Triaxial creep characteristics and constitutive relation of gas containing coal

The characteristics of conventional triaxial creep are obtained via a conventional triaxial creep test of gas containing coal under different stress levels. Mathematical fitting research regarding an experimental curve is done. A constitutive model of conventional triaxial creep of gas containing coal is developed by improving a generalized K B model. The results show that the characteristics of conventional triaxial creep of gas containing coal are related to the load step, and creep deformation is limited by the gas and confining pressures. Curves of the order 3 ε(t) describe the three steps of conventional triaxial creep tests of gas containing coal very well; the improved generalized K B model can represent a constitutive model of the conventional triaxial creep of gas containing coal.     

A visco elasto plastic creep model of outburstprone coal under triaxial compression

Using self made creep seepage experimental apparatus, triaxial creep tests on outburst prone coal are carried out with coal samples taken from Songzao Coal Mine. It is found that the attenuation creep property is shown in the conditions of less than its long term strength load, while non attenuation creep property for the greater conditions. By introducing a visco elasto plastic body which can describe the deformation property of non Newtonian fluids, the visco elasto plastic creep model of outburst prone coal is put forward combined with a Poyting Thomson model. The model parameters were fitted out with Matlab programmer and validation is employed. It is found that the test results were in good agreement with those by the proposed model.     

Experiments on triaxial mechanical properties of soft coal containing gas

Taking briquette specimens composed by soft coal with outbursting trends as research object, systematic research of the influence on triaxial mechanical properties of soft coal containing gas caused by confining pressure and gas pressure is done. The results show that confining pressure can significantly affect mechanical properties of soft coal containing gas, the compression phase of soft coal containing gas is obvious and the triaxial strength is increased gradually as confining pressure is increased; the influence on elastic modulus and shape of stress-strain curve is not obvious. The triaxial strength of soft coal containing gas shows decrease trend as gas pressure is increased; the influence on strength of soft coal containing gas caused by gas pressure is obvious when the confining pressure is smaller, but it is not obvious when the confining pressure is bigger. the influence caused by gas pressure on elastic modulus and deformation of yield step is not obvious; gas pressure has not only obvious mechanical effects,but also have chemical action on soft coal containing gas; stress-strain curve of soft coal containing gas can be expressed by conic, the influence caused by gas pressure can be considered as equivalent confining pressure effects.     

Uniaxial Experimental Analysis on Creep Properties of Deep Limestone

The uniaxial creep tests were carried out for deep limestone from Chongqing adoptal by the MTS815 servo controlled rock mechanical test system. Meanwhile, the DISP 2 based AE system was used to test acoustic emission (AE) signal. It is found that AE can better reflect rock microstructure such as diffusion of lattice defects, opening and close of the pore and fracture, compatible deformation among particles, the generation and connection of micro ruptures. Through different creep model parameter inversion, 〖JP2〗it is concluded that either the logarithmic function creep model or the exponential function can better reflect deep rock creep deformation behavior and the logarithmic function creep model is much better. 〖JP〗     

The effect of original moisture content in coal beds on coal and gas outburst risk level

By using self-made “Coal Containing Gas Thermal-Hydrological-Mechanical (THM) Coupling Experimental Equipment”, an experimental study is carried out to investigate the gas seepage law of containing-gas coal during the complete stress-strain process with different original water contents. Under the conditions of constant gas pressure and confining pressure, and the experimental results are compared with the field test of the gas emission variation of coal seams before and after being infused. The research results show that the triaxial compressive strength and the elastic ratio of coal decrease with the increase of original water content in coal, but the axial strain of coal increases and the absolute value of transverse strain and volume strain of coal increases at the triaxial compressive strength; the methane effective permeability of coal decreases throughout the complete stress-strain process. Outburst prevention by coal seam infusion attributed is analyzed from two aspects: the mechanics of coal and the gas flow in the coal seams. The higher original water content in coal seams is, the smaller the risk of coal and gas outburst becomes. So the original water content in coal seam can be treated as a significant criterion for the judgments of coal and gas outburst criticality.     

Study on Methane Flow Regularity and Secondary Stress and Deformation of Coal Bady around Penetration Boreholes in Coal Seam

A new mathematical model for the coefficient of variable permeability affected by coal body, secondary stress & pressure in coalseam & a constitutive equation for gas migration are improved on the basis of the secondary stress & deformation of coal body around boreholes in coalseam. Using this model and equation,the gas flow field belongs to the 20102 experimental zone in BaiJiao coal Mine is computed & this verifies that the model & constitutive equation are both agreeable to practical results. Simultaneously, a comparison between the variable and constant permeabilities is carried out.     

Experiments of coal’s gas seepage with outburst dangers

The experiments of outburst coal’s gas seepage are carried out by triaxial permeameter which is independently developed. Under the condition of fixed gas pressure, different confining pressures and axial pressures acting on raw coal test samples are tested. It shows that as the increasing of confining pressure, coal sample seepage velocity tends to decline, and this phenomenon is more obvious for the coal with serious original injury. The relationship between seepage velocity and confining pressure can be described as a quadratic function,and its general expression is established. Coal sample’s seepage velocity-strain path is similar to stress-strain path, therefore, the changes in coal seepage velocity indirectly reflects the process of the evolvement of coal damage. The influence of effective stress on coal sample seepage velocity can be concluded as hypersensitive stage and smooth stage, and in the hypersensitive stage, seepage velocity of the coal sample vary quickly, while in smooth stage, its variety is not obvious. In different stress conditions, the degree of development for hole and cranny of coal affect coal gas energy and elastic deformation energy in coal body as well as the form of coal destruction.     

Experimental analysis of matrix shrinkage’s influence on mechanical permeability of gas filled coal

Based on the coal seam of Zhao zhuang coal mine and Si he coal mine belong to Jin cheng anthracite Mining Group in Shanxi province, by using the self developed three axial servo controlled seepage equipment for hot fluid solid coupling of coal containing methane, parallel experiments for permeability of helium and methane under different gas pressures are carried out to quantitatively study the influence of Klinkenberg effects on the permeability of coal. Meanwhile, the problem of permeability decreasing caused by fluid pressure decreasing and gas desorption is eliminated by controlling the effective stress. The results show thatthe Klinkenberg effect is obvious under low gas pressure. With the increase of gas pressure, the permeability of coal decreases at first and then increases. The inflection points of gas pressure for Klinkenberg effect are about 0.9 MPa both for coal samples in Zhao zhuang coal mine and Si he coal mine. When the effective stress is constant, with the increase of gas pressure, the permeability increment caused by Klinkenberg effect decreases, while permeability increment caused by coal matrix shrinkage increases. Coal matrix shrinkage has a close relationship with the mechanical properties of coal rock mass. For the coal sample with a poor mechanical property, coal matrix shrinkage is more obvious.     

Correlation dimension analysis of AE sequence under failure process of gas-filled coal

The correlation dimension evolution characteristics of acoustic emission(AE) time-series during the instability and failure of coal with and without gas are analyzed base on fractal theory. The results show the coal rock’s AE time-series of deformation stages have obviously fractal characteristics under uniaxial loading. The processes of AE of deformation have different self-similarity, and the correlation dimension can describe the mechanical state of coal. During the triaxial loading process, the stress-varying trend of AE time-series of coal with and without gas is similar to the trend of AE correlation dimension. The sharp decrease of AE time-series’ correlation dimension can be considered as a “precursor” for failure. This feature doesn’t change with the change of confining pressure and gas pressure.     

Characteristics of triaxial creep and constitutive relationship of outburst coal

The creep characteristics of coal and rock under confining pressure in Songzao, P.R. China were studied. The experimental data were analyzed and discussed in detail. The characteristics of creep could be described by the generalized K B model after the experimental data were fitted. The parameters of the model were obtained. The results show that when the creep load was σ≥σs, the curve of coal and rock under confining pressure displayed three typical phases. In the first and second phases, the fitted curve was consistent with the test curve. Thus, the generalized K B model can describe the whole process of coal creep under confining pressure.     

Two-dimension Thermal Elasto-plastic and Creep Stress Model for Thin Slab Strand in Continous Casting

The thin-slab continuous casting is an important new technology in the world. The major key of this technology is to assure the good quality and the less crack of thin slab. Thus in this paper a two-dimension thermal elasto-plastic and creep stress (TEPC) model for thin slab strand is developed. The equations between stress and strain are given on thermal creep condition for elastic area, plastic area and the transitional area of thin-slab strand section.The formula of equivalent plasto-strain increment is obtained. The boundary conditions of the model given is fairly accurate. Therefore, the TEPC model is possessed of higher precision, and the model is also used to analyze the stress and strain distribution of strand in continuous casting.     

THE STUDY OF THE RELATION BETWEEN COAL-GAS OUTBURST AND PHYSIC AND MECHANIC PROPERTIES OF COAL SEAM IN A COAL MINE

The experimental study of physic and mechanic properties of coal seam in a coal mine is presented, and the relation between coal-gas outburst and the strength, the elastic modulus, as well as the deformation property of coal seam is analysed on the basis of the experimental results.     

Numerical simulation of carbon dioxide storage in unmineable coalbed

The injection of carbon dioxide (CO2) in deep unmineable coalbeds（CO2-ECBM） is a very attractive option for geologic CO2storage. The CO2 is stored and at the same time the recovery of coalbed methane is enhanced. The revenue of methane (CH4) production can offset the expenditures of the storage operation. The processes involving in the CO2-ECBM include multi-components gas adsorption,diffusion,and gas-water two phase flow in coalbed. On the basis of above processes,a model considering adsorption induced coal matrix swelling is developed, which can be used to describe multi-components gas adsorption/desorption-diffusion-flow in coalbed. The finite difference method is used to solve this model,and the comparison of the results with the existed model suggests the model is valid. Then the model is applied to evaluated the influence of permeability of coal seam,different gas adsorption induced swelling,CO2/CH4 adsorption capacity ratio,pore model considering matrix swelling,and gas composition of injection gas on CO2 sequestration and methane production, and these results are very important for site selection and technological parameters optimum of CO2-ECBM project.     

Experimental study of coalbed methane seepage in fuxin basin

Taking raw coal samples collected from the wangying colliery in Fuxin basin as the research object, the experiment of coalbed methane seepage law is conducted by the triaxial osmoscope and the relationship between effective stress and coal permeability is presented. The experiment results indicate that the coal permeability is of stress sensitivity and tended to decline in exponent with the increment of effective stress, which is consistent with the former research results. The seepage flow curves from experiments demonstrate the nonlinear characteristics under different confining pressures and moisture contents, and it accurately reflecte the effect of coal body deformation on the coalbed methane seepage flow in the experimental process. Therefore considering the influence of effective stress on the coal body deformation, a new motion equation for the nonlinear percolation characteristic of coalbed methane is established. Moreover, the motion equation is correlated well with the experimental data, and the fitting correlation coefficients are above 99.6%.     

Damage Constitutive Relationship of Coupled Plasticity-creep for Magnesium Alloy

Based on a simple mechanical model and an appropriate definition of generalized time,a constitutive equation is obtained for coupled plasticity and creep behavior of materials.A damage evolution based on a spherical void model for mixed hardening materials is obtained by means of Gurson's model.The damage evolution law is embedded in the adopted constitutive equation,and a damage constitutive equation is obtained for coupled plasticity and creep behavior of die-casting magnesium alloy.The response of magnesium alloy subjected to cyclic loads is analyzed.The validity of the proposed model is demonstrated by the satisfactory agreement between the experimental and calculated results.     

Numerical simulation of seepage field of gas extraction drilling of single bedding of mining-coal bed

Aiming at the problem of reasonable decorate of coal gas extraction drilling, combining with the initial and boundary condition of borehole extraction, the solid and gas coupling mathematical model of gas seepage of borehole extraction is deduced based on established control equations of seepage field and deformation field. Taking gas extraction of stone coal slit as an example, according to the characteristics of coal seam in the research, the numerical simulation method is used to obtain the distribution rule of gas pressure, coalbed gas permeability, and coalbed gas seepage velocity as well as seam deformation. The effective radius of gas extraction drilling of single bedding about mining-coal bed is determined, which provides the basis for optimum arrangement of gas extraction drilling of single bedding. The results show that the effective radius of stone coal is about 4 m. If the prolonging drilling drainage time is less than 20%, the drilling quantities would reduce by about 50%.     

Experimental Study Creep Characteristics on Creep and Cyclic of Saturated Soft Clay

Several long term creep and cyclic creep tests of Shanghai soft clay are performed in the laboratory. Test results have been obtained under various combinations of confining stresses, deviation static stress ratios and deviation cyclic stress ratios. The results reveal that the axial creep strain increases with the increase of deviation static stress ratio under a constant confining stress, while it also increases with the increase of confining stress under a constant deviation stress ratio. Such a phenomenon is similar to that from the cyclic creep tests, but the following creep strain increase will be significantly reduced after a period of cyclic loading. The results will benefit the expressway, railroad and underground constructions.     

Testing and modeling studies on creep characters of supports for tunnel using nonlinear constitutive model

Creep characters of tunnel supports structure is analyzed according to triaxial creep test of concrete specimen of deep tunnel supports structure adopting MTS815 hydraulic servo system. The creep parameters of nonlinear creep model are presented based oncreep curve. The finite element numerical model has been established considering tunnel construction process, creep characters of surrounding rock and support structure. The displacement characteristics, the effective stress and the maximum shear stress in different position in primary support and second lining are obtained through creep characters analysis of surrounding rock and supports structure. The features of displacement and stress dependent time of tunnel supports has visible difference comparing with discard creep effect and calculation displacement and practical monitoring results is basic concordance. The creep characters of surrounding rock and tunnel supports should be considered for deep tunnel.     

The Application of Gene Expression Programming in Concrete Creep Analysis

The creep analysis of concrete is of great importance in stress calculation and deformation analysis of concrete structures. Since many factors affect concrete creep, the creep function is highly nonlinear, making it difficult to obtain satisfactory results using the traditional regression method. The gene expression programming (GEP) method, which combines the advantages of GA and gene programming, shows great promise and capability of data mining in nonlinear fitting. Based on theoretical analysis of GEP, the data mining model of the creep function was established by using GEP. Examples indicate this model is more accurate than functions obtained by conventional methods.