High-temperature mechanical, physical and Thermal properties of granitic rocks— A review

Analysis of physical and mechanical behaviors and microscopic mineral characteristics of thermally damaged granite

Temperature's influence on the physical and mechanical properties of rocks is a crucial concern for the rational design of deep rock engineering structures and the assurance of their long-term stability. To systematically comprehend the impact of the evolution of mineral composition and micro characteristics on the physical and mechanical behavior of thermally damaged granite, we observed the microscopic structural defects inside the rocks with a polarizing microscope and revealed the thermal damage mechanism of granite from a microscopic perspective by combining ultrasound detection and XRD phase characteristic analysis. The results show that the physical properties of the specimens changed significantly at three characteristic temperature points: 400 °C, 800 °C, and 1000 °C. Under high temperature conditions, the diffraction intensity of all minerals in granite, except for quartz, generally decreased, and stable minerals decomposed. Albite and potash feldspar decomposed to form anorthoclase, thereby reducing the structural stability of the rock material. In addition, the peak width of various minerals decreased to varying degrees with increasing temperature. The increase in mineral volume further damaged the internal structure of the rock material while promoting the transformation from grain boundary to intergranular cracks and from intragranular cracks to transgranular cracks, ultimately forming a interconnected crack network. Thermal damage significantly reduced the longitudinal wave velocity, uniaxial compressive strength, and elastic modulus of the specimens, while the stress-strain curve relationship indicated that the specimens underwent two opposite processes of transformation from brittleness to ductility and then from ductility to brittleness. The thermal damage threshold of granite in this study was 600 °C.

Simulation Study of Coal Seam Gas Extraction Characteristics Based on Coal Permeability Evolution Model under Thermal Effect

The permeability evolution law of high temperature and high stress coal seam is determined by the influence of multiphase coexistence and multifield coupling. In an environment greatly affected by disturbance and high temperature, the coal permeability model under the coupling of thermal and mechanical creep is not only a vital framework from which to examine gas migration law in multiphase and multifield coal seams but also an important theoretical foundation for gas control in coal seams. The influence of high-temperature environment on creep deformation and permeability is analyzed by several creep seepage tests under different temperature conditions.A mathematical model for the evolution of coal permeability considering the influence of temperature is established through the theory of matrix-crack interaction based on gas adsorption and desorption and thermal expansion deformation. Based on the permeability model under the coupling of thermal and mechanical creep, the numerical model of gas migration, seepage field, diffusion field, stress field, and temperature field is constructed, and the law of gas migration in coal seam under multifield coupling is explored. The influence law of thermal effect on gas extraction characteristics is analyzed, in which the time-varying mechanism of temperature field, the relationship between creep deformation and temperature and pressure, the influence of creep deformation on permeability, the dynamic distribution of gas pressure, and the change of gas extraction quantity are described in detail. It is concluded that the influence of temperature on permeability is much greater than that of creep deformation and that a high initial coal seam temperature is beneficial to gas extraction. It provides theoretical basis and technical guidance for the study of multifield coupled gas migration and coal seam gas treatment.

Damage Characteristics of Thermally Deteriorated Carbonate Rocks: A Review

This review paper summarizes the recent and past experimental findings to evaluate the damage characteristics of carbonate rocks subjected to thermal treatment (20–1500 °C). The outcomes of published studies show that the degree of thermal damage in the post-heated carbonate rocks is attributed to their rock fabric, microstructural patterns, mineral composition, texture, grain cementations, particle orientations, and grain contact surface area. The expressive variations in the engineering properties of these rocks subjected to the temperature (>500 °C) are the results of chemical processes (hydration, dehydration, deionization, melting, mineral phase transformation, etc.), intercrystalline and intergranular thermal cracking, the separation between cemented particles, removal of bonding agents, and internal defects. Thermally deteriorated carbonate rocks experience a significant reduction in their fracture toughness, static–dynamic strength, static–dynamic elastic moduli, wave velocities, and thermal transport properties, whereas their porous network properties appreciate with the temperature. The stress–strain curves illustrate that post-heated carbonate rocks show brittleness below a temperature of 400 °C, brittle–ductile transformation at a temperature range of 400 to 500 °C, and ductile behavior beyond this critical temperature. The aspects discussed in this review comprehensively describe the damage mechanism of thermally exploited carbonate rocks that can be used as a reference in rock mass classification, sub-surface investigation, and geotechnical site characterization.

3D Numerical Prediction of Thermal Weakening of Granite under Tension

This paper deals with numerical prediction of temperature (weakening) effects on the tensile strength of granitic rock. A 3D numerical approach based on the embedded discontinuity finite elements is developed for this purpose. The governing thermo-mechanical initial/boundary value problem is solved with an explicit (in time) staggered method while using extreme mass scaling to increase the critical time step. Rock fracture is represented by the embedded discontinuity concept implemented here with the linear (4-node) tetrahedral elements. The rock is modelled as a linear elastic (up to fracture by the Rankine criterion) heterogeneous material consisting of Quartz, Feldspar and Biotite minerals. Due to its strong and anomalous temperature dependence upon approaching the α-β transition at the Curie point (~573 °C), only Quartz in the numerical rock depends on temperature in the present approach. In the numerical testing, the sample is first volumetrically heated to a target temperature. Then, the uniaxial tension test is performed on the cooled down sample. The simulations demonstrate the validity of the proposed approach as the experimental deterioration, by thermally induced cracking, of the rock tensile strength is predicted with a good accuracy.

Study on Damage Statistical Constitutive Model of Triaxial Compression of Acid-Etched Rock under Coupling Effect of Temperature and Confining Pressure

Based on Lemaitre's strain equivalence hypothesis theory, it is assumed that the strength of acid-etching rock microelements under the coupling effect of temperature and confining pressure follows the Weibull distribution. Under the hypothesis that micro-element damage meets the D-P criterion and based on continuum damage mechanics and statistical theory, chemical damage variables, thermal damage variables and mechanical damage variables were introduced in the construction of damage evolution equations and constitutive models for acid-etching rocks considering the coupled effects of temperature and confining pressure. The required model parameters were obtained by theoretical derivation, and the model was verified based on the triaxial compression test data of granite. Comparing the experimental stress-strain curve with the theoretical stress-strain curve, the results show that they were in good agreement. By selecting reasonable model parameters, the damage statistical constitutive model can accurately reflect the stress-strain curve characteristics of rock in the process of triaxial compression. The comparison between the experimental and theoretical results also verifies the reasonableness and reliability of the model. This model provides a new rock damage statistical constitutive equation for the study of rock mechanics and its application in engineering, and has certain reference significance for rock underground engineering.

Physical Alteration and Color Change of Granite Subjected to High Temperature

Cylindrical specimens obtained from the monzogranite host rock of the National Radioactive Waste Repository of Hungary were tested at room temperature and 250 °C, 500 °C, and 750 °C of heat treatment. Reflectance spectra (color), bulk density, Duroskop surface hardness, and ultrasound-wave velocity values were measures before and after thermal stress. According to CIE L*a*b* colorimetric characteristics, the specimens’ color became brighter and yellower after the heat treatment. At 750 °C, a significant volume increase was recorded linked to the formation of macro-cracks, and it also led to the drop in bulk density. Smaller temperature treatment (250 °C) caused a minor decrease in density (−1.3%), which is higher than the reduction of density at 500 °C (−0.8%). Duroskop surface strength showed a slight decrease until 500 °C, and then a drastic decline at 750 °C. P- and S-wave velocity values tend to decrease uniformly and significantly from room temperature to 750 °C. P-wave velocity and Duroskop values have a high exponential correlation at elevated temperatures. Physical alterations originated from the differential thermal-induced expansion of minerals, the formation of micro-cracks. Mineralogical changes at higher temperatures also contribute to the volume change and the loss in strength.

Numerical Modeling of Temperature Effect on Tensile Strength of Granitic Rock

The aim of this paper is to numerically predict the temperature effect on the tensile strength of granitic rock. To this end, a numerical approach based on the embedded discontinuity finite elements is developed. The underlying thermo-mechanical problem is solved with a staggered method marching explicitly in time while using extreme mass scaling, allowed by the quasi-static nature of the slow heating of a rock sample to a uniform target temperature, to increase the critical time step. Linear triangle elements are used to implement the embedded discontinuity kinematics with two intersecting cracks in a single element. It is assumed that the quartz mineral, with its strong and anomalous temperature dependence upon approaching the α-β transition at the Curie point (~573 °C), in granitic rock is the major factor resulting in thermal cracking and the consequent degradation of tensile strength. Accordingly, only the thermal expansion coefficient of quartz depends on temperature in the present approach. Moreover, numerically, the rock is taken as isotropic except for the tensile strength, which is unique for each mineral in a rock. In the numerical simulations mimicking the experimental setup on granitic numerical rock samples consisting of quartz, feldspar and biotite minerals, the sample is first heated slowly to a target temperature below the Curie point. Then, a uniaxial tension test is numerically performed on the cooled down sample. The simulations demonstrate the validity of the proposed approach as the experimental deterioration of the tensile strength of the rock is predicted with agreeable accuracy.

Thermal jet drilling of granite rock: a numerical 3D finite-element study

This paper presents a numerical study on thermal jet drilling of granite rock that is based on a thermal spallation phenomenon. For this end, a numerical method based on finite elements and a damage-viscoplasticity model are developed for solving the underlying coupled thermo-mechanical problem. An explicit time-stepping scheme is applied in solving the global problem, which in the present case is amenable to extreme mass scaling. Rock heterogeneity is accounted for as random clusters of finite elements representing rock constituent minerals. The numerical approach is validated based on experiments on thermal shock weakening effect of granite in a dynamic Brazilian disc test. The validated model is applied in three-dimensional simulations of thermal jet drilling with a short duration (0.2 s) and high intensity (approx. 3 MW m^-2) thermal flux. The present numerical approach predicts the spalling as highly (tensile) damaged rock. Finally, it was shown that thermal drilling exploiting heating-forced cooling cycles is a viable method when drilling in hot rock mass. This article is part of the theme issue 'Fracture dynamics of solid materials: from particles to the globe'.

The Use of Infrared Thermography on the Measurement of Microstructural Changes of Reservoir Rocks Induced by Temperature

A variation of temperature produces a change in the microstructure of the rock due to the mineral thermal expansion and its residual strain. Depending on the temperature cycle and texture, microstresses may lead to the development of preexistent cracks or the creation of a new and irreversible cracking. The effect of temperature on reservoir rocks is an important topic since it conditions the permeability and the fluid flow. Two main questions arise from this: the first is if an irreversible cracking threshold is attained in the reservoir rocks at low temperature geothermal systems (around 100 °C); the second one is about the influence of thermal fatigue by the repetition of heating–cooling cycles on the different rock types. To answer these questions, four reservoir rocks (chalk, sandstone, fresh granite, and weathered granite) were submitted to two different thermal regimes. The first test was conceived to detect the irreversible cracking threshold, and for that, the rocks were submitted to progressive heating (90°, 100°, 110°, 120°, and 130 °C). The second test consisted of doing cycles of fast heating of the samples up to 200 °C. The microstructure variation was assessed by means of a scanning electron microscope, mercury porosimetry, and capillary water uptake combined with passive infrared thermography. Infrared thermography is an emerging tool in the field of rock study, used to detect water masses or determine thermal properties. The water transfer during the capillary tests of the rocks, before and after the tests, was monitored with this technique. In addition, the cooling rate index, a non-destructive parameter to detect cracking development, was calculated. The results made it possible to differentiate the behaviours in relation to the rock type, with a chalk and a weathered granite less susceptible to thermal stresses than a fresh granite and sandstone. In addition, infrared thermography resulted in being a very useful indirect technique to detect the changes on the surface, although they do not always correlate to the bulk microstructural changes.

Modeling and Numerical Analysis in 3D of Anisotropic and Nonlinear Mechanical Behavior of Tournemire Argillite under High Temperatures and Dynamic Loading

This work proposes a model that takes into account the anisotropy of material with its inhomogeneity and geometrical and material nonlinearities. According to Newton's second law, the investigations were carried out on the simultaneous effects of mechanical load and thermal treatment on the Tournemire argillite material. The finite difference method was used for the numerical resolution of the problem by the MATLAB 2015a software in order to determine the peak stress and strain of argillite as a function of material nonlinearity and demonstrated the inhomogeneity parameter Ω. The critical temperature from which the material damage was pronounced is 500°C. Indeed, above this temperature, the loss of rigidity of argillite reduced significantly the mechanical performance of this rock. Therefore, after 2.9 min, the stress reduction in X or Y direction was 75.5% with a peak stress value of 2500 MPa, whereas in Z direction, the stress reduction was 74.1% with a peak stress value of 1998 MPa. Meanwhile, knowing that the material inhomogeneity was between 2995 and 3256.010, there was an increase in peak stress of about 75%. However, the influence of the material nonlinearity was almost negligible. Thus, the geometrical nonlinearity allows having the maximal constant strain of about 1.25 in the direction of the applied dynamic mechanical force.

Effects of Cyclic Heating and Water Cooling on the Physical Characteristics of Granite

This paper aims to study the effect of cyclic heating and flowing-water cooling conditions on the physical properties of granite. Ultrasonic tests, gas measured porosity, permeability, and microscope observations were conducted on granite after thermal treatment. The results showed that the velocity of P- and S-waves decreased as the number of thermal cycles increased. The porosity increased with the number of the thermal cycles attained at 600 °C, while no apparent changes were observed at 200 and 400 °C. The permeability increased with the increasing number of thermal cycles. Furthermore, microscope observations showed that degradation of the granite after thermal treatment was attributed to a large network of microcracks induced by thermal stress. As the number of thermal cycles increased, the number of transgranular microcracks gradually increased, as well as their length and width. The quantification of microcracks from cast thin section (CTS) images supported the visual observation.

Effect of High Temperature and Inclination Angle on Mechanical Properties and Fracture Behavior of Granite at Low Strain Rate

Comprehensive understanding of the effects of temperature and inclination angle on mechanical properties and fracture modes of rock is essential for the design of rock engineering under complex loads, such as the construction of nuclear waste repository, geothermal energy development and stability assessment of deep pillar. In this paper, a novel inclined uniaxial compression (inclined UCS) test system was introduced to carry out two series of inclined uniaxial compression tests on granite specimens under various inclination angles (0–20°) and treated temperatures (25–800 °C) at 5° inclination. Experimental results revealed that the peak compression stress and elastic modulus gradually decreased, while peak shear stress increased nonlinearly with the increasing inclination angle; the peak compression and shear stress as well as elastic modulus slightly increased from 25 to 200 °C, then gradually decreased onwards with the increasing temperature. The effect of temperature on peak axial strain was the same as that on peak shear displacement. Acoustic emission (AE) results suggested that the relationship between crack initiation stress, inclination angle and treated temperature followed a similar trend as that of the peak compression stress and elastic modulus. Particularly, the crack initiation (CI) stress threshold and shear stress corresponding to CI threshold under 800 °C were only 7.4% of that under 200 °C and revealed a severe heat damage phenomenon, which was consistent with the results of the scanning electron microscopy (SEM) with the appearance of a large number of thermal pores observed only under 800 °C. The failure modes tended to shear failure with the increasing inclination angle, indicating that the shear stress component can accelerate sliding instability of rocks. On the other hand, the failure patterns with different temperatures changed from combined splitting-shear failure (25–400 °C) to single shear failure (600 and 800 °C). The study results can provide an extremely important reference for underground thermal engineering construction under complex loading environment.

Thermophysical Measurements in Liquid Alloys and Phase Diagram Studies

Towards the construction of pressure-dependent phase diagrams of binary alloy systems, both thermophysical measurements and thermodynamic modeling are employed. High-accuracy measurements of sound velocity, density, and electrical resistivity were performed for selected metallic elements from columns III to V and their alloys in the liquid phase. Sound velocity measurements were made using ultrasonic techniques, density measurements using the gamma radiation attenuation method, and electrical resistivity measurements were performed using the four probe method. Sound velocity and density data, measured at ambient pressure, were incorporated into a thermodynamic model to calculate the pressure dependence of binary phase diagrams. Electrical resistivity measurements were performed on binary systems to study phase separation and identify phase transitions in the liquid state.

Cracking and segregation in high-alloy steel 0.4C1.5Mn2Cr0.35Mo1.5Ni produced by thick continuous casting

Based on our innovative application of using thick continuous casting slab 0.4C1.5Mn2Cr0.35Mo1.5Ni (high alloy) for the production of high-quality mould steel, the present study investigated the high cracking susceptibility of high-alloy steel and segregation in continuous casting slab. The thermal expansion and the continuous cooling transformation (CCT) curve measurement, together with a high temperature in situ observation, confirmed the martensite phase transition happening at approximately 583 K that would result in an increase in the hardenability and cracking susceptibility. The cracking susceptibility zone was determined by high-temperature mechanical properties measurement. The high-alloy mould steel has no II brittle zone, and III brittle zone is 973–1148 K. As a conclusion, the straightening temperature should be above 1148 K to avoid the cracking during the continuous casting. Moreover, the elemental segregation of carbon, sulfur, chromium, and molybdenum along the cracking was examined by electron probe microanalysis (EPMA) quantitative analysis that might be another reason for the steel crack formation. It shows that Martensite phase transition happened at approximately 583 K that would result in an increase in the hardenability and cracking susceptibility.

Influence of High Pressure and Temperature on the Mechanical Behavior and Permeability of a Fractured Coal

Understanding mechanical behavior and permeability of coal at ambient and high temperature is key in optimizing high-temperature in-situ processes such as underground coal gasification. The main objectives of this study were to characterize thermal deformation, stress-strain behavior, and gas permeability of coal samples acquired from the Genesee coal mine in Central Alberta, Canada under various temperatures and confining stresses. These measurements were conducted in a high-pressure high-temperature triaxial apparatus. Initial thermal expansion of the coal was followed by contraction in both axial and lateral directions at about 140 °C. This temperature corresponds to occurrence of pyrolysis in the coal. All specimens showed brittle behavior during shear while forming complex shear planes. The specimens exhibited compressional volumetric strain responses at all temperatures. Deformation localization initiated at various stage during shearing. Specimens sheared at 200 °C showed higher peak stresses and larger axial strains compared to those tested at room temperature (24 °C). Fluctuations of permeability were observed with confining stress and temperature. Permeability dropped at 80 °C due to thermal expansion of coal and closure of initial fractures; however, it increased at 140 and 200 °C due to a combined response of thermal expansion and pyrolysis. Small axial strain during shear was observed to reduce permeability.