Search Results (80)

The paper studies the behavior of homogeneous isotropic materials by performing appropriate numerical analyses and utilizing suitable FEMs to reproduce the Brazilian splitting test. Starting with a theoretical approach and adopting suitable numerical simulations, a new formula that is able to characterize the Young’s modulus is presented. To this end, in addition to the analysis of the specimen’s response in terms of stresses and strains, the real displacement field resulting from the real kinematical constraints on the specimen is determined. Therefore, the Brazilian test is taken as a reference test and the specimen’s behavior is derived by taking advantage of both the theoretical approach and numerical simulations developed in the ANSYS 2021 R1 environment. The latter allows us to define a new mathematical relation representing the missing part of the kinematical field. Furthermore, a new formula which explicitly relates the Young’s modulus of the material to the geometrical characteristics of the specimen, to the acting force, and to a measured selected displacement is proposed. Future developments will include adopting the proposed formulas for the identification of other mechanical parameters of the material, e.g., by adopting a full-field contactless approach to displacement measurement and studying the behavior of specimens with different geometrical characteristics. Full article

This study presents an innovative solution to improve the mechanical performance of traditional cemented tailings backfill (CTB) by incorporating 3D-printed polymer lattice (3DPPL) reinforcements. We systematically investigated three distinct 3DPPL configurations (four-column FC, six-column SC, and cross-shaped CO) through comprehensive experimental methods including Brazilian splitting tests, digital image correlation (DIC), and scanning electron microscopy (SEM). The results show that the 3DPPL reinforcement significantly enhances the CTB’s tensile properties, with the CO structure demonstrating the most substantial improvement—increasing the tensile strength by 85.6% (to 0.386 MPa) at a cement-to-tailings ratio of 1:8. The 3DPPL-modified CTB exhibited superior ductility and progressive failure characteristics, as evidenced by multi-stage load-deflection behavior and a significantly higher strain capacity (41.698–51.765%) compared to unreinforced specimens (2.504–4.841%). The reinforcement mechanism involved synergistic effects of macroscopic truss behavior and microscopic interfacial bonding, which effectively redistributed the stress and dissipated energy. This multi-scale approach successfully transforms CTB’s failure mode from brittle to progressive while optimizing both strength and toughness, providing a promising advancement for mine backfill material design. Full article

Coal gasification slag (CGS) is a solid byproduct generated during coal gasification. Stacking and land-filling of CGS wastes substantial land resources and has significant environmental risks. In this paper, based on the Ca/Si and Si/Al ratios of the raw materials, the mix design of alkali-activated CGS concrete was optimized using a pure center-of-gravity design method. The compressive and flexural strengths of geopolymer concrete with varying mix proportions were measured to investigate the effects of sodium silicate modulus, material content, and dry density on its mechanical properties. Specimens of different sizes were prepared to analyze the influence of testing methods on the compressive, flexural, and tensile properties. The results indicate that the mechanical properties of geopolymer concrete are significantly influenced by the raw material composition and the modulus of the activator. With increasing curing age, both compressive and flexural strengths exhibit varying degrees of improvement. The stress-strain behavior of alkali-activated CGS concrete aligns closely with that of ordinary concrete. A comparative analysis of 100 mm length and 20 mm length cubic specimens revealed a compressive strength size conversion coefficient of approximately 0.456, while the flexural specimen exhibited a coefficient of 0.599. For tensile strength evaluation, both the Brazilian splitting method and the double punch test method yielded consistent and reliable results, demonstrating their suitability for assessing CGS-based concrete. Full article

Seawater Coral Aggregate Concrete (SCAC), made using coral aggregates from marine environments, is gaining attention as a promising material for marine and coastal engineering applications. This study investigates the dynamic tensile behavior of SCAC under both dry and saturated conditions, with an emphasis on the effects of free water on its mechanical properties. The dynamic Brazilian splitting (DBS) tests were conducted to evaluate the dynamic tensile strength, strain rate sensitivity, failure modes, and fracture morphology of SCAC specimens. The results show that saturated SCAC specimens exhibit a reduction in dynamic tensile strength compared to dry specimens, with this difference becoming more pronounced at higher strain rates. The maximum reduction can be observed to be 17.87%. Additionally, saturated SCAC specimens demonstrate greater strain rate sensitivity than dry specimens, which highlights the significant influence of moisture on the material’s mechanical behavior. The failure modes of SCAC were found to be less severe under saturated conditions, suggesting that moisture suppresses crack propagation to some extent, thereby reducing brittleness. Numerical simulations based on the finite element analysis were conducted to simulate the dynamic tensile response; the comparison of numerical and experimental data indicates that adjusting material model parameters effectively simulates the behavior of saturated SCAC. Full article

Existing studies have not considered the impact of interface bond strength on the ease of crack propagation at the cement sheath interface. Through Brazilian splitting and direct shear tests, the normal and shear bond strengths at interfaces I and II of a cement sheath were quantified. Based on this, a crack propagation model for the cement sheath interface was established using cohesive zone elements. The propagation characteristics of cracks along the axial and circumferential directions at interfaces I and II of a cement sheath during hydraulic fracturing were analyzed, along with their influencing factors. The results show that, due to the difference in interface bond strength, the crack propagation rate and length at interface I in the axial direction are greater than those at interface II, while the interface II crack is more likely to propagate in the circumferential direction. The elastic modulus of the cement sheath is a key factor affecting the integrity of the cement seal. Both excessively low and high elastic moduli can lead to different forms of failure in the cement sheath. It is recommended to control the elastic modulus of the cement sheath between 7 and 8 GPa. As the internal casing pressure increases, the axial propagation length of cement sheath interface cracks also increases. During fracturing, reducing pump pressure can reduce the axial crack propagation length in the cement sheath, alleviating or preventing the risk of fluid migration between stages and clusters. The findings of this study provide theoretical references and engineering support for the control of cement sheath seal integrity. Full article

Fractures and voids are widely distributed in slope rock masses. These defects promote crack initiation and propagation, ultimately leading to rock mass failure. Investigating their damage evolution mechanisms and strength characteristics is of significant importance for slope hazard prevention. A numerical simulation study of Brazilian splitting tests on disk samples containing prefabricated holes and fractures was conducted using the Finite Element Method with Cohesive Zone Modeling (FEM-CZM) in ABAQUS by embedding zero-thickness cohesive elements within the finite element model. This 2021 study analyzed the effects of fracture angle and length on tensile strength and crack propagation characteristics. The results revealed that when the fracture angle is small, cracks initiate near the fracture and propagate and intersect radially as the load increases, ultimately leading to specimen failure, with the crack coalescence pattern exhibiting local closure. As the fracture angle increases, the initiation location of the crack shifts. With an increase in fracture length, the crack initiation position may transfer to other parts of the fracture or near the hole, and longer fractures may result in more complex coalescence patterns and local closure phenomena. During the tensile and stable failure stages, the load–displacement curves of samples with different fracture angles and lengths exhibit similar trends. However, the fracture angle has a notable impact on the curve during the shear failure stage, while the fracture length significantly affects the peak value of the curve. Furthermore, as displacement increases, the proportion of tensile failure undergoes a process of rapid decline, slow rise, and then rapid decline again before stabilizing, with the fracture angle having a significant influence on the proportion of tensile failure. Lastly, as the fracture angle and length increase, the number of damaged cohesive elements shows an upward trend. This study provides novel perspectives on the tensile behavior of fractured rock masses through the FEM-CZM approach, contributing to a fundamental understanding of the strength characteristics and crack initiation mechanism of rocks under tensile loading conditions. Full article

To investigate the dynamic compressive and tensile mechanical properties and failure modes of shale, split Hopkinson pressure bar (SHPB) and high-speed imaging and digital image correlation (DIC) technologies were adopted. Dynamic impact compression and Brazilian splitting tests of shale samples at five different bedding angles of 0°, 30°, 45°, 60°, and 90° (angles between the dynamic compressive loading direction or the actual dynamic tensile loading direction and the normal direction of the bedding planes) were conducted to reveal the influence of the bedding angle, strain rate, and impact velocity on the dynamic compressive and tensile mechanical properties and failure modes of shale. The experimental results indicate that the dynamic compressive and tensile strengths, as well as the failure modes, of shale exhibit significant anisotropy. The dynamic strength of the shale increased with the strain rate and impact velocity, while it decreased initially and then increased with the increase in the bedding angle. The failure modes of shale under dynamic compressive and tensile loads are closely related to the bedding angle, strain rate, and impact velocity. Full article

Hydraulic fracturing of gas and oil reservoirs is the primary stimulation method for enhancing production in the field of petroleum engineering. The hydraulic fracturing technology plays a crucial role in increasing shale gas production from shale reservoirs. Understanding the effects of reservoir and fracturing conditions on fracture propagation is of great significance for optimizing the hydraulic fracturing process and has not been adequately explored in the current literature. In the context of shale reservoirs in Yibin, Sichuan Province, China, the study selects outcrops to prepare samples for uniaxial compression and Brazilian splitting tests. These tests measure the compressive and tensile strengths of shale in parallel bedding and vertical bedding directions, obtaining the shale’s anisotropic elastic modulus and Poisson’s ratio. These parameters are crucial for simulating reservoir hydraulic fracturing. This paper presents a numerical model utilizing a finite element (FE) analysis to simulate the process of multi-cluster hydraulic fracturing in a shale reservoir with natural fractures in three dimensions. A numerical simulation of the intersection of multiple clusters of 3D hydraulic fractures and natural fractures was performed, and the complex 3D fracture morphologies after the interaction between any two fractures were revealed. The influences of natural fractures, reservoir ground stress, fracturing conditions, and fracture interference concerning the spreading of hydraulic fractures were analyzed. The results highlight several key points: (1) Shale samples exhibit distinct layering with significant anisotropy. The elastic compressive modulus and Poisson’s ratio of parallel bedding shale samples are similar to those of vertical bedding shale samples, while the compressive strength of parallel bedding shale samples is significantly greater than that of vertical bedding shale samples. The elastic compressive modulus of shale is 6 to 10 times its tensile modulus. (2) The anisotropy of shale’s tensile properties is pronounced. The ultimate load capacity of vertical bedding shale samples is 2 to 4 times that of parallel bedding shale samples. The tensile strength of vertical bedding shale samples is 2 to 5 times that of parallel bedding shale samples. (3) The hydraulic fractures induced by the injection well closest to the natural fractures expanded the fastest, and the natural fractures opened when they intersected the hydraulic fractures. When the difference in the horizontal ground stress was significant, natural fractures were more inclined to open after the intersection between the hydraulic and natural fractures. (4) The higher the injection rate and viscosity of the fracturing fluid, the faster the fracture propagation. The research findings could improve the fracturing process through a better understanding of the fracture propagation process and provide practical guidance for hydraulic fracturing design in shale gas reservoirs. Full article

This study investigates the tensile properties of granite subjected to cyclic thermal treatment under cyclic loading-unloading conditions, which is of great significance for the modification of hot dry rock reservoirs. Brazilian splitting tests under cyclic loading-unloading were conducted on granite samples exposed to 400 °C cyclic water-cooling shock (applied for 1, 3, 5, and 7 cycles) at different preset load upper limits (65%, 70%, 75%, and 80% of the peak load). The experimental results reveal the evolution of the tensile properties of granite under the combined effects of 400 °C cyclic water-cooling shock and cyclic loading-unloading. The findings indicate that the tensile strength of granite decreases with an increasing number of cyclic water-cooling shocks and further declines as the preset load upper limit decreases. Under typical conditions, the peak displacement of granite exhibits three distinct stages with increasing loading-unloading cycles: rapid increase, slow increase, and eventual failure. During the slow increase stage, peak displacement decreases due to an increase in elastic stiffness. Initially, elastic stiffness increases with the number of cycles, followed by a stabilization phase, and subsequently declines. After granite failure, macroscopic failure cracks gradually deviate from the center as additional cyclic water-cooling shocks are applied. In contrast, cyclic loading-unloading has a minimal effect on macroscopic cracks. Furthermore, as the number of cycles increases, microcrack evolution transitions from intergranular to transgranular cracking. Under cyclic loading-unloading conditions, these cracks continue to propagate, ultimately forming a fracture network. The findings of this study provide a theoretical foundation for the fracturing and modification of hot dry rock reservoirs. Full article

An accurate understanding of the tensile mechanical behavior of shale rock is essential for optimizing shale gas drilling and hydraulic fracturing operations. However, the mechanical behavior of shale is significantly influenced by its anisotropy. Therefore, this study investigated the tensile mechanical behavior of layered shale by combining acoustic emission (AE) monitoring with two testing methods: the Brazilian splitting test (BST) and a novel direct tensile test (DTT). The impact of anisotropy on the tensile mechanical behavior and failure modes of layered shale under different test methods was evaluated. Additionally, seven anisotropic tensile strength criteria were compared and validated using the experimental results. The results show that: (1) As the loading angle (β) increased, the tensile strength measured by both BST and DTT increased. Both methods exhibited maximum tensile strength at β = 90° and minimum tensile strength at β = 0°. The anisotropy ratios for BST and DTT were 1.52 and 2.36, respectively, indicating the significant influence of the loading angle on tensile strength. (2) The AE results indicated that both DTT and BST specimens exhibited brittle failure characteristics. However, the DTT specimens demonstrated more pronounced progressive failure behavior, with failure modes categorized into four types: tensile failure across the bedding plane, shear failure along the bedding plane, and two types of tensile–shear mixed failure. In contrast, the BST specimens primarily exhibited tensile–shear mixed failure, except for tensile failure along the bedding plane at β = 0° and tensile failure across the bedding plane at β = 90°. (3) Neither of the two test methods could fully eliminate the influence of anisotropy, but three anisotropic tensile criteria, the Lee–Pietruszczak criterion, the critical plane approach criterion, and the anisotropic mode I fracture toughness criterion based on the stress–strain transformation rule demonstrated high accuracy in predicting tensile strength. Furthermore, in alignment with previous studies, the indirect tensile strength of various rock types was found to range between one and three times the direct tensile strength, and a linear correlation between the two variables was established, with a coefficient of approximately 1.11. Full article

To investigate the stratification effect on rock splitting and Mode I fracture characteristics, standard Brazilian splitting disk specimens and straight-crack disk specimens were subjected to splitting loading tests, and a high-speed camera system and acoustic emission (AE) system were used to study the rocks’ mechanical properties, fracture parameters, and AE characteristics. The results demonstrate the following: (1) The tensile strength and fracture toughness of the layered rock exhibit significant stratification effects, gradually decreasing with the increase in the number of layers and the layer angle. (2) The different angles of the stratification planes lead to the diversity of failure modes in the disk specimens. (3) The S-value and the cumulative AE count curve of specimens without prefabricated cracks show two types of pattern during loading: fluctuating increase mode, and “gentle–steep” increase mode. (4) Layered rock specimens exhibit a low ratio of rise time to voltage amplitude (RA) value and high average frequency (AF) characteristics during fracture, and the shear failure mainly occurs during the stable propagation phase after the initiation of macroscopic cracks. (5) The fracture process zone (FPZ)’s length at the peak point of the specimens decreases exponentially with the increase in the number of layers, but this reduction does not go on indefinitely, and there exists a minimum value. Within the range of 0° to 60°, the FPZ length decreases linearly with increasing stratification angle. Full article

Over the past 30 years, China has emerged as the country with the world’s largest engineering construction industry. However, rockbursts induced by tunnel excavation in rock engineering have resulted in a substantial number of casualties and extensive property damage. Understanding the brittle failure behavior of rock masses and identifying the mechanism of rockbursts have become critical challenges in the field. Physical model tests can provide a more intuitive simulation of the rockburst process. The selection and proportioning of materials similar to brittle rocks are crucial factors for the success of these model tests. This study selected refined iron powder, barite powder, quartz sand, gypsum powder, and a rosin–alcohol solution to prepare rockburst simulation materials characterized by a low strength and high brittleness. The rockburst tendency and brittleness indices were introduced, and an orthogonal experimental design was used to establish 25 different formulation schemes. The influence of the material component proportions on the physical and mechanical properties of the specimens, as well as their brittleness characteristics, was systematically analyzed. A multiple linear regression analysis was conducted to derive linear regression equations for the physical and mechanical parameters of the brittle rock simulation materials. In addition, simulation materials and standard specimens of Jinping marble were prepared. The brittle failure modes and acoustic emission characteristics of the specimens under uniaxial compression and Brazilian splitting conditions were analyzed. The results indicate that component proportions significantly affected the physical and mechanical properties of the specimens. The refined iron powder–barite powder ratio, as well as the concentration of the rosin–alcohol solution, played a primary role in controlling the physical and mechanical parameters of the brittle rock simulation materials. Full article

A universally applicable direct tension test method is proposed in this paper based on the concept of “compression-to-tension”. Using this method, one or two typical rocks were selected for each of the three types of rocks. The testing results of the direct tension method proposed were compared with the internationally recommended Brazilian splitting method to validate the feasibility of the direct tension method. Results showed that the tensile strengths of six typical rocks were consistent using the direct tensile test method proposed in this study and the Brazilian splitting method recommended internationally. The direct tensile strength deviation coefficient (C_v) of the six types of rocks was less than 0.1, indicating very small variability. In this study, the deviation coefficient (C_v) of the axial displacement corresponding to the tensile strength in both the direct tensile and indirect tensile tests was also less than 0.1, reflecting minimal variability. This shows the consistency of the two tensile test results to a certain extent, and also shows that the direct tensile test method is feasible to determine the tensile strength of rock. Full article

This study investigates the effect of thermal cracking on the tensile strength of granite through a combination of experimental testing and numerical simulations. The primary objective is to understand how thermal stress, induced by heat treatment at various temperatures (25 °C to 600 °C), influences crack initiation, propagation, and tensile strength changes. The granite specimens were subjected to Brazilian splitting tests after heat treatment, and the load–displacement curves and tensile strength variations with heat treatment temperature were analyzed. A grain-based model (GBM) was developed to simulate the complex cracking behavior, incorporating the mineral compositions and thermal expansion properties of the granite. The fractal dimension of the cracks was quantified using the box-counting method, and the relationship between fractal dimension and tensile strength was discussed. The results show that the GBM can effectively simulate the microcracking behavior and tensile fracture properties of heat-treated granite, accounting for mineral composition and thermal expansion. Thermal cracks are mainly intergranular tensile cracks, which increase in number with higher temperatures, while under mechanical loading failure is primarily due to intragranular tensile cracks. Higher heat treatment temperatures lead to denser crack networks with greater fractal complexity, reducing tensile strength and creating more tortuous crack propagation paths. Full article

To investigate the fracture mechanism of rock–concrete (R–C) systems with an interface crack, Brazilian splitting tests were conducted, with a focus on understanding the influence of the interface crack angle on failure patterns, energy evolution, and RA/AF characteristics. The study addresses a critical issue in rock–concrete structures, particularly how crack propagation differs with varying crack angles, which has direct implications for structural integrity. The experimental results show that the failure paths in R–C disc specimens are highly dependent on the interface crack angle. For crack angles of 0°, 15°, 30°, and 45°, cracks initiate from the tips of the interface crack and propagate toward the loading ends. However, for angles of 60°, 75°, and 90°, crack initiation shifts away from the interface crack tips. The AE parameters RA (rise time/amplitude) and AF (average frequency) were used to characterize different failure patterns, while energy evolution analysis revealed that the highest percentage of energy consumption occurs at a crack angle of 45°, indicating intense microcrack activity. Moreover, a novel tensile strength prediction model, incorporating macro–micro damage interactions caused by both microcracks and macrocracks, was developed to explain the failure mechanisms in R–C specimens under radial compression. The model was validated through experimental results, demonstrating its potential for predicting failure behavior in R–C systems. This study offers insights into the fracture mechanics of R–C structures, advancing the understanding of their failure mechanisms and providing a reliable model for tensile strength prediction. Full article