Mechanical characteristics and microcosmic mechanisms of granite under temperature loads

JOURNAL OF CHINA UNIVERSITY OF

MINING & TECHNOLOGY J China Univ Mining & Technol 18 (2008) 0413–0417 www.elsevier.com/locate/jcumt

Mechanical characteristics and microcosmic mechanisms of granite under temperature loads XU Xiao-li1,2, GAO Feng1, SHEN Xiao-ming1, XIE He-ping3 1

2

School of Sciences, China University of Mining & Technology, Xuzhou, Jiangsu 221008, China School of Architecture and Civil Engineering, Nantong University, Nantong, Jiangsu 226019, China 3 Sichuan University, Chengdu, Sichuan 610065, China

Abstract: The relationships between mechanical characteristics of rock and microcosmic mechanism at high temperatures were investigated by MTS815, as well as the stress-strain behavior of granite under the action of temperatures ranging from room temperature to 1200 °C. Based on a micropore structure analyzer and SEM, the changes in rock porosity and micro structural morphology of sample fractures and brittle-plastic characteristics under high temperatures were analyzed. The results are as follows: 1) Mechanical characteristics do not show obvious variations before 800 °C; strength decreases suddenly after 800 °C and bearing capacity is almost lost at 1200 °C. 2) Rock porosity increases with rising temperatures; the threshold temperature is about 800 °C; at this temperature its effect is basically uniform with strength decreasing rapidly. 3) The failure type of granite is a brittle tensile fracture at temperatures below 800 °C which transforms into plasticity at temperatures higher than 800 °C and crystal formation takes place at this time. Chemical reactions take place at 1200 °C. Failure of granite under high temperature is a common result of thermal stress as indicated by an increase in the thermal expansion coefficient, transformation to crystal formation of minerals and structural chemical reactions. Key words: granite; mechanical characteristics; temperature effect; microcosmic mechanism

1

Introduction

A number of new topics have been raised, related to rock mechanics, given an increasing demand for underground storage of nuclear waste, natural gas and petroleum, as the rate of exploration for energy resources on a worldwide scale accelerates. A wide range of temperature variations has to be considered in the building of underground chambers for nuclear waste (100~300 °C) and in the study of geothermal fields (about 200 °C) as well as the high temperature (maybe as high as 1000 °C) at which thermal cracking of rocks occurs with mechanical drilling. These considerations require detailed information about changes in physical and mechanical properties of rocks as a function of temperature. In the last few years, some investigations have been carried out in this field[1–6], concerning, for example, changes in the mechanical characteristics of heat-treated granite, the destructive process of heated rocks, the variation in breaking strength, Young's modulus and the Poisson ratio of heated granite and the characteristic acoustic emission during rock thermal cracking. However,

rock failure and instability are complicated because the rock types, temperatures, confining pressure, strain rates and plastic components of rocks directly affect rock failure and modes of instability[7–9]. Therefore, the problem under consideration is that mechanical behavior, macroscopic structures and microscopic mechanisms should be completely understood. Mechanical characteristics and microcosmic mechanisms of granite under temperatures ranging from normal levels to temperatures of 1200 °C have been investigated by us with MTS 815 hydraulic servo system, micropore structure analyzer and SEM.

2 2.1

Experimental Specimen and preparation

In our investigation, we have used cylindrical rock specimens, 50 mm long with diameters of 25 mm. The two ends of the specimens were ground flat and parallel to each other at a level of accuracy of approximately 0.05 mm. There were 45 specimens in total, divided into nine groups with 3 specimens to

Received 12 December 2007; accepted 15 May 2008 Projects 50579042 supported by the National Natural Science Foundation of China, 2002CB412705 by the National Basic Research and Development Program of China and ok060122 by the Young Foundation of China University of Mining & Technology Corresponding author. Tel: +86-13813451842; E-mail address: [email protected]

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each group. First, the specimens of each group were put into a MTS 653.04 high temperature furnace in which the temperature reached 25, 50, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200 and 1300 °C separately. In each case, the temperature was kept constant for 20 min so that the specimens could be heated to the assigned value from the outside to the inside and then the specimens were taken out and cooled down in the air. 2.2 Equipment and processes 1) Experiments on granite under uniaxial compression after high temperature Experiments on granite under uniaxial compression after high temperatures (normal~1300 °C) were investigated with MTS815 hydraulic servo system. Stress-strain behavior was obtained and mechanical characteristics such as load, deformation and time were also measured in the process of rock deformation and destruction after exposure to high temperatures. 2) Mercury injection experiment Mercury injection experiments were carried out using 9310 micropore structure analyzer, produced by Micromerities, Corp. Georgia, USA. The operating pressure range was from 0.0 MPa to 207 MPa, with a low pressure resolution of ±0.001 MPa and a high pressure resolution of ±0.01 MPa. The determination of pore sizes ranged from 0.006~360 µm. The mercury injection experiment had nine temperature points: room temperature, 50, 100, 200, 300, 500, 800, 1000 and 1200 °C. At each temperature point three samples were tested for a total of 27 samples. 3) SEM experiment The SEM experiment was used to observe the micro structural morphology of rock sample fractures and brittle-plastic characteristics. The fracture scanning experiment had also the same nine temperature points as the previous experiment, i.e., room temperature, 50, 100, 200, 300, 500, 800, 1000 and 1200 °C, but each temperature point had only one sample for a total of nine samples.

3 3.1

Results and discussion Results of mechanical characteristics in granite after high temperature

Because the stress- strain curves of each rock group has a similar form of distribution, we just preTable 1

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sent some representative samples in Fig. 1.

Stress (MPa)

414

200 25ć 180 160 200ć 140 120 500ć 100 80 60 40 20 0

800ć

900ć 1000ć 1200ć

0.003 0.006 0.009 0.012 0.015 0.018 Strain e

Fig. 1 Stress-strain curves of granite after high temperature

From Fig. 1, we can draw two conclusions. 1) The strength decreases suddenly and the granite presents plastic and post-peak behavior after 800 °C, which indicates that a phase change behavior of brittle-ductile transition appears around 800 °C. 2) The experimental results also show that after high temperatures, the sample failure mode changes from abrupt instability to a gradual failure with an increase in temperature. The failure type is a brittle or semi brittle shear fracture below 800 °C and a semi ductile shear fracture at temperatures over 800 °C, showing crystal plasticity deformation has occurred, which changes to plasticity at 1200 °C. 3.2 Characteristics of pore structure of granite after high temperature The study of pore structures necessarily involves the classification of pores, but there is still no unified classification standard, either at home or abroad. We have used the classification, based on a standard of pore size considering the origin pores and the level of seepage, established by Wu et al. in 2005[10]. Pores are divided into four kinds: 1) ultramicropores with an aperture less than 1 µm, where, in general, liquids cannot seep through; 2) micropores with an aperture in the range of 1~10 µm, where liquid can seep through under higher pressure, but permeability is low; 3) low pores with an aperture between 10~100 µm, which permits water to seep through at a water head pressure in a natural state and 4) macropores, with an aperture over 100 µm, allowing groundwater to seep through freely. Porosity of granite under different temperatures measured during our experiments is shown in Table 1.

Rock porosity at different temperatures

Temperature (°C)

25

50

100

200

300

500

800

1000

1200

Porosity of rock 1 (%)

0.953

0.729

0.866

0.912

1.49

1.044

2.619

3.554

3.973

Porosity of rock 2 (%)

0.945

0.805

0.681

1.064

1.369

1.114

2.417

4.191

4.631

Porosity of rock 3 (%)

0.721

0.703

0.66

1.048

1.373

1.823

2.678

3.708

4.766

Average value (%)

0.873

0.746

0.736

1.008

1.411

1.327

2.571

3.818

4.457

XU Xiao-li et al

Mechanical characteristics and microcosmic mechanisms of granite …

Rock porosity increases with rising temperature. The change in porosity has a mutation process, with a threshold temperature of about 800 °C (see Table 1). The amplitude of growth in porosity suddenly becomes large at about 800 °C, which is 2.945 times compared with that at room temperature. This tem0.35 0.30 0.25 0.20 0.15 0.10 0.05 0

0.01

10 0.1 1 Aperture (µm)

100

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0.01

415

perature is the same as that where the strength decreases abruptly. Phase pressure-mercury curves and accumulated pressure-mercury curves of granite at different temperatures are shown in Figs. 2–3. 2.0 1.5 1.0 0.5

10 0.1 1 Aperture (µm)

(a) 25 °C

100

0

0.01

(b) 800 °C

10 0.1 1 Aperture (µm)

100

(c) 1200 °C

Fig. 2 Phase pressure-mercury curves of granite at different temperatures

(µm)

Fig. 3

(µm)

(µm)

Accumulated pressure-mercury curves of granite at different temperatures

In terms of these curves, the connectivity is good although rock porosity is small. The coexistence of the characteristics of different widths of micro-fissures is displayed in phase pressure-mercury test curves. The accumulated pressure-mercury test curves show a stepwise shape. Ultramicropores account for 48.39% of all types of pores at room temperature, micropores for 16.13%. At 800 °C ultramicropores account for 38.82% and micropores for 28.24%. At 1200 °C, ultramicropores account for 23.70% and micropores for 47.41%. Ultramicropores gradually convert to micropores and connectivity increases when temperatures are over 800 °C. The major change in rock sample is a change in their capacity to adsorb water and a change in the interlayer water found in micropores when the heating temperature is below the threshold temperature, where rock porosity changes little. Activated and plastic composition of the rock medium increases with rising temperatures, which promote rock transformation from a brittle to a ductile phase, and changes its mineral structure and composition. When temperatures are over the threshold temperature, mineral composition appears as dehydration and phase transition. Hydrogen groups, hydroxyl and water produce intracrystalline diffusion and microcracks. Hydrolysis and water aggregation occurs and other physical and chemical reactions appear. All these factors cause microcracks to extend rapidly, lead to changes in pore structures and increase and improve

the channel of fluid flow, hence rock porosity increases rapidly. 3.3 Analysis of fracture image with SEM Granite is to typical brittle rock, which displays typical brittle failure under room and lower temperature. Failures begin to be transformed from brittle to plastic deformation when temperatures are over 800 °C. Several representative pictures are presented in Fig. 4. Analyses show that a fracture of a surface is the common effect of temperature and stress. Fracture surfaces of granite are smooth and fractures often appear as candy and river like patterns of brittle, tensile fractures at temperature below 800 °C. The fracture surface possesses mixed-rupture characteristics of cleavage steps, minute lacerated ridges and tough dimple features. Phase transitions from brittle to ductile and crystal formation have appeared at temperature higher than 800 °C. At 1200 °C, the number of dimples and micropores clearly increases, which indicates that chemical reactions take place. Fracture surfaces form shear fracture zones and show plastic shear fractures. Failure of granite under high temperature is a common result of thermal stress indicated by different crystal thermal expansion coefficients, transformation of crystal forms of minerals and chemical structural reactions.

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(a) Candy pattern, river let pattern (25 °C)

(b) Intergranular fracture pattern (200 °C)

(c) Cleavage step, dimple (800 °C)

Fig. 4

4

(d) Dimple, micro porous, shear fracture zone (1200 °C)

Fracture images with SEM of granite under different temperature

Analysis of brittle-plasticity transfer mechanism under temperature effect [11]

Temperature (T) is an important factor affecting material that turns easily to brittle fractures. Materials will experience brittle fractures when temperatures are lower than the brittle transition temperature. In general, the brittle-plasticity transfer phenomenon is the reason for material yield stress (σs) decreases with increasing temperatures obviously, while the critical stress (σc) of unstable crack extensions is relatively insensitive to temperature. The two stresses will be equal at a certain temperature (TK), as Fig. 5 shows. When T is less than TK, σc is smaller than σs, i.e., at TK critical stress σc , is attained at this point and stress meets the necessary condition for unstable crack extension, so that brittle fractures do occur. When T is greater than TK, σc is also greater than σs and the material first yields and then fractures, so that plastic fractures occur under this condition. σs

σ

σc

Fig. 5

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Relationships between temperature, σc and σs

5

Conclusions

1) Mechanical characteristics of granite do not show clear variation before 800 °C. Its strength decreases suddenly after 800 °C and almost loses bearing capacity at 1200 °C. 2) Rock porosity increases with rising temperatures. Its threshold temperature is about 800 °C. Ultramicropores gradually convert to micropores and connectivity increases when temperatures are higher than 800 °C. The reason for this is that at high temperatures rock minerals experience processes such as dehydration, lattice recombination, mineral shrinkage and decomposition and increased crack connectivity. Simultaneously, different crystals are formed, the thermal expansion coefficient increases and the heterogeneity of rock particles produces new cracks. The crack extensions then form a connected network structure. 3) Analyses show that fractured surfaces are a common effect of temperature and stress. Fracture surfaces of granite are smooth and the appearance of fractures is a candy and river like patterns of brittle tensile fractures at temperatures below 800 °C. The fracture surface possesses mixed-rupture characteristics of cleavage steps, minute lacerated ridges and tough dimples. Transition from a brittle to a ductile phase and crystal formations appear at temperatures higher than 800 °C. At 1200 °C, the number of dimples and micropores clearly increases, which indicates chemical reaction are taking place. Fracture

XU Xiao-li et al

Mechanical characteristics and microcosmic mechanisms of granite …

surfaces form shear fracture zones and show plastic shear fractures. The temperature at which granite strength decreases rapidly is basically uniform with the formation of internal rock fractures and a phase transition to a crystal state. It shows that the change of the ingredients and the crystal state of rock is a major factor which causes sudden changes in mechanical properties of rock subjected to high temperatures.

Acknowledgements Financial support for this work, provided by the National Natural Science Foundation of China (Project No.50579042) and the National Key Basic Research and Development (973) Program of China (Project No.2002CB412705) is gratefully acknowledged.

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