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Water is a crucial factor that influences the mechanical behavior of rock and can induce failure. The aim of this study was to investigate the effects of water content and water distribution on the mechanical behavior of sandstone subjected to triaxial compression. Sandstone samples under different water states, including dry, unsaturated, saturated, and long-term saturated, were prepared. Some saturated samples were dried in air for different periods to prepare samples that were dry on the outside but remained wet on the inside. The triaxial compression tests were performed on the samples under four confining pressures: 5, 10, 15 and 25 MPa. The results indicated that the water state of a sandstone sample can be characterized by its water content, soaking time, water distribution, and long-term saturation. The different water states can affect remarkably the strength, deformation, and failure pattern of the sandstone samples subjected to triaxial compression. Several empirical equations were established to describe the evolution of the mechanical behavior of sandstone with increasing water content and soaking duration. The nonuniform distribution of water promotes the generation of tensile cracks, while increasing confining pressure promotes the development of shear cracks. The strength of sandstone samples with dry outside and wet inside are lower than that of samples with wet outside and dry inside. What is more, a long-term saturated state increases the strain-softening of the sandstone samples, thereby increasing their deformation and lowering their strength.HighlightsInfluence of water state including water content and water distribution on sandstone is comprehensively investigated by triaxial experiments.A series of predictive empirical equations are established to describe the evolutions of the mechanical properties of sandstone with water content/soaking duration.The strengths of the sandstone samples in the dry outside but wet inside state are smaller than those of samples in the wet outside but dry inside state.A long-term saturated state increases the softening of the rock, thereby increasing the deformation and lowering the strength.Water especially nonuniformly distributed water promotes the propagation of tensile cracks near the surface of the rock.

Water inrush represents a significant hazard in karst tunnels, and the water-resistant rock mass plays a pivotal role in water inrush incidents. It is of great significance to explore the coupled hydro-mechanical failure process of water-resistant rock mass under excavation. A series of triaxial compression tests with different pressure conditions and loading rates were conducted in this study to investigate progressive failure characteristics and permeability evolution of limestone specimens. The experimental findings demonstrate that loading rate and confining pressure significantly influence progressive failure characteristics and permeability evolution. Additionally, a decreasing-then-increasing trend is observed in the crack initiation threshold (σci), and the crack damage threshold (σcd) exhibits a decreasing tendency. The resulting σci/σp ratio increases while the σcd/σp ratio decreases, indicating a diminishing stable fracture development stage. In general, permeability experiences an increase within an order of magnitude during the progressive failure of rock specimens, and the permeability of the rock mass decreases with rising initial confining pressure. The rate of deformation induced over the duration of permeability measurement exhibits a positive correlation with the stress ratio. Lastly, we propose a potential hysteresis water inrush mechanism, drawing upon the aforementioned observations. It is deduced that the creation, expansion, and ultimate penetration of fractures within the rock mass, resulting from the combined effects of excavation stress disturbance and changed pore pressure, give rise to water inrush events in karst tunnels.HighlightsStudy of progressive failure characteristics and permeability evolution of limestone.Loading rate and confining pressure significantly influence progressive failure characteristics.Permeability experiences an increase within an order of magnitude.Deformation induced by permeability measurement exhibits a positive correlation with the stress ratio.Investigation of water inrush mechanism linking construction disturbance and pore pressure.

Under the action of a load, internal pores and cracks expand, and irreversible plastic deformations occur. Compared with conventional rock mechanics tests, nuclear magnetic resonance (NMR) can characterize the size and distribution of pores at the microscopic scale. In this study, a series of low-confining-stress triaxial compression tests were performed on different types of sandstone samples using real-time T2-weighted NMR spectra and imaging. It was found that the area of macropores in sandstone significantly increased only during the initial loading stage, but played an opposite role in the damage evolution process. This phenomenon is contrary to our expectations and provides a new basis for understanding the evolution of damage in rocks. Furthermore, during the linear deformation stage, the number of pores and mesopores increased, whereas the number of macropores decreased. A damage model based on the NMR results is proposed. The value of Dn sharply increases during the initial stage due to the expansion of pores, then decreases, and finally begins to increase again before the failure stage and until the sample fractures owing to the development of macroscopic cracks. In conclusion, the structure of micropores has a significant influence on the failure mode of sandstone rocks in low-confining-pressure triaxial compression tests.HighlightsThe area of macropores in sandstone significantly increased only during the initial loading stage and played an opposite role in the damage evolution process.During the linear deformation stage, the number of pores and mesopores increased, whereas the number of macropores decreased.The structure of micropores has a significant influence on the failure mode exhibited by sandstone during low-confining-pressure triaxial compression tests.

Effective methane extraction in underground coal seams can improve the efficient utilization of fossil energy and reduce the risk of safety accidents in coal mines. Pulsating nitrogen fatigue fracturing technology is proposed as a novel and effective method to enhance gas production in low-permeability coal seams and improve gas extraction efficiency. In this study, a pulsating gas test system was established to apply fatigue fracturing of pulsating nitrogen to low-permeability coal. Mercury intrusion tests, wave velocity tests, and triaxial compression tests were used to reveal the changes in microstructure and mechanical properties of low-permeability coal under fatigue fracturing tests. Results show that the residual deformation of the coal changes considerably under fatigue fracturing. The strain of the coal is characterized by a periodic “expansion–contraction” variation with the intrusion and discharge of the pulsating nitrogen, and the residual strain increases gradually in this process. After fatigue fracturing, facilitates the seepage of gas and the development of micropores and transition pores toward mesopores and macropores enhances the permeability of the coal. The specific surface area of the pores is considerably improved in the transition pores and mesopores. The pore fractal dimension of the coal tends to decrease under fatigue fracturing, resulting in a more uniform distribution of pores and enhanced interpore connectivity within the coal. The fatigue period is negatively correlated with the strength and wave velocity evolution of the coal, and the peak intensity first decreases rapidly and then gradually stabilizes. The fatigue fracturing results in a substantial increase in the spatial complexity and connectivity of the fracture distribution throughout the specimen. On the basis of the evolution characteristics of residual volumetric strain, a fatigue damage model was constructed to analyze the characteristics of fatigue deformation and failure of coal.HighlightsThe pulsating nitrogen fatigue test of low-permeability coal was carried out.The change of residual deformation of coal under pulsating nitrogen fatigue is obtained.The influence mechanism of pulsating nitrogen fatigue on pore structure is analyzed.The macromechanical parameters and damage evolution characteristics under pulsating nitrogen fatigue are evaluated.

The increasing demand for a wider access to additive manufacturing technologies is driving the production of metal lattice structure with powder bed fusion techniques, especially laser-based powder bed fusion. Lattice structures are porous structures formed by a controlled repetition in space of a designed base unit cell. The tailored porosity, the low weight, and the tunable mechanical properties make the lattice structures suitable for applications in fields like aerospace, automotive, and biomedicine. Due to their wide-spectrum applications, the mechanical characterization of lattice structures is mostly carried out under compression tests, but recently, tensile, bending, and fatigue tests have been carried out demonstrating the increasing interest in these structures developed by academy and industry. Although their physical and mechanical properties have been extensively studied in recent years, there still are no specific standards for their characterization. In the absence of definite standards, this work aims to collect the parameters used by recent researches for the mechanical characterization of metal lattice structures. By doing so, it provides a comparison guide within tests already carried out, allowing the choice of optimal parameters to researchers before testing lattice samples. For every mechanical test, a detailed review of the process design, test parameters, and output is given, suggesting that a specific standard would enhance the collaboration between all the stakeholders and enable an acceleration of the translation process.

Microbially induced calcite precipitation (MICP) is used increasingly to improve the engineering properties of granular soils that are unsuitable for construction. This shows MICP technique significant advantages such as low energy consumption and environmentally friendly feature. The objective of the present study is to assess the strength behaviour of bio-cemented sand with varying cementation levels, and to provide an insight into the mechanism of MICP treatment. A series of isotropic consolidated undrained compression tests, calcite mass measurement and scanning electron microscopy tests were conducted. The experimental results show that the strength of bio-cemented sand depends heavily on the cementation level (or calcite content). The variations of strength parameters, i.e. effective friction angle φ ′ and effective cohesion c ′, with the increase in calcite content can be well evaluated by a linear function and an exponential function, respectively. Based on the precipitation mechanism of calcite crystals, bio-clogging and bio-cementation of calcite crystals are correlated to the amount of total calcite crystals and effective calcite crystals, respectively, and contributed to the improvement in the effective friction angle and effective cohesion of bio-cemented sand, separately.

Dense topologically interlocked panels are made of well-ordered, stiff building blocks interacting mainly by frictional contact. Under mechanical loads, the deformation of the individual blocks is small, but they can slide and rotate collectively, generating high strength, toughness, impact resistance, and damage tolerance. Here, we expand this construction strategy to fully dense, 3D architectured materials made of space filling building blocks or “grains.” We used mechanical vibrations to assemble 3D printed rhombic dodecahedral and truncated octahedral grains into fully dense face-centered cubic and body-centered cubic “granular crystals.” Triaxial compression tests revealed that these granular crystals are up to 25 times stronger than randomly packed spheres and that after testing, the grains can be recycled into new samples with no loss of strength. They also displayed a rich set of mechanisms: nonlinear deformations, crystal plasticity reminiscent of atomistic mechanisms, geometrical hardening, cross-slip, shear-induced dilatancy, and microbuckling. A most intriguing mechanism involved a pressure-dependent “granular crystal plasticity” with interlocked slip planes that completely forbid slip along certain loading directions. We captured these phenomena using a three-length scale theoretical model which agreed well with the experiments. Once fully understood and harnessed, we envision that these mechanisms will lead to 3D architectured materials with unusual and attractive combinations of mechanical performances as well as capabilities for repair, reshaping, on-site alterations, and recycling of the building blocks. In addition, these granular crystals could serve as “model materials” to explore unusual atomic scale deformation mechanisms, for example, non-Schmid plasticity.

Experimental results provide strong evidence that the deformation and strength of rocks are closely related to the damage suffered during loading. The classic constitutive models such as Mohr–Coulomb criterion and the Drucker–Prager criterion are usually unable to describe the non-linear deformation behavior of rocks, including strain hardening and softening. Their applications in numerical analysis and practical engineering are limited. For this purpose, an elastoplastic damage constitutive model that takes into account the competition mechanism between damage and strain hardening or softening during rock compression is proposed herein. This model is used to investigate the deformation behaviors and damage evolution of rocks. More importantly, this model has been implemented by finite element programming code and verified by a series of triaxial compression tests. The comparison results between theoretical analysis and experimental data indicate that this model can well describe the stress–strain curves and damage–strain curves of the investigated rocks (sandstone and salt rock), especially the characteristics of softening, hardening and residual strength. Based on parametric analysis, the influence of confining pressure, scale parameter and shape parameter on rock damage is revealed. It is found that the rock scale parameter in this model has a power function relationship with confining pressure. The ultimate plastic deformation that rocks can withstand is related to the scale parameter. The shape parameter controls the residual strength and deformation of rocks. Model results demonstrate that the strength and deformation vary with rock properties, and are strongly dependent on the stress-induced damage and strain characteristics.

The purpose of the present study is to investigate the initial irreversible critical damage state of red mudstone during the elastic stage of the stress–stain curve. First, a series of uniaxial and triaxial compression tests were carried out using a digital image correlation (DIC) based high spatial and temporal resolution 3D visualization test system of multi-field coupling damage of rocks. The time-dependent strain on the surface of rock samples during the test process was captured by DIC technology. The initial irreversible critical damage state of red mudstone was then investigated by analyzing damage process in macroscopic and mesoscopic scales in an integrated manner through developing theoretical models based on critical deceleration theory. It demonstrates that the developed models can recognize the initial irreversible critical damage point of the rocks by analyzing the DIC-measured deformation behavior of the samples in both macroscopic and mesoscopic level.HighlightsStudied the variance increase characteristics of mesoscopic strain at the crack site of red mudstone during the elastic stageProposed the initial irreversible critical damage state of red mudstone under the influence of strength and confining pressureStudy results enrich the theory of rock mechanics and can be applied to tunnel design or slope stability investigation, etc.

The failure and deformation mechanism of sandstone and mudstone has always been highlighted in research on mining engineering. To further investigate the failure and deformation mechanism of sandstone and mudstone, a damage definition was proposed to describe the failure mechanism of a rock specimen with micro-defects and inhomogeneity; the Weibull distribution function was used to illustrate the dispersion of mechanical properties (i.e. damage extent) of rock; the nonconstant terms of Z–P yield function was employed to describe the strength of rock elements. Based on the framework of the continuum damage mechanics and strain equivalence hypothesis, a continuous damage constitutive model was established. Finally, triaxial compression tests on sandstone and mudstone taken from the Chensilou coal mine were conducted to verify the reliability of the proposed model. The results show that the damage evolution curve presents the shape of a square root sign, The damage evolution of the rock specimens can be divided into six stages: (1) initial damage stage, (2) damage-weakening stage, (3) slight-increased damage stage, (4) rapidly-increased damage stage, (5) rock failure stage and (6) rock slippage stage. The proposed damage evolution contributes to establishing the constitutive model of the stress–strain relationship of sandstone and mudstone in the mining field.HighlightsProcessing a continuous damage statistical constitutive model for sandstone and mudstone based on triaxial compression tests.The damage evolution curve presents the shape of a square root sign.The damage evolution of the rock: initial damage-damage weakening-slight increased-rapidly increased-rock failure-rock slippage.