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2017 Vol. 38, No. 6
Published: 2017-12-28
483
Nonlocal peridynamic modeling for split of notched Brazilian disk
The split of Brazilian disk has attracted serious concern of researchers from the fields of elasticity and rock mechanics and engineering. In this paper a modified two-parameter micro-elastic prototyped brittle peridynamic constitutive model was proposed considering the relative rotation of the interactive bonds of material points based on the recently developed nonlocal peridynamic theory, and the mechanical behavior and fracture characteristics of rock-like materials were also taken into account. By introducing a series of physical algorithms including local damping, force boundary, equilibrium and dynamic relaxation of particle system, the failure process of a pre-notched rock-like Brazilian disk with different central cracks were studied with the proposed peridynamic simulation method. The crack propagation paths and failure mode agree well with experimental observation, which provides a promising alternative for numerical simulation of crack growth and fracture.
2017 Vol. 38 (6): 483-491 [
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277
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492
ANALYSIS OF FEM CUBIC REPRESENTATIVE VOLUME ELEMENT ALONG SPECIFIED STRESS PATH
The representative volume element (RVE) established on the mesoscopic scale, by which the analysis can reflect the microstructure of materials and macroscopic statistical mechanical properties, is an effective method to predict the mechanical properties. In this paper, the method of simulating a normal hexahedron RVE under arbitrary stress state with arbitrary stress path to calculate the macroscopic stress and strain is theoretically analyzed. The numerical calculation process is realized by applying the FEM software ABAQUS. The multi-scale analysis of the special positions of notched round bar under cyclic loading is carried out. The results show that: (1) This method can accurately control and realize the loading of RVE in arbitrary stress state and stress state path; (2) The simulation associated with RVE can obtain the microstructure evolution when the macrostructure under complex loading.
2017 Vol. 38 (6): 492-502 [
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453
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503
Laboratory investigations on the mechanical properties of sandstone in chemical corrosion under and freeze-thawing cycles
The physical and mechanical characteristics of sandstone specimens eroded in different chemical solutions （acidic Na2SO4 solution、neutral Na2SO4 solution and alkaline NaOH solution）and subjected to different freeze-thawing cycles is experimentally studied. The variation of physical and mechanical characteristics of sandstone specimens subjected to different chemical solutions for 30ds and then different freeze-thawing cycles is analyzed. Meanwhile the damage of microstructure of sandstone specimens is observed by means of stereo microscope and SEM scanning electron microscope. The damage variable based on the change of porosity was defined to analyze quantitatively damage degree of sandstone specimens. The test results show that for different chemical solutions，the peak strength and elastic modulus of sandstone specimens decrease by exponential function with the increase of freeze-thawing cycles，but its peak axial strain increases by exponential function with the increase of freeze-thawing cycles. With the increase of freeze-thawing cycles，the damage of sandstone specimens increased gradually soaked in different chemical solutions. The damage degree of sandstone specimens soaked in acidic chemical solution is bigger than that in neutral chemical solution and alkaline chemical solution，that is the smallest under alkaline NaOH solution. Acidic chemical solution intensified the damage and degradation degree of sandstone specimens under freeze-thawing cycles，but there has some mitigative effect on that of sandstone specimens soaked in neutral solution and alkaline solution. Chemical corrosion and together with freeze-thawing cycles can give mutual promotion and mutual influence on the degradation and damage of sandstone specimens.
2017 Vol. 38 (6): 503-520 [
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217
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521
Modeling for Normal Contact of Rough Surface Based on Stress Distribution of Asperity Contact
An elastic-plastic mechanical model is proposed to characterize the contact load and area of rough surface. FEM is used to analyze the elastic-plastic contact of asperity against the rigid mass, with several different plastic constitutive equations. According to the contact stress distribution of different contact load, the asperity contact regime is divided into the elastic regime, the mixed elastic plastic regime and the fully plastic regime. The increasing change of contact stress is considered to obtain the relation between contact load with contact deformation. Statistical analysis of GW model is then applied to integrate the formulations of contact load over the rough surface. The model is compared with the previous elastic model, CEB model, ZMC model, KE model and JG model, and the effect of plastic index on the relation between contact load and mean contact separation of rough surface is also investigated. The results show that the proposed model can better model the relations of contact load and deformation, and agree well with KE model and ZMC model. With the increasing of mean contact separations, the total contact load will become less and less. The plastic index has a great effect on the contact load of different model, and larger plastic index will induce more difference.
2017 Vol. 38 (6): 521-529 [
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287
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530
Fracture analysis of cracked equilateral triangle hole with surface effect under antiplane shear
Based on the surface elasticity theory and conformal mapping technique, a theoretical study is conducted on the fracture behavior of cracked equilateral triangle hole with surface effect under antiplane shear. An exact solution of the whole-field stress around triangle hole is presented. An analytical solution for the stress intensity factors at the tips of crack is obtained. Numerical examples are provided to discuss the variations of the dimensionless stress intensity factor with the size of triangle hole, the length of crack and the surface property. The major results are: The stress intensity factor of the crack tip is dependent dramatically when the size of the triangle hole is on the order of nanometer. The present solution approaches to the classical fracture theory when the triangle hole has large characteristic dimensions. With the increase of crack length, the stress intensity factor first decreases, then increases. When the relative length of the crack is small, surface effect of the defect is quite weak. The size-dependent effect of the stress intensity factor is significantly affected by the surface property.
2017 Vol. 38 (6): 530-536 [
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205
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537
Exact analysis and adjustment for radial vibration of functionally graded piezoelectric spherical shells filled with elastic medium
An exact solution is obtained for the radial vibration of a functionally graded piezoelectric sphere filled with an elastic medium by means of the change of variables technique. This solution can be applied to functionally graded piezoelectric spheres of any thickness, with the material properties varying along the thickness direction in power law forms. Especially, the gradient index of the density can be different from that of the other material parameters. Thus, the limitation that all material gradient indices have been assumed to be the same in the literature is then overcome. Numerical examples show that the radial vibration characteristics of the functionally graded piezoelectric spherical shell can be effectively tuned by adjusting either the gradient index or the stiffness of the filling elastic medium.
2017 Vol. 38 (6): 537-543 [
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225
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544
Dynamic Indentation Experimental Investigations of Quasi-brittle Single Crystal
A dynamic indentation test device consisting of a titanium alloy pressure bar and a Vickers diamond indenter was designed. A momentum trap is introduced to avoid multiple loading due to a single pressure bar. This device can deliver an indentation load with pulse width of 80 μs and then is used to investigate the high-strain-rate response of quasi-brittle single crystal materials such as sugar and RDX crystals. Comparative experiments between single and multiple indentation loading were carried out for sugar crystals. the dynamic hardness of crystal sugar is approximately 5.18MPa. RDX single crystal is very prone to fracture and fragment under indentation loading and its dynamic hardness is 1.304MPa. Compared with RDX crystal, plastic deformation ability of sugar single crystal is better. Radial and circumferential crack of the indentation pit are observed using optical microscope. The crack growth mode of quasi-brittle crystal under dynamic indentation load is presented.
2017 Vol. 38 (6): 544-550 [
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214
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551
Modeling the Dynamical Hysteretic Behavior in Magnetostrictive Materials Based on Phenomenological Theory of Phase Transition
In the current paper, a coupled nonlinear differential equation is proposed to model the hysteretic dynamics of one-dimensional magnetostrictive materials based on the modified Landau phenomenological theory of phase transition. A non-convex free energy function is constructed to model the irreversible magnetization orientation switchings and magneto-strain. Each of its minima is associated with one of the magnetization orientations in materials. Nonlinear constitutive laws accounting for magnetostriction effects are obtained by using thermodynamic equilibrium conditions. The hysteretic loops and butterfly-shaped behaviors in the magnetic and mechanical fields are both successfully modeled. Comparison of the model results with its experimental results reported in literatures is presented, capability of the model is approved.
2017 Vol. 38 (6): 551-557 [
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178
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558
The coupling method of hydraulic fracturing base on poroelasticity concepts and numerical research of fracturing state parameters
In the paper we completely and systematically elaborate the controlling equations and FEM coupling methods for numeric simulation of hydraulic fracturing. The framework is consist of: (1) The concept of poroexpansibility is employed to couple the fluid pore pressure with the stress field; (2) An formula of hydraulic fracture opening is proposed based on the concepts of damage localization;(3) Deduce the formula for calculation of anisotropic properties such as damage, permeability, poroelasticity, etc; (4) Propose a coupling loading scheme using full flow rate. As a validation, a three-Dimensional hydraulic fracturing simulation is performed. The comparison of numeric solution and analytical solution of the state parameters of the hydraulic fracture is performed and proved the validation and correction of the numeric simulation.
2017 Vol. 38 (6): 558-569 [
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208
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570
Identification of the Creep Model and Its Parameters of Soft Rock on the Basis of Disturbed State Concept
Contrary to hard rocks, the plastic deformation of soft rocks during the initial creep stage occurs more obviously. The traditional component models can be used to describe the visco-plastic behaviour of the geomaterials, but only when the stress value is beyond the yield point. This paper proposes the plastic deformation as a variable in the disturbance function, in the framework of the Disturbed State Concept, to describe the evolution characteristics of elastic-plastic states during loading and unloading. In addition, the evolution of the disturbance equation in time is further investigated and analyzed according to the relationship between plastic deformation and time. The Burgers model is applied to the relative intact state, while the Bingham model is employed to describe the fully adjusted state. The disturbance function is utilized to combine the two states. Following this approach, the creep constitutive model for soft rocks are presented. Comparison with the creep test results of argillaceous shale indicates that the proposed disturbance state constitutive model accurately captures the temporal evolution of the transformation process from relative intact state up to fully adjusted state. Furthermore, the proposed model can better handle both nonlinearity and stage coordination, while it provides a more profound description of the initial, stable and accelerating creep stage of soft rock
2017 Vol. 38 (6): 570-578 [
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313
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579
The triple-shear elasto-plastic constitutive model for saturated over-consolidated clay
The extended failure stress ratio connecting with the different stress state and the cohesion is given out based on the triple-shear unified failure criterion and coordinate translation for the saturated over-consolidated clay, in order to overcome the shortcomings of the original sub-loading yield surface modified cam-clay model with the definite failure stress ratio without considering the soil cohesion, which can't reflect the practical differences of strength properties under different stress states and cohesions for soils. The advantage is that the extended failure stress ratio could show the practical effects of different stress states acting on the saturated over-consolidated clay with different cohesions better. Depending on the viewpoint above, a new triple-shear elasto-plastic constitutive model for the saturated over-consolidated clay is set up. Its features are that it can describe the effects of intermediate principal stress, rang changes of stress, difference between tension and compression and different cohesions. In addition, the ABAQUS is developed secondly based on the new constitutive model and the mechanical properties are simulated under the true and the conventional triaxial tests in the drainage and the undrained conditions for the saturated over-consolidated clay. Comparisons between the numerical simulations and experimental results under conventional triaxial tests show that the proposed model could reflect the changes of deformations, shear dilatations and pore water pressures for the saturated over-consolidated clay with different OCR.
2017 Vol. 38 (6): 579-590 [
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275
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591
THE 3D NUMERICAL SPECIMEN OF GRANITE BASED ON CT TECHNOLOGY AND APPLICATION
In this paper, the granite specimen is taken as an example, and the computerized tomography (CT) technology is employed to capture a serial of sectional images of the granite specimen, and achieved the identification and characterization of granite specimen structure based on CT image using digital image processing technology. After that, the 3D digitized solid model of material structure is reconstructed based on the vectorization?method and 3D Rock Failure Process Analysis (RFPA3D), and then the uniaxial compression process is simulated. By comparison, it is found that the failure modes of the numerical granite specimen are similar to that obtained from the experiment, which provides a new tool for deeply studying the mechanical behaviors of quasi-brittle materials such as rock and composite materials.
2017 Vol. 38 (6): 591-600 [
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302
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