Cracked Straight Through Brazilian Disc Research Articles

Effect of high-temperature thermal fatigue on mode I fracture toughness of gabbro

Investigating variations of rock fracture toughness after high-temperature thermal fatigue is greatly significant for developing deep geothermal resources. This study investigated the mode I fracture toughness of gabbro after high-temperature fatigue treatment by heating a cracked straight-through Brazilian disc (CSTBD) gabbro specimen to 200, 400, 500, and 600 °C, followed by 100 thermal cycles. Additionally, acoustic emission (AE) and digital image correlation (DIC) were used to monitor the failure process of gabbro. The results showed that the peak load, fracture toughness, and fracture energy of gabbro decreased with the increased temperature and number of cycles, which was most significant at 500 and 600 °C. Under mode I loading, cracks developed along the loading direction and connected with the prefabricated crack. Additionally, the regions with higher von Mises equivalent strain (Evm) values were concentrated at the loading points and prefabricated crack of the rock sample. The degree of bending of the primary cracks increased, the number of secondary cracks increased, and the Evm values changed significantly as the temperature increased. The phase transformation of plagioclase feldspar at 450 °C and the oxidation of iron- and magnesium-containing pyroxene at 600 °C changed the internal structure of the rock. Cyclic treatment further exacerbated the thermal damage, developing microcracks within the rock and reducing fracture toughness significantly.

Investigation of fracture characteristics of cracked granite suffered from different thermal treatments and water-cooling time

Rock mass may be exposed to high temperature and cooled by water in fire, geothermal resource exploitation and deep underground engineering, which affects the safety of these engineering. In this study, cracked straight through Brazilian disc (CSTBD) specimen was used to study the fracture properties of granite treated with different water-cooling time from 25 °C to 800 °C. A camera was used to record the fracture process of the specimen, the evolution of strain at the specimen surface was obtained by using digital image correlation technology, and the morphological characteristics of fracture surface were observed by using scanning electron microscopy. The results show that water-cooling time has some effects on fracture properties of granite, but its effect is less than the temperature. With the increase of water-cooling time, the mode I fracture toughness decreases first, then increases and finally decreases at the same temperature, and the fracture toughness ranges from 0.419 MPa m1/2 to 0.567 MPa m1/2 at 400 °C. The crack tip opening displacement (CTOD), crack propagation path and fracture surface are related to the energy released during the specimen failure. When the temperature is 25 °C or 200 °C, the energy released is large and the fracture surface is relatively flat with irregular sharp edges, the main cracks are straight lines. At 600 °C, the main crack is zigzag, the CTOD and the maximum principal strain in order can reach 0.143 mm and 0.731 %. This study can help to understand the effect of water-cooling time on engineering rock mass.

Influence of microwave irradiation and water-based cooling on the fracturing behavior and failure mode transition of CSTBD granite

Utilizing microwave heating in conjunction with water-based cooling technology can effectively reduce the fracture toughness of hard rocks. However, the fracturing characteristics and failure modes of hard rocks following microwave heating and water-based cooling remain undisclosed. Granite, being one of the most common types of hard rocks, was selected for this study, employing Cracked Straight Through Brazilian Disc (CSTBD) specimens for laboratory testing. CSTBD specimens with various crack angles (0, 30, 60, and 90°) underwent treatment with varying microwave durations (30, 60, 180, 300, and 420 s). Subsequently, Brazilian splitting tests were conducted on the damaged specimens. To simulate the entire process of microwave heating and mechanical testing of CSTBD specimens, we developed a coupled Finite Element Method – Distinct Lattice Spring Model (FEM-DLSM) approach. The results indicate that microwave heating with water-based cooling significantly influences the fracture toughness of CSTBD granite. Mode-I fracture toughness decreases with microwave heating temperature at a low crack angle (pure tensile failure and tensile–shear mixed failure) and increases at a high crack angle (compressive–shear failure and compressive failure). Mode II fracture toughness decreases with microwave heating temperature at any crack angle. Microwave heating with water-based cooling causes a decrease in the transition angles from tensile–shear failure to compressive–shear failure and from compressive–shear failure to compressive failure. Thermally induced microcracks in the rock specimen are responsible for changes in fracture toughness and failure mode transitions. After microwave heating and water-based cooling, numerous microcracks are induced in the rock interior. While macroscopic cracks predominantly control failure, growing microcracks, by weakening the rock matrix, shift the initiation position of the failure crack from the tip of the macroscopic crack to other positions, following the principle of the weakest failure path.

Mechanical behavior and fracture mechanism of red-bed mudstone under varied dry-wet cycling and prefabricated fracture planes with different loading angles

Red-bed mudstone is a common geological material in engineering construction in the Southwest region in China. Its initial fractures are easily influenced by the combined effects of dry-wet cycling and external loads. Therefore, evaluating the mechanical behavior and fracture mechanism of red-bed mudstone under dry-wet cycling and prefabricated fracture planes with different loading angles is of great significance. In this study, the cracked straight through Brazilian disc (CSTBD) specimens with a prefabricated crack were tested using an RMT testing machine. Multiple-scale analysis methods, including digital image correlation (DIC) and acoustic emission (AE) techniques, 3D scanning, and scanning electron microscopy (SEM), were employed to comprehensively analyze the mechanical evolution and fracture damage mechanism of mudstone under dry-wet cycling. The experimental results indicate that as the loading angle increases, the peak load gradually increases, while an increase in the number of dry-wet cycles leads to a decrease in peak load. At the same time, dry-wet cycling causes the expansion and connection of initial microcracks, forming weak layers that affect the morphology of the main crack and the distribution of secondary cracks. Furthermore, with an increasing number of dry-wet cycles, the acoustic emission count significantly decreases, and the relative high value range of acoustic emission count expands. 3D scanning results show that an increase in the number of dry-wet cycles results in rougher fracture planes, while an increase in loading angle generally increases the roughness of the fracture planes. Additionally, SEM observations reveal that with an increasing number of dry-wet cycles, intragranular fracture patterns and microcrack distribution become more widespread, and tensile and shear fracture patterns are also observed in the center of the fracture plane. These research findings have important practical value for evaluating the stability and reliability of red-bed mudstone under dry-wet cycling and prefabricated fracture planes with different loading angles.

Fracture behavior of yellow sandstone under freeze–thaw cycles with varied saturation states: An investigation of mode I fracture

In cold regions, the behavior of fissured rock in rock engineering is significantly affected by freeze–thaw (F–T) cycles. To investigate the impact of freeze–thaw cycles on rock fracture behavior, freeze–thaw cycle treatment, and fracture tests were conducted on Cracked Straight Through Brazilian Disc (CSTBD) specimens. The freeze–thaw treatment notably degraded the physical and mechanical properties of the rock, marked by a downward trend in fracture load, toughness, and energy. A considerable volume of data suggests that the impact of freeze–thaw (F–T) on yellow sandstone was predominantly during the initial stages of cycling, with this trend stabilizing as the number of cycles increased. Microscopic analysis via Scanning Electron Microscopy (SEM) uncovered the patterns of freeze–thaw damage in yellow sandstone. Dry rocks exhibited crack propagation, while water-saturated rocks displayed pore and crack expansion. The Digital Image Correlation (DIC) method illuminated the evolution of the high strain zone during the fracturing process. The mode I fracture process revealed a gradual expansion of the high strain zone originating from the crack tip, intensifying with increased load. As freeze–thaw cycles progressed, damage accumulation was gradual, and the path of mode I crack propagation grew increasingly intricate from a macroscopic perspective. Additionally, the quantification of fractal dimensions, achieved through the analysis of binarized SEM images, corroborates these observations at a microscopic level.

Effects of thermal treatment on the fracture behavior of rock-concrete bi-material specimens containing an interface crack

The study of the fracture characteristics of rock-concrete structure containing an interface crack under high temperature is vitally significant for the durability and stability assessment of concrete construction projects on rock foundations. Brazilian splitting tests were performed on rock-concrete cracked straight-through Brazilian disc (CSTBD) specimens after different heat treatment temperatures. The crack extension process of the specimen was analyzed using digital image correlation (DIC) technique. The effects of temperature, interface crack inclination and concrete strength on fracture characteristics were investigated. The results indicate that at high temperatures, free water and structural water escape from rock and concrete, and crystal structure of the mineral composition is altered, which induces the development of internal pores and microcracks in the material and weakens the mechanical properties. The specimen displays brittle fracture behavior at 25 °C-300 °C, and plastic fracture behavior at 500 °C-700 °C. The wing crack extension in concrete (weak material) occurs preferentially, and this phenomenon becomes more apparent with raising temperature. When α ≤ 45°, the length of crack extension along the interface increases with increasing temperature, while when α > 45°, the increase in temperature makes the crack initiation closer to the disc center. The fracture energy and the difference in fracture energy between varying inclinations decrease with raising temperature. Furthermore, temperature has an essential impact on fracture toughness. As temperature increases, the fracture toughness generally decreases, the difference in mixed fracture toughness between different inclination angles gradually increases, and the fracture propagation resistance weakens.

Design of rock material parameters by cracked straight through Brazilian disc

Rock is one of the main construction materials for engineering structures. Its tensile strength (ft) and fracture toughness (KIC) are the key characteristics significantly influencing the safety of engineering structures, and determining these parameters is critical for designing underground engineering structures, such as mining, tunnels, and water conservancy. Therefore, this study focuses on developing a theoretical model based on the cracked straight through Brazilian disc (CSTBD) test, which allows for the simultaneous determination of ft and KIC for rocks. The proposed model provides a comprehensive understanding of the material properties by deriving expressions for the nominal stress (σn) and equivalent crack length (ae) using the CSTBD approach. Further, the research employed the normal distribution method to assess the variability of individualized fracture toughness and tensile strength parameters. By applying this model, statistical values of fracture toughness and tensile strength can be obtained, facilitating comparative analysis of test results for different rock types and validating the efficacy of the proposed model. Furthermore, the relationship between material parameters (ft &KIC) and peak load (Pmax) was established through CSTBD, enabling reliable prediction of experimental peak load with 95% reliability.

Mixed mode I/II fracture behavior and surface morphology of hard rock under dynamic loading

To investigate the dynamic mixed mode I/II fracture properties of hard rock, split-Hopkinson pressure bar (SHPB) was utilized to conduct dynamic fracture tests on cracked straight through Brazilian disc (CSTBD) diorite specimens with different mode mixities. The dynamic fracture load and fracture toughness for different crack angles or mode mixities were determined. The dynamic fracture process and crack mouth opening displacement (CMOD) of CSTBD specimens were monitored by high-speed camera and digital image correlation (DIC) technique, and the fractal and morphology characteristics of fracture surfaces were further analyzed using laser 3D scanner and scanning electron microscope (SEM). The test results indicate that the dynamic CMOD behavior is interrelated closely to the force acting on the crack surface, and the allowable CMOD corresponding to the dynamic peak load decreases with increasing crack angle. In addition, the stress-based and strain-based fracture criteria were introduced to analyze the mixed mode fracture mechanism of diorite. It is found that the GMTS criterion with small fracture process zone (FPZ) size and the EMTSN criterion have the best prediction effect for fracture toughness and crack initiation angle respectively. Observation results show that the macro fracture trajectories of primary cracks and secondary cracks are controlled by the prefabricated crack angle, while the variation in mode mixity has little impact on the meso-roughness and micro-morphology of the fracture surface. The fracture surface of CSTBD diorite specimen under dynamic load exhibits the SEM morphology features of mixed tensile-shear cracks.

Evaluation of equivalent crack propagation length and fracture energy of two commonly used rock fracture toughness test configurations based on Bažant’s size effect law

Purpose of this research is to apply the Bažant’s size effect law to investigate the effects of material properties, specimen size, normalized crack length and loading span on the equivalent crack propagation (ECP) length and fracture energy test results of cracked straight through Brazilian disc (CSTBD) specimens and notch semi-circular bend (NSCB) specimens, and to clarify the relationship between ECP length and fracture process zone (FPZ) length. The results indicate that the ECP length is positively correlated with the specimen radius, cf (the ECP length of the infinite-size specimen) and the loading span, and the influence of the normalized crack length on the ECP length has the dependence of specimen configuration. Both the apparent fracture energy and the equivalent fracture energy have size effect, and the fracture energy tested by the specimen with smaller cf are more reliable. Based on the analysis of the influence of normalized crack length and loading span on fracture energy test results, CSTBD specimen with normalized crack length of 0.4 is recommended for rock fracture toughness test. In addition, the FPZ length determined by digital image correlation technique is more accurate than that determined by stress intensity factor method and finite element model method, and the ratio of ECP length to FPZ length decreases with the increase of specimen size.

Experimental study on acoustic emission and surface morphology characteristics of concrete under different fracture modes

In this work, experiments were employed to study on fracture behaviors of concrete under different fracture modes using cracked straight-through Brazilian disc (CSTBD) specimens, focusing on revealing the characteristics of acoustic emission (AE) and surface morphology. The results show the failure modes of CSTBD specimens under different fracture modes contain tensile coalescence, mixed coalescence, as well as shear coalescence. Based on the AE count, peak frequency and stress, the loading process can be dived into four stages (i.e., microcrack closure, linear elastic deformation, plastic deformation, post-peak failure). The b value can be considered as a precursor index of concrete structure instability. A novel method is proposed to distinguish the diving line of RA-AF distribution based on the splitting tensile test and direct shear test, and the evolution trends of the microcrack propagation modes with different curing regimes are consistent. Meanwhile, as the fracture mode moves from pure mode I towards mixed-mode I-II and pure mode II, the fracture surfaces become uneven and rough, the fractal dimension presents an increasing tendency, the standard deviation of asperity height and slope angle exhibits an increasing tendency, and the distribution of the asperity direction frequency becomes disorganized and fluctuates widely. In addition, according to the proposed 2D micro-element model, tangential deformation plays a crucial role in the fracture process, and a larger tangential deformation leads to a rougher fracture surface.

Static and dynamic fracture behavior of rock-concrete bi-material disc with different interface crack inclinations

The fracture behavior of rock-concrete bi-material structures with interface crack is of great significance to the stability of construction projects in rock engineering. Dynamic testing of cracked straight-through Brazilian disc (CSTBD) samples was performed using a split-Hopkinson pressure bar (SHPB) system. The fracture development of rock-concrete bi-material was monitored by the digital image correlation (DIC) technique. A static fracture test using the MTS322 hydraulic servo-controlled testing system was also carried out for comparison. The results show that three typical fracture patterns can be identified for CSTBD samples under both static and dynamic loading. At a smaller interface inclination angle, the disk samples fracture along the interface, while at a larger interface inclination angle, the samples suffer tensile fracture along the loading direction. The increase of strain rate leads to an increase in the number of secondary cracks. As the inclination of interface increases from 0° to 75°, the dissipated energy increases gradually. When the inclination exceeds 75°, the dissipated energy turns to weaken. The difference in peak dissipated energy between different inclination angles weakens as the strain rate increases. The dimensionless stress intensity factors (SIFs) at the two crack tips are different, and increasing the crack length-diameter ratio will enlarge the difference. The interface inclination angle has a significant effect on both the static and dynamic fracture toughness of CSTBD samples. As the inclination of the interface increases, the difference between mixed-mode static and dynamic fracture toughness also increases. The dependence of dynamic fracture toughness on inclination is significantly higher than that of static fracture toughness, and the ratio between dynamic and static fracture toughnesses increases with increasing loading rate.

Mixed mode fracture parameters and fracture characteristics of diorite using cracked straight through Brazilian disc specimen

In this study, the cracked straight through Brazilian disc (CSTBD) specimen was applied to study the I/II mixed mode static fracture of diorite. The fracture load and fracture toughness for different prefabricated crack angles β were determined. The fracture and morphological characteristics were analyzed using digital image correlation (DIC), scanning electron microscope (SEM) and 3D topography scanning techniques. In addition, the key fracture parameters such as fracture energy, crack mouth opening displacement (CMOD) and fracture process zone (FPZ) were characterized, and a fracture energy calculation method considering the primary crack growth was proposed. The results show that the fracture mechanical behavior and fracture trajectories are controlled by the crack angle. At the millimeter measurement scale, the fractal dimensions of fracture surfaces do not vary with crack angle. Further SEM observation shows that intergranular fracture and transgranular fracture are the dominate micro fracture modes of fracture surfaces under mode I and mode II loading, respectively. With the increase of crack angle, the fracture energy first increases and then decreases, while the allowable CMOD decreases monotonically and even appears negative when β ≥ 45°. Based on the displacement gradient method, the calculated width of FPZ for the mode I fracture specimen at peak load is 0.5–1.4 mm and the length is 2.66 mm. Finally, the stress-based and strain-based fracture criteria were modified. It is found that the proposed EEMTSN criterion has the best prediction effect on the mixed mode fracture toughness of diorite.

Investigation on Energy Evolution and Storage Characteristic of CSTBD Red Sandstone during Mixed-Mode Fracture

To investigate the energy evolution and storage characteristics of the rock during the mixed-mode fracture failure, several single loading-unloading tests under preset different unloading levels were conducted on the cracked straight-through Brazilian disc (CSTBD) red-sandstone specimen with crack inclination angles of 10° and 20°. The input energy, elastic energy, and dissipated energy parameters were obtained by calculating the area integral of the load-displacement curve. Test results display that the total input energy, elastic energy, and dissipated energy of rock specimens increase in quadratic function with the increase of unloading level at different unloading levels. It was found that there are significant linear energy storage and dissipation laws of rock during the mixed-mode fracture process. The concepts of fracture energy storage coefficient (FESC) and fracture energy dissipation coefficient (FEDC) were proposed to express the energy variation of rock in the prepeak stage. Under the same loading conditions, FESC and FEDC remain unchanged regardless of crack inclination angles, indicating that the linear energy storage and dissipation laws are an inherent property of the rock. Besides, the peak fracture elastic-dissipation index W ed (the ratio of peak elastic energy to peak dissipation energy) was obtained through quantitative analysis and demonstrated its invariant feature.

Dynamic fracture behavior and fracture toughness analysis of rock-concrete bi-material with interface crack at different impact angles

The crack initiation and propagation behavior of interface crack under impact are of great significance for evaluating the stability of rock-concrete system and other layered structures. Dynamic test using split-Hopkinson pressure bar (SHPB) system was performed on bi-material cracked straight-through Brazilian disc (CSTBD) specimens, and the fracture processes of specimens were monitored by means of a high-speed camera. The effects of the impact angle, concrete strength and strain rate on the dynamic fracture behavior were investigated. The results show that three typical fracture types can be identified: interface fracture, combined fracture (interface/shear and tension fracture) and tensile fracture. The interface fracture appears under small impact angle, while the combined fracture occurs with the further increase of impact angle. The crack initiation time of wing cracks and the number of secondary cracks are mainly affected by the strength of concrete in the bi-material specimen. Crack initiates earlier and more secondary cracks are generated in concrete material (weak material), and the number of secondary cracks increases with increasing strain rate. The dissipated energy increases gradually as the impact angle increases from 0° to 75°, while it decreases when the impact angle exceeds 75°. As the concrete strength increases, the difference in dissipated energy for different impact angles decreases. Furthermore, the effects of impact angle, concrete strength and loading rate on the dynamic fracture toughness for different loading modes are also discussed. The results indicate that the normalized stress intensity factors (SIFs) at the two tips of the interface crack are distinct and the difference increases with increasing crack length-to-diameter ratio. The impact angle, concrete strength and loading rate influence the dynamic fracture toughness significantly.

Combined effects of microwave irradiation and water cooling on the deformation and failure behaviours of CSTBD granite

The low cutting rate and high cutting tool wear rate in hard rock tunnel excavation by a tunnel boring machine (TBM) are critical problems. By integrating microwave technology with water cooling into the TBM cutterhead, microwave-assisted mechanical rock breaking provides a promising solution. To reveal the deformation and failure behaviours of hard rocks after microwave heating and water cooling, this work experimentally conducted a series of tests on cracked straight-through Brazilian disc (CSTBD) granite specimens, where the crack angle was set to 0, 30, 60, and 90° and the microwave heating duration was set to 0.5, 1, 3, 5, and 7 min. During the experiment, digital image correlation (DIC) technology was employed to measure the deformation and failure process. The results show that microwave heating and water-based cooling facilitate the failure of CSTBD granite. The mode-Ⅰ fracture toughness linearly decreases with heating duration for pure tensile failure (crack angle: θ=0°), while it linearly increases with heating duration at other crack angles (30°, 60°, and 90°). The mode-Ⅱ fracture toughness linearly decreases with heating duration in the crack angle range of 0° to 90°. The mode-Ⅰ fracture toughness linearly decreases with the crack angle. The mode-Ⅱ fracture toughness increases with a crack angle of <24° and decreases in the crack angle range of 24° to 90°. The failure patterns are divided into five categories by different crack angles: pure tensile failure (θ = 0°), tensile-shear mixed failure (0° <θ < 24°), shear failure (θ = 24°), compressive-shear mixed failure (24° <θ < 90°), and compressive failure (θ = 90°). The reasons for the main and second cracks during the failure process are revealed. The experimental results have significant implications for microwave-assisted mechanical rock breaking.

Thermal effects on fracture toughness of cracked straight-through Brazilian disk green sandstone and granite

Cracked straight-through Brazilian disc (CSTBD) samples prepared using two rock materials were used for thermal treatment from room temperature to 700 °C. Uniaxial splitting experiments were performed using an automatic electro-hydraulic servo press to study the evolution laws of physical and fracture properties of different deep rock materials under high-temperature geological conditions. The fracture characteristics were measured using an industrial camera and digital image correlation technology to analyze the effect of high temperature on fracture properties and failure modes of the CSTBD samples after different thermal treatments. The micro-damage properties of green sandstone and granite materials were obtained using a scanning electron microscope (SEM). The following conclusions were drawn from the test results: (1) With the increasing temperature, the fracture characteristics of green sandstone and granite change from brittle fracture to plasticity fracture, the longitudinal wave velocity of granite decreases sharply at 600 °C, and the damage factor reaches 0.8748 at 700 °C. (2) The fracture toughness of green sandstone and granite decreases with increasing temperature; however, the decreasing range of granite is larger than that of green sandstone. (3) As the temperature increases, the fracture morphologies of green sandstone and granite materials become rougher, whereas thermal damage cracks of granite and intergranular fractures inside sandstone as well as pores of sandstone increase. (4) The crack tip opening displacement and peak strain corresponding to peak load increase with the temperature. • The effect of thermal treatment on the fracture characteristics of granite and green sandstone materials was studied. • The evolution law of fracture toughness of rock with temperature was studied. • The fracture morphology of granite and green sandstone after thermal treatment was analyzed using SEM. • The CTOD and strain field curves of CSTBD specimens after thermal treatment were calculated using DIC technology.

Research on the effect of loading rate on the fracture characteristics of granite using cracked straight through Brazilian disc specimens

The loading rate effect is an important mechanical property affecting the failure mechanism of rock materials. In this paper, the effect of loading rate on the mode I fracture characteristics of granite was studied by using Cracked Straight Through Brazilian Disc (CSTBD) specimen. The acoustic emission (AE) and Digital Image Correlation (DIC) techniques were used to monitor the fracture behavior of the specimen. Due to the nonlinearity of specimen deformation, the tested fracture toughness was corrected by equivalent energy method. The test results show that the equivalent fracture toughness has obvious rate dependency effect. Meanwhile, the crack tip opening displacement increases with the increase of the loading rate at low loading rate, and decreases with the increase of the loading rate when the loading rate reaches a certain value. The AE results show that the relationship between the cumulative energy of AE signals and the loading time can be well fitted by the power-law function, and the power exponent gradually decreases with the increase of loading rate. Moreover, b value decreases with the increase of loading rate, and there is a negative linear correlation between b value and fracture toughness. These indicate that with the increase of loading rate, the cracking process of the specimen gradually changes from that dominated by small-scale failure to that dominated by large-scale failure, and more fracture energy is needed for instability failure of specimens. In addition, the relationship between b value and fracture toughness can provide the possibility for estimating the equivalent fracture toughness of the material by the b value obtained from the AE test.

Research on dynamic cracking properties of cracked rock mass under the effect of thermal treatment

Deep fractured rock mass is more prone to failure with the coupling effect of suddenly dynamic disturbances loads (explosions, seismic waves, etc.) and high temperature environment conditions, these factors have a great effect on the dynamic stability of fractured rock mass. In the present study, some traditional cracked straight through Brazilian disc (CSTBD) specimens made of granite and sandstone were heated from 25 °C to 700 °C to obtain dynamic fracture properties of high temperature soft and hard rocks. A modified traditional split Hopkinson pressure bar (SHPB) was applied for dynamic fracture experiments under impact loads, high speed camera, digital image correlation (DIC) and fracture surface scanning technology were used to study the fracture morphology of the specimens, the dynamic damage mechanism of high temperature soft and hard rocks under impact loads was obtained. The test results indicate that high temperature can greatly affect the roughness of the fracture surfaces under impact loads during crack formation and propagation, and the fractal dimension increases with the increase of temperature. Observing the dynamic fracture strength and fracture toughness of the CSTBD specimens, it can be found that the strength of the granite decreases gradually with high temperature treatment, and the dynamic fracture strength of the green sandstone increases firstly and then gradually decreases. At 500 °C–600 °C, the dynamic fracture strength of the two types of rocks decreases rapidly. Dynamic critical crack tip opening displacement of the specimens with different thermal temperatures increases with temperature, but the fracture process was similar and the cracks would follow the loading direction.

Prediction of Mode I Fracture Toughness of Shale Specimens by Different Fracture Theories Considering Size Effect

In this study, mode I fracture tests on cracked straight-through Brazilian disc (CSTBD) and notched semi-circular bend (NSCB) shale specimens with different sizes were conducted to investigate the difference between maximum tangential stress fracture criterion and the size effect law (SEL) model in predicting apparent fracture toughness (Ka) of shale. In addition, the effects of specimen size and geometry on the Ka and the selection of fracture criterion on the prediction of the inherent fracture toughness (KIc) were also studied. The results show that the Ka increases with the increase of specimen size, and the difference between KIc of shale specimens with different sizes predicted by the fracture process zone length determined by the further improved maximum tangential stress (FIMTS) criterion is the smallest. For the prediction of Ka of NSCB specimen, the results predicted by the FIMTS criterion are the closest to the tested fracture toughness. However, the effect of SEL model applied to the prediction of Ka of NSCB specimens is poor. The effective establishment of SEL model requires high accuracy for test data, especially for the configuration with large variation of the dimensionless stress intensity factor (Y*) with normalized crack length (α).

Fracture toughness weakening effect of high static pre-loaded CSTBD granite subjected to low frequency dynamic disturbance during the rock fracture

Highly static pre-loaded rock with cracks will be generated during the deep rock engineering excavation, is subjected to low-frequency dynamic disturbance (Fd) to induce fracture failure. To investigate the fracture mechanisms of the high static pre-loaded rock with cracks, subjected to low-frequency dynamic disturbances, several tests were conducted on cracked straight through Brazilian disc (CSTBD) granite specimens using the MTS system. Based on the average static peak fracture load (Fmax) of the CSTBD specimens, three high static pre-load levels, Fp (70%, 80%, and 90% of Fmax), and the corresponding disturbance amplitude, Fa (30%, 20%, and 10% of Fmax), and low-frequency disturbance frequency, f (1, 5, 10, 30, and 50 Hz), were determined. The results revealed that the fracture toughness of the static pre-loaded CSTBD specimens subjected to low-frequency dynamic disturbance was lower than that under static load. The higher the Fp, the more significant is the decrease in fracture toughness with f. Comprehensive analysis indicates that the degree of fracture toughness weakening is highly dependent on the Fp, Fd induces strength weakening, and f affects the weakening degree.