scholarly journals Elastic-Plastic Threshold and Rational Unloading Level of Rocks

2019 ◽  
Vol 9 (15) ◽  
pp. 3164 ◽  
Author(s):  
Ming Ji ◽  
Hongjun Guo
Keyword(s):  
Stress Level ◽  
Triaxial Compression ◽  
Confining Pressure ◽  
Confining Stress ◽  
Elastic Plastic ◽  
Conventional Triaxial Compression ◽  
Confining Pressures ◽  
Pressure Increment ◽  
Loading And Unloading ◽  
Plastic Transformation

During loading and unloading test, various rocks manifest different stress values of elastic-plastic transformation. This study proposes to include axial pressure increment ratio in the conventional triaxial compression test to evaluate different variables (nominal elastic modulus, nominal Poisson’s ratio, strain, and energy). The relationships among various factors including variables, the stress level of initial confining stress and axial pressures, were analyzed by analyzing the stress–strain plot record obtained from testing various rocks. The extreme value point of the deformation parameter, also known as the elastic-plastic threshold, was analyzed. In addition, the elastic-plastic thresholds were later used as unloading points during the unloading tests. Under the same confining condition, different rocks demonstrated different unloading levels. Furthermore, a linear correlation was observed between unloading levels and changing confining pressures, and the gradient is mainly related to the types of rocks. During the unloading tests of rocks, the rational unloading level is recommended to be no higher than the stress level at the elastic-plastic threshold under the corresponding confining pressure.

scholarly journals Acoustic Emission Monitoring and Failure Precursors of Sandstone Samples under Various Loading and Unloading Paths

2017 ◽  
Vol 2017 ◽  
pp. 1-11 ◽  
Author(s):  
Jie Xu ◽  
Jingdong Jiang ◽  
Lingling Zuo ◽  
Yufeng Gao
Keyword(s):  
Acoustic Emission ◽  
Triaxial Compression ◽  
Confining Pressure ◽  
Sandstone Sample ◽  
Mode Decomposition ◽  
Conventional Triaxial Compression ◽  
Confining Pressures ◽  
Loading And Unloading ◽  
Precursor Information ◽  
Failure Precursors

To explore the failure precursors of hard rock, a series of triaxial loading and unloading experiments were carried out on sandstone sample using the acoustic emission systems. The extreme-point symmetric mode decomposition method (ESMD method) was used to denoise and reconstruct the AE data. The AE quiet period in Scheme I becomes much more obvious with the confining pressure increasing, which can be regarded as the precursor information of the sample failure under conventional triaxial compression. Unlike Scheme I, there are no obvious precursory characteristics before failure in Schemes II and III, and the count rate reaches the maximum at the peak point. When the stress ratio ranges from 0.8 to 1.0, the fractal values of acoustic emission can be used to investigate the failure precursors of samples at a lower confining pressure. When the time ratio is greater than 0.8 under higher confining pressures, the fractal values of sandstone samples under unloading paths are rapidly reduced, which can be used to predict rock failure at higher confining pressures.

scholarly journals Researches on the Constitutive Models of Artificial Frozen Silt in Underground Engineering

2014 ◽  
Vol 2014 ◽  
pp. 1-8 ◽  
Author(s):  
Yugui Yang ◽  
Feng Gao ◽  
Hongmei Cheng ◽  
Yuanming Lai ◽  
Xiangxiang Zhang
Keyword(s):  
Mechanical Characteristic ◽  
Stress Level ◽  
Strain Softening ◽  
Triaxial Compression ◽  
High Stress ◽  
Constitutive Models ◽  
Confining Pressure ◽  
Frozen Soil ◽  
Conventional Triaxial Compression ◽  
Shear Process

The researches on the mechanical characteristic and constitutive models of frozen soil have important meanings in structural design of deep frozen soil wall. In the present study, the triaxial compression and creep tests have been carried out, and the mechanical characteristic of frozen silt is obtained. The experiment results show that the deformation characteristic of frozen silt is related to confining pressure under conventional triaxial compression condition. The frozen silt presents strain softening in shear process; with increase of confining pressure, the strain softening characteristic gradually decreases. The creep curves of frozen silt present the decaying and the stable creep stages under low stress level; however, under high stress level, once the strain increases to a critical value, the creep strain velocity gradually increases and the specimen quickly happens to destroy. To reproduce the deformation behavior, the disturbed state elastoplastic and new creep constitutive models of frozen silt are developed. The comparisons between experimental results and calculated results from constitutive models show that the proposed constitutive models could describe the conventional triaxial compression and creep deformation behaviors of frozen silt.

Post-Critical Mechanical Properties Of Sedimentary Rocks In The Upper Silesian Coal Basin (Poland)

2015 ◽  
Vol 60 (2) ◽  
pp. 517-534 ◽  
Author(s):  
Mirosława Bukowska
Keyword(s):  
Triaxial Compression ◽  
Residual Deformation ◽  
Confining Pressure ◽  
Compression Tests ◽  
Functional Relationships ◽  
Conventional Triaxial Compression ◽  
Upper Silesian Coal Basin ◽  
Confining Pressures ◽  
Waste Rocks ◽  
Triaxial Compression Tests

AbstractIn this paper, we present the results of a study of the Upper Carboniferous sedimentary rocks of the Upper Silesian Coal Basin (USCB) in Poland. We examined the hard coals, which belong to various stratigraphic units of Upper Carboniferous coal-bearing strata, and waste rocks, i.e., sandstones, mudstones, claystones. We present the results of tests of their post-critical mechanical properties. These results are from tests of the post-critical modulus, residual stress and residual deformation from experiments using a servo-controlled testing machine (MTS) with uniaxial compression and conventional triaxial compression. We applied confining pressures of up to 50 MPa at a strain rate of 10−5− 10−1s−1(0.003-6.0 mm/sec). The confining pressure applied in the triaxial compression tests reflected the conditions of current and future mining activities in the USCB at depths exceeding 1.300 metres. The strain rate applied in the tests reflected the values observed in the rockmass surrounding the mine workings and the rate of certain geodynamic phenomena occurring in the Carboniferous rockmass in the USCB, e.g., rock bursts. We present the values of the sub-critical modulus of coals and waste rocks, the functional relationships between the post-critical modulus and uniaxial compression strength, which are described using an exponential function of high correlation coefficients of the given rocks, and an exponential relationship between the post-critical modulus and the longitudinal elasticity modulus (Young’s modulus). Based on the results of tests of the post-critical properties of the Carboniferous rocks under triaxial compression and at various strain rates, we devised the functional relationships between the properties of the rocks and the confining pressure. The dependence of the post-critical modulus of the sandstones and claystones on the confining pressure is described using a polynomial function of degree 2, and that of the coals is described using an exponential function. The relationship between the residual stress and residual deformation in the rocks and the confining pressure was described using a linear function. The obtained results of tests have a practical application in forecasting behaviour of rocks located deep, and designing safe exploitation of mineral deposits. Confining pressures of up to 50 MPa used in the conventional triaxial compression tests allowed us to predict the behaviour of the rock mass at large depths. These data provide general knowledge of the tendencies in behaviour of rocks at substantial depths and the ability to design safe methods of mining deposits of various raw materials, including energy sources. These deposits are mined from increasingly great depths as the reserves are gradually exhausted and collieries of the largest European coal basins are continuously reconfigured.

Mechanical behaviour of powders using a medium pressure flexible boundary cubical triaxial tester

2003 ◽  
Vol 217 (3) ◽  
pp. 233-241 ◽  
Author(s):  
F Li ◽  
V M Puri
Keyword(s):  
Shear Modulus ◽  
Mechanical Behaviour ◽  
Triaxial Compression ◽  
Three Dimensional ◽  
Volumetric Strain ◽  
Confining Pressure ◽  
Alumina Powder ◽  
Medium Pressure ◽  
Mean Pressure ◽  
Confining Pressures

A medium pressure (<21 MPa) flexible boundary cubical triaxial tester was designed to measure the true three-dimensional response of powders. In this study, compression behaviour and strength of a microcrystalline cellulose powder (Avicel® PH102), a spray-dried alumina powder (A16SG), and a fluid-bed-granulated silicon nitride based powder (KY3500) were measured. To characterize the mechanical behaviour, three types of triaxial stress paths, that is, the hydrostatic triaxial compression (HTC), the conventional triaxial compression (CTC), and the constant mean pressure triaxial compression (CMPTC) tests were performed. The HTC test measured the volumetric response of the test powders under isostatic pressure from 0 to 13.79MPa, during which the three powders underwent a maximum volumetric strain of 40.8 per cent for Avicel® PH102, 30.5 per cent for A16SG, and 33.0 per cent for KY3500. The bulk modulus values increased 6.4-fold from 57 to 367MPa for Avicel® PH102, 3.7-fold from 174 to 637 MPa for A16SG, and 8.1-fold from 74 to 597MPa for KY3500, when the isotropic stress increased from 0.69 to 13.79 MPa. The CTC and CMPTC tests measured the shear response of the three powders. From 0.035 to 3.45MPa confining pressure, the shear modulus increased 28.7-fold from 1.6 to 45.9MPa for Avicel® PH102, 35-fold from 1.7 to 60.5MPa for A16SG, and 28.5-fold from 1.5 to 42.8MPa for KY3500. In addition, the failure stresses of the three powders increased from 0.129 to 4.41 MPa for Avicel® PH102, 0.082 to 3.62 MPa for A16SG, and 0.090 to 4.66MPa for KY3500, respectively, when consolidation pressure increased from 0.035 to 3.45MPa. In addition, the shear modulus and failure stress values determined from the CTC test at 2.07, 2.76, and 3.45MPa confining pressures are consistently greater than those from the CMPTC test at the same constant mean pressures. This observation demonstrates the influence of stress paths on material properties. The CTT is a useful tool for characterizing the three-dimensional response of powders and powder mixtures.

Experimental Study of Remolded Unsaturated Soil Subjected to Wetting Load under Constant Compression

2013 ◽  
Vol 291-294 ◽  
pp. 2657-2661
Author(s):  
Xiu Mei Qiu ◽  
Han Bing Bian
Keyword(s):  
Unsaturated Soil ◽  
Water Saturation ◽  
Confining Pressure ◽  
Earth Dam ◽  
Confining Stress ◽  
Initial Water ◽  
Volumetric Deformation ◽  
Deformation Properties ◽  
Confining Pressures ◽  
Unsaturated Clay

The mechanical behavior of a compacted unsaturated clay soil was experimentally investigated. Volume changes were investigated using a conventional odometer cell under a series of constant confining pressures, following a wetting path. The special loading paths were utilized to reflect field conditions associated with the compacted earth structure in earth filled embankment. The soils used in the experiments were taken from an earth dam. The compacted specimens were consolidated under k0-oedometer conditions. The volume change and the water content variation were measured during the tests. The influence of the confining pressure and the initial water saturation were taking into considerations. The experimental results show that the volumetric deformation properties of the remolded unsaturated soil could be expansive and/or contractive, depending on the confining pressure and the initial water saturation. It is also observed that for the mediate confining stress, there volumetric deformation of specimen applied to wetting loads has a transition from dilation to contraction.

scholarly journals The Effect of Confining Pressure on the Mechanical Properties of Sand–Ice Materials

1973 ◽  
Vol 12 (66) ◽  
pp. 469-481 ◽  
Author(s):  
Bernard D. Alkire ◽  
Orlando B. Andersland
Keyword(s):  
Axial Strain ◽  
Triaxial Compression ◽  
Confining Pressure ◽  
Silica Sand ◽  
Creep Tests ◽  
Ice Volume ◽  
Coulomb Failure ◽  
Confining Pressures ◽  
Sand Material ◽  
Cylindrical Samples

Cylindrical samples containing 0.59 mm to 0.84 mm diameter silica sand at about 97% and 55% ice saturation (the ratio of ice volume to sand pore volume) were tested at a temperature of −12° C in triaxial compression. Both constant axial strain-rate tests and step-stress creep tests provide information on the influence of confining pressure on the shear strength and creep behavior of the sand–ice material. Changes in the degree of ice saturation help show the influence of the ice matrix versus the sand material on the mechanical behavior. Data are discussed in terms of the Mohr–Coulomb failure law and creep theories. It is shown that the cohesive component of strength depends on response of the ice matrix, whereas the frictional component of strength responds in a manner very similar to unfrozen sand tested at high confining pressures. Experimental data show that creep rates decrease exponentially and creep strength increases with an increase in confining pressure.

scholarly journals Ice triaxial deformation and fracture

1994 ◽  
Vol 40 (135) ◽  
pp. 305-318 ◽  
Author(s):  
M.A. Rist ◽  
S.A.F Murrell
Keyword(s):  
Shear Fracture ◽  
Triaxial Compression ◽  
Confining Pressure ◽  
Maximum Principal Stress ◽  
Brittle Strength ◽  
Independent Evidence ◽  
Polycrystalline Ice ◽  
Confining Pressures ◽  
Pressure Independent ◽  
Ice Failure

AbstractAn experimental investigation into the mechanical behaviour of polycrystalline ice in triaxial compression has been conducted using conditions generally favourable to brittle fracture and microcracking. Under triaxial stresses at high strain rate, ice failure occurs by abrupt shear fracturing, generally inclined at about 45° to the maximum principal stress. At −20°C, such failure is suppressed by the imposition of a small confining pressure, allowing a transition to ductile-type flow accompanied by distributed microcracking, but at —40°C shear fracture persists under confinement of up to at least 50 MPa. For low confining pressures (< 10 MPa), brittle strength is strongly pressure-dependent; above this it is pressure-independent. Evidence is presented that suggests this may reflect a change from a fracture process influenced by friction to fracture initiated by localized yielding. Ductile yield strength is found to be little influenced by confining pressure despite the inhibition of cracking that leads to greatly contrasting observed crack densities. Flow conforms to the well-known power law for ice withQ= 69 J mol−1andn= 4.2 over the temperature range −20° to −4-5° C Under these conditions, microcracking in ice appears to remain remarkably stable and non-interacting.

scholarly journals Investigation on the relationship between CT numbers and marble failure under different confining pressures

2021 ◽  
Vol 60 (1) ◽  
pp. 846-852
Author(s):  
Yang Yan-Shuang ◽  
Li Kai-Yue ◽  
Zhou Hui ◽  
Tian Hao-Yuan ◽  
Cheng Wei ◽  
...  
Keyword(s):  
Damage Mechanics ◽  
Triaxial Compression ◽  
Ct Scanning ◽  
Confining Pressure ◽  
Triaxial Tests ◽  
Compression Tests ◽  
Ct Number ◽  
Confining Pressures ◽  
Loading Tests ◽  
Rock Specimens

Abstract Computed tomography (CT) scanning technology is helpful in investigating rock materials as it can demonstrate the micro structure of rock clearly. Conventional triaxial compression tests and the corresponding graded triaxial loading tests were carried out to investigate the complex failure mechanism of the marble at the Jinping Hydropower Station. After that CT-scanning tests were done on the loaded marble specimens. The test results show that (1) the CT numbers of the specimens have a certain statistical regularity, that is, the CT numbers of the specimens under different confining pressures satisfy the Weibull distribution, as the confining pressure increases, the mean values rise while variances decrease; (2) in the two groups of tests, the average CT numbers corresponding to the conventional triaxial tests are higher than those corresponding to the graded loading tests, but the CT number variances are lower than those of the graded loading tests; and (3) according to meso-damage mechanics, the damage variables of the rock specimens were established based on the definition of CT numbers. The calculation results show that the damage variables decrease with the increase in confining pressure, the damage variables of the rock specimens in the graded loading tests are higher than those in the conventional triaxial test, and the differences between the two loading tests have grown with the increase in confining pressure.

scholarly journals A Nonlinear Strength Criterion for Frozen Sulfate Saline Silty Clay with Different Salt Contents

2018 ◽  
Vol 2018 ◽  
pp. 1-8 ◽  
Author(s):  
Yanhu Zhao ◽  
Yuanming Lai ◽  
Jing Zhang ◽  
Chong Wang
Keyword(s):  
Triaxial Compression ◽  
Strength Criterion ◽  
Experimental Results ◽  
Saline Soils ◽  
Silty Clay ◽  
Compression Tests ◽  
Frozen Soils ◽  
Conventional Triaxial Compression ◽  
Confining Pressures ◽  
Pressure Melting

It has been proven that the mechanical properties of frozen saline soils are different from frozen soils and unfrozen saline soils. In this paper, in order to study the effects of the salt contents on the strength characteristics of frozen soils, a series of conventional triaxial compression tests are carried out for frozen saline silty clay with Na2SO4 contents 0.0, 0.5, 1.5, and 2.5% under confining pressures from 0 MPa to 18 MPa at −6°C, respectively. The experimental results show that the strength of frozen saline silty clay presents obvious nonlinearity, the strength of frozen saline silty clay increases with increasing confining pressures at first, but with a further increase in confining pressures, the strength decreases because of pressure melting and crushing phenomena under high confining pressures, and salt contents have an important influence on strength of frozen saline silty clay. A strength criterion is proposed on the basis of the experimental results. The strength criterion could well reflect the nonlinear strength characteristic of frozen saline silty clay and the influence of salt contents on frozen saline silty clay.

scholarly journals Experimental Investigation on the Influence of Water Content on the Mechanical Properties of Coal under Conventional Triaxial Compression

2020 ◽  
Vol 2020 ◽  
pp. 1-11
Author(s):  
Meichang Zhang ◽  
Rongshan Nie
Keyword(s):  
Mechanical Properties ◽  
Elastic Modulus ◽  
Internal Friction ◽  
Water Content ◽  
Triaxial Compression ◽  
Friction Angle ◽  
Conventional Triaxial Compression ◽  
Influence Of Water ◽  
Confining Pressures ◽  
Water Contents

The presence of water is one of the most important factors in coal mining, and it has a dual influence on the mechanical behavior of rock. To study the influence of water content on the mechanical properties of coal under complicated stress conditions, dry coal specimens and wet coal specimens with water contents of 1.8% and 3.6% were conducted by uniaxial and conventional triaxial compression tests. The relations between the uniaxial compressive strength, deformation, and water content were observed. The reductions in the strength and elastic modulus under different confining pressures were obtained. The mechanical properties of coal specimens with different water contents under triaxial compression were studied. The influences of water content on the microstructure, clay minerals, internal friction angle, and cohesive force of coal were discussed. The results show that the strengths and elastic moduli of wet specimens are clearly lower than those of dry specimens under different confining pressures. The water content has a significant influence on the postfailure mechanical behavior of coal. The loss rates of strength and elastic modulus decrease with increasing confining pressure. The water content has almost no effect on the internal friction angle, while the cohesive force of the saturated specimens is 36.5% lower than that of the dry specimens. The results can provide a reference for inhibiting the occurrence of disasters during coal mining and exploiting coal efficiently.

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