Uniaxial Compression Conditions Research Articles

Study on mechanical properties and acoustic emission characteristics of composite rock mass with different thicknesses of weak interlayer.

To study the influence of the thickness of a weak interlayer on the deformation and failure characteristics of composite rock mass. Based on the measured data of sand-mudstone interbedded floor rock mass in the Yima mining area, Henan Province, China. Using quartz sand, gypsum, and cement as similar materials make composite rock-like specimens with weak interlayers. The mechanical properties, deformation evolution process, failure characteristics, and acoustic emission laws of rock samples with different interlayer thicknesses under uniaxial compression were studied using the digital image correlation method and acoustic emission monitoring technology. The results show that: (1) With an increase in the thickness of the weak interlayer, there is a corresponding decrease in the compressive strength of the specimen. When the thickness of the weak interlayer exceeds 5cm, the compressive strength of the composite rock specimen is even less than that of the single weak rock-like. (2) Under uniaxial compression conditions, the strain concentration zone first appears in the weak interlayer part of the composite rock-like specimen. As the pressure continues to increase, the specimen is the first to fail at the position of the maximum strain concentration zone. (3) The thickness of the weak interlayer is an important factor affecting the failure mode of composite rock-like specimens. With the increase of the thickness of the weak interlayer, the composite rock-like specimens change from overall failure to local failure. (4) With the increase of the thickness of the weak interlayer in the middle of the composite rock specimen, the maximum value of the energy released by the single acoustic emission event and the maximum value of the cumulative acoustic emission energy decrease during the compression failure process. The research results can provide a reference for the manufacture of composite rock mass with a weak interlayer and the deformation and failure analysis of composite rock mass.

Mechanical properties and energy evolution of outburst coal seams under different load regimes

AbstractCoal elasticity and gas expansion are important factors for coal and gas outburst. During the outburst process, the elastic strain energy of coal is mainly released from the stress region, and the gas expansion energy near the working face is larger, and it is not a continuous release process. To reveal the mechanical characteristics and energy evolution of outburst coal seam, uniaxial and triaxial compression tests were carried out on outburst coal seam samples under different loading methods. The experimental results show that the elastic characteristics become more obvious with the increase of loading rate, the peak strain increases, the elastic modulus is linearly related with the loading rate，and the overall degree of fragmentation increases with the increase of loading rate, which is consistent with the severity of macroscopic coal failure. The failure mode of coal under uniaxial compression conditions is often manifest as brittle failure. The strength characteristics of coal under different loading rates comply with the Mohr‐Coulomb criterion, and the peak strength is linearly related to the failure time and loading rate. With the increasing confining pressure causes the failure of coal samples to transition from ductile to brittle, and the failure mode develops from local shear to overall splitting. The elastic energy evolution curve is consistent with its stress‐strain curve. With the increase of confining pressure, the limiting elastic energy and peak total energy increase in a quasi‐linear manner. The accumulated limit elastic energy plays an important role in the failure of coal samples, and the macroscopic manifestation thereof is that the coal samples fail more severely under high confining pressure conditions than under low confining pressure conditions. The research results are of great significance for the comprehensive prevention and control of coal and gas outburst.

Experimental determination of the coefficient of viscosity of the dry snow

Snow compaction is a natural process in the evolution of any snow cover, and this obviously changes the physical and mechanical properties of a snow thickness. The quantitative estimate of this process is a coefficient of the snow viscosity, which is linearly related to the rate of the snow cover deformation The technique for preparation and testing of samples with the fine-grain snow under condition of uniaxial compression at negative temperatures is described. It was determined that values of the snow viscosity coefficient fall within the range from 0.10 to 0.30 g/cm³ for three temperature intervals. The viscosity coefficient of the fine-grain snow varies from 10⁸ до 10¹⁰ Па. The data obtained made possible to find the limits of variability of this coefficient for winter conditions on the Russian plain. A dependence of the viscosity coefficient on the snow density and temperature is proposed, and it describes the series of the above shown experimental data. It is shown that influence of changes in the viscosity coefficient of fine-grain snow on the snow properties depending on its density are comparable in scale to the influence of the temperature. The result may be useful for modeling the evolution of snow cover.

Experimental Investigation on Rock Failure Characteristics of Large-Span Goafs Using Digital Image Correlation Analysis and Acoustic Emission Monitoring

Rockmass in deep mining is highly susceptible to large-scale collapses under high stress and blast-induced disturbances, leading to casualties and economic losses. To investigate the evolution characteristics of goaf instability and the types of seismic sources that induce instability, an experiment on goaf instability was designed under uniaxial compression conditions based on actual mining operations. The entire experimental process was monitored using digital image correlation analysis and acoustic emission monitoring. By calculating the digital speckle field on the surface of the rock specimen during the experiment, the evolution characteristics of the deformation and strain fields from the beginning of loading to complete failure were analyzed. The study explored the dynamic behavior of cracks from initiation to propagation and eventually inducing large-scale collapse. The results show that the instability process of the goaf begins with the formation of tensile cracks. As stress increases, shear cracks occur in the specimen, leading to macroscopic failure. Furthermore, based on the differences in overall microfracture types measured by RA-AF characteristic parameters during specimen failure, large amplitude acoustic emission events corresponding to the formation of dominant macroscopic cracks were selected, and the focal mechanisms of these events were inverted. The results indicate that shear failure sources are significantly more prevalent than tensile failure sources in acoustic emission events leading to goaf instability. These findings can provide useful guidance for the support design and the prevention and control of rockmass instability disasters.

Progressive failure of frozen sodium sulfate saline sandy soil under uniaxial compression

The progressive failure characteristics of geomaterial are a remarkable and challenging topic in geotechnical engineering. To study the effect of salt content and temperature on the progressive failure characteristics of frozen sodium sulfate saline sandy soil, a series of uniaxial compression tests were performed by integrating digital image correlation (DIC) technology into the testing apparatus. The evolution law of the uniaxial compression strength (UCS), the failure strain, and the formation of the shear band of the frozen sodium sulfate saline sandy soil were analyzed. The test results show that within the scope of this study, with the increase of salt content, both the UCS and the shear band angle initially decrease with increasing salt content before showing an increase. In contrast, the failure strain and the width of the shear band exhibit an initial increase followed by a decrease in the samples. In addition, to investigate the brittle failure characteristics of frozen sodium sulfate saline sandy soil, two classic brittleness evaluation methods were employed to quantitatively assess the brittleness level for the soil samples. The findings suggest that the failure characteristics under all test conditions in this study belong to the transition stage between brittle and ductile, indicating that frozen sodium sulfate saline sandy soil exhibits certain brittle behavior under uniaxial compression conditions, and the brittleness index basically decreases and then increases with the rise in salt content.

Study on influencing factors of controllable mechanical behavior of rock-like porous materials.

Due to the discrete and non-homogeneous of similar materials and the inability to realize large-size and original scale modeling, it is difficult to restore the structure and stress state of underground coal and rock mass in similar simulation tests. To solve this problem, a lightweight and suitable for large-scale modeling similar material, rock-like porous material has been developed. The quasi-static uniaxial compression experiment was carried out by using the large tonnage multi-module electronic control test system. And the influencing factors of controllable mechanical behavior of rock-like porous materials were studied. The results showed that, under uniaxial compression conditions, the material stress-strain curve exhibits three phases: elastic stage, failure stage, and platform stage. The uniaxial compressive strength, elasticity modulus, stress drop, and softening modulus of rock-like porous materials basically increase with the increase of density. The stress after peak strength changes from a slow decrease to a "stepped" or even "cliff like" downward trend. Polypropylene fibers have the effect of enhancing the uniaxial compressive strength, elasticity modulus, stress drop, softening modulus, shear deformation, and residual strength stability of rock-like porous materials. The rock-like porous material has a critical loading velocity, and it increases with density. At the critical loading velocity, the material shows obvious shear failure, and the shear inclination angle is the largest, and so is the uniaxial compressive strength. Through the experimental research, the influence laws of density, polypropylene fiber, and loading velocity on the failure mode, mechanical parameters, and mechanical behavior of the material are clarified, and the quantitative relationship between density and each mechanical parameter is obtained. The research is helpful to realize the accurate control of mechanical behavior of rock-like porous materials and further inverts the deformation and failure mechanism of underground coal and rock structures through indoor similar simulation tests.

Understanding a new wrinkling behavior of annular grooved panel during flexible free incremental sheet forming

Wrinkling behavior of sheet metal by inappropriate process parameters and the resultant variation of thickness and microstructure during incremental sheet forming (ISF) have significant impact on forming stability and geometric profile. The wrinkling-related mechanism is yet to be well understood, which make it difficult to effectively control wrinkling defects during ISF. In the present work, the wrinkling behavior and related mechanism of a typical annular grooved panel are investigated during a novel flexible free ISF process. Experiments demonstrate that during this ISF process, the sheet material sequentially undergoes 4 deformation modes: contacting state under plane strain condition, contracting state with only elastic deflection, thickening state under uniaxial compression condition, and wrinkling state where the circumferential compressive stress exceeds the critical wrinkling stress. The evolution of these deformation modes is dominantly caused by the increasing of edge contraction and circumferential stress, which are induced by the bending moment from the forming tool and auxiliary sheets. Based on the mechanism, an analytical model is developed to predict the wrinkling behavior, by which the corresponded sheet thickness, bending moment, circumferential compressive stress and critical wrinkling stress during the wrinkling process could be comprehensively calculated. Experimental results validate that the analytical model can accurately predict the wrinkling behavior under different process parameters, material properties and auxiliary sheet materials. Through microstructural characterization, numerical simulation and analytical modeling, the influence of process parameter and material property on wrinkling behavior is systematically investigated, along with the variation of sheet thickness and microstructure under different wrinkling modes. This work enhance a deep understanding of wrinkling behavior during ISF process, thereby providing effective methods for process optimization and quality improvement.

Mechanical properties and multi-field ferroelastic response of Nb/Mn Co-doped CaBi4Ti4O15 high-temperature ferroelectric ceramics

CaBi4Ti4O15 (CBT) ceramics are promising piezoelectric materials that have been widely studied for high-temperature applications. Despite significant advancements in the electrical performance of CBT ceramics, the understanding of their mechanical behaviors remains limited, which is unfavorable for designing ceramics with high stability and reliability during high-temperature service. This work investigated the mechanical properties and fracture behaviors of Nb/Mn co-doped CaBi4Ti4O15 (CBTNM) with various doping levels, focusing on stress-strain responses and ferroelastic deformation behaviors under uniaxial compression and multi-field coupling conditions. HRTEM analysis reveals small-scale layered domain wall structures on the surface of plate-like grains. The fracture and compressive strengths of CBTNM ceramics initially decrease and then increase with an increase in doping content, and the underlying mechanisms are related to grain size, defects, and densification. CBTNM ceramics exhibit nonlinear stress-strain responses due to ferroelastic deformation under compressive loading, and the resultant irreversible domain switching strain increases with an increase in doping content, while poling can further increase the residual strain. Under multi-field loading conditions, CBTNM ceramics undergo more ferroelastic deformation events and exhibit larger residual strain. The micro-cracks, pores, complicated fracture modes, and degraded fracture surfaces with fragmental and rough features are mainly responsible for the lower elastic modulus and inferior mechanical response. This work enhances our understanding of the mechanical behaviors of high-temperature piezoelectric ceramics and provides guidance for designing high-performance piezoelectric materials for complex environmental applications.

A New Method for the Dynamics Analysis of Super-Elastic-Plastic Foams under Inhomogeneous Loading and Unloading Conditions.

In this research, a new computational method was proposed for describing the mechanical behavior of super-elastic-plastic foams under inhomogeneous compressive impacts. The method regarded the foam material as composed of two typical mechanical properties superimposed multiple times: one was the hyper-elastic layer, and the other was the elastoplastic layer. The hyper-elastic layer and the elastoplastic layer were interwoven and overlapped, divided into double-layer, four-layer, and six-layer configurations to characterize the foam material. After the equivalent layering of the foam, by comparing the results of the four-layer and six-layer divisions, it was found that when the layering reached four layers, the foam performance curve had already converged. The study utilized the HYPERFOAM model and Mullins effect in the ABAQUS software to establish the constitutive relationship of the hyper-elastic layer. It adopted the Crushable foam model to develop the constitutive relationship of the elastoplastic layer. Under uniaxial compression conditions, quasi-static and intermediate strain rate compression tests were performed on polyethylene (PE) foam materials with three different densities. Based on the experimental results, the parameter values of the hyper-elastic-plastic foam model in the ABAQUS code were determined. By comparing the computational results and the experimental results, the established finite element (FE) model was validated using the mechanical behavior of indentation and compression tests. The results showed that this method could effectively describe the complex mechanical behavior and residual deformation of hyper-elastic-plastic foam packaging materials under non-uniform compression, and the experimental and simulation results agreed well, proving the reliability of this method.

Design of hybrid multi-crosslinked hydrogel with a combination of high stiffness, elastic applied in high temperature and high salinity reservoirs

The mechanical strength of conventional hydrogels is inadequate for preventing leakage in high-temperature and high-salinity reservoirs, primarily due to the increased mobility of polymer chains and the degradation of crosslinked networks. Herein, we employed the strategy of ’network nesting and layer reinforcement’ to design a hybrid multi-site crosslinked hydrogel. This approach enhances the interconnections between polymer chains and fortifies the crosslinked network. The shear rheological behavior and temperature sensitive characteristics of the profile control working fluids were evaluated based on rheological dynamics. In the high temperature (140 ℃) and high salinity (21.81 × 104 mg/L) preparation environment, the effects of different components to the mechanical strength of the hydrogel were investigated under dynamic shear and uniaxial compression conditions. The incorporation of N-(hydroxymethyl)acrylamide (NMA) in the copolymerization with acrylamide (AM) resulted in an increase in the covalent cross-linking density, leading to a rise in the shear elastic modulus from 105 Pa to 238 Pa. The use of pre-gelatinized cassava starch (PGCM) as the grafting framework enhanced the bonding between the polymer chains, as evidenced by the increase in uniaxial compressive strength from 0.018 MPa to 0.081 MPa and the rise in peak strain from 55.0 % to 72.9 %. Furthermore, the synergistic effect of fumed silica (AEROSIL), which exploits mechanical coupling between inorganic nano-fillers and organic polymers, increased the compressive strength from 0.081 MPa to 0.418 MPa and raised the peak strain from 72.9 % to 81.8 %. It provides an effective approach to enhance the plugging performance of hydrogels in high temperature and high salinity reservoirs.

Microscopic interface deterioration mechanism and damage behavior of high-toughness recycled aggregate concrete based on 4D in-situ CT experiments

High-toughness recycled aggregate concrete (HTRAC), as an innovative green building material, showcases outstanding performance and environmental attributes. To investigate its microscopic structural characteristics and the mechanisms of damage evolution under loading, a series of 4D in-situ X-ray computed tomography (CT) experiments were conducted under uniaxial compression conditions. During the experimental process, detailed scans were performed on six key loading points (0 %, 30 %, 60 %, and 100 % of the peak load, 70 % and 40 % of the post-peak load of the specimen). Advanced three-dimensional image processing techniques were utilized to conduct a comprehensive analysis of internal features within the specimen, including pores, steel fibers, and cracks. The evolution of damage in HTRAC was characterized by both qualitative 2D crack slice analysis and quantitative 3D reconstruction analysis of cracks. Changes in the average grayscale values of CT cross-sectional images were also examined. The deterioration mechanism of recycled aggregate concrete interface cracks was elucidated by the combination of CT images and modeled HTRAC. Furthermore, a statistics model illustrating the interaction mechanism between damage, cracks, and strain was proposed. This study provides technical and theoretical support for the further promotion and application of high-toughness recycled aggregate concrete.

Rock damage and fracture characteristics considering the interaction between holes and joints

The work studied the damage and fracture characteristics of granite samples with circular holes and parallel fractures of different lengths under uniaxial compression conditions. Acoustic emission (AE) monitoring and digital image correlation (DIC) were used to analyze the dynamic changes of AE parameters, the evolution characteristics of the deformation field on the sample surface, and the correlation between the fractal dimension and the fracture geometry parameters. A computational model was proposed for evaluating the intensity factor of the fracture tip in a double-fracture structure with holes. The interaction between the fractures was considered to reveal the complex law of the intensity factor and the fracturing angle with the fracture geometry. The changes in AE parameters and fractal dimension indicated the influence of fracture extension on the damage mode of samples. Besides, the fracture length affected the micro-fracture behavior. The surface displacement and strain characteristics of samples revealed the modulation effect of the fracture length on the failure mode. The inclination angle, length, holes, and size of the fractures’ friction coefficient significantly affected the evolution of the intensity factor, which in turn regulated the changes in the fracture angle. The work offers a novel quantitative analytical approach and a theoretical framework for comprehending the damage and failure mechanisms of porous and fractured rocks through extensive experimentation and theoretical analysis. It holds significant practical relevance in geological engineering, mining, and tunnel construction.

Quantitative calculation of rock strain concentration and corresponding damage evolution analysis

Understanding the critical strain and damage evolution process of sandstone can help engineers more accurately predict the failure modes and critical states of sandstone in actual engineering structures. Therefore, this article quantitatively analyzes the strain field data obtained through digital image correlation (DIC), and for the first time establishes a strain concentration calculation model (SCCM) to analyze the critical strain, compaction, and damage evolution process of sandstone under uniaxial compression conditions. The experimental results show that both elastic and plastic strain zones exist on the specimen surface. The proportion curve of strain concentration calculated by SCCM indicates that the strain field ε1 generally undergoes two stages: the elastic strain fluctuation stage and the plastic strain development stage. In contrast, the strain field ε2 exhibits roughly three stages: a rapid change phase, a slow change phase, and a stability phase. Microscopic strain analysis reveals an overlap between the compaction and damage processes of the specimen, and the critical strain value εmd for micro-damage in the specimen is significantly smaller than values obtained by traditional discrimination methods. Specifically, when the defect width of the specimen is 15 mm and 40 mm, the mean εmd vaules are approximately 0.006016 and 0.00539, respectively. In contrast, the mean critical strain values determined by traditional discrimination methods are 0.0180 and 0.0178, respectively. The above research results provide a new method for analyzing the strain field data of geotechnical materials, serving the optimization of engineering structure design.

Study on the mesoscopic mechanical behavior and damage constitutive model of micro-steel fiber reinforced recycled aggregate concrete

This study employs in-situ 4D CT experimental techniques to investigate the mesostructured changes and mechanical behavior of micro steel fiber-reinforced recycled aggregate concrete (MSF-RAC) under uniaxial compression and cyclic loading conditions. The research aims to address the scientific problem of limited understanding of how micro steel fibers affect the mesoscopic mechanical behavior and structural damage characteristics of recycled aggregate concrete. High-resolution CT scanning technology was utilized for real-time observation of crack initiation and propagation, revealing the micro-mechanisms by which micro steel fibers enhance RAC. The study found that incorporating micro-steel fibers significantly improves the density of the interfacial transition zone (ITZ), inhibits crack development, and enhances the mechanical properties of the concrete. Based on these observations, an improved stiffness degradation damage constitutive model was proposed, which comprehensively considers the distribution orientation of micro steel fibers and ITZ characteristics. The model effectively predicts the stress-strain relationship of MSF-RAC under various conditions, demonstrating high accuracy and practicality. These findings provide important theoretical support and experimental evidence for the optimal design of RAC, showcasing significant engineering application value by promoting the sustainable use of recycled materials in construction.

Dualistic effect of the deformation of protective structures made of broken rock in mine workings under static load

Purpose is to reveal physical essence of the dualistic (double) nature of deformation effects and their influence on the mechanical properties of protective structures made of broken rock while unloading coal-bearing mass to ensure stability of side rocks and operational conditions of the development mine working within the working areas of coal mines. Methods. The deformation properties of protective structures made of broken rock was modeled on experimental samples during their static load in terms of uniaxial compression with the possibility of lateral expansion of the original material or its compressive stress. Findings. A dualistic effect of deformations for the conditions of uniaxial compression of protective structures was revealed. Under the effect, a complex transformation of volume and shape occurs in the structures caused by the process of relative changes in the backfill material volume. The observed phenomenon occurs within the range of 0.12 ≤δV ≤ 0.32 and the values of the compaction coefficient of broken rock being 1.13≤k con. ≤ 1.47, depending on the granulometric composition of the source material and its bulk density. It was established that under conditions when broken rock is compressed, there is an effect of forming the bearing capacity of protective structures, which is observed when a relative change in the volume of backfill material is 0.14 ≤ δV ≤ 0.38, and its compaction factor reaches 1.16 ≤ kcon. ≤ 1.59. Depending on the homogeneity degree of the backfill material of protective structures, when comparing the values of kcon., a dualistic (double) effect is manifested in the difference of deformation characteristics of broken rock under uniaxial and compressive stress, reaching two or more times. Originality. A regularity was established that determines the relationship between compaction factor kcon. of broken rock and a relative change in the volume (δV) of backfill material, which makes it possible to evaluate bearing capacity of protective structures, being under static load in a stress-strain state. Practical implications. The research results can be used to substantiate the selection of a method to protect development mine working with flexible supports made of broken rock. To clarify the assessment of bearing capacity of such structures, it is advisable to carry out specific field studies in mine conditions.

Mechanical Properties and Mesoscopic Numerical Simulation of Local Weakening in High-Performance Concrete after 10 Years of Alkali Solution Immersion

The natural environment in the high-altitude regions of Northwest China is extremely harsh, characterized by numerous salt lakes. The high concentrations of chloride salts, sulfates, and alkali metal ions in these areas can induce alkali–silica reactions (ASRs) in concrete. These reactions generate harmful gel within the concrete, causing expansion and cracking, which significantly impacts the durability of concrete structures. This study investigates the evolution of the mechanical properties in high-performance concrete (HPC) under long-term ASR by incorporating different admixtures and varying the equivalent alkali content. A three-dimensional random aggregate mesoscopic model was used to simulate static compression tests under various operational conditions. Non-destructive testing methods were utilized to determine the expansion rate, internal, and surface damage variables of the concrete. The experimental results indicate that the 10-year expansion rate differs from the 1-year rate by approximately 1%, and under long-term ASR mitigation measures, the internal damage in the HPC is minimal, though the surface damage is more severe. As the equivalent alkali content increases, the compressive strength of the concrete cubes decreases, initially rising before falling by 5–15% over time. The HPC with only air-entraining agent added exhibited better mechanical performance than the HPC with both air-entraining and corrosion inhibitors added, with the poorest performance observed in the HPC with only a corrosion inhibitor. A relationship was established between the surface and internal damage variables, with the surface damage initially increasing rapidly before stabilizing as the internal damage rose. Numerical simulations effectively describe the damage behavior of HPC under static uniaxial compression. Comparisons with actual failure morphologies revealed that, in the cube compression tests, crack propagation directly penetrated both coarse and fine aggregates rather than circumventing them. The simulations closely matched the experimental outcomes, demonstrating their accuracy in modeling experiments. This study discusses the compressive mechanical properties of concrete under prolonged ASR through a combination of experimental and simulation approaches. It also delves into the impact of surface damage on the overall mechanical performance and failure modes of concrete. The findings provide experimental and simulation support for the concrete structures in regions with high alkali contents.

Study on delamination characteristics and mechanical properties of cemented waste rock backfill with rubber aggregation

Since some phosphorus mines use raw ore as a product and do not conduct beneficiation around the mines. This makes it difficult for the amount of waste rock to prepare enough backfill for the filling of underground mined-out areas. Using rubber particles made of waste tires to replace part of the waste rock aggregate (WRA) can not only solve the problem of insufficient WRA, but also improve the environmental problems caused by the large accumulation of waste tires. However, due to the small density of rubber and high density of waste rock, the phenomenon of segregation delamination may occur in the cemented backfill, thus affecting the mechanical properties of cemented waste rock backfill (CWRB). To this end, the effect of rubber aggregate (RA) on the segregation delamination of CWRB was determined by using the section image analysis method of specimens, and the quantitative index of segregation delamination of cemented waste rock backfill with rubber (CWRB-R) was constructed. The mechanical properties and failure characteristics of CWRB-R were studied by acoustic emission (AE) test under uniaxial compression condition. The experimental results showed that the segregation delamination characteristics of CWRB-R were related to the mass concentration. The higher the mass concentration, the weaker the degree of segregation stratification of the aggregate, the more uniform the distribution of the aggregate, and the less the strength loss of CWRB caused by the incorporation of RA. The incorporation of RA in CWRB not only reduced the uniaxial compressive strength (UCS) of the backfill, but also caused more cracks to be produced during the failure process of the backfill, and the failure form changes from brittle failure to ductile failure.

Conventional triaxial loading and unloading test and PFC numerical simulation of rock with single fracture

Take the metamorphic sandstone as the reference object, by making rock like samples with fractures, the conventional triaxial loading and unloading test and PFC numerical simulation of rock like sample with single fracture were conducted, and the effects of the loading path, inclined angle of fracture, axial stress level during unloading, initial confining pressure during unloading on the compressive strength, peak strain and crack propagation evolution of the samples were considered. The compressive strength of the specimen under triaxial unloading is smaller than that under triaxial loading. The peak strain of the specimen under triaxial unloading is also smaller than that under triaxial loading. The specimen is more prone to brittle failure. When the axial stress level is the same during unloading, with the increase of the initial confining pressure during unloading, the difference of the compressive strength of the specimens with different inclined angles of fracture gradually decreases. Under the condition of uniaxial compression and triaxial compression, the failure of all specimens is tensile failure, and the shear failure is the main one during unloading.

Исследование физико-механических свойств ийолит-уртита в условиях одноосного и трехосного сжатия

The study of deformation and failure processes in rock masses is often carried out in laboratory conditions on samples. Widely used test equipment is used for this purpose, which makes it possible to create loading conditions similar to those inside the rock mass. Basically, the loading mode is implemented when forces are applied only to one side of the sample (along one axis), that is, uniaxial compression or tension. Test equipment that allows creation of loading along three axes, i.e. triaxial compression, is used less frequently. Such equipment is often very expensive, and testing requires a lot of time and resources. Nevertheless, this loading mode is one of the most interesting, since it is close to real conditions in the rock mass. This paper presents the results of experimental studies of ijolite-urtite samples (rocks from the Khibiny massif, the Kola Region) in conditions of uniaxial and triaxial compression. The purpose of the research is to establish how the values of the compressive strength of this rock change, to assess the critical values of the specific strain energy, as well as the tendency to dynamic destruction during the transition from the uniaxial loading mode to the triaxial one. The experimental studies revealed that there exists a sharper increase in the values of compressive strength and the critical values of specific strain energy of the ijolite-urtite samples in conditions of triaxial compression than under uniaxial compression. It has been established that the studied rock is prone to dynamic failure under uniaxial compression and preserve its tendency to this type of failure under triaxial compression.

Numerical investigation of mechanical responses and failure features of coal measure strata using combined finite-discrete element method

Investigating the mechanical responses and failure features of the deep coal measure strata (CMS) is essential for designing hydraulic fracturing treatments and preventing geological dynamic hazards in underground engineering. In this paper, a three-dimensional CMS model is established by the combined finite-discrete element method (FDEM). The mechanical properties, stress evolutions and failure features of the CMS with different lithological combinations (sandstone-coal-sandstone (SCS), sandstone-coal-mudstone (SCM) and mudstone-coal-mudstone (MCM)) and thickness ratios are investigated under the uniaxial compressive condition. Results indicate that the SCS combination exhibits the highest uniaxial compressive strength (UCS) and elastic modulus, followed by the SCM and MCM combinations. The mechanical properties of the CMS are predominantly determined by the softer coal layer. The mechanical contrasts in elastic modulus and Poisson’s ratio between adjacent layers cause the generation of additional compressive stress in the softer layer and additional tensile stress in the stiffer layer. Therefore, the softer coal layer exhibits the shear-dominated failure pattern while the stiffer sandstone/mudstone layer displays the tensile-dominated failure pattern in the CMS. Furthermore, as the thickness ratio of the coal layer increases, the UCS, elastic modulus, and interfacial constraint stress decrease in the CMS. Additionally, the UCS and stress–strain relationship in the post-peak stage exhibit the noticeable size effect, whereas the mechanical behaviors in the pre-peak stage and the ultimate fracture patterns show the opposite. The elastic modulus and interfacial constraint stress obtained by the FDEM simulation match well with those calculated by theoretical models. The key findings of this paper can provide fundamental understanding and reliable parameters for multi-scale geomechanical modeling in petroleum and mining engineering.