Granite Specimens Research Articles

Investigating the Macro-Micromechanical Properties and Failure Law of Granite under Loading and Unloading Conditions

The excavation of rock significantly changes the initial stress state of rock slopes, which makes rock in complex loading and unloading conditions. However, the failure mechanisms and macro-micromechanical properties of rock under loading and unloading conditions are not very clear. This study investigates the macro-micromechanical properties and failure law of granite under loading and unloading conditions through traditional laboratory tests and particle flow simulations. Granite specimens are taken from Shuichang iron mine, and stress unloading experiments are designed based on the engineering practices. The stress-strain curves and failure modes under different confining pressures and unloading paths are obtained to analyze the granite properties from the viewpoint of the macroscopic mechanism. Moreover, numerical models are established in PFC software. The microcracks developments, failure characteristics, and energy evolutions under loading and unloading conditions are obtained and discussed. Results show that compared with the loading tests, the brittle failure characteristics of specimens under unloading tests are more obvious. When the confining pressure reduces to about 66% of the initial confining pressure, the specimen loses its load-bearing capacity and destroyed due to the lateral expansion. For loading tests, an inclined plane can be produced as the main failure surface. While for confining pressure unloading tests, there are many damage zones parallel to or intersecting with the main failure surface.

Analysis of size effects on the Hoek-Brown failure criterion of intact granite samples

Scale or size effects of the uniaxial strength response in rock samples have been studied in detail in the past, so a good number of studies on different rocks are available. However, analyses on triaxial strength scale effects in rocks are scarce and they seldom address failure criteria (i.e. Hoek-Brown) evolution with specimen size. This obvious lack of data can be attributed to the difficulties of having available Hoek’s cell of different sizes. With the aim of filling this void, the authors have carried out sets of around 25 stress-strain triaxial tests on intact 30, 38, 54 and 84 mm diameter granite specimens, with various confinements (0.2 to 15 MPa), so reliable estimates of Hoek-Brown strength were obtained for every scale. We compare results with previous studies on UCS scale effects, showing a good correlation. Results suggest the studied granite undergoes a reverse size effect in terms of strength at low confinements. Indeed, the UCS increases as sample diameter increases up to around 50 mm, but decreases thereafter. However, results obtained put forward that this strength variation with scale tends to be mitigated for higher confinements where the scale effect may not be clearly recognised. So increased confinement can be associated with a decreased scale dependency component of strength.

Experimental study on the physical and mechanical variations of hot granite under different cooling treatments

Geothermal energy arrested in HDR (hot dry rock) is commonly harvested from the reservoir fractured by artificial stimulations. Understanding the physical and mechanical behaviors of HDR after different cooling treatments is critical for selecting most efficient fluid to fracture the rock mass. Here we experimentally examine the physical and mechanical variations of a typical type of HDR, granite, after natural, water, and liquid-nitrogen cooling treatments, followed by a multi-scale investigation on the evolution of cracks. We find that the effect of cooling treatment on the physical and mechanical properties of heated granite specimens is increasingly remarkable as the peak temperature grows. The difference among the influences of different cooling treatments is unnoticeable when the specimens are heated to peak temperatures of 200 °C and 400 °C, but becomes highly pronounced as the peak temperature ascends to 600 °C and 800 °C. Liquid-nitrogen cooling treatment induces most alterations in the granite properties. The cracks in the granite specimens cooled by liquid-nitrogen coalesce together and form complex fracture networks, which are more intensive than those by natural and water cooling treatments. Previous studies claimed that the liquid-nitrogen cooling treatment had weaker influence on the granite specimen than water, possibly due to the Leidenfrost effect that impeded the heat transfer between rock and liquid-nitrogen. Our findings demonstrate that liquid-nitrogen is superior in cracking and thus enhancing the permeability of the granite specimens, particularly for those heated to a temperature over 400 °C. The good performance of liquid-nitrogen is attributed to that the granite specimen is easily cracked by the high tensile stress created jointly by the high temperature difference caused by its ultra-low temperature and large convective heat transfer coefficient induced by violent boiling.

Study on the Relationship between Fracture Morphology and Splitting Strength of Granite Based on Fabric Distribution

Because of the requirement of the mechanical properties of the damaged surrounding granite rock and the existence of the fracture water in hydraulic fracturing engineering, the strength of granite is related to the fabric, and the roughness of the section is also related to the liquid flow rate, a method of roughly determining the strength of the specimen by observing the failure mode of rock is needed. Considering that the physical and mechanical properties of granite are obviously affected by the fabric, the discrete element numerical simulation method was used to reconstruct the granite fabric based on the spatial correlation function to simulate the splitting experiment to investigate the failure mode of the specimen. The relationship between strength, the fractal value of cross-section, and the fabric was analyzed, which was verified through experiment. The results show that (1) the Voronoi GBM model with spatial correlation function of the discrete element can effectively simulate the controllable granite fabric and carry out micromechanical analysis. (2) The strength of the granite specimen and the fractal value of the cross-section have an obvious linear relationship with the fabric; besides, there is also a certain linear relationship between the strength of the specimen and the fractal value, which is influenced by granite fabric. (3) The predicted strength of the specimen according to the fractal value of the section is in good agreement with the actual strength with the error rate of 30%. In a word, this method can predict the strength of the specimen through the failure section and analyze the hydraulic fracture section and water pressure.

Effect of thermal treatment on the fracture toughness and subcritical crack growth of granite in double-torsion test

An understanding of the fracture behavior and subcritical crack propagation of hard rock under different temperatures is necessary for the design and stability of nuclear waste disposal projects. The aim of this study was to understand the thermal effect on the fracture behavior of granite, which had been heated at 25 (untreated), 100, 200, 400, and 600 °C, respectively. A series of uniaxial compressive tests and double-torsion tests were performed on granite specimens. The three-dimensional digital image correlation technique was used to investigate the strain evolution and fracture pattern in granite specimens during subcritical crack propagation. According to the results, the uniaxial compression strength, Young’s modulus and fracture toughness of the granite specimens increased and then decreased with an increase in treatment temperature. These parameters dropped sharply as the treatment temperature exceeded 400 °C, mainly because of the thermally-induced destruction of the quartz crystal structure and a generation of wider intergranular fractures. A comparison of the subcritical crack length by theoretical calculations and experimental observations from digital image correlation showed that the two values were consistent. At a treatment temperature of 400 °C and 600 °C, the velocity of the subcritical crack propagation was accelerated, so the length of the subcritical crack during the relaxation test was 4.14 times and 5.25 times that at 25 °C, respectively. The subcritical crack growth and fracture process of the double-torsion tests were visible by the three-dimensional digital image correlation system. A subcritical crack growth index (n) can be used to characterize the susceptibility of the subcritical crack propagation in rock relaxation tests, which helps to evaluate the long-term stability of nuclear waste disposal projects.

Geothermal reservoir stimulation through hydro-shearing: an experimental study under conditions close to enhanced geothermal systems

Injection-induced shear stimulation (commonly known as ‘hydro-shearing’) is often implemented in Enhanced Geothermal Systems (EGS) to enhance the reservoir permeability. Hydro-shearing occurs when a critically stressed pre-existing fracture (or fault) slips at an injection pressure below the minimum principal stress in the reservoir, which opens the fracture due to self-propping of asperities on the fracture surface. In this study, six triaxial flow through tests were conducted on rough fracture and saw-cut granite specimens at temperatures of 20°C, 65°C, and 130°C in order to investigate the impact of hydro-shearing on fracture response to coupled thermal-hydrological-mechanical-chemical (THMC) processes. Hydro-shearing results in a permanent shear-slip in a rough fracture, which opens the fracture due to self-propping of asperities and contributes to retainable permeability enhancement. Thermal stress induced shear-slip was observed in case of saw-cut fracture tested at 130 °C. Thermal stress at elevated temperature could have contributed to the mechanical crushing of asperities and weak fractures, which resulted in higher amount of gouge particles. Increase in temperature also resulted in an enhanced mineral dissolution rate in post-hydro-shearing stage compared to pre-hydro-shearing stage. X-Ray micro CT image showed new micro-cracks propagated from rough surface during hydro-shearing at 130°C, which could contribute to the fracture permeability during post-hydro-shearing stage of the specimen. Gradual decrease in fracture permeability observed during post-hydro-shearing affects the long-term production of the reservoir. The result of the experiments presented here helps to strengthen the understanding of the fracture response to coupled THMC processes during EGS stimulation through hydro-shearing.

How do thermally induced microcracks alter microcracking mechanisms in Hong Kong granite?

Thermally induced microcracks alter the mechanical properties of rocks. Mode I loading prevails in nature, while rocks are typically weak in tension. Understanding the effects of pre-existing thermal microcracks on the microcracking mechanisms of granite under mode I loading is of both scientific and practical value. We conducted three-point bending tests on pre-notched semi-circular bend Hong Kong granite specimens, in which the acoustic emissions (AEs) were monitored. To introduce the thermal microcracks, the specimens have been thermally treated to different temperature levels (50 °C, 100 °C, 150 °C, 200 °C, and 400 °C) before the loading tests. The microcracking mechanisms are analyzed by deciphering the AE signals recorded from the start of loading until the macrocrack initiation. We find that the presence of the thermal microcracks aggravates the microcrack damage. Our detailed AE analysis reveals that the thermal microcracks do not significantly affect the proportions of three types of AE events (i.e. tensile, shear and mixed-mode) as well as their energy distributions. The tensile events dominate and the proportions of the three types are independent of event magnitude. The temporal evolution features of the event-type ratio are different between the specimens with few thermal microcracks and those with substantial thermal microcracks. For the former, the tensile event ratio rapidly increases to its peak, while the shear and mixed-mode event ratio decreases, which becomes a signature precursor prior to the initiation of a macrocrack. In contrast, this precursor is absent for the latter.

Dynamic properties of granite rock employed as coarse aggregate in high-strength concrete

The dynamic properties of a granite rock employed in crushed form as coarse aggregate in high-strength concrete, were experimentally characterised, and the average values obtained were assumed to be representative of the properties of the coarse aggregate. Cylindrical specimens were core-drilled from the granite rock, then cut into various lengths for different tests, i.e. static and dynamic uniaxial compression, as well as static and dynamic splitting tension (Brazilian tests). From the data obtained, the effects of strain rate on the mechanical properties (e.g. stress-strain relationship, tensile and compressive strengths) were analysed. Significant rate dependence was observed for both tensile and compressive responses, and this was quantified via a dynamic increase factor (DIF, the ratio between the dynamic and static strength). For compression, the DIF follows the CEB-FIP 2010 equation frequently used to describe the rate sensitivity of normal-strength concrete. The tensile response shows a stronger rate dependence than the compressive response, and its DIF deviates from predictions based on the CEB-FIP equation, which underestimates the tensile DIF, and defines a much higher strain rate at which transition from a gradual to a rapid increase in the DIF occurs. An empirical DIF formula, derived from fitting of experimental data, is proposed to describe the tensile rate sensitivity. Apart from experimental characterisation, efforts were also directed at identifying a constitutive model to describe the dynamic behaviour of the granite coarse aggregate, and the Johnson-Holmquist 2 (JH-2) model, commonly used to capture the impact response of brittle materials (e.g. rock, ceramic) was chosen. The dynamic tensile and compressive properties obtained from experiments, were employed to determine the material parameters in the JH-2 model. Finite element modelling of the dynamic compression and splitting tension tests was performed, using the JH-2 model and the material constants derived, to describe the behaviour of the granite specimen. The simulation results correlate well with experimental measurements.

Strain gauge experimental study on mode I rock fracture characteristics under impact loading

Based on the analysis of the strain field of the dynamic crack tip under impact loading, a specific acute (obtuse) angle multi-point strain gauge method is proposed to study the mode I rock crack dynamic fracture. Compared with the dynamic caustics method, the feasibility of the strain gauge method for calculating the crack propagation velocity and dynamic stress intensity factor (SIF) is verified. The rock specimen mode I crack dynamic fracture behaviour under impact loading was studied by the strain gauge method and assisted by high-speed photography and scanning electron microscopy. The results showed that the mode I crack propagation behaviour calculated by two strain gauge methods under impact loading was close to the caustics experimental results. Compared with the dynamic caustics method, the multi-point strain gauge method could roughly record the crack propagation behaviour of rock under impact loading, which provides an experimental basis for studying non-transparent impact fracture. The rock section mainly experienced tensile stress failure under impact loading, manifested as mode I cracking, and the granite specimen fracture damage occurred in white spot micro-crack propagation. Under impact loading, cracks in both polymethyl methacrylate (PMMA) and granite specimens propagated stably in the middle stage of propagation, the average granite specimen crack propagation velocity was 647.9 m/s, and the average SIF value of was 5.37 MN/m3/2, which were higher than the PMMA specimen experimental results.

Crack growth properties in granite specimens consisting non-persistent joint under punch shear through test

Experimental and numerical methods were used to investigate the effect of echelon notchs on the shear behaviour of joint’s bridge area in granite. A punch-through shear test was used to model the granite cracks under shear loading. Granite samples with dimension of 20 mm×150 mm×40 mm were prepared in the laboratory. Within the specimen model and near the edges, four edge notches were provided. Nine different configuration systems were prepared for notches. In these configurations, the length of each notch was taken as 3 cm, 4cm and 5 cm. Assuming a plane strain condition, special rectangular models were prepared with dimensions of 100 mm×100 mm using particle flow code in two dimensions. similar to those for joints configuration systems in the experimental tests i.e. 9 models with different rock bridge lengths and rock bridge joint angle were prepared. The axial load was applied to the punch through the central portion of the model. This testing showed that the failure process was mostly governed by the rock bridge length and rock bridge angle. The shear strengths of the specimens were related to the fracture pattern and failure mechanism of the discontinuities. It was shown that the shear behaviour of discontinuities is related to the number of the induced tensile cracks which are increased by increasing the rock bridge angle. The strength of samples decreases by increasing the joint length. The failure pattern and failure strength are similar in both methods i.e. the experimental testing and the numerical simulation methods.

Study on Nonuniform Evolution Characteristics of Rock Friction and Sliding

By means of experimental research and theoretical analysis, the nonuniform evolution characteristics of rock friction and sliding were studied. Using digital speckle correlation (DIC) as observation method, the whole process of friction and sliding of a granite specimen in double-sided shear experiment is studied. A spring slider model considering the microscopic characteristics of interface asperities was established to simulate the microscopic process of rock friction and sliding. By comparing the theoretical analysis results with the experimental results, the effect of interface nonuniformity on rock friction sliding instability is studied. The results demonstrated that, with the increase of nonuniformity of sliding interface, the degree of local instability before stick-slip decreases, the stick-slip period shortens, and the value of shear drop during stick-slip period decreases. The nonuniformity of sliding interface will increase after local instability.

Effect of high temperature on mode-I fracture toughness of granite subjected to liquid nitrogen cooling

Liquid nitrogen (LN2) ultra-low temperature fracturing technology has broad application prospects in the storage and transformation of geothermal wells. Therefore, it is of great significance to investigate the fracture toughness of granite samples after elevated temperature treatment and LN2 cooling. In this paper, the Cracked Straight Through Brazilian Disc (CSTBD) granite specimen was heat-treated at different temperatures to study the mode-I fracture toughness of granite after rapid cooling by LN2. Meanwhile, acoustic emission (AE) and digital image correlation (DIC) techniques were used to monitor the deformation and failure process of granite. Our results show that when the heating temperature is more than 400 °C, after LN2 cooling, the internal thermal damage of granite is intensified, the plastic characteristics are gradually enhanced, and the fracture toughness decreases significantly. The DIC results show that, with increasing stress, cracks start to form at the two ends of the pre-crack and propagate to the contact point between the press and rock sample along the loading direction under mode-I loading. Near the peak load, the Von-Mises equivalent strain (Evm) gradually increases, and a high strain band appears along the loading direction. When the heating temperature exceeds 400 °C, the density and width of thermally induced cracks in granite gradually increase after LN2 cooling, the fracture track gradually deviates from the straight line, secondary cracks extend, and the plastic characteristics are enhanced. High temperatures cause thermal damage to the rock, and rapid cooling exacerbates this damage. It can be considered that 400 °C is the critical temperature threshold at which the physical–mechanical behavior and fracture characteristics of CSTBD granite change significantly under the condition of rapid LN2 cooling.

Determination of the mixed mode I/II fracture characteristics of heat-treated granite specimens based on the extended finite element method

Previous researches mainly concentrate on investigating the mixed mode I/II fracture characteristics of heat-treated rocks through laboratory tests. This paper makes an effort to obtain the mixed mode fracture characteristics of heat-treated granite by means of the extended finite element method (XFEM), which is based on the cohesive zone model (CZM). The experimentally obtained unstable fracture toughness, tensile strength and tensile elastic modulus of the granite after being subjected to different high temperatures are integrated into the numerical simulation models to estimate the fracture behaviours of the cracked straight-through notch Brazilian disc (CSTBD) specimens under mixed mode loading. Findings and observations from this study indicate that by applying the unstable fracture toughness as the mode independent cohesive zone fracture parameter into the numerical models, although some limitations still exist, the numerical models can successfully predict the fracture loads and the crack propagation paths of the CSTBD specimens under mixed mode loading within the temperature range of 25–600 °C. This paper provides a numerical approach to investigate the mixed mode I/II fracture characteristics of thermally treated rocks.

Cracking mechanism and strength criteria evaluation of granite affected by intermediate principal stresses subjected to unloading stress state

Extensive true-triaxial unloading tests are conducted on rectangular Miluo granite specimens to investigate their crack propagation behavior and peak unloading strength properties. Experimental results demonstrate that their failure mode transforms from shear to slabbing with the increase in intermediate principal stresses σ2. Under a lower value of σ2, the dominant failure mode represented by the macro shear band is composed of several meso-extensional cracks parallel to the maximum principal stress σ1, and the fracturing process indicates intensive dislocation and slide owing to the normal force acting on extensional cracks. However, the increase in σ2 exacerbates the extensional cracks and further propagates them toward the end of the specimen along the direction of σ1, if σ3 is unloaded to 0. This is primarily due to the enhanced ability to resist mutual slide and squeeze among extensional cracks in the shear band, and the promotion of crack initiation and propagation in the unloading direction (σ3) owing to the provided free surface. The transformation of the cracking mode is revealed via three-dimensional combined finite/discrete element modeling, scanning electron microscope observations, and acoustic emission signal feature (average frequency, rise angle, Acoustic emission (AE) counts, and accumulated AE energy) analysis. The peak unloading strength exhibits an initial increase followed by a decrease as a function of σ2. Four rock-strength criteria are employed to evaluate the true-triaxial data with emphasis on predictability in practical applications and accuracy between best-fitting functions (experimental data) and theoretical (predicted) solutions. It is demonstrated that modified the Wiebols–Cook criterion and modified Lade criterion cannot accurately estimate the unloading strength throughout the true-triaxial data. Although the Mogi 1971 criterion fits the experimental data (higher correlation coefficients) well, it cannot be correlated to strength parameters (cohesion and internal frictional angle). Hence, it fails to predict the real strength values if complex true-triaxial unloading tests are not performed. The linear Mogi criterion is recommended for predicting the true-triaxial unloading strength of granite.

Study on the Tensile Strength and Crack Development of Granite Crystal Models with Different Fabrics

According to the relationship between the development of hydraulic fracture and the distribution of different rock and mineral fabrics, digital image technology was used in this study based on the discrete element particle flow method. An SCGM numerical model, which can control fabric and refine mesoscopic crystal defects, was constructed. The tensile strength test of granite specimens with different component contents and polymerization conditions was conducted, with the monitorization of damaged cracks. The test results showed that (1) the SCGM model could construct a model of a crystal structure containing variable components and polymerization, to achieve numerical experiments of different parameters; (2) compared with previous models, the SCGM model can reflect internal stress change of a specimen with the variation of the structure. At the same time, the contained crystal structure can characterize different failure modes such as intergranular cracks of different components, grain boundary cracks of the same component, and transgranular cracks, which were in good agreement with the actual failure state of granite; (3) the mica content of the specimen and its strength presented a linear relationship of y = − 14.723 x + 12.642 . The degree of polymerization of the components and the strength of the specimens were both affected by the accumulation of microcracks and stress concentration, which appeared to increase first and then decrease. When the specimen was broken, the number of microcracks and the degree of polymerization of the components also increased first and then decreased. The SCGM model can construct a numerical model of granite with arbitrary fabric and crystal structure and provide a hydraulic fracturing analysis method.

Effect of microwave heating on fracture behavior of granite: An experimental investigation

Hard rock breaking is a great challenge for deep drilling and mining engineering, and the microwave-assisted rock breaking technique offers a promising solution. However, the lack of knowledge of the effects of microwave irradiation on hard rocks limits the further development of this technology. Laboratory tests have been conducted to investigate the variations of fracture properties of granite subjected to microwave heating. The semi-circular bend (SCB) specimens were prepared, and mode I fracture tests were conducted on the microwave treated granite specimens. The acoustic emission (AE) events was recorded and the strain field was calculated by the digital image correlation (DIC) technique. The test results demonstrated that the temperature of the sample surface increased linearly as the increasing heating time, and increasing microwave power contributes to enhancing the heating rate. According to the results of scanning electron microscope (SEM), intergranular cracks were the main form of cracks, and the transgranular cracks usually appeared in samples with high microwave power or longer heating time. The generation of micro-cracks can be attributed to the high temperature and the thermal shock. With the increasing heating time, the fracture toughness decreased exponentially, the AE active period prolonged, and the fractal dimension of the fracture surfaces (FSD) continuous increased. Compared to an untreated sample, the maximum principal strain and fracture process zone (FPZ) size of the microwave heated sample were larger at the same loading level, which indicated the enhancement of ductility. According to the experimental results, the relationships between FPZ size, fracture toughness and FSD were empirically regressed. Understanding the effects of microwave heating on the fracture properties is of significant to improve hard rock breaking efficiency.

The influence of microwave treatment on the mode I fracture toughness of granite

Microwave assisted mechanical rock breaking is a hybrid rock breaking method that combines microwave heating and mechanical rock breaking technologies and is proven to have potential applications in mineral processing, mining, and geotechnical engineering. It is, thus, important to understand the effect of microwave-induced damage on rock fracture characteristics. In this study, granite specimens of the Cracked Straight Through Brazilian Disc (CSTBD) were used to carry out the mode I fracture test of the samples irradiated by microwave for different time and thermal cycles. The results showed that the heating rate of the surface temperature of granite is about 1 °C/s, under microwave power of 0.9 kW. When the heating time exceeds the 60 s and after 3 cycles, the fracture toughness of the granite hardly changes with the heating time and cycles. Before 100 °C, the fracture toughness and fracture energy of granite decrease by about 40% with the increase of temperature. Under microwave irradiation, the medium absorber (biotite) in granite is heated and expanded, which results in thermal damage to the rock. Moreover, the polar materials in fluid inclusions move more violently than under conventional heat conduction heating, resulting in a lower decrepitation temperature of the inclusions than under conventional heating and decrepitation degree is more severe, which further leads to an increase in the density of microcracks in mineral crystal and a significant decrease in fracture toughness. When the surface temperature is 300 °C, the bending degree of the crack increases and deviates from the straight line obviously.

Experimental study of rock fragmentation under different stemming conditions in model blasting

Nine blasts with cylindrical granite specimens were carried out under different stemming conditions. All rock specimens had a diameter of 240 mm and a length of 300 mm. Explosive Pentaerythritol tetranitrate (PETN) with specific charges 0.2 and 0.3 kg/m3 were used, and the blasts were carried out in a large explosion chamber. Each rock specimen was enclosed by a steel box in the chamber during blasting, without constraint or confinement to the specimen from lateral and top sides. During blasting a high-speed camera was used to film the blasting process and monitor the gas ejection. After each blast all fragments were collected, sieved and analyzed. The experimental results showed that: (1) the blasts with full sand stemming yielded better (finer) fragmentation than the blasts with partial steel stemming at a constant specific charge, and (2) gas ejection from the collars of blastholes occurred much earlier in the blasts with partial steel stemming than in the blasts with full sand stemming. Based on the experimental results, it can be concluded that it is extremely important to prevent gas ejection from the collars in order to achieve desirable rock fragmentation.

Effect of thermal shock on laboratory hydraulic fracturing in Laizhou granite: An experimental study

This paper aims to investigate the effect of thermal shock on hydraulic fracturing in hot dry rock (HDR). To simulate the thermal shock effect caused during the injection of cold water, several natural granite specimens were slowly heated to various target temperatures and then rapidly cooled down by flowing water. Laboratory hydraulic fracturing experiments were performed on granite specimens under triaxial stress condition and room temperature. After that, computed tomography (CT) scanning technique, scanning electron microscope (SEM) observation, and surface profile scanning (SPS) tests were combined to analyze the influence of thermal shock on fracture morphology from micro-scale to macro-scale. Moreover, the characteristics of injection pressure curve were discussed. Experimental results show that a single main hydraulic fracture with large aperture is created and approximately propagates along the direction perpendicular to the minimum principal stress before 300 °C. When the thermal treatment temperature is higher than 300 °C, multiple hydraulic fractures with branches tend to be created due to the existence of thermally-induced damage zone near the well hole. Experimental results of SPS tests indicate that the maximum height difference of the hydraulic fracture surface increases with increasing thermal treatment temperature. The tortuosity of hydraulic fracture significantly increases when the thermal treatment temperature is higher than 300 °C. Additionally, the analysis of injection pressure response shows that apparent non-linear pressurization, and decrease in pressurization rate and breakdown pressure are observed after 300 °C.

High-fidelity and high-resolution phase mapping of granites via confocal Raman imaging

In physical sciences such as chemistry and earth sciences, specifically for characterization of minerals in a rock, automated, objective mapping methods based on elemental analysis have replaced traditional optical petrography. However, mineral phase maps obtained from these newer approaches rely on conversion of elemental compositions to mineralogical compositions and thus cannot distinguish mineral polymorphs. Secondly, these techniques often require laborious sample preparations such as sectioning, polishing, and coating which are time-consuming. Here, we develop a new Raman imaging protocol that is capable of mapping unpolished samples with an auto-focusing Z-mapping feature that allows direct fingerprinting of different polymorphs. Specifically, we report a new methodology for generating high fidelity phase maps by exploiting characteristic peak intensity ratios which can be extended to any multi-phase, heterogenous system. Collectively, these enhancements allow us to rapidly map an unpolished granite specimen (~ 2 × 2 mm) with an exceptionally high accuracy (> 97%) and an extremely fine spatial resolution (< 0.3–2 µm).