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2014 Vol. 35, No. 2
Published:
109
Cluster Analysis of Acoustic Emission Signals of 2D-C/SiC under Tensile Loading
Static tensile test of two dimensional C/SiC composite material was carried out at room temperature and the damage processes was on-line monitored by an acoustic emission instrument. From the SEM observation, five damage modes were found during the tension, which are matrix cracking, interfacial debonding, delamination, individual fiber breakage, and fiber bundle breakage. Acoustic emission (AE) signals were analyzed by using K-means cluster method to identify these five classes of damage modes and to determine the center value of the AE parameters of these five classes damage modes. Then damage evolution is described through analyzing the cumulative count of AE events and cumulative energy change with loading time.
2014 Vol. 35 (2): 109-114 [
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115
Experimental study of granite under combined dynamic and quasi-static compression-shear loading
Rocks are natural complex medium with micro cracks, micro-holes and other defects. It is significant to study their performance under complex stress state, especially under combined compression-shear loading for their engineering applications. Quasi-static and dynamic experiments of granites under combined compression- shear loading were carried out by adding a set of double beveled device in MTS for quasi-static experiments and in Split Hopkinson Pressure Bar (SHPB) experimental devices for dynamic experiments. When changing the inclined angle of cushions, equivalent load-displacement curves of granite samples under different loading paths were obtained. By decoupling these curves, compression and shear properties of granite samples were discussed, respectively. Failure surfaces were further analyzed. The results showed that failure strength of granite had obvious loading rate effects and compression-shear coupling effects. The technique was helpful for the study of the mechanical properties for rock under complex stress states.
2014 Vol. 35 (2): 115-123 [
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163
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124
ANALYSIS OF DIRECT SHEAR TEST OF SOIL-ROCK MIXTURE BASED ON DISCRETE ELEMENT MODEL
Soil-rock mixture is a special engineering geological material, which is composed with high strength rock and low strength soil. Direct shear test is an effective approach to investigate the mechanical characteristics of soil-rock mixture. In this paper, we adopt spherical elements to simulate soil material, and irregular overlapped clumps to simulate the rock rubble with Discrete Element Model (DEM). The shear strength of soil-rock mixture is obtained with direct shear tests based on DEM simulation under different rock contents and inter-particle bonding strengths. The results show that the shear strength of soil-rock mixture increases with increasing either of the rock content and the bonding strength. Moreover, the internal friction angle and the cohesive strength of soil-rock mixture are determined with DEM simulation under various rock contents. The results above will be benefit to reveal the shear strength of soil-rock mixture.
2014 Vol. 35 (2): 124-134 [
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204
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135
Analytic solutions of two collinear fast propagating cracks of a stip in one-dimensional hexagonal piezoelectric quasicrystals
Based on the construction of conformal mapping, the antiplane problem of two collinear semi-infinite rapid propagation cracks in a symmetrical strip of one-dimensional hexagonal piezoelectric quasicrystals is analyzed by the complex variable method. Under the electrically impermeable and electrically permeable conditions, the analytic solutions of the dynamic stress intensity factors are obtained. When the crack velocity tends to zeros, the analytic solutions of the dynamic stress intensity factors can be degenerated to the existing solutions of the static solutions of correspondence. These solutions have a certain theoretical value for the engineering application of quasicrystal materials.
2014 Vol. 35 (2): 135-141 [
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222
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142
A DDA based complete and high order polynomial displacement approximation method in elastic mechanics and its cases verification
According to Weierstrass theorem for polynomial approximation, and based on elastic mechanics derivation where the complete and high order polynomial displacement function used in traditional DDA is employed, a DDA based complete and high order polynomial function approximation method is presented in this paper. In the method mentioned above, the complete and high order polynomial function is used as displacement function of the elastic domain, the coefficients of the basic functions of coordinate variables in polynomial are used as unknowns, and the simplex integration method is used as the analytic integration, and the simultaneous equations are established and solved in order to approach the elastic solution. Based on the study of the calculating process for simplex integration, a diagram interpretation in three-dimensional condition is presented, which is helpful to illustrate the logical relationship among the variables in the simplex integration formula and the whole calculation process for integration in a given tetrahedron. At the end of this paper, the corresponding calculation code is developed and two calculation cases, one is a three-dimensional simply supported beam with uniform loading while the other is a boundary fixed elastic plate with point loading, verify the feasibility of this method. It can be indicated by the calculation result that, with the increase of the order of polynomial function, the result of polynomial function calculated by this numerical method approximate the analytical solution monotonously. According to the polynomial approximation function with six-order in this paper, the error of displacement for simply supported beam is less than 0.2%, while the error of displacement for elastic plate is less than 0.91%. In addition, in two cases, the displacement difference, compared to the analytical solution, is within micrometer.
2014 Vol. 35 (2): 142-149 [
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284
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150
The EXPERIMENTAL RESEARCH ON CONSTITUTIVE RELATIONSHIP OF FROZEN SILTY CLAY BASING ON THE DUNCAN-ZHANG MODEL
Abstract In order to research the strength and deformation characteristics of artificial freezing frozen silty clay, the soil samples which carried from the construction of connecting passages in DK11+395 of Shenyang subway are taken as the research sample. The strength and deformation characteristics of frozen silty clay are studied through triaxial shear test for frozen soil under different confining pressures, different temperatures and different initial moisture contents. The results indicate that confining pressures, temperatures and initial moisture contents are the most important influence factors for the strength and deformation characteristics. The deviatoric stress versus strain curves of frozen silty clay is hardening. The failure strength (σf) and elasticity modulus (Et) are increasing with confining pressures (σ3c) and initial moisture contents (ω), but decreasing with the temperatures (T). The value of deviator stress ratios are between 0.79~0.96. The Duncan-Chang Model which considered the confining pressures, temperatures and initial moisture contents are established based on the triaxial shear test data. The regress equations between the model’s parameter a, b and σ3c，T and ω are built by using the regression analysis. The calculated deviatoric stress versus strain curves which obtained from the model and the contrast tests are compared. The comparison results show that the calculated results fit well with the test results. The results also show that the model which has been established is efficacious. It can reflect the strength and deformation characteristics of frozen silty clay in different confining pressures, different temperatures and different initial moisture contents. The research results provide a reference to the study of the deformation and strength properties of frozen silty clay and the guidance for the artificial freezing construction. It has not only important theory value, but also has important application value of engineering.
2014 Vol. 35 (2): 150-159 [
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171
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160
STUDY ON TOPOLOGY OPTIMIZATION METHOD OF ADAPTIVE EFG METHOD BASED ON NODAL DENSITY
This study proposed a topology optimization method of adaptive EFG method based on the node density, in response to the problems of huge number of variables and low efficiency in the topology optimization of the meshless method. The adaptive EFG topology optimization model was established with minimizing the compliance of the structure from using nodal density for design variables in the topology optimization. The adaptive node refinement criteria, the node refinement scheme and the added nodes variables updating rule were detailedly described. Numerical examples prove that the proposed method can extremity reduce the number of the design variables, and improve the efficiency of optimization.
2014 Vol. 35 (2): 160-166 [
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277
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167
Molecular dynamics simulations of atomic-scale friction behavior
The friction behaviors between rigid spherical tips with different atomic scale rough topography and the elastic flat substrate are simulated using large-scale molecular dynamics method .The relationship between friction and load, real contact area and load, friction and real contact area are analyzed for non-adhesion and adhesion to study the friction behavior laws on nanometer scale. The results show that the relations between real contact area and load of the studied systems are all agree with the corresponding continuum contact models, such as Hertz smooth contact model, Greenwood-Williamson rough contact model and Maugis-Dugdale adhesive contact model. For the non-adhesive contacts, the friction-load relations are linear for both smooth surface system and rough surface system, while there is no simple relationship between friction and real contact area. However, the relationships between friction and real contact area are linear for the adhesive contacts, while the friction-load relations are sublinear and agree with Maugis-Dugdale model prediction. Our research shows that when changed from non-adhesion to adhesion, the decisive factor of friction will transfer from load to contact area, and the friction behavior will transfer from load-controlled friction to adhesion-controlled friction.
2014 Vol. 35 (2): 167-172 [
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173
DAMAGE AND PERFORMANCE OF THE BRIDGEWIRE ELECTRIC DETONATOR UNDER HIGH SHOCK OVERLOAD
The influences of peak acceleration, loading direction, and product structure on the damage and performance of two standard bridgewire electric detonators, one leg and two legs structure, were studied by gas gun at high overload of 60 000g, 80 000g, 100 000g and 120 000g. The damage laws and damage modes were obtained, which were further analyzed by dynamics theory in conjunction with the product structures. The results show that there are three types of damage modes, namely, local plastic deformation, relative displacement, and brittle damage, which are indicated by impact end deformation, electrode plug moving inward, electrode and fuses incline, electrode plug crack, resistance increase, bridgewire break, and increase of output power scattering. The total length change rate increases with the acceleration raising under output loading mode, and the total length decrease for one leg detonator is caused by electrode plug moving inward, while the total length decrease for two legs detonator results from impact deformation. The resistance of one leg detonator changes obviously under lateral loading mode, with the bridgewire break and detonator explosion occurring occasionally. The output power scattering increases after high-g loading.
2014 Vol. 35 (2): 173-181 [
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198
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182
Fatigue Reliability Study on T-welded Component of Shiled Machine Considering Load Shedding
T-welded component is one of the key structures of the shield machine cutterhead, and sub-critical crack in the weld extensions is a key factor in its fatigue faliure. A new structural fatigue reliability analysis method is proposed, considering load shedding, by the use of probablistic fracture mechinics based on FORM method. The method is applied to a T-welded shield structure, and results of structural fatigue reliability are gotten at the cases of with and without considering load shedding. The method’s validity is verified using Monte Carlo method. And a curve that load shedding impacts on the T-welded structure fatigue reliability is fitted via least square method, which is divided in three regions: the high, medium and low fatigue failure zone. The results showed that the fatigue failure probabilities were 8.2644e-05 and 3.4314e-04 when with and without considering load shedding, which load shedding improved the structure fatigue reliability was reputed. The thickness of T-welded structure was thought as a design variable in the practical project design of fatigue reliability, in which low fatigue failure zone may be used as a reliability constraint. The results have a certain theoretical guidance for the maintenance, inspection plan and design of shield machine cutterhead in the practical project.
2014 Vol. 35 (2): 182-188 [
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180
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189
Vector cooperative response surface method for structural reliability analysis based on optimal probabilistic collocation points
Response surface approximation and collocation point selection are two important problems of response surface method for structural reliability analysis. In this paper, a vector cooperative response surface for structural reliability analysis was constructed by the cooperative basis vectors, while the optimal probabilistic collocation points were slelected from large numbers of candidate points according to the full row rank criterion of collocation point matrix. The preconditioned stochastic Krylov subspace was defined using the global stiffness matrix and force vector, and the stochastic nodal displacement vector was expanded subsequently in the subspace to develop vector cooperative response surface hierarchically. The stochastic row vector was constructed by the combination of basic random variables in accord with cooperative basis vectors. The collocation point matrix was constructed by the stochastic row vector with particular collocation points and the optimal probabilistic collocation points were slelected according to the full row rank criterion of collocation point matrix. Finally the unknown coefficients of cooperative response surface were determined by the regression analysis. The results show that the proposed method can reduce the number of probabilistic collocation points significantly on the condition of ensuring high accuracy, global applicability and fast convergence, comparing with the traditional response surface method and stochastic response surface method.
2014 Vol. 35 (2): 189-198 [
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213
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199
GENERAL ANALYTICAL METHOD FOR COMPOSITE BEAMS’ BENDING USING REDDY’S HIGHER ORDER BEAM THEORY
Based on the displacement mode of Reddy’s higher-order beam, and taking account of the slip deformation at the interface of the composite beam, the governing equations and boundary conditions for bending of the Reddy’s composite beam are formulated using the principle of minimum potential energy. The governing equations are transformed into an ordinary differential equation system consisting of 12 fundamental unknowns, and the solution expressions and the general analytical method for solving the ODE system are presented. Then, the bending of the simply-supported Reddy composite beam subjected to a uniform transversal load is studied, and the obtained results are in good agreement with those of the finite element method, which demonstrates the effectiveness and reliability of the general analytical method presented in the paper. Finally, the influences of the slip shear stiffness kcs at the composite beam interface, the Young’s modulus to shear modulus ratio E/G, beam span to depth ratio L/h and the layer’s depth ratio hs/hc, etc. on the bending of the Reddy composite beam are examined numerically. It is revealed that the slip stiffness has a great influence on the distribution of the stresses on the cross-section of composite beam, and the influence of the shear effect on the composite beam’s deflection becomes more remarkable with the beam span to depth ratio L/h decreasing and the the Young’s modulus to shear modulus ratio or the slip stiffness increasing. In such cases, the shear effect can not be neglected.
2014 Vol. 35 (2): 199-208 [
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272
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209
Unsteady temperature fields in the 2D-FGM plate under different constant temperatures of initial and boundary
To study the plane unsteady temperature field and its effect factors in the 2D-FGM plate, assuming that the thermal conductivity and volumetric specific heat capacity of the plate are the distributions of same type of exponential functions along the direction of the height and width in the plate, based on the unsteady heat conduction basic equation of the plate, the series analytical solution of the plane unsteady temperature field in the plate subjected to the different constant temperatures of initial and boundary was derived by the variable separation method. The correctness of the results of this study was verified by using the comparison of two methods. The results show as follows. (1) The isotherms in the square plate are symmetrical to the 450 diagonal of the plate when the gradient parameter ALFA=BETA, and symmetrical to the center of the plate when ALFA=BETA=0. (2) The isotherms in any plate are not symmetrical when ? ALFA is not equal to BETA. (3) With the increase of cooling time, the isotherms in the non-homogenous plate move to the lower left (or upper right) corner, and the isotherms spacing in the upper right (or lower left) corner becomes wider, and the isotherms spacing in the lower left (or upper right) corner becomes more narrow, and the cooling process of the rectangular plate is faster than that of the square plate. Thus, the suitable selection of gradient parameters and geometry can meet the need of the design, application and analysis of the thermal stress of the plate, and the analytical solution can be used as a reference standard for other approximate methods.
2014 Vol. 35 (2): 209-216 [
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175
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