Triaxial Loading Research Articles

Comparison Study on Damage Characteristics of Deeply Buried Granite Under Triaxial Cyclic Loading and Unloading

Comparison Study on Damage Characteristics of Deeply Buried Granite Under Triaxial Cyclic Loading and Unloading

Asymmetric deformation and failure behavior of roadway subjected to different principal stress based on biaxial tests

The magnitude and direction of stress will affect the failure of roadway. The impact of stress magnitude and direction on roadway failure was investigated using a rock failure system equipped with true triaxial loading, acoustic emission (AE), and digital image correlation (DIC) technologies. The system allowed for the simulation of rock sample failures under various three-dimensional stress conditions, while real-time monitoring was conducted using a high-precision microseismic system. By prefabricating rectangular holes at different angles in sandstone specimens, roadways under different stress effects in engineering can be simulated. Biaxial loading tests were then performed on these prefabricated sandstone cube specimens to observe external fracture characteristics and internal fracture information using acoustic emission equipment. The peak strength of samples with holes is approximately 38–56% of that of intact samples. In the early and middle loading stages, the strain on the surface is on the order of 10-3. In the later loading stage, the strain on the surface is on the order of 10-2. The findings indicate that roadway failure primarily involves local particle ejection, fragment spalling, large-scale particle ejection, plate crack buckling, and eventual failure. The ultimate failure mode varies based on stress deflection angles: with no included angle, the roadway exhibits a ’V’ shaped failure zone on both sides, predominantly experiencing tensile failure. At included angles of 30°, 45°, and 60° between stress and roadway axis, the roadway displays a “—” failure pattern along the diagonal, characterized by a combination of tension and shear failure. The extension angle of the“—” failure zone varies depending on the stress deflection angle. At a 90° included angle, the roadway shows a ’V’ shaped failure in the roof and floor, primarily undergoing tensile failure. Real-time monitoring of the strain field evolution around the roadway during failure using DIC method revealed valuable insights. AE events at different deflection angles reflect the progression of micro cracks in the specimen, showing a strong correlation with macro failure. The research outcomes elucidate the mechanisms behind various forms of roadway failure induced by stress deflection and shed light on the failure mechanism at different stress deflection angles.

Numerical analysis of failure mechanics of concrete under true dynamic triaxial loading using a four-phase meso-model

In the present paper, the failure mechanics of concrete under true triaxial dynamic loading is investigated. A four-phase concrete meso-model is developed. The meso-model consists of aggregate, mortar, interfacial transition zone (ITZ) and macropores. A finite element (FE) model of a split Hopkinson pressure bar is also created to apply true triaxial dynamic loading. The development of cracks in concrete is presented for the uniaxial, biaxial and triaxial loading cases. The presence of local discontinuity is modelled, and its effect on the failure of concrete is extensively investigated and discussed. In the last, the effect of aggregate distribution is also analysed. It is found that the internal structure of concrete changes the stress distribution from true triaxial to compression or extension triaxial, which is the prime cause of internal cracking.

Determination of the REV size for heterogeneous rocks with different grain sizes: Deep learning and numerical approaches

Accurate determination of the representative elementary volume (REV) size plays a pivotal role in analysing the mechanical properties and failure processes of heterogeneous rocks in complex engineering environments. In this study, a novel microstructure modelling strategy (NMMS) for determining the REV size is proposed by combining deep learning and an improved phase-field method (PFM). Micro- and macroscale experiments are systematically conducted to determine the real microstructural characteristics and mechanical properties of heterogeneous rocks with different grain sizes. On the basis of this experimental evidence, geometric models of different sizes were reconstructed through deep learning to avoid the limitations of human-based methods, and an improved PFM was used for numerical calculations. These models were then employed to perform numerical tests under uniaxial loading conditions, and the coefficient of variation was introduced to determine the REV size of heterogeneous rocks with different grain sizes. The research findings indicate that the final REV size is the maximum value of the REVs defined by the evaluation properties within an acceptable coefficient of variation. At a criterion of 5% for the coefficient of variation, the REV sizes are 60 mm×60 mm, 70 mm×70 mm, and 90 mm×90 mm for fine-medium-grained (FMG), medium-grained (MG), and coarse-grained (CG) rocks, respectively. Furthermore, the REV determined by the NMMS was applied to investigate the effects of microstructure on macromechanical properties and damage evolution under triaxial loading conditions. The numerical results show that the NMMS can accurately predict the macromechanical properties and microcracking patterns of heterogeneous rocks, especially the intracrystalline cracks in feldspar, the interfacial cracks in gravel, and the “voids” of cracks in biotite. This research can provide some basic references for the optimal choice of the REV size of heterogeneous rocks.

Magnitude Clustering During Stick‐Slip Dynamics on Laboratory Faults

AbstractWe present an analysis of magnitude clustering of microfractures inferred from acoustic emissions (AEs) during stick‐slip (SS) dynamics of faulted Westerly granite samples in frictional sliding experiments, with and without fluids, under triaxial loading with constant displacement rate. We investigate magnitude clustering in time across periods during, preceding and after macroscopic slip events on laboratory faults. Our findings reveal that magnitude clustering exists such that subsequent AEs tend to have more similar magnitudes than expected. Yet, this clustering only exists during macroscopic slip events and is strongest during major slip events in fluid‐saturated and dry samples. We demonstrate that robust magnitude clustering arises from variations in frequency‐magnitude distributions of AE events during macroscopic slip events. These temporal variations indicate a prevalence of larger AE events right after (0.3–3 s) the SS onset. Hence, magnitude clustering is a consequence of non‐stationarities.

Characteristics of coal crack development and gas desorption in the stress affected zone of rock pillar

Coal (Rock) pillar retaining in the mining of protective layer would cause gas dynamic disaster in the protected layer. Based on the gas geological conditions of the two-layer coal seam in Jinhe Coal Mine of Yaojie Mining District, the stress evolution law of coal seam in the rock pillar affected area was studied by theoretical analysis and numerical simulation, and the crack development law of coal seam in different loading stages under conventional triaxial loading was studied by CT scanning technology. With the analysis of the stress evolution and crack development of coal in rock pillar affected area, the gas extraction effect under different stress states and the gas desorption law after pressure relief antireflection were studied on site. The results showed that the stress of coal in the rock pillar affected area is in the approximate elastic stage of the conventional triaxial stress-strain curve, and the cracks of coal are mainly closed at this stage. Meanwhile, the increase of stress leaded to the decrease of coal permeability and the poor gas extraction effect. CT scanning tests under conventional triaxial loading were carried out in the laboratory, and three-dimensional visual models of coal sample cracks were constructed at different loading stages. When loading to the linear elastic stage, the crack volume and surface area were reduced by 74% and 71% compared with the ones in initial state. At the same time, the expression between stress σ and crack density T was further established. After comprehensive control measures such as intensive drilling discharge, presplitting blasting and coal water injection were taken to the coal in rock pillar affected area, the crack density T could reach the crack development level of the conventional triaxial loading softening stage, realizing the crack development of the coal under low stress. The enclosed gas in front of the coal could desorption flow during the roadway driving. And the predict index value K1 also decreased from 0.57 mL/(g·min1/2) to 0.17 mL/(g·min1/2) continuously. The safety of coal roadway in rock pillar affected area was realized, and the accuracy of numerical simulation and laboratory test results was verified, which had certain reference significance for coal roadway excavation under this similar conditions.

Effects of Fatigue Load Amplitude on the Mechanical and Energy Evolution Characteristics of Stratified Rock Mass under Triaxial Cyclic Loading and Unloading Conditions

Effects of Fatigue Load Amplitude on the Mechanical and Energy Evolution Characteristics of Stratified Rock Mass under Triaxial Cyclic Loading and Unloading Conditions

Damage index correlation in massive granite-porous backfills under hydromechanical triaxial cyclic loading using acoustic emissions and X-ray computed tomography

The characterization accuracy of damage indices in massive and porous composite biomaterials under complex osmotic conditions has long been a challenge in engineering materials science. Establishing correlations between damage indices calculated using different methodologies is crucial to validate and improve defects characterizations. In this study, we investigated the damage index correlation of cylindrical massive granite-porous backfill structural composites under both dry and seepage conditions. A series of triaxial cyclic loading and unloading tests were conducted on specimens under hydromechanical conditions with varying osmotic pressures (0, 1, 3, 5, 7, and 9 MPa) and a confining pressure of 9 MPa involving acoustic emission monitoring and phase-based 3D nondestructive CT scanning. The results revealed the AE energy was lower under osmotic conditions compared to dry conditions. The AE Damage Index (DAE), defined by separating tensile and shear microcracking signals, significantly increased from 0.32 to 0.55 with rising osmotic pressure. 3D mesoscopic fracture visualization provided detailed insights into the fractures in granite and backfill interfaces. The number of small-angle cracks increased with osmotic pressure, and the crack inclination distribution range expanded from 63°- 85° to 49°–80°. The average crack angle showed a significant monotonic decrease with increasing osmotic pressure. An Energy Dissipation Damage Index (DED) was defined, and its correlation with the DAE under dry conditions was modeled using a reciprocal function DED=a/(DAE+bσ3)+c. This novel approach offers insights into the transition points between tensile and shear-dominated failure stages. Furthermore, an exponential function DV=aDAEb(b<0) was used to model the correlation between AE Damage Index and Visual Damage Index (DV) under osmotic conditions. The damage indices derived from different methods can be mathematically correlated, providing a comprehensive evaluation of material damage states. The study found the flow transition between different natural-human medias results in different damage mechanisms, which are all obey the correlation law. These damage mechanisms offered constructive recommendations for engineering applications based on varying ratios of osmotic pressure to confining pressure.

Damage evolution and acoustic emission characteristics of sandstone under different true triaxial cyclic loading and unloading modes

In the process of deep coal mining, the cyclic mining disturbance behavior produces significant cyclic loading and unloading effects, to ensure safe production in the mining area, it is necessary to study the damage characteristics and acoustic emission characteristics of the rock body under different cyclic loading and unloading effects. This study developed a TAWZ-5000/3000 rock true triaxial (disturbance) test system and used it to carry out true triaxial cyclic loading and unloading tests in different modes, with acoustic emission (AE) monitoring of the deformation and damage processes. The test results proved that a large number of cycles weakens the rock’s ability to control deformation, and this weakening ability is also related to the types of cyclic loading and unloading modes. The hysteresis loop characteristics and mechanical parameters show obvious path dependence. Based on the damage equivalence method, it is found that the absolute damage parameter increases with the increase of the number of cycles in the same mode, and the accumulation rate of the cumulative damage parameter is faster; the cumulative damage parameter in the equal amplitude cyclic loading and unloading (EAC) mode is always higher than that in the other two modes. The normalized AE ring count rate in the rock samples under cyclic loading and unloading has a significant relationship with the damage percentage, and the crack extension scale function (b-value) shows the “steady increase – strong fluctuation − significant decline” trend. By analyzing the AE characteristic parameters, namely rise time/amplitude (RA) and ringing count/duration (AF), it is found that the sandstones in EAC, graded cyclic loading and unloading (GC), and stepped cyclic loading and unloading (SC) modes are mainly damaged by tension-shear and shear, respectively. By analyzing the values of the acoustic emission parameters RA-AF, it is found that the sandstone is mainly damaged by tensile shear, tensile tension, and shear under EAC, GC, and SC modes, respectively, corresponding to the rock samples’ damage patterns. The study results have important guiding significance for preventing and controlling disasters in deep mines.

Study of the Micro-Vibration Response and Related Vibration Isolation of Complex Traffic Load-Induced Experimental Buildings

In view of the high-sensitivity vibration effect of precision instrument laboratory buildings under the influence of surrounding traffic loads, field micro-vibration tests under various working conditions were carried out based on actual projects. Combined with numerical simulation, measured data served as input loads to simulate the vibration effect of various traffic loads on the floor of a building structure, and the structural vibration laws under the comprehensive action of various loads were analyzed. The vibration isolation effect of the isolation ditch on the oblique orthogonal load was investigated according to the corresponding safety index. The results show that the main frequency components of the site vibration frequency caused by various traffic loads are approximately 25 Hz and that the root-mean-square speed value is stable below VC-E, which meets the design requirements. Under the comprehensive action of multiple loads, the site structure will produce a vibration amplification effect, which is obvious when all types of loads are distributed symmetrically and the curve distribution is controlled by load factors with large amplitudes. The isolation effect of a small isolation ditch is best when it is located close to the source and the building. The depth of the isolation ditch must be greater than the maximum depth of the source to achieve better results, and the width has little influence. The effect of a small isolation trench on vertical vibration is poor, and the oblique orthogonal triaxial load has a counteracting effect on the vertical component. Thus, additional structural vibration isolation measures are needed. The research results provide a reference for engineering vibration isolation, damping measures, and structural design.

Study of triaxial loading of segregated granular assemblies through experiments and DEM simulations

Study of triaxial loading of segregated granular assemblies through experiments and DEM simulations

Cavitation damage in rubber-like silicone adhesives

In this work, the failure of rubber-like materials by cavitation is investigated. Under local triaxial loading states, as they can occur in confined geometries of adhesive bonds, the instantaneous formation of voids can be observed. Analytical results of cavitation in a commercial silicone adhesive are provided. The results show that cavitation voids can be distinguished from manufacturing voids by the inner surface of the void. The surface of the cavitation void is rugged and shows a complex fracture pattern. Pancake experiments have been conducted for different ratios of radius and thickness. To assess the fracture process of cavitation onset one macroscopic and two asymptotic solutions of the energy release rate of cavitation voids are proposed. The solutions are compared and their limitations for the analysis of cavitation onset are discussed.

Creep Failure Mechanism and Model of Granite under True Triaxial Loading and Unloading Conditions

Creep Failure Mechanism and Model of Granite under True Triaxial Loading and Unloading Conditions

A Numerical Study of the Mechanical Behavior of Jointed Soft Rocks under Triaxial Loading Using a Bonded Particle Model.

In order to master the strength and deformation characteristics, including the macro-micro failure mechanism of soft rock samples with penetrating joints under triaxial loading, a series of numerical triaxial tests have been carried out. The strength and deformation characteristics, failure modes, crack propagation, distribution of force chains, and the influences of joint dip angles and confining pressures have been analyzed and compared with the laboratory test results. The results show that (1) the residual strength ratio of jointed rock samples generally increases first and then decreases with the increase in joint dip angles under the same confining pressure and reaches the maximum value around 23-24°. Poisson's ratio increases with the increase in the confining pressure or the joint dip angle. The elastic modulus increases with the increase in the confining pressure and decreases with the increase in the joint dip angle. (2) The jointed rock samples with different joint dip angles compact with relatively small volumetric strains and then dilate up to failure with relatively large volume expansions. Lower confining pressure and smaller dip angles will lead to a more pronounced dilation phenomenon and less obvious volume shrinkage rules. (3) The low-angle jointed rock samples all exhibit the X-type shear failure. The jointed rock samples with a joint dip angle of 45° exhibit hybrid failure with both slippage and shearing, which are controlled by both the matrix and the joint. (4) The change in the number of cracks includes three stages: the slow crack initiation stage, rapid growth stage, and crack coalescence stage. The total number of shear or tensile cracks all decrease with an increase in the joint dip angles, with the number of tensile cracks being approximately twice that of shear cracks. The tension cracks are mostly horizontal, and the shear cracks are mostly vertical. (5) The number of force chains shows a decreasing trend after the cracks begin to grow. The jointed rock samples for the intact, 15° and 30° cases all form a main force chain during the failure process, while there is no main force chain for the 45° case.

Characteristics of Energy Evolution and Failure Mechanisms in Sandstone Subject to Triaxial Cyclic Loading and Unloading Conditions

Characteristics of Energy Evolution and Failure Mechanisms in Sandstone Subject to Triaxial Cyclic Loading and Unloading Conditions

A new approach on constitutive modeling for quasi-brittle materials under multiaxial loading

Abstract This work introduces a new method for expanding unidimensional models to predict material behavior under multiaxial loading. The concept of six-dimensional mechanical space is adopted, and in each basis direction of the space, there is one fiber-bundle model (FBM), so these six FBMs are independent amongst one another but follow the same damage evolution rule. Under triaxial loading, these FBMs have different stress and strain states, so six corresponding damage variables have different values. The anisotropic damage is thus represented by the six damage variables. When it is under uniaxial loading, the six-dimensional space reduces to one-dimensional space, so the proposed model can be calibrated using uniaxial loading test data, and the calibrated model can predict the material behavior under multiaxial loading conditions. Biaxial loading and triaxial loading test data are used to verify the proposed model, and parametric analyses are performed to analyze model predictions under different loading conditions. It shows that model predictions agree well with the test data and are consistent with experimental observations.

Analysis of failure properties of red sandstone with structural plane subjected to true triaxial stress paths

In order to study the mechanical behavior and failure characteristics of rock mass with the structural plane in complex deep stress environment, the QKX-YB200 true triaxial rockburst test system, acoustic emission, and digital video recorder were adopted to conduct true triaxial loading and unloading indoor tests on the red sandstone cube specimens. The results show that: (1) Under true triaxial loading conditions, all specimens propagate in the form of anti-wing crack, and the dip angle of the structural plane is linearly related to the average value of the anti-wing crack propagation angle; under true triaxial unloading conditions, the specimens with structural plane dip angles of 30°, 45°, and 60° propagate in the form of wing crack, while those with a 90° angle propagate anti-wing cracks, and the closure of structural plane is related to the the crack type at the tips of structural plane. (2) The unstable failure stages in the stress–strain curve under true triaxial unloading are longer and exhibit more pronounced fluctuations; the peak strength of the specimens under the conditions of true triaxial loading and unloading decrease first and then increase with the increase of the dip angle, reaching the lowest at 45°. (3) During the true triaxial unloading tests, the rockburst degree of the intact rock specimens is more violent than that of the specimens with a structural plane. Additionally, the dip angle of the structural plane influences the severity of rockbursts under the same stress path. (4) The crack propagation mechanism and the closure degree of structural planes under different true triaxial stress paths and dip angles are discussed. It is observed that the crack initiation mode around the structural plane tips and the crack mode near free surface are closely related to the stress environment in different areas of the specimens. (5) Based on the indoor test results and previous experimental data, the differences in peak strength characteristics between open structural planes and closed structural planes are preliminarily analyzed and compared.

Consequences of repeated hyperbaric hydrogen exposures on mechanical properties and microstructure of polyamide 11

The objective was to evaluate the impact of repeated exposure to hyperbaric hydrogen (90 MPa; 30 °C) and pressure release on the microstructure and mechanical behavior of PA11. Samples were analyzed after 1, 2, 5 and 10 cycles, by SAXS, WAXS, DMA, DSC, and a series of mechanical tests with variable triaxiality ratio. The most visible change in the residual state after desorption was a stiffening of the amorphous phase. It mainly originated from the first cycle, especially the first pressurization. The crystalline phase was slightly affected and no evidence of nano-voiding was brought in the residual state up to 10 cycles. Similar analyses were conducted during the first cycle's desorption transient. They showed a reversible plasticizing effect and a trend of nano-voiding vanishing after desorption but might promote damage upon further triaxial loading.

Elastoplastic analysis on deformation and failure characteristics of surrounding rock of soft-coal roadway based on true triaxial loading and unloading tests

Since accidents such as roof caving, rock fragmentation, and severe deformation are particularly likely to occur during roadway excavation in soft and thick coal seams, grasping the range and distribution of deformation and fracturing of surrounding rock is of crucial for evaluating roadway stability and optimizing support design in such coal seams. In this study, based on the stress paths encountered during roadway excavation, true triaxial loading and unloading tests were carried out on soft coal, and the deformation and strength evolutions of soft coal under different intermediate principal stress conditions were analyzed. The test results show that the stress–strain relationship in the pre-peak plasticity-strengthening and post-peak plasticity-weakening stages follows a quadratic function, and the strengeth evolution conforms to the Mogi–Coulomb criterion. Moreover, analytical solutions for the displacement of surrounding rock, the radius of the broken zone, and the radius of the plastic zone of soft-coal roadways under excavation stress paths were derived after taking the nonlinear hardening and softening characteristics of the strain of soft coal, the Mogi–Coulomb criterion, the intermediate principal stress, and the dilatancy characteristics of surrounding rock into comprehensive consideration. Finally, in accordance with a practical engineering case, the influences of the intermediate principal stress coefficient, the lateral pressure coefficient, and the support force on the deformation and failure characteristics of the soft-coal roadway were analyzed. The analysis reveals that an increase in intermediate principal stress aggravates the deformation of surrounding rock and enlarges the plastic and broken zones; variations in the lateral pressure coefficient alter the shape of the broken zone and the distribution of surface displacement; and an increase in the support force effectively reduces the plastic zone, broken zone, and surface displacement of the roadway. The research results can provide valuable theoretical basis for the stability evaluation and support design of soft-coal roadways.

Mechanical Properties of Latex-Modified Cement Stone under Uniaxial and Triaxial Cyclic Loading.

During the cyclic injection and extraction process in underground storage wellbores, the cement sheath undergoes loading and unloading stress cycles. In this study, we investigated the mechanical properties of latex-modified cement stone (LMCS), widely used in oil and gas wells, through uniaxial and triaxial cyclic loading and unloading tests. The aim of the study was to determine the effect of various loading conditions on the compressive strength and stress-strain behavior of LMCS. The results show that the stress-strain curve of LMCS exhibits a hysteresis loop phenomenon, with the loop intervals decreasing throughout the entire cyclic loading and unloading process. As the number of cycles increases, the cumulative plastic strain of the LMCS increases approximately linearly. Under uniaxial cyclic loading and unloading conditions, the elastic modulus tends to stabilize. However, under triaxial conditions, the elastic modulus increases continuously as the number of cycles increases. This result provides data for engineering predictions. Furthermore, a comparison of the uniaxial and triaxial cyclic loading and unloading of LMCS shows that its cumulative plastic strain develops rapidly under uniaxial conditions, while the elastic modulus is larger under triaxial conditions. These findings provide a valuable reference for constructing underground storage wellbores.