Search

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. [ABSTRACT FROM AUTHOR]

Rockburst is a kind of rock failure phenomenon during which the internal elastic strain energy of surrounding rock mass is released dynamically under external load, and the loading rate is an essential influencing factor of potential for bursting. To investigate the effects of loading rate on rockburst proneness from energy storage and surplus perspectives, conventional uniaxial compression tests are conducted on granite under four orders of magnitude loading rate. The failure process and mode of granite specimens were recorded in real time with a high-speed camera with microsecond shooting speed. The variation trend of the internal elastic strain energy of granite specimens under four loading rates was obtained by performing the single-cycle loading–unloading uniaxial compression test. The experimental results show that the elastic strain energy linearly increases as the input strain energy increases under each loading rate, which meet the linear energy storage law. Based on the linear energy storage law, the peak elastic strain energy of each granite specimen can be accurately obtained. According to the mass and range of ejected rock debris after specimen failure, the bursting liability of each specimen was evaluated by the far-field ejection mass ratio (MF) from a qualitative point of view. Meanwhile, the residual elastic energy index (AEF) and the other three criteria were used to evaluate the potential for bursting of granite specimens under different loading rates. The comparison results show the rockburst proneness of granite specimens increases with the loading rate and that the evaluation results of MF and AEF are unified from qualitative and quantitative aspects, respectively. The fundamental reason for the consistent results is that these two indexes have a common essence of elastic strain energy release. Highlights: The mechanical properties of granite are significantly influenced by loading rate. The loading rate does not affect the existence of linear energy storage law and the compression energy storage coefficient is independent of loading rate. The rockburst liability of granite increases with the loading rate. At different loading rates, the far-field ejection mass ratio and residual elastic energy index offer consistent qualitative and quantitative evaluations for the rockburst proneness of granite. [ABSTRACT FROM AUTHOR]

To investigate the internal mechanism of drilling pressure relief (DPR) to reduce the elastic strain energy of deep rock, SG4500 equipment that can realize real-time drilling under high stress was independently developed. Real-time drilled (RD) uniaxial compression experiments were conducted on red sandstone specimens under different stress levels, and then the influence of real-time drilling on the weakening of rockburst proneness was studied. For comparison purposes, uniaxial compression tests were also performed on non-drilled (ND) and prefabricated drilled (PD) rock specimens. The results demonstrate that both prefabricated drilling and real-time drilling under high stress can effectively weaken the mechanical behavior of rock and reduce the storage capacity of the elastic strain energy and rockburst proneness of rock. Compared with ND specimen, the energy storage coefficients of PD and RD specimens were reduced by 1.01% and 5.16%, and the rockburst proneness index was reduced by 5.15% and 19.32%, respectively. The failure intensity of PD and RD specimens was also weaker than that of ND specimen. However, compared with prefabricated drilling, real-time drilling under high stress can more accurately reflect the change in the rock energy state. The energy storage coefficient and rockburst proneness index of RD specimen were decreased by 3.8% and 14.9% compared to those of PD specimen, respectively. Furthermore, it is also found that drilling energy relief (DER) is a significant feature of real-time drilling under high stress to prevent rockburst, which further demonstrates the effectiveness of DPR in reducing rockburst proneness from an energy perspective. Highlights: Real-time drilling pressure relief experimental equipment SG4500 was developed autonomously. Real-time drilling pressure relief under high stress accurately reflects the change in the rock energy state. Drilling energy relief is a significant feature of drilling pressure relief to prevent rockburst. Real-time drilling pressure relief under high stress effectively reduces the rockburst proneness of rock. [ABSTRACT FROM AUTHOR]

To explore the mechanical behaviors and energy evolution characteristics of rock materials with hole defects, eight types of red sandstone specimens with different hole numbers and arrangements were subjected to a series of single-cycle loading-unloading uniaxial compression tests. The experimental result revealed that the mechanical behaviors and energy evolution characteristics of the red sandstone specimens are significantly influenced by hole defects. As the number of boreholes increases, the compressive strength of rock specimens decreases. When the angle between drilling hole arrangement and loading direction changes from 0° to 27° and then to 90°, the compressive strength first decreases and then increases, and shear-splitting, splitting, and shear failure modes appear respectively. The experimental results also show that the energy evolution characteristics (linear energy storage and dissipation laws) of the rock specimens with hole defects were basically consistent with that of intact rock specimens, while the energy storage coefficient is different in various cases. There was a good correspondence between the compressive strength and energy storage coefficient. The larger the energy storage coefficient is, the greater the compressive strength. The hole arrangement angle shows a stronger effect on energy storage coefficient and compressive strength than the number of holes. Among all the arrangements, the rock specimens containing three holes with an angle of 27° between borehole arrangement and loading direction have the lowest energy storage coefficient and compression strength. The above research results provide a scientific basis for rock fracturing induced by drilling. [ABSTRACT FROM AUTHOR]

The elastic energy stored in deep rock in three-dimensional stress environment is the energy source of rockburst. To investigate the energy storage characteristics of deep rock under different confining pressures, a series of triaxial single-cyclic loading-unloading compression tests were conducted on red sandstone specimens under eight confining pressures. The input energy density, elastic energy density, and dissipative energy density of the specimen in axial, circumferential, and total directions can be obtained by the area diagram integration method. The results show that the input energy density in the axial direction accounts for the largest logarithmic proportion of the total input energy density, and the relationship between all energy density parameters and unloading level can be described by quadratic function. In the axial direction, there is a linear function relationship among elastic energy density, dissipative energy density, and input energy density. In the circumferential direction, there is a quadratic function relationship among elastic energy density, dissipative energy density, and input energy density. For the total energy density parameters of the rock specimen, the relationship among elastic energy density, dissipative energy density, and input energy density conforms to the quadratic function. According to the above correlation function, the elastic energy stored in deep rock under different confining pressures can be accurately obtained, which provides a foundation for studying the mechanism of rockburst under three-dimensional unloading from the energy perspective. [ABSTRACT FROM AUTHOR]

To investigate the energy evolution and storage performances of rock under uniaxial cyclic compression, a series of uniaxial cyclic loading and unloading compression tests were conducted on Green sandstone and Yueyang granite. Two methods for calculating the total input energy of the specimen under each cycle were proposed. One is based on the actual stress-strain curve of the specimen (ASC method); the other is based on the stress-strain envelope curve during the loading process (SEC method). The experimental results show that, for those two methods, the total input energy, elastic energy, and dissipated energy of the specimen show a quadratic function increasing trend with the increase of stress levels. Besides, the elastic energy increases linearly with the increase of total input energy for both methods, which confirms that the linear energy storage law is also applicable to rock materials under uniaxial cyclic loading and unloading compression conditions. Moreover, the uniaxial compression energy storage coefficient calculated by the SEC method is highly close to that obtained based on the single cycle loading and unloading test, which indicates that the uniaxial compression energy storage coefficient of rock can also be calculated by multiple cyclic loading and unloading test. In conclusion, the linear energy storage law is a basic physical property of rock materials, and the uniaxial compression energy storage coefficient is a physical index reflecting the energy storage capacity of rock materials. [ABSTRACT FROM AUTHOR]

To investigate the specimen shape effect on rockburst proneness of rock materials, a string of conventional and single-cycle loading-unloading uniaxial compression tests was performed with cylindrical and cuboid red sandstone specimens. Despite similar development paths on stress-strain curves for the specimens with two shapes, the cuboid specimens generally show a higher uniaxial compressive strength than the cylindrical specimens. The energy evolution laws inside the two shaped specimens were explored. The results show that the input energy density (IED), elastic energy density (EED), and dissipated energy density (DED) of the two shaped specimens increased in a quadratic relationship with the increment of unloading level. Moreover, the linear relationships between the EED, DED, and IED were further confirmed for two shaped specimens, which were defined as the linear energy storage and dissipation laws, respectively. The energy storage coefficients and energy dissipation coefficients (the slopes of the linear relationships between the EED, DED, and IED, respectively) are almost independent of the specimen shape. According to the linear energy storage and dissipation laws, the peak EED and peak DED of every specimen can be calculated accurately. Finally, combining the failure process of rock specimens recorded by a high-speed camera, the elastic energy index ( W ET ), the residual elastic energy index (AEF), and the far-field ejection mass ratio (MF) of each specimen were adopted to assess the rockburst proneness of the red sandstone sampled in cylindrical and cuboid. The results show that cuboid specimens exhibited stronger rockburst proneness than cylindrical ones, which favorably agreed with the actual failure phenomena. [ABSTRACT FROM AUTHOR]

To investigate the energy evolution characteristics of rock materials under uniaxial compression, the single-cyclic loading–unloading uniaxial compression tests of four rock materials (Qingshan granite, Yellow sandstone, Longdong limestone and Black sandstone) were conducted under five unloading stress levels. The stress–strain curves and failure characteristics of rock specimens under the single-cyclic loading–unloading uniaxial compression tests basically corresponded with those of under uniaxial compression, which indicates that single-cyclic loading–unloading has minimal effects on the variations in the loading–deformation response of rocks. The input energy density, elastic energy density and dissipated energy density of four rocks under five unloading stress levels were calculated using the graphical integration method, and variation characteristics of those three energy density parameters with different unloading stress levels were explored. The results show that all three energy density parameters above increased nonlinearly with increasing unloading stress level as quadratic polynomial functions. Meanwhile, both the elastic and dissipated energy density increased linearly when the input energy density increased, and the linear energy storage and dissipation laws for rock materials were observed. Furthermore, a linear relationship between the dissipated and elastic energy density was also proposed. Using the linear energy storage or dissipation law, the elastic and dissipated energy density at any stress levels can be calculated, and the internal elastic (or dissipated) energy density at peak compressive strength (the peak elastic and dissipated energy density for short) can be obtained. The ratio of the elastic energy density to dissipated energy density with increasing input energy density was investigated using a new method, and the results show that this ratio tends to be constant at the peak compressive strength of rock specimens. [ABSTRACT FROM AUTHOR]

To fully understand the influence of water on rockburst proneness from energy storage viewpoint of rock and to check the performance of energy-related rockburst proneness indexes, several groups of uniaxial compression tests at different stress levels are performed on three types of red sandstone specimens (i.e., the saturated, natural, and oven-dried specimens). The method of combining strain energy calculations and experimental observations is used to assess the rockburst proneness. The progressive failure of the three types of specimens is recorded by a high-speed (H-S) camera, and the rockburst proneness is comprehensively assessed by considering the failure features of the rock specimens (including the fracture sound, fragment ejection, fragment distribution, and far-field ejection mass ratio). The laboratory results reveal that the rockburst proneness of the natural and oven-dried sandstone specimens is similar. However, compared with the natural and oven-dried specimens, the water-saturated specimens exhibit a visible weaker rockburst proneness. It is also evident that the linear energy storage and dissipation laws are applicable to the saturated, natural, and oven-dried specimens. Most interestingly, the energy storage and energy dissipation coefficients are nearly immune to the presence of water in the red sandstone. Based on this, the definitions of absolute energy storage capacity and relative energy storage capacity are adopted to describe the influence of water on the energy storage performance of rocks. It is found that water dramatically weakens the absolute energy storage capacity, but has little and negligible effect on the relative energy storage capacity of sandstone. Considering the direct and indirect indexes, the accuracy of several representative energy-related rockburst proneness indexes is comprehensively compared in terms of the far-field ejection mass ratio, and their performance is discussed from the relative and absolute energy storage aspects. According to their performance, it is strongly recommended to use the direct index for determination of rockburst proneness when energy-related indexes are used. • The rockburst proneness of oven-dried, natural, water-saturated sandstone specimens is analyzed. • The linear energy storage and dissipation laws are applicable for the sandstone under different contained water conditions. • The concepts of relative energy storage and dissipation capacities are proposed. • The performance of several representative energy-related rockburst proneness indexes is discussed [ABSTRACT FROM AUTHOR]

Judgement of rock burst proneness of rock materials is one of the basic problems in the field of rock burst research. In this study, a peak-strength strain energy storage index is proposed for estimating and classifying the rock burst proneness of rock materials. The method for determining this index is also introduced in this paper. The peak-strength strain energy storage index is defined as the ratio of the elastic strain energy density to the dissipated strain energy density corresponding to the peak compressive strength of rock specimen. In order to obtain the elastic strain energy density and the dissipated strain energy density at the peak strength, a series of single cyclic loading-unloading uniaxial compression tests on nine rock materials were conducted. The relationships between the total input energy density and the elastic strain energy density at different unloading stress levels were investigated. The results show that, for every rock material, the elastic strain energy density increases linearly with the increase of the total input energy density. Based on this linear storage energy law, the elastic & dissipated strain energy density at the peak strength can be calculated for each specimen, and the peak-strength strain energy storage index can be obtained accordingly. A new criterion for rock burst proneness of rock materials is proposed. The rock burst proneness of nine rock materials estimated with the proposed criterion agreed well with the laboratory test results. [ABSTRACT FROM AUTHOR]

To estimate the energy storage and release performances of rock pillars in high stress and gain insights into the prevention and control of rockburst hazards from an energy aspect, several series of single-cyclic loading-unloading uniaxial compression tests with different stress levels were conducted on five types of rocks sampled in standard cylinders and prisms. The concepts of relative energy storage potential and ultimate energy storage potential were proposed to evaluate the energy storage performance of the two shaped specimens, and the energy release potential was determined using the residual elastic strain energy index. The results showed that, under similar loading conditions, the relative energy storage potential of rocks was insensitive to the specimen shape, whereas the ultimate energy storage potential and energy release potential were sensitive to both the specimen shape and rock type. Based on the lab results, a theoretical approach of assessing the rockburst (or instability) risk of isolated rock pillars subjected to high geostress was illustrated by estimating its current energy state and ultimate energy storage potential. Importantly, the residual elastic energy of an isolated pillar can also be estimated based on the lab results, according to which the number of supporting devices such as the energy-absorbing bolts needed for scientific and sufficient support of pillar rockburst was quantitatively estimated. This study provides a new method for estimating the desired number of energy-absorbing bolts during rockburst support in engineering practice. [ABSTRACT FROM AUTHOR]

• The u t , u e , and u d increase nonlinearly with the increase of the i. • The u e , and u d have a significant linear relationship with the u t. • The A E increase linearly with the increase of age, while A D is opposite. • A novel method was proposed to calculate the u d and u e at ultimate strength point. To investigate the influence of age on energy storage and dissipation laws, uniaxial compression (UC) and single–cyclic loading–unloading uniaxial compression (SCLUC) tests were conducted on C35 concrete specimens with ages of 3 d, 7 d, 15 d, 28 d. The experimental results showed that the input strain energy (ISE), elastic strain energy (ESE), and dissipated strain energy (DSE) increased nonlinearly with the stress level during the pre-peak loading stage. Analyzing the relationship between the three strain energies, both ESE and DSE increase linearly with the increase of ISE in the pre-peak loading stage at different ages, namely, linear energy storage and dissipation laws exist in every ages. Based on the linear energy storage and dissipation laws, a novel method was proposed to calculate the DSE and ESE at the ultimate strength point of the concrete at different ages. In addition, we found that the DSE was greater than the ESE of concrete in the loading stage, because the energy was mainly dissipated in forming cracks, grain dislocation, and producing plastic displacement during compression. The value of compression energy storage coefficient represents the capacity of the concrete to store ESE, whereas energy dissipation coefficient represents the strain energy dissipation capacity of concrete. In the future, based on the linear energy storage and dissipation laws of concrete, the new method for calculating the damage variable may be proposed. [ABSTRACT FROM AUTHOR]

The investigation of rock damage behaviour is an important requirement for ensuring stability control and safety prediction in rock engineering. In this study, based on the linear energy dissipation law and energy dissipation coefficient, a theoretical method is introduced for characterising the damage of intact rocks under uniaxial compression conditions. The existing uniaxial compression test data of 14 kinds of rock materials (including 6 types of granite, 4 types of sandstone, 2 types of marble, 1 type of slate, and 1 type of limestone) were used to verify the effectiveness of the damage characterization method. The results indicate that the damage evolution process of rocks under uniaxial compression can be fully expressed by the proposed theoretical damage characterisation method, and the damage variable exhibits a quadratic functional relationship with the loading stress level. Moreover, with the linear energy storage law serving as a theoretical basis, the peak dissipated strain energy in a rock damage expression under uniaxial compression can be accurately calculated. The new theoretical damage characterisation method overcomes the shortcomings of conventional damage characterisation methods (e.g. estimating the peak dissipated strain energy through a hypothesis), and provides a new means for analysing rock damage from an energy viewpoint. [ABSTRACT FROM AUTHOR]

• The mechanical properties of red sandstone increase with the increase of pre-existing single crack angle. • The linear energy storage and dissipation laws of pre-existing single crack rock are obtained. • The peak strain energy densities are determined using the linear energy storage law. • The linear energy dissipation law is applied to express the damage of rock with single crack. To investigate the effects of the pre-existing single crack angle on the mechanical behaviors and energy storage characteristics of red sandstone, a series of uniaxial compression and single-cyclic loading–unloading uniaxial compression tests were conducted on red sandstone specimens with five crack angles. The relationships among the crack angle and the mechanical properties, failure modes, and energy storage characteristics of red sandstone were analyzed using the acoustic emission (AE), stress–strain curve and curvilinear integral. The results indicate that the peak compressive strength, peak strain, and elastic modulus of specimens increase with the crack angle increasing, and the specimens with five different angles all present "anti-N-type" shear failure. Throughout the loading process, the total input energy density, elastic strain energy density, and dissipated energy density of the red sandstone at five different crack angles exhibit significant quadratic growth in response to the actual unloading stress. Importantly, the elastic strain energy density and dissipated energy density of red sandstone with different crack angles all show highly linear relationships with the total input energy density, and the linear energy storage and dissipation laws of pre-cracked rocks are obtained. Based on the linear energy storage law, the energy densities at peak strength of red sandstone with different crack angles are determined quantitatively. In addition, the linear energy dissipation law is applied to the expression of rock damage, and the damage constitutive equations of uniaxial compression specimens with different crack angles are obtained. Finally, the effects of crack geometric parameters on the energy storage and dissipation characteristics were discussed. [ABSTRACT FROM AUTHOR]

• The linear energy storage and dissipation laws during rock flexure are identified. • The elastic and dissipation energies at rock failure are determined using the linear energy storage and dissipation laws. • The relationship between the energy properties and mechanical properties is obtained. • The influence of pre-existing flaws on the linear energy storage and dissipation laws is discussed. To investigate the energy storage and dissipation characteristics during rock flexure fracturing, a series of single cyclic loading–unloading flexural fracture tests was conducted on rectangular rock beams using the three-point loading technique. The results show that under different experimental unloading levels, the elastic and dissipation energies increase linearly as the input energy increases. On this basis, the linear energy storage and dissipation laws were obtained, which were immune to the experimental unloading level. The flexural energy storage coefficient and flexural energy dissipation coefficient were subsequently introduced to characterize the potentials of the rock for storing and dissipating energy, respectively. A positive or negative relationship between the two coefficients and the rock mechanics properties (the brittleness index and average flexural strength) was observed. The influence of pre-existing rock flaws on the linear energy storage and dissipation laws was discussed. In addition, highly quadratic increases in the elastic energy, dissipation energy, and input energy were observed with an increasing experimental unloading level. For the energy characteristics at rock failure (the failure point of the rock specimen), the peak input energy, peak elastic energy, and peak dissipation energy varied considerably under different experimental unloading levels, while the associated peak elastic dissipation index (the ratio of peak elastic energy to peak dissipation energy) remained constant. Similar to the flexural energy storage coefficient, the mean value of peak elastic dissipation index is also positively related to the rock mechanics properties. The linear evolution law of energy storage and dissipation not only permits a quantitative determination of the energy parameters at rock failure (the peak elastic energy and peak dissipation energy), but also relates the energy properties with the rock mechanics properties. [ABSTRACT FROM AUTHOR]