaxial cell. Chin J Geophys 1990;33(2):202–7 (in Chinese). [7] Mei, JY, Fu, BJ, Kang, WF The development and current state of rock mechanics in China. In: Proc 5th Int Congress of Rock Mechanics, Melbourne, 1983; 2: 251. [8] Beijing: Science Press; 2006 (in Chinese). [9] Wan, ZJ Study on thermal-mechanical coupling process of heterogeneous rock mass and Channel’s stability of underground coal gasification, PhD Thesis. China University of Mining and Technology, Xuzhou; 2006. (in Chinese). [10] Ren, AH Experimental research of inner heating apparatus of rock triaxial testing system, In: Proc 1st Symposium High Temperature and High Pressure Rock Mechanics. Beijing, Science Press, 1988, pp. 166–170. (in Chinese). [11] Liu CL, San Y, Wang ZC, et al. Uniformity simulation and model of temperature of well-shape heater’s hearth. Ind Heat 2000;29(6):12–5 (in Chinese). [12] Gao XM. Application of graphite in high temperature vacuum electric heater. Ind Heat 2005;34(1):39–41 (in Chinese). [13] Song ST, Pan HY, Wang SZ. Study on heat element made by zirconia and super high temperature electric heater. J Iron Steel Res Int 1996;8(6):53–7 (in Chinese). [14] Zhou JX, Wang GL, Shao ZJ. Coal deformation under high temperature and confining pressure. J China Coal Soc 1994;19(3):324–32 (in Chinese). [15] Jiang B, Qin Y, Jin FL. Coal deformation test under high temperature and confining pressure. J China Coal Soc 1997;22(1):80–4 (in Chinese). [16] Brace WF, Paulding BW, Scholz C. Dilatancy in the fracture of crystalline rocks. J Geophys Res 1966;71(16):3939–53. [17] Bieniawski ZT. Mechanism of Brittle fracture of rocks. Int J Rock Mech Min Sci 1967;4(4) 395–30. [18] Duchane DV. Hot dry rock: a realistic energy option. Geotherm Resour Counc Bull 1990;19(3):83–8. [19] Zhao YS, Wan ZJ, Kang JR. Introduction of Geothermal Extraction of Hot Dry Rock. Beijing: Science Press; 2004 (in Chinese). [20] Wan ZJ, Zhao YS, Kang JR. Simulation and forecast method of geothermal resources in hot dry rock. Chin J Rock Mech Eng 2005;24(6):945–9 (in Chinese). [21] Wang SZ. High-temperature/high-pressure rock mechanics: history, state-ofart and prospect. Prog Geophys 1995;10(4):1–31 (in Chinese). [22] Liu QS, Xu XC. Yamaguehi Tsutomo et al. testing study on mechanical properties of the three Gorges granite concerning temperature and time. Chin J Rock Mech Eng 2001;20(5):715–9 (in Chinese). [23] Xu XC, Liu QS. A preliminary study on basic mechanical properties for granite at high temperature. Chin J Geotech Eng 2000;22(3):332–5 (in Chinese). [24] Zhu HH, Yan ZG, Deng T, et al. Testing study on mechanical properties of tuff, granite and breccia after high temperatures. Chin J Rock Mech Eng 2006;25(10):1945–50 (in Chinese). [25] Du SJ, Liu H, Zhi HT, et al. Testing study on mechanical properties of posthigh-temperature granite. Chin J Rock Mech Eng 2004;23(14):2359–64 (in Chinese). [26] Wang JT, Zhao AG, Huang MC. Effect of high temperature on the fracture toughness of granite. Chin J Geotech Eng 1989;11(6):113–9 (in Chinese). [27] Zhang ZR, He SX, Xi XS. Experimental deformation of Wangxiang granite at high temperature and pressure. J Cent South Univ Tech 1999;30(3):221–4 (in Chinese). [28] Schulmann K, Mlcoch B, Melka R. High-temperature microstructures and rheology of deformed granite, Erzgebirge, Bohemian Massif. J Struct Geol 1996;18(6):719–33. [29] Tullis J, Yund RA. Experimental deformation of dry westerly granite. J Geophys Res 1977;82(36):5705–18. [30] Qiu YP, Lin ZY. Testing study on damage of granite samples after high temperature. Rock Soil Mech 2006;27(6):1005–10 (in Chinese). [31] Aizawa Y, Ito K, Tatsumi Y. Compressional wave velocity of granite and amphibolite up to melting temperatures at 1 GPa. Tectonophysics 2002;351(3): 255–61. [32] Ross JV, Wilks KR. Effects of a third phase on the mechanical and microstructural evolution of a granulite. Tectonophysics 1995;241(3–4):303–15. [33] Skoczylas F, Henry JP. A study of the intrinsic permeability of granite to gas. Int J Rock Mech Min Sci 1995;32(2):171–9. [34] Wang HF, Bonner BP, Carlson SR, Kowallis BJ, Heard HC. Thermal stress cracking in granite. J Geophys Res 1989;94(B2):1745–58. [35] Darot M, Gueguen Y, Baratin ML. Permeability of thermally cracked granite. Geophys Res Lett 1992;19(9):869–72. [36] Geraud Y, Mazerolle F, Raynaud S. Comparison between connected and overall porosity of thermally stressed granites. J Struct Geol 1992;14(8/9): 981–90. [37] Chen Y, Wang CY. Thermally induced acoustic emission in westerly granite. Geophys Res Letts 1980;7(12):1089–92. [38] Heuze FE. High-temperature mechanical, physical and thermal properties of granitic rocks - a review. Int J Rock Mech Min Sci 1983;20(1):3–10. [39] Lin MZ. Rock Thermophysics and Its Engineering Application. Chongqing: Chongqing University Press; 1991 (in Chinese). [40] Griggs DT, Turner FJ, Heard HC. Deformation of rock at 500 1C to 800 1C. Geol Soc Am Mem 1960;79:39–104. [41] Xu, X.H., Yu, J. Rock Crushing Technology, Beijing: Coal Industry Press, 1984. (In Chinese).
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Technical Note
Triaxial compression system for rock testing under high temperature and high pressure Yangsheng Zhao a,n, Zhijun Wan b, Zijun Feng a, Dong Yang c, Yuan Zhang d, Fang Qu e a
College of Mining Engineering, Taiyuan University of Technology, Taiyuan, Shanxi 030024, China School of Mining and Safety Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221008, China c Mining Technology Institute, Taiyuan University of Technology, 030024, China d Huaiyin Institute of Technology, Huaiyin, Jiangsu 223003, China e China Jiliang University, Jinan, Hangzhou, Zhejiang 310018, China b
a r t i c l e i n f o Article history: Received 9 April 2010 Received in revised form 5 February 2012 Accepted 26 February 2012 Available online 5 April 2012
1. Introduction Rock mechanics always develops with improvement of rock testing technique, especially the improvement of rock testing machines. The latter has been gradually separated from material mechanics and has evolved to an independent subject. In early stages of its development, rock testing was carried out by ordinary material testing machines. Then, the discovery of the principles of stiff testing machine [1], the developments of rock triaxial testing machine with confining pressure [2,3], true triaxial tests machine [4], high temperature and high pressure triaxial testing machine [5–7,21], large-scale true triaxial testing machine, as well as other subsequent experimental improvement, have revealed many features of rock samples that were little known, and have greatly promoted the development of rock mechanics. The rapid growth of deep resources and energy exploitation is greatly relying on the knowledge of the mechanical properties of deep rock [8,9]. For example, the depth of oil drilling has reached 8000 m; the exploitation depth of the geothermal resources for high-temperature rock is greater than 5000 m [18–20]; the exploitation depth of the metal mining reached 3000 m; the temperature of underground coal gasification and underground fire is up to 1000 1C. However, most of the current existing high temperature and high pressure test machines in the world, due to their limited sample size (often 20 mm 40 mm) and normal experiment condition, can only test for the rock properties such as deformation and elastic waves, and hardly provide further data
n
Corresponding author. E-mail address: [email protected] (Y. Zhao).
1365-1609/$ - see front matter & 2012 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.ijrmms.2012.02.011
regarding rock engineering behavior. Therefore, our research group developed XPS-20 MN, a servo-controlled triaxial rock testing system of high temperature and high pressure, for high temperature and high pressure rock testing. This paper intends to describe the structure, functions, and technical considerations of the test machine. Examples of measured deformation properties and thermal expansion coefficients of coal and granite samples at high temperature and high pressure are also presented.
2. Main functions and parameters 2.1. Main functions The testing machine XPS-20 MN can be used to study rock mechanical behavior in a wide range of engineering fields, such as deep mining, underground coal liquefaction and gasification, geothermal exploitation, coal-bed methane recovery, deep oil and gas extraction, waste disposal, etc. The testing machine is designed to study rock properties including deformation, strength, solid-fluid-thermal coupling, rheology, permeability and thermal conductivity at high temperature and high pressure as well as at normal temperature; to study rock properties and laws in phase transitions, melting, heat and mass transfer, liquefaction, gasification, chemical reactions of solid minerals (coal, oil shale, etc.) at the mutual action of heat and stress; and to study the interaction between the deformation of rock mass and the drilling rig and to study hydraulic fracturing at high temperature and high pressure.
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3. Structures of the test machine and key technologies of each component The testing machine consists of four parts: host loading system, high-temperature triaxial pressure chamber with temperature controlling system, auxiliary installing system, and testing system. The host loading system is the force source of the testing machine. The high-temperature triaxial pressure chamber with temperature controlling system is used for holding pressuring and heating the specimen. The auxiliary installing system is specifically designed for installing coal (rock) specimens in the high-temperature triaxial pressure chamber. The testing system can measure the temperature, pressure, deformation, permeability and acoustic emission, etc. of coal (rock) specimens. 3.1. Host loading system The host loading system consists of the following three parts: (1) The host framework consists of four-vertical columns with ‘‘bottom-push style’’, i.e., the piston moves upwards to press the specimen. Its vertical and lateral forces can both reach 10,000 kN. The vertical and lateral pressers are designed differently to ensure the stiffness of the testing machine. Inside the pressure chamber, the diameter of the vertical presser cylinder is 200 mm; while the pressers for lateral pressure are hollow cylinders with an inner diameter of 210 mm and outer diameter of 300 mm. Outside the pressure chamber, the pressers with larger diameter is transited to a smaller diameter gradually in order to ensure the overall stiffness of the testing machine satisfying the requirements for stiff testing machine. (2) The hydraulic pressure system is the power source of the host machine. The pressure is generated by three hydraulic pumps. The working pressure of the pump station is 25 MPa. For the purpose of servo control, we used the precisely proportioned servo valve produced by MOGO, which can accurately control the movements of the cylinders for axial pressure and lateral pressure. (3) The main console controls the movements of the host system. Its operation is fulfilled through a computer combined with a control panel. Incremental loading process is controlled using a computer program; while the adjustment of the presser and the movements of the material holding cylinder are controlled by the control panel. Axial and lateral loads are applied separately. The loading methods include constant load, constant load rate, constant displacement, constant displacement rate, saw tooth wave load, etc.
The high-temperature triaxial pressure chamber is not only a core component of the testing machine but also an important component producing the required temperature and pressure in the entire experiment. It is a thick-walled cylinder made of thermoplastic die steel (H13). The phase transition temperature of the cylinder is 800 1C. Its structure is like an inner–outer double-layer bucket. The two layers in the shrink sleeve structure are nested together by tight fit after a special treatment. This ensures that the cylinder will not damage at a high-pressure up to 250 MPa. The cylinder height is 960 mm, with outer diameter of 1060 mm and inner diameter of 300 mm. The heating, temperature adjustment and temperature measurements in the high-temperature triaxial pressure chamber are implemented by the temperature-controlled cabinet. The operation diagram of the high-temperature triaxial pressure chamber is shown in Fig. 1. The surface of the coal (rock) specimen is wrapped by heat-resistant alloy films. The heatresistant alloy film is connected with the H13 presser placed on the top and bottom of the coal (rock) specimen. The heat-resistant alloy films are charged with direct current from the temperaturecontrolled cabinet and produce heat to heat up the coal (rock) specimen and its surrounding salt ring (pyrophyllite ring) to the required temperature. The high temperature triaxial pressure chamber is heated using direct current that is generated by 380 V three-phase electricity, transformed and rectified in the temperature-controlled cabinet. The designed maximum output voltage is 30 V and the maximum output current is 1700 A with a maximum output power of 51 kW. 3.3. Auxiliary installing systems The auxiliary installing system is designed for setting up coal (rock) specimens. The designed maximum force is 2000 kN. The maximum pressure of the hydraulic pump is 25 MPa. There are
Pyrophyllite powder Press head Thermocouple Resistance chip Copper sleeve Sample H13 Tank
Salt
mica plate
H13 Tank
Thermocouple
Lead of thermocouple
The maximum axial load is 10,000 kN, the maximum lateral load is 10,000 kN, the maximum axial pressure on rock specimen is 318 MPa, the maximum lateral pressure on the rock specimen is 250 MPa (pseudo-triaxial confining pressure), the maximum pore pressure is 250 MPa, the specimen size is f200 mm 400 mm. The parameters of the drill are as follows: maximum travel is 450 mm, maximum loading is 200 kN, maximum torque is 500 Nm, tha maximum temperature is 600 1C, the holding time of pressures is over 360 h, and the offset of axial pressure and lateral pressure is less than 70.3%. The high-temperature triaxial pressure chamber has a highprecision control, of which the temperature sensitivity is less than 0.3%. Stress, deformation, pore pressure of inlet and outlet, temperature, torque and other parameters can be recorded automatically. The stiffness of the whole machine is greater than 9 1010 N/m.
3.2. High-temperature triaxial pressure chamber with temperaturecontrolled heating system
Lead of thermocouple
2.2. Parameters of the equipment
133
Press head Pyrophyllite powder Lateral press head Axial press head
Lateral strain/Axial stress/Lateral strain Fig. 1. Schematic diagram of high temperature triaxial compression chamber.
134
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three hydraulic cylinders, namely center cylinder, side-pressure cylinder and lower cylinder, working together to set up the specimen, the loose salt with ring resonator, and the pyrophyllite ring.
industries. Its main technical parameters include maximum temperature of 1100 1C, density of 7.9 g/cm3, resistivity 1.0470.05 O mm2/m, melting point of 1390 1C, tensile strength of 637–784 MPa, and elongation greater than 20%.
3.4. Testing system 4.2. Insulation and sealing The deformation of a specimen can be calculated by measuring the positions of the axial and lateral presser. The measuring instrument is a grating ruler. The measuring precision is 70.005 mm. The specimen load is calculated by the pressures converted from hydraulic pressures measured at the upper and lower chambers of the hydraulic cylinders with pressure gauges. The surface temperature of a specimen is measured by three thermocouples deployed with 1201 between each other at the upper, middle and lower parts of the circumference near the specimen. In order to increase measurement accuracy and reliability, an additional thermocouple is placed at the middle part of the specimen galvanic symmetrically. The real-time data of the thermometers can be transferred to a computer through the communication port. Based on the temperature value set in one of the four thermometers, the temperature in the heat stress chamber can be controlled automatically by the temperaturecontrolled cabinet. The permeability-measuring instrument consists of inflatable tools, high-pressure gas cylinders and flow meters, etc. The medium used for testing is nitrogen (N2). The flow-meters are soap-bubble meter and rota-meter, which are used for the small and large gas flow rate cases respectively. The drainage gas sampling method can also be used to measure the amount of exhaust gas (mainly used in measuring the gas emitted from coal samples). An AE-04 four-channel acoustic emission detection system is used. Its preamplifier gain is 40 dB; bandwidth is 0.005–1 MHz; the main gain is 9–60 dB; the sensor sensitivity is no less than 60 dB; and its standard resonant frequency is 140 kHz. The system is designed to handle multiple parameters, such as the located functions of linear, regional, and square. It can collect and process the AE parameters of each event, including ring counting, energy counts, event duration, and amplitude, etc.
4. Technical considerations 4.1. Heating elements The appropriate heating element and heating mode can generate temperature environmental in the high-temperature triaxial pressure chamber. The carbon fiber tape used at early heating stage is found to be easy to burn and break, so it is substituted by the alloy resistance film. The alloy resistance film has the following advantages: (1) a wide range of elongation and tensile resistance capacity; (2) longer usage duration; and (3) high temperature durability. The alloy resistance films can produce nearly uniform temperature distributions around the specimen. We changed the distribution of the heating power of the varistor by thickening the varistor in appropriate part of the sample and changing the resistance distribution of the varistor winding around the specimen to produce a uniform temperature field [10–13]. The alloy resistance used is nickel-chromium alloy film named Ni80Cr20. It has good deformation characteristics, thermal stability and mechanical strength. It also has advantages such as better seismic and antioxidant capacity in thermal states. It has been widely used for electric heating and heat treatment in chemical industry, machinery, metallurgy, national defense and other
In the development of this machine, the insulation and sealing between the upper part of the high-temperature pressure chamber and the upper presser of the test machine, the insulation and sealing between the axial presser at the lower part of pressure chamber and the axial presser of testing machine and those between H13 pressers above the specimen in the pressure chamber are important not only for the safe operation of the apparatus, but also for determining the accuracy and reliability of the experimental results. The materials for sealing and insulating are required to have highly stable performance at the conditions of high pressure and temperature. A layer of high temperature insulation pads is placed in the gap between the high-temperature pressure chamber and the platform underneath it. A layer of asbestos or mica plate is placed between the upper part of the high temperature and pressure chamber and the upper presser of the testing machine. Insulating plastic cast for high temperature is pasted around the ventilation (water) hole on the upper presser of the host machine play a role in sealing and insulating. To prevent the leakage of N2, seal is required between the axial presser of the host machine and the lower axial presser in hightemperature pressure chamber. A seal groove is manufactured on the surface of the axial presser of the host machine and a copper pad placed in it to make good sealing and insulation. Similarly, a seal groove plus copper pad are used to seal between two H13 pressers above the specimen in the pressure chamber.
5. Deformation and failure characteristics of coal samples at high temperature 5.1. Specimen preparation The coal specimens tested are taken from three gas coal seams in Xinglongzhuang Coal Mine in Yanzhou, China. Large lump coal samples were cut from mining face of 500 m deep. Each lump sample was cut into a cylindrical blank and then grinded carefully to meet the size requirements. The final dimensions of the samples were f200 mm 400 mm. The coal samples were in good integrity without obvious cracks. Potholes and broken corner were repaired by filling with blended pyrophyllite powder and silicate. 5.2. Deformation of coal samples [14,15] Fig. 2 shows the stress–strain relationship of a coal specimen at 600 1C. When g1–ga was lower than 20 MPa, both the axial and the volumetric strain increased linearly and the sample showed good linear-elastic characteristics. When g1–ga exceeded 20 MPa, the deformation of the coal sample went into plastic strengthening phase. Though the axial stress increased with the increasing axial and lateral deformation, the resistance for deformation is significantly reduced and the slope of the stress–strain curve is greatly reduced compared with the elastic stage. When g1–ga passed 75 MPa, the sample’s capacity to resist deformation still remained. Fig. 3 shows the failed coal specimen. The coal specimen was severely compressed from 400 mm to 280 mm in axial direction with an axial strain of about 30%, which is a unique
Y. Zhao et al. / International Journal of Rock Mechanics & Mining Sciences 52 (2012) 132–138
90
15 Gas coal from Shandong during thermal decomposition brown coal from Neimeng Xinao after thermal decomposition
80
Gas coal after pyrolysis 300-600°C
12
60
Young's modulus / GPa
stress (σ1-σa)/MPa
70
50 40 30 axial strain lateral strain volume strain
20 10 -0.01
135
0
9
6
3 0
0.01
0.02
0.03 0.04 strain
0.05
0.06
0.07
0.08
Fig. 2. Stress–strain curve of gas coal at 600 1C (sa ¼ 15 MPa).
0 0
100
200
300 400 Temperature/°C
500
600
Fig. 4. Young’s modulus of coal at variable temperature.
gas was expelled from the coal sample and left great amount of newly created voids. The voids were closed gradually with increased pressure and resulted in the contraction of the sample. With the increase of axial pressure to 120 MPa and a lateral confining pressure remaining at 15 MPa, the coal sample experienced significant axial deformation compared to only little lateral deformation. However, the deformation characteristics, due to thermal decomposition reaction, of the gas coal sample under high temperature and high pressure should be distinguished from the plastic deformation characteristics of other rocks. It should also be pointed out that the compression of the H13 presser at 600 1C, with Young’s modulus of 110 GPa at that temperature, was calculated only 0.27 mm, about 1.1% of that of the coal sample. The deformation of the coal sample was adjusted with consideration of the presser deformation.
5.3. Young’s modulus of coal samples at high temperature and high stress
Fig. 3. Coal sample after failure.
feature not found in other rock. At high temperature and high pressure, the coal sample showed high plasticity and its particle composition also showed the capacity of re-crystallization. Fig. 4 shows that the volume strain of the coal specimen is always positive in the entire loading process, that is, it is always in a state of volume contraction even though the axial strain of the sample reached about 30%. As a comparison, a drastic volume expansion was found for other rocks, such as granite [16], and quartzite [17]. The coal specimen deformed with sharp axial contraction and very small lateral expansion at 600 1C is very different from the deformation properties at normal temperatures. This is due to the thermal decomposition reaction bursted when the gas coal sample was heated from 300 1C to 600 1C. Approximately 1500 l
Fig. 4 shows Young’s modulus of the coal samples at different temperatures. At low temperature and the confining pressure less than 15 MPa, Young’s modulus of the coal samples was high (14.3 GPa for 50 1C). At higher temperatures, Young’s modulus dropped drastically. The data can be fitted by exponential functions with the correlation coefficient of 0.94. The fitted equation is Ec ¼ 19:6 expðT=199:2Þ1:55 GPa
ð1Þ
Fig. 4 shows the variation of Young’s modulus when the temperature of gas coal and brown coal samples was increased from room temperature to 600 1C. Young’s modulus of gas coal sample was 14.3 GPa at 50 1C and gradually decreased when sample temperature rose. The reduction of Young’s modulus slowed down at about 250 1C and accelerated sharply when the sample was heated from 300 1C to 400 1C. Due to the thermal decomposition reaction occurred during heating of the gas coal sample, great amount of gas was released and left numerous new voids and fissures. Young’s modulus of the gas coal sample dropped from 3.0 GPa at 300 1C to 0.4 GPa at 400 1C and then slowly dropped to 0.2 GPa at 600 1C.
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14
stress (σ1-σa)/MPa
12 10 8 6 20°C 100°C 200°C 300°C 400°C 500°C
4 2 0
0
0.002 0.004 0.006 0.008 strain
0.01
0.012 0.014 0.016
Fig. 5. The stress–strain curve of brown coal upon completion of thermal decomposition.
For each temperature, upon completion of the thermal decomposition reaction, additional axial load was applied to the sample and the corresponding deformation was measured. Young’s modulus at that temperature is then calculated based on the observed stress– strain curve. When the sample temperature is higher than 300 1C, Young’s modulus calculated using this method is high than the one measured when the thermal decomposition reaction was still ongoing. Fig. 2 shows the stress–strain curve of gas coal at 600 1C. Fig. 5 shows the stress–strain curves of brown coal at different temperatures upon completion of the thermal decomposition reaction. The calculation of Young’s modulus is described as follows. In cylindrical coordinate system, the stresses are expressed as:
sz ¼ s1 , sr ¼ sy ¼ s2
stage from room temperature to 120 1C, the thermal expansion of granite is small and the thermal expansion coefficient is 0.5 10 5/1C (Fig. 6). Middle temperature stage in the range of 120–460 1C: the thermal expansion rate of the granite continues to increase. Fig. 7 shows the thermal deformation characteristics of the 2 granite specimen at this stage. High-temperature section above 460 1C, the coefficient of thermal expansion of the granite is rapidly reduced and the deformation of its thermal expansion also decreases (Fig. 7). This is due to changing of some mineral crystals in granite at high temperature and melting or coming into phase transition of some minerals. In comparison with the linear expansion coefficient and temperature of the granite presented by Xu and Yu in [41], we found that at temperatures ranging 120–460 1C, the thermal expansion coefficient of granite increases non-linearly, and its rate of increase gets higher with temperature. Xu and Yu indicated that the thermal expansion coefficient of granite increases non-linearly with a slightly different temperature range of 150–560 1C. The maximum of the thermal expansion coefficient of the Luhui granite occurred at 460 1C compared with 560 1C presented in [41]. Both results show a peak point, i.e., a critical temperature, exceeding which the thermal expansion coefficient drops rapidly and the heat deformation becomes very gentle. (3) In general, the thermal deformation characteristics of granite presented in the present work and in [41] are very similar. The difference of temperature range and threshold temperature may be due to the differences of the structure and components of granite samples. Our test results show that under stresses corresponding to about 1000 m deep below ground surface, the thermal expansion
ð2Þ 0
where s1, s2 are the principal stresses, sz is the axial stress, and sr and sy are the radial and tangential stresses. According to Hooke’s law, ð3Þ
where E is Young’s modulus, n is Poisson’s ratio, and e1, e2 are the principal strains. Comparing the two equations from Hooke’s law, Young’s modulus E is expressed as E¼
ðs1 s2 Þðs1 þ 2s2 Þ ðs1 þ s2 Þe2 2s2 e2
Thermal strain
1 E
6.1. Specimen preparation The granite sample, Luhui Granite, was collected from Pingyi in Shandong Province in China. The collected lump granite sample was cut into a cylindrical blank. Then it was refined to meet the experimental requirements. The processed granite specimen was a gray, dense and crack-free cylinder with the diameter of 200.3 mm and height of 414.5 mm. It was in a state of natural water-bearing with the density of 2.71 g/cm3. 6.2. Thermal expansion of the granite sample at hydrostatic pressure of 25 MPa At all-around hydrostatic pressure of 25 MPa, the thermal deformation of granite can be divided into three stages: Low-temperature
Temperature /°C 300 400
500
600
700
-0.004 -0.006 -0.008 axial strain lataral strain volume strain
-0.01 -0.012
ð4Þ
6. The deformation properties of granite at high-temperature and high pressure
200
-0.002
-0.014 Fig. 6. Thermal strain of granite at all-around hydrostatic pressure of s1 ¼ sa ¼25 MPa.
Linear expansion coefficient /°C-1
1 E
e1 ¼ ðs1 2ns2 Þ, e2 ¼ ðs2 nðs1 þ s2 ÞÞ
100
0
1.8E-05 1.5E-05 1.2E-05 9.0E-06 6.0E-06 3.0E-06 0.0E+00 0
100
200
400 300 Temperature /°C
500
600
700
Fig. 7. Linear thermal expansion coefficient of granite at different temperatures.
Y. Zhao et al. / International Journal of Rock Mechanics & Mining Sciences 52 (2012) 132–138
coefficient of granite changes between 0.2 10 5/1C and 1.25 10 5/1C. However, Xu and Yu indicated that the thermal expansion coefficient of the granite is between 0.5 1C and 12 10 5/1C, which is about ten times higher. This is attributed to different confining pressures applied to the sample. When there is little to no confining pressure, thermal expansion coefficient is larger because thermal cracking may occur and micro-cracks may develop. However, when high confining pressure is applied, as the 200 400°C 300°C 500°C 200°C
stress (σ1-σa)/MPa
160 120 80 40 0 0
0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 0.0045 strain
137
tests presented in this paper, thermal cracking is restricted and resulted in a smaller thermal expansion coefficient [33–36]. 6.3. Deformation characteristics of granite at high temperature [22– 32,37–40] Fig. 8 shows the curve of axial stress vs. axial strain derived by increasing the differential stress (s1–s2) of granite at 200–500 1C, and at a constant confining pressure (sa ¼25 MPa). When the axial pressure is increased, axial compression became larger. When the differential stress (s1–s2) reached 128 MPa, the rock gets into a yield/flowing state. The rock showed greater lateral expansion deformation which increases rock total volume. Fig. 7 shows that the granite sample gradually turned to plastic after temperature reached 200 1C. The sample behaved apparent plastic when the temperature reached 400 1C and the deviate stress reached 110 MPa. The transition can also be seen on Fig. 9, at which stage Young’s modulus reduced significantly. In comparison, the samples showed linear elastic behavior at temperature 200 1C, even with a high deviatoric stress of 170 MPa. When the temperature reached 300 1C and the deviate stress reached 100 MPa, the sample entered a non-linear deformation stage, which is a plastic strengthen stage and features with a significant decrease of secant modulus.
Fig. 8. Axial strain–stress curve of granite at different temperatures (sa ¼ 25 MPa).
6.4. Failure mode of granite 70 60 Young's modulus/GP
Fig. 10 is a photograph of granite after failure at temperature 300 1C. After the granite failed at high temperatures, the main rupture appeared ‘‘X’’ shape and the two ends showed regular ‘‘top cones’’, which is a typical mode of shear failure.
2# sample zhu et al test results
50
6.5. Young’s modulus at different temperatures
Low temperature
Medium high temperature
40
high temperature
Fig. 9 shows that Young’s modulus of granite decreases with increasing temperature [24]. Their relationship can be expressed by the following exponential function:
30
E ¼ 60 expð0:0006TÞ 20 0
100
200
300 400 temperature/°C
500
600
Fig. 9. Young’s modulus of granite at different temperatures.
700
ð5Þ
where T is the temperature of granite, in 1C. In our test, when the temperature increases from room temperature to 600 1C, Young’s modulus of granite dropped by 25.1%. This is also shown in Fig. 11.
Fig. 10. Failure of granite specimen at 300 1C; (a) shear plane (b) full view.
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35
stress (σ1-σa)/MPa
30 25 20 15 20°C 100°C 200°C 300°C 400°C 500°C 600°C
10 5 0
0
0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 0.0007 axial strain
Fig. 11. Stress–strain curve of granite at different temperatures.
7. Conclusions The performance and technological innovations of a self-developed 20 MN rock triaxial test machine XPS-20 MN for high temperature and high pressure are presented in this paper. Compared with the common testing machine for high temperature and high pressure in geophysical exploration, the test machine has the following features: (1) Specimen of larger dimensions can be tested. The diameter of the specimen is up to 200 mm and height is up to 400 mm. Compared to ordinary rock testing machine, this system can test rock specimen with volume 80–170 times larger, which may improve the representativeness of the sample. (2) The test machine uses servo-control loading; therefore, the deformation of rock samples at a variety of temperatures can be easily examined. (3) The test machine can be used to study rock permeability and deformation characteristics in the thermal decomposition process. (4) Holes can be drilled in the specimen at high temperature and the creep characteristics can be tested. This paper also presents the test results of coal specimens at high temperature and high pressure. The tests revealed the stress–strain characteristics of the coal specimens at high temperature, particularly the enhanced plastic features. Young’s modulus decreases in a negative exponential function in regarding to the temperature. Test results revealing the thermal deformation and failure mode of the large-size granite specimens at high temperature and high pressure, and the changing of the thermodynamic parameters of the specimen, such as Young’s modulus, with temperature, are also presented as examples of tests carried out using the testing machine. The following findings are drawn from the test results: (1) The ‘‘X’’ shape fractures during failure of the granite specimen at high temperature and high pressure indicate a shear failure; (2) The linear thermal expansion coefficient of granite increases with temperature while its Young’s modulus decreases with the temperature. The elasticity modulus decreases with the temperature in an exponential function with the temperature. References [1] Vutukuri VS, Lama RD, Saluja SS. Handbook of Mechanical Properties of Rocks. Clausthal. Trans Tech Publications; 1974. [2] Tao ZY. The Theory and Practice of Rock Mechanics. Beijing: China Water Conservancy Press; 1981 (in Chinese). [3] Metal Mine 1976;6:1–7 . (in Chinese). [4] Zhang JZ, Lin TJ. Stress conditions and the variation of rupture characteristics of a rock as shown by triaxial tests. Acta Mech Sin 1979;15(2):3–10 (in Chinese).
[5] Ren AH. 800 t high temperature and high pressure servo-controlled triaxial rock rheology testing system. Chin J Geophys 1988;31(2):9 (in Chinese). [6] Shi ZQ, Zhou MQ. Designing of 800 MPa high temperature and high pressure triaxial cell. Chin J Geophys 1990;33(2):202–7 (in Chinese). [7] Mei, JY, Fu, BJ, Kang, WF The development and current state of rock mechanics in China. In: Proc 5th Int Congress of Rock Mechanics, Melbourne, 1983; 2: 251. [8] Beijing: Science Press; 2006 (in Chinese). [9] Wan, ZJ Study on thermal-mechanical coupling process of heterogeneous rock mass and Channel’s stability of underground coal gasification, PhD Thesis. China University of Mining and Technology, Xuzhou; 2006. (in Chinese). [10] Ren, AH Experimental research of inner heating apparatus of rock triaxial testing system, In: Proc 1st Symposium High Temperature and High Pressure Rock Mechanics. Beijing, Science Press, 1988, pp. 166–170. (in Chinese). [11] Liu CL, San Y, Wang ZC, et al. Uniformity simulation and model of temperature of well-shape heater’s hearth. Ind Heat 2000;29(6):12–5 (in Chinese). [12] Gao XM. Application of graphite in high temperature vacuum electric heater. Ind Heat 2005;34(1):39–41 (in Chinese). [13] Song ST, Pan HY, Wang SZ. Study on heat element made by zirconia and super high temperature electric heater. J Iron Steel Res Int 1996;8(6):53–7 (in Chinese). [14] Zhou JX, Wang GL, Shao ZJ. Coal deformation under high temperature and confining pressure. J China Coal Soc 1994;19(3):324–32 (in Chinese). [15] Jiang B, Qin Y, Jin FL. Coal deformation test under high temperature and confining pressure. J China Coal Soc 1997;22(1):80–4 (in Chinese). [16] Brace WF, Paulding BW, Scholz C. Dilatancy in the fracture of crystalline rocks. J Geophys Res 1966;71(16):3939–53. [17] Bieniawski ZT. Mechanism of Brittle fracture of rocks. Int J Rock Mech Min Sci 1967;4(4) 395–30. [18] Duchane DV. Hot dry rock: a realistic energy option. Geotherm Resour Counc Bull 1990;19(3):83–8. [19] Zhao YS, Wan ZJ, Kang JR. Introduction of Geothermal Extraction of Hot Dry Rock. Beijing: Science Press; 2004 (in Chinese). [20] Wan ZJ, Zhao YS, Kang JR. Simulation and forecast method of geothermal resources in hot dry rock. Chin J Rock Mech Eng 2005;24(6):945–9 (in Chinese). [21] Wang SZ. High-temperature/high-pressure rock mechanics: history, state-ofart and prospect. Prog Geophys 1995;10(4):1–31 (in Chinese). [22] Liu QS, Xu XC. Yamaguehi Tsutomo et al. testing study on mechanical properties of the three Gorges granite concerning temperature and time. Chin J Rock Mech Eng 2001;20(5):715–9 (in Chinese). [23] Xu XC, Liu QS. A preliminary study on basic mechanical properties for granite at high temperature. Chin J Geotech Eng 2000;22(3):332–5 (in Chinese). [24] Zhu HH, Yan ZG, Deng T, et al. Testing study on mechanical properties of tuff, granite and breccia after high temperatures. Chin J Rock Mech Eng 2006;25(10):1945–50 (in Chinese). [25] Du SJ, Liu H, Zhi HT, et al. Testing study on mechanical properties of posthigh-temperature granite. Chin J Rock Mech Eng 2004;23(14):2359–64 (in Chinese). [26] Wang JT, Zhao AG, Huang MC. Effect of high temperature on the fracture toughness of granite. Chin J Geotech Eng 1989;11(6):113–9 (in Chinese). [27] Zhang ZR, He SX, Xi XS. Experimental deformation of Wangxiang granite at high temperature and pressure. J Cent South Univ Tech 1999;30(3):221–4 (in Chinese). [28] Schulmann K, Mlcoch B, Melka R. High-temperature microstructures and rheology of deformed granite, Erzgebirge, Bohemian Massif. J Struct Geol 1996;18(6):719–33. [29] Tullis J, Yund RA. Experimental deformation of dry westerly granite. J Geophys Res 1977;82(36):5705–18. [30] Qiu YP, Lin ZY. Testing study on damage of granite samples after high temperature. Rock Soil Mech 2006;27(6):1005–10 (in Chinese). [31] Aizawa Y, Ito K, Tatsumi Y. Compressional wave velocity of granite and amphibolite up to melting temperatures at 1 GPa. Tectonophysics 2002;351(3): 255–61. [32] Ross JV, Wilks KR. Effects of a third phase on the mechanical and microstructural evolution of a granulite. Tectonophysics 1995;241(3–4):303–15. [33] Skoczylas F, Henry JP. A study of the intrinsic permeability of granite to gas. Int J Rock Mech Min Sci 1995;32(2):171–9. [34] Wang HF, Bonner BP, Carlson SR, Kowallis BJ, Heard HC. Thermal stress cracking in granite. J Geophys Res 1989;94(B2):1745–58. [35] Darot M, Gueguen Y, Baratin ML. Permeability of thermally cracked granite. Geophys Res Lett 1992;19(9):869–72. [36] Geraud Y, Mazerolle F, Raynaud S. Comparison between connected and overall porosity of thermally stressed granites. J Struct Geol 1992;14(8/9): 981–90. [37] Chen Y, Wang CY. Thermally induced acoustic emission in westerly granite. Geophys Res Letts 1980;7(12):1089–92. [38] Heuze FE. High-temperature mechanical, physical and thermal properties of granitic rocks - a review. Int J Rock Mech Min Sci 1983;20(1):3–10. [39] Lin MZ. Rock Thermophysics and Its Engineering Application. Chongqing: Chongqing University Press; 1991 (in Chinese). [40] Griggs DT, Turner FJ, Heard HC. Deformation of rock at 500 1C to 800 1C. Geol Soc Am Mem 1960;79:39–104. [41] Xu, X.H., Yu, J. Rock Crushing Technology, Beijing: Coal Industry Press, 1984. (In Chinese).