Failure Of Sandstone Research Articles

Application of the Extension Strain Criterion for Sandstone Failure Evaluation under Tension and Shear Stress Conditions

Application of the Extension Strain Criterion for Sandstone Failure Evaluation under Tension and Shear Stress Conditions

Experimental investigation of failure mechanism of fissure-filled sandstone under hydro-mechanical conditions

Fissure fillings are critical to the hydro-mechanical properties of jointed rock masses in rock engineering. In this study, triaxial seepage tests were performed on standard cylindrical fissure-filled sandstone. The characteristics of stress–strain relationships, absorption and consumption of energy, variations in deformation resistance, and permeability evolution during the experimental process, along with the crack development observed in post-failure computed tomography scan images of the sandstone specimens were analyzed. The results demonstrate that the fillings improve the energy capacity and reduce the damage accumulation of sandstone specimens, with sand-filled specimens performing better than mud-filled specimens, especially at lower bridge angles. The fillings can reduce the depth of crack extension and lessen the influence of prefabricated fissures on sandstone failure, with this effect diminishing as the rock bridge angle increases. Permeability decreases in the pre-peak failure stage as the fillings improve the deformation resistance of the sandstone specimens. In the post-peak failure stage, the fillings and rock debris generated by the sandstone failure move within the developed fractures, causing significant fluctuations in permeability. These findings deepen the understanding of the hydro-mechanical properties of jointed rocks and provide a scientific basis for stability analysis in rock engineering.

Study on fatigue characteristics of red sandstone under extremely high stress in the hydro-chemical environment

To study the failure of red sandstone under extremely high stress during the service life of tunnels, an in-depth study was conducted on the mechanical properties of red sandstone under uniaxial loading and cyclic loading and unloading processes at different pH values using the AG-250kNIS electronic precision material testing machine and MTS815 mechanical testing machine. The results show that as the acidity and alkalinity increase, the peak stress under uniaxial loading decreases and the axial strain increases,The peak stress at failure is 9.40, 12.37, 7.18, and 5.36 MPa, respectively, accounting for 74.19%, 68.91%, 40.38%, and 36.21% of the uniaxial compressive strength; The number of cycles significantly decreases during cyclic loading and unloading fatigue failure, and the stress required for sandstone failure gradually decreases. The peak strength and elastic modulus of sandstone show a decreasing trend, indicating that the hydrochemical environment plays an accelerating role in rock degradation. During the cyclic loading and unloading process of sandstone, there is a continuous increase in dissipated energy and finally a sudden increase, the Ud/U and Ue/U ratios at the peak point of sandstone in the natural state are 0.399 and 0.601, respectively, while the overall elastic energy shows an increasing trend; and a damage evolution model was established based on dissipative energy, which can better describe the degradation process of red sandstone.

A B-spline based gradient-enhanced micropolar implicit material point method for large localized inelastic deformations

The quasi-brittle response of cohesive-frictional materials in numerical simulations is commonly represented by softening plasticity or continuum damage models, either individually or in combination. However, classical models, particularly when coupled with non-associated plasticity, often suffer from ill-posedness and a lack of objectivity in numerical simulations. Moreover, the performance of the finite element method significantly degrades in simulations involving finite strains when mesh distortion reaches excessive levels. This represents a challenge for modeling cohesive-frictional materials, given their tendency to experience strongly localized deformations, such as those occurring during shear band dominated failure. Hence, accurate modeling of the response of cohesive-frictional solids is a demanding task. To address these challenges, we present an extension of the material point method (MPM) for the unified gradient-enhanced micropolar continuum, aiming at the analysis of finite localized inelastic deformations in cohesive-frictional materials. The generalized gradient-enhanced micropolar continuum formulation is employed to tackle challenges related to localization and softening material behavior, while the MPM addresses issues arising from excessive deformations. The method utilizes a B-spline formulation for the rigid background mesh to mitigate the well-known cell crossing errors of the MPM. To demonstrate the performance of the method, 2D and 3D numerical studies on localized failure in sandstone in plane strain compression and triaxial extension tests are presented. A comparison with finite element results confirms the suitability of the formulation. Moreover, an efficient numerical implementation of the formulation is presented, and it is demonstrated that the additional MPM specific overhead is negligible.

Investigate on the mechanical properties and microscopic three-dimensional morphology of rock failure surfaces under different stress states

The macro and micro morphology of rock failure surfaces play crucial roles in determining the rock mechanical and seepage properties. The morphology of unloaded deep rock failure surfaces exhibits significant variability and complexity. Surface roughness is closely linked to both shear strength and crack seepage behavior. Understanding these morphology parameters is vital for comprehending the mechanical behavior and seepage characteristics of rock masses. In this study, three-dimensional optical scanning technology was employed to analyze the micromorphological properties of limestone and sandstone failure surfaces under varying stress conditions. Line and surface roughness characteristics of different rock failure surfaces were then determined. Our findings reveal a critical confining pressure value (12 MPa) that influences the damage features of Ordovician limestone failure surfaces. With increasing confining pressure, pore depth and crack formation connecting the pores also increase. Beyond the critical confining pressure, the mesoscopic roughness of the failure surface decreases, and the range of interval-distributed pore roughness diminishes. Additionally, we conducted a detailed investigation into the water conductivity properties of rocks under different stress states using Barton's joint roughness coefficient (JRC) index and rock fractal theory. The roughness features of rock failure surfaces were classified into three categories based on mesoscopic pore and crack undulation forms: straight, wavy, and jagged. We also observed significant confining pressure effects on limestone and sandstone, which exceeding the critical confining pressure led to increased water conductivity in both rocks, albeit through different mechanisms. While sandstone exhibits fissures running across it, limestone shows shear abrasion holes. Beyond the critical confining pressure, the rock failure surface becomes smoother, leading to decreased water flow blocking capacity. The fractal dimension of Ordovician limestone increases significantly under critical confining pressure, leading to a more complex mesoscopic crack extension route.

Mechanical and acoustic characteristics of sandstone failure at different immersion heights under tailings water erosion conditions

To investigate the influence of tailings water immersion height on the failure process of rocks, intact yellow sandstone samples and yellow sandstone samples with prefabricated fissures were prepared. Uniaxial compression tests were conducted on these sandstone samples with varying immersion heights, and the failure processes were simultaneously monitored using a camera and an acoustic emission monitoring system. Based on the test results, the crack propagation and failure characteristics of the yellow sandstone samples under uniaxial loading at different immersion heights were determined. The relationships between the immersion height and peak strength and between the immersion height and elastic modulus were characterized. Additionally, the intrinsic connection between sandstone damage and acoustic emission signal parameters was revealed. The experimental results indicate that the uniaxial compressive strength and elastic modulus of the sandstone samples decrease continuously with increasing immersion height, but the rate of decrease gradually slows. The presence of water is the primary reason for the deterioration of mechanical properties, and the chemical components in the tailings water are significant factors that cannot be ignored. The initiation location and propagation of cracks in intact samples depend on the immersion height, while the fracture behaviour of fissured samples after immersion is mainly influenced by the prefabricated fissures. As the immersion height increases, the longitudinal wave velocity increases, while the number of acoustic emission events, peak frequency, and amplitude exhibit a decreasing trend that is initially rapid and then slows. A damage evolution model based on acoustic emission ringing counts at different immersion heights was established. It was also found that the acoustic emission signals of these yellow sandstone samples mainly show the characteristics of abrupt signals under any immersion water height.

Study on the Failure Process and Acoustic Emission Characteristics of Freeze–Thawed Sandstone under Cyclic Loading and Unloading

In order to investigate freeze–thawed red sandstone failure processes under cyclic loading and unloading conditions, real-time acoustic emission (AE) and scanning electron microscopy (SEM) techniques were used to reveal the fracture process of the saturated red sandstone after cyclic loading and unloading tests using uniaxial compression. The results show that the stress–strain curves of the freeze–thawed sandstones show signs of hysteresis and exhibit a two-stage evolution of “sparse → dense”. In the cyclic loading and unloading process, the modulus of elasticity in the loading process is always larger than that in the unloading process, while the Poisson’s ratio is the opposite, and the radial irreversible strain and cumulative irreversible strain are larger than those in the axial direction. As the number of freeze–thaw cycles increases, the rock specimens need more cycles of loading and unloading to make the crack volume compressive strain Δεcv+ reach the maximum value and tend to stabilize, while the crack volume extensional strain Δεcv− tends to decrease gradually. This study also shows that the growth phase of the cyclic loading and unloading process has more ringing counts and a shorter duration, while the slow degradation phase has more ringing counts with loading and less with unloading. In addition, the F-T cycle gradually changes the internal microcracks of the red sandstone from shear damage, which is dominated by shear cracks, to tensile damage, which is dominated by tensile cracks. This study’s findings contribute to our knowledge of the mechanical characteristics and sandstone’s degradation process following F-T treatment, and also serve as a guide for engineering stability analyses conducted in the presence of multiphysical field coupling.

Experimental Study on Energy Release Mechanism and Crack Propagation Evolution of Sandstone under True Triaxial Loading

The instability of hard and brittle rock often leads to disastrous consequences in underground engineering. Under various surrounding rock pressure conditions, in situ stress induces corresponding deformation and damage to the floor post-mining. Therefore, it is crucial to examine the effects of mining under different confining pressures on rock disturbance, damage characteristics, and their distribution. Consequently, triaxial loading experiments under varying intermediate principal stress conditions were conducted on red sandstone specimens, using an acoustic emission monitoring system to track energy changes during rock damage and failure. This approach aids in studying crack generation, propagation, and fracture damage evolution. The results indicate that rock deformation results in axial compression and dilatancy, aligned with the direction of minimum and intermediate principal stresses. Ductility in rock failure becomes more pronounced with increased stress, primarily manifesting as shear failure. Internal cracks in the specimen lead to stress concentration and marked plastic deformation under compression, yet do not result in macroscopic surface cracks. The fracture angle θ of specimens post-failure generally exceeds 45° and varies with stress changes; at consistent burial depths, the angle of the sandstone failure surface increases with intermediate principal stress. This paper preliminarily establishes the informational linkage between rock failure and energy release, analyzing the rock samples over time and space. This research offers insights for analyzing and mitigating sudden rock instability.

Effect of internal fractures on mechanical properties and failure of sandstone under multi-physical fields

Under deep mining conditions, rocks are subjected to complex multi-physical fields and can contain numerous pores and fractures. To explore the influence and correlation of these factors on the physical and mechanical properties of fractured rock samples, this study conducted triaxial compression tests on sandstone specimens under various physical conditions using a rock full stress multi-field coupling triaxial tester. Additionally, a random fracture model for multi-field coupling numerical simulation was established. This allowed the study to obtain the mechanical parameters, failure mode, and internal fracture development of rocks under multi-physical field conditions. By analyzing the complete stress-strain curve, mechanical characteristic points, and permeability, a combination of laboratory tests and numerical simulations was used to examine how temperature, seepage, and stress fields affect the development of pores and fractures in rocks. It was found that the temperature field, under conventional geothermal conditions, generates tensile force through thermal expansion and the presence of fluid, thereby promoting fracture development within the rocks. This mechanism is similar to that of seepage. The confining pressure caused by deep geo stress uniformly inhibits the expansion of pores and fissures within the rocks.

Parameter calibration of the tensile-shear interactive damage constitutive model for sandstone failure

The tensile-shear interactive damage (TSID) model is a novel and powerful constitutive model for rock-like materials. This study proposes a methodology to calibrate the TSID model parameters to simulate sandstone. The basic parameters of sandstone are determined through a series of static and dynamic tests, including uniaxial compression, Brazilian disc, triaxial compression under varying confining pressures, hydrostatic compression, and dynamic compression and tensile tests with a split Hopkinson pressure bar. Based on the sandstone test results from this study and previous research, a step-by-step procedure for parameter calibration is outlined, which accounts for the categories of the strength surface, equation of state (EOS), strain rate effect, and damage. The calibrated parameters are verified through numerical tests that correspond to the experimental loading conditions. Consistency between numerical results and experimental data indicates the precision and reliability of the calibrated parameters. The methodology presented in this study is scientifically sound, straightforward, and essential for improving the TSID model. Furthermore, it has the potential to contribute to other rock constitutive models, particularly new user-defined models.

Experimental and numerical investigation on the dynamic shear failure mechanism of sandstone using short beam compression specimen

In this study, a novel testing method is proposed to characterize the dynamic shear property and failure mechanism of rocks by introducing the short beam compression (SBC) specimen into the split Hopkinson pressure bar (SHPB) system. Firstly, the stress distribution of SBC specimen is comprehensively analyzed by finite element method (FEM), and the results show that the optimal notch separation ratio of SBC specimen is C/H = 0.2 to achieve successful dynamic simple-shear tests. Then, dynamic shear tests are conducted on sandstone using the SBC-SHPB method. Via careful pulse shaping technique, the dynamic force balance is guaranteed for SBC specimens, and the testing results show that the dynamic shear strength of sandstone is significantly rate-dependent. Combining the results of dynamic compression and tension tests, the failure envelopes of sandstone under different loading rates are obtained in the principle stress plane. It is found that the failure envelope of sandstone constantly expands outwards with increasing loading rate. Moreover, the energy partition of SBC specimen is quantified by virtue of high-speed digital image correlation (DIC) technique. The results show that the kinetic energy portion is non-negligible, and the shear fracture energy increases with increasing loading rate. In addition, the microscopic shear cracking mechanism of SBC specimen is analyzed by the thin section observation: the intra-granular (TG) fracture of minerals dominates the dynamic shear failure of sandstone, and the portion of TG fracture increases with increasing loading rate. This study provides a convenient and reliable method to measure the dynamic shear property and failure mechanism of rocks.

Investigation on Prediction of Sandstone Failure Under Uniaxial Compression Based on Supervised Deep Learning

Investigation on Prediction of Sandstone Failure Under Uniaxial Compression Based on Supervised Deep Learning

Study on microscopic failure mechanism and numerical simulation of sandstone under different saturated pressure

X-ray diffraction and SEM scanning are conducted to examine the alterations in sandstone under different saturation conditions to reveal the water-rock softening effect on sandstone from the Qilicun tunnels in China under varying saturation pressures. The mechanisms underpinning the strength softening of sandstone are analyzed using uniaxial compression tests. The experimental results demonstrated that after immersion in water, the internal cementing material within the sandstone dissolves, and the mineral particles fragment or disintegrate, increasing porosity. In the presence of water, the macroscopic compressive strength of sandstone exhibits a declining trend. Concurrently, as the saturation pressure escalates, the compressive strength diminishes by approximately 10%, the elastic modulus decreases by about 30%, and Poisson's ratio incrementally falls by about 25%. The sandstone's failure is characterized by both axial multiple splitting surface failure and shear failure surface. Finally, a strain-softening numerical model is employed to simulate the failure behaviors of sandstone under various saturation pressures. The findings indicated that the sandstone sample exhibits plastic failure characteristics under high saturation pressure.

Size effect of mechanical characteristics of sandstone and granite under uniaxial compression

The accurate evaluation of the mechanical properties of rocks is crucial to the design of large-scale projects, such as the construction of dams and bridges, the excavation of tunnels, and petroleum engineering research. In this study, experiments pertaining to the size effect of sandstone and granite were performed on small square samples of 10–25 mm. It was found that the elastic modulus of sandstone and granite decreases significantly with increasing size. The uniaxial compressive strength, however, follows a different trend. When the size of the sample increases from 10 to 25 mm, the uniaxial compressive strength of sandstone increases by approximately 12%. However, the strength of granite decreases by 35% as the size increases. The discrepancies in the failure of sandstone and granite are apparent, and the failure patterns of the same rock do not vary with size. Sandstone mostly suffers from split damage, whereas granite mainly suffers from shear failure. Contemporary studies have found that the mechanical properties of rocks are closely related to the sizes of the samples. However, these studies have primarily focused on the size effect of rocks, whose samples sizes were greater than 30 mm in diameter. The understanding constructs the relationship between the strength transformation of different sizes of rocks, which is of great significance for practical engineering applications.

Mechanical and Energy Evolution Characteristics of Sandstone under True Triaxial Cyclic Loading

To study the mechanical and energy evolution characteristics of sandstone under true triaxial cyclic loading, a sandstone mechanical test with different intermediate principal stress under true triaxial loading was conducted using the rock true triaxial disturbance unloading test system. The influence of axial load on the deformation, energy evolution, and macroscopic failure characteristics of sandstone under different intermediate principal stress in a true triaxial test was systematically analyzed, and the damage evolution law of sandstone under true triaxial cyclic load was revealed. Results showed that the failure mode of sandstone under true triaxial compression changed from tension–shear composite failure to tension failure. Grading cyclic load σ1 greatly influenced maximum principal strain ε1 and minimum principal strain ε3 but had little influence on intermediate principal strain ε2. Under the same σ2 condition, the input energy and elastic energy in σ1, σ2, and σ3 directions increased nonlinearly. Under different σ2 conditions, the dissipated energy in σ1, σ2, and σ3 directions decreased with the increase in σ2. With the increase in σ2, graded cycles σ1, ε2, and ε3 decreased considerably, and the failure mode changed from tensile failure to shear failure. When the cyclic loading rate increased, the σ1, ε1, ε2, ε3, and volume strain εv of sandstone failure decreased, but the expansion point increased. Under true triaxial grading cyclic loading and unloading, the total dissipated energy of sandstone increased exponentially. The larger σ2 was, the smaller the damage variable was.

Evolution Law of Acoustic–Thermal Effect of Freeze–Thaw Sandstone Failure Based on Coupling of Multivariate Monitoring Information

The instability and failure of rock that has been frozen and thawed cause serious rock engineering accidents in cold regions. Exploring the precursor information of freeze–thaw rock failure is of great theoretical value and engineering significance. Real-time uniaxial compression acoustic thermal monitoring experiments were conducted on freeze–thaw sandstone, and non-contact rock fracture precursor warning indicators were proposed. According to the coupled analysis of acoustic–thermal monitoring information, a precursor information chain for freeze–thaw rocks was established in time and space, and the spatiotemporal evolution of damage and acoustic thermal effects of freeze–thaw sandstone under compressive load was studied. The freeze–thaw cycle enhances the sensitivity of acoustic–thermal precursor information. Significant synchronous changes in ring count often occur during the rapid expansion period of damage, which can provide an essential reference for the occurrence and intensification of damage. The sequence of precursor warning information during the process of freeze–thaw sandstone compression failure is in the order of thermal infrared temperature → acoustic emission ringing count → acoustic emission energy → infrared thermal image. Thermal infrared temperature and acoustic emission precursor information can help in prioritizing early warning of rock damage in terms of time. At the same time, thermal image anomalies can predict potential fracture areas of rocks.

Investigation on the anisotropy of mechanical properties and brittleness characteristics of deep laminated sandstones

Rock brittleness is a crucial mechanical property and essential for fracability evaluation and fracturing scheme design in unconventional reservoirs. However, the influence of inherent anisotropy on deep laminated sandstone’s mechanical properties and brittleness characteristics is rarely investigated. The energy transformation and damage evolution reflected by complete stress–strain curves are analyzed during the entire process of rock rupture under compressions. A new brittleness index is established based on energy evolution during sandstone failure. Its advantages involve comprehensively considering the energy transformation characteristics at both pre-peak and post-peak stages and the capability to characterize the effect of confining pressure and bedding plane (BP) geometry on sandstone brittleness. The triaxial compression tests on sandstones are conducted to validate the reliability and accuracy of the new brittleness index. Numerical simulations are then performed to further investigate the manner in which BP angle, BP density, and confining pressure control the brittleness anisotropy of deep laminated sandstones based on the finite element method. Then the acoustic emission (AE) characteristics of anisotropic sandstone and correlations between AE mode and brittleness index are discussed. The results indicated that the anisotropy of mechanical properties and brittleness of deep laminated sandstones were significantly affected by BP angle, BP density, and confining pressure. With the increase of BP angle, the brittleness index of deep laminated sandstone decreases first and then increases, showing a U-shape variation law, whose maximum and minimum values are obtained at φ = 0° and φ = 45°, respectively. The AE characteristics were closely related to rock brittleness, which was jointly controlled by BP geometry and confining pressure. The results provide a basis for the brittleness and fracability evaluation and optimum hydraulic fracturing design in deep laminated sandstones.

The influence of water on the failure characteristics of sandstone under uniaxial compression conditions by acoustic emission and NMR observation

Revealing the failure characteristics and damage process of rocks soaked in water is particularly critical for preventing and controlling geological disasters. To this end, we employed nuclear magnetic resonance (NMR) and an ultrasonic system to capture the variation in the T2 spectrum, different pore waters and P-wave velocity during soaking (from 0 to 120 h). Furthermore, an acoustic emission (AE)-uniaxial compression test was carried out to analyse the AE characteristics (e.g., hits, absolute energy, and events), and strength of sandstone under different soaking times. The AF (the ratio of AE counts to duration)-RA (the ratio of the AE rise time to amplitude) is used to distinguish the tensile failure shear failure of sandstone during soaking. The results showed that the increase in soaking time decreased the bound water percentage and strength while increasing the P-wave velocity and capillary water percentage. An interesting phenomenon was that the failure mode was transformed due to soaking based on the evolution of the AE response. From the above results and classical water-weakening mechanisms, we suggest that the weakening process after loading can be divided into three stages. In the initially soaking, the reduction in fracture toughness and frictional more serious caused the increase in the occurrence of tensile failure. The quartz hydrolysis and chemical deterioration may aggravate the shear failure and intergranular fracture during long-term soaking. This study clarifies the transformation of weakening mechanisms during soaking and is of great significance for understanding the microscopic damage processes in rock failure.

Precursor Development of Sandstone Failure Using IR to Measure the Fractal Characteristics

Precursor Development of Sandstone Failure Using IR to Measure the Fractal Characteristics

Mechanical and fractal characteristics of sandstone with Pre-existing fissures of different lengths under varying impact loads in SHPB tests

The Split Hopkinson Pressure Bar (SHPB) with a 100 mm diameter was used to investigate the impact of pre-existing fissure length on the mechanical properties of sandstone under different launch pressure (P = 0.3 MPa, P = 0.45 MPa, and P = 0.6 MPa). Subsequently, CT scanning was conducted on sandstone samples with pre-existing fissure after failure to examine the relationship between pre-existing fissure length (Lc) and fractal characteristics under different launch pressures. Our findings indicate that, compared to intact sandstone, the proportion of plastic deformation in the stress–strain curve of sandstone with pre-existing fissure increases. As Lc increases, the initial plastic deformation stress and secant modulus decrease. After CT scanning, sandstone failure was classified into three conditions for different Lc: (1) Axial-splitting-tensile failure is caused by the development of natural defects, and in this case, the degree of development of pre-existing fissure is low, (2) Radial-splitting -tensile-failure is caused by the development of pre-existing fissure, in which the degree of development of pre-existing fissure is high and dominates the macroscopic failure of sandstone, while the degree of development of natural defects is low, and; (3) Combined-splitting-tensile failure: The failure of sandstone includes both axial-splitting-tensile failure and radial –splitting-tensile failure. At this time, both the degree of development of natural defects and pre-existing fissure are high, which jointly dominate the macroscopic failure of sandstone. With increasing launch pressure (P) and Lc, the failure mode of sandstone transitions from axial splitting-tensile failure to radial splitting-tensile failure, and eventually to combined splitting-tensile failure. However, local compression-shear failure occurs with increased launch pressure. Moreover, Lc significantly affects the fractal characteristics of sandstone. Under 0.3 MPa launch pressure, the fractal dimension (DH) on different height (H) slices initially increases and then decreases with increased Lc. Under 0.45 MPa launch pressure, the DH on different H slices exhibits two changes with increasing Lc. For H = 70 mm and H = 100 mm, the variation of DH is decreasing-increasing–decreasing-increasing, while for H = 0 mm and H = 30 mm, the variation of DH is increasing–decreasing-increasing. Under 0.6 MPa launch pressure, the DH on different H slices increases first and decreases secondly with the increase of Lc, but Lc is different when DH reaches its maximum.