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2017 Vol. 38, No. 5
Published: 2017-10-28
379
A modified boundary element method for scattering problem of Rayleigh wave
Boundary element method (BEM) has the advantage in solving infinite field problems such as those concerning half-spaces. However, for elastic wave propagation problem, spurious reflected waves will occur at the truncation points by the conventional BEM if full space Green’s function is adopted, which directly affects the accuracy in solving scattering fields. In light of this, a modified BEM method is proposed to analyze scattered fields of the two dimensional Rayleigh waves in 2-D half-plane based on the conventional BEM. We calculated the correction factors by reciprocity relations between the unit Rayleigh wave and the full plane Green’s function. Meanwhile, by substituting the correction factors into the modified BEM method, we can calculate the scattered field of Rayleigh wave. The method is varified by comparing the numerical results with analytical solutions and calculating scattering fields of different flaws. Those comparison results show that the modified BEM proposed in the paper can accurately solve the scattering problem of Rayleigh waves, which provides an effective forward simulation way for further investigating inverse problem of flaw reconstruction based on surface waves.
2017 Vol. 38 (5): 379-390 [
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391
Study on resistance of lightweight sandwich curve plates with metal foam core to blast loadings
Sandwich structures have been widely used in engineering structures to resist the intense blast loadings due to the high specific strength, high specific stiffness, superior energy absorption, easily shaped to curved configurations, easily formed with integral face-sheets, and multifunctionality. Compared with the sandwich flat structures, the sandwich curve plates have different performances and will provide much more choices for the design of structures. In the past decades, more attentions focused on the dynamic response of sandwich flat structures. Investigations of the sandwich curve structures are few. In this paper, finite element analysis is performed to study on the dynamic response of fully clamped sandwich curve plates subjected to blast loading. The maximum deflections and maximum kinetic energy of the sandwich curve plate, sandwich flat plate, solid monolithic curve plate and solid monolithic flat plate of the same mass are compared. Evolution process of blast wave produced by TNT explosive on the surface of the sandwich flat and curve plates are presented. Central velocity, central deflection of inner and outer face sheets, and kinetic energy of structure versus time of the sandwich curve plate under TNT explosive loading are analyzed. Effects of different curvature and asymmetric factor on dynamic response of the sandwich curve structures are discussed. It is shown that the resistance of sandwich curve plate is the best among the plates subjected to blast loadings. Increasing the curvature of the sandwich curve plate can result in increasing of the blast resistance of the structure. In the certain range, increasing the thickness of the outer face sheet can decrease the deflection of the inner face sheet, and the resistance of the sandwich curve plate can be increased under the blast loading. The optimal asymmetric factor making the best blast resistance of the sandwich curve plate is obtained.
2017 Vol. 38 (5): 391-399 [
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281
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400
A NUMERICAL SIMULATION METHOD BASED ON ENTIRE DISPLACEMENT MODE FOR PLAIN HOLE STRUCTURE
There are a large number of structures with discontinuities such as holes and inclusions in engineering, the existence of voids and other defects plays an important role in the integrity and stability of the structure, and affects the macro mechanical property of the material. Therefore, developing numerical simulation methods for such structure has great directive significance for engineering application. At present, the main numerical simulation methods for such structure mainly include FEM, XFEM and meshless methods, each of these methods has its advantages and disadvantages such as blending elements in XFEM and displacement boundary condition in meshless methods. This paper proposes a new kind of numerical simulation method based on entire displacement mode for plain hole structure. In the paper, we introduce level set functions for hole boundary and force boundary by level set method, construct boundary trial function with the aid of boundary level set function, and express the trial space as the linear combination of power series and boundary trial functions. Meanwhile, this paper also presents a new displacement boundary processing method based on level set method, which can be hopefully applied in other existing meshless methods. In the method, we involve the displacement boundary condition into the of approximate displacement field, the displacement boundary level set function is introduced to construct the approximate displacement field satisfying displacement boundary condition, and consequently we obtain the formula of balanced equation, stiffness matrix and load matrix. Comparing with FEM, XFEM and element free methods, this method doesn’t need to scatter the solution area, it can obtain the reversible stiffness matrix directly, avoid the ill condition problem of stiffness matrix, and decrease the difficulty of solving the linear equations, furthermore it is also suitable for hole structure with more complicated shape. Finally a numerical case is presented to testify the validity of the method.
2017 Vol. 38 (5): 400-407 [
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322
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408
AN AMENDED MOONEY MODEL OF CARBON BLACK FILLED RUBBER-LIKE MATERIALS
A phenomenological constitutive model for carbon black filled rubber materials is investigated in this paper. Considering the Mooney model cannot describes stress-strain relations of rubber-like materials at large deformation regions, characteristics of the Mooney model is analyzed by using experimental data. The relationship of carbon black content and strain range size of Mooney model accurate characterization of rubber materials is discussed, then the prediction ability of the model account for the stress-strain relations under multiaxial loading conditions and the influence of model parameter are analyzed. Finally, on the basis of Mooney model, a correction term is added, and the amended Mooney model must satisfy the six assumptions proposed by Treloar and Ogden. By comparing with the experimental data, the new model can well describe the rubber material stress-strain relationship of the large deformation stage, and can improve the ability of predicting the complex deformation of the rubber-like materials.
2017 Vol. 38 (5): 408-415 [
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278
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416
Experiment and DEM simulation on the viscoelastic properties of rubber bonded particulate material
The specimens of rubber bonded particulate material with regular square particle arrangement and hexagonal arrangement are fabricated, and the stress relaxation curves and stress-strain curves under different strain rates are tested for the specimens under the uniaxial tensile loads. The Burger’s viscoelastic model is used to characterize the mechanical properties of the tested rubber bonded particulate material, and the model parameters are determinedthrough curve-fitting to the stress relaxation curve. The discreteelement model (DEM) with the same particle arrangement as the tested specimens are used to simulate the viscoelastic behaviour of the rubber bonded particulate material under uniaxial tensile loads. The rubber binder is represented by the micro Burger’s viscoelastic bond model at contacts between the elements, and the parameters of the micro Burger’s viscoelastic bond model are obtained from the macro Burger’s viscoelastic model of the tested specimens. The stress relaxation and tensile stress-strain curves are calculated by the DEM models, and the DEM predictions are compared with the experiment results. The simulation results show that the DEM predictions are consistent with the experiment results for the specimens with regular particle arrangements. But theBurger’s model parameters fitted from the stress relaxation curve can notaccurately represent the strain rate dependence of the rubber binder under high strain rate deformation.
2017 Vol. 38 (5): 416-425 [
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348
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426
Dynamic characteristics of carbon nanotubes conveying fIuid in multi-physics fields
Based on the nonlocal elastic theory, a Euler–Bernoulli beam model is utilized to investigate the effect of longitudinal magnetic and temperature field on the transverse vibration of a single-walled carbon nanotubes (SWCNTs) conveying fluid. The slip boundary conditions of CNT conveying fluid are considered based on Knudsen number (Kn). The equation of motion and associated boundary conditions of a clamped-clamped SWCNT conveying fluid are derived by using Hamilton’s principle. In the solution part the differential transformation method (DTM) is uesed to solve the higher-order differential equations of motion. The effects of Knudsen number, nonlocal parameter, temperature and longitudinal magnetic field on the vibration frequency and divergence instability of SWCNT conveying fluid are investigated. Numerical results from the model show that the fundamental natural frequency and critical flow velocity for the SWCNT are affected by the small scale effect, Knudsen number, longitudinal magnetic and temperature field.
2017 Vol. 38 (5): 426-432 [
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433
Instantaneous Optimal Control algorithm of Connected Structures based on Time Domain Explicit Method
A new instantaneous optimal control algorithm based on time domain explicit method for the connected structures subjected to non-stationary random excitations is proposed. The proposed algorithm has the advantage of establish control model by applying impulse load upon finite element model, also avoided the problem of system state partially observable, and can adjust the control effects of every sub-structure as needed under different circumstances. A connection tall building structures is taken as the example to illustrate the proposed method, and compared the proposed method with the traditional instantaneous optimal control algorithm (ISO). It is found that the proposed method has good control effect.
2017 Vol. 38 (5): 433-441 [
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292
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442
Separation theory of ballistic separation for coal and gangue underground considering plastic yielding
Aimed to complete the theoretical system of ballistic separation for coal and gangue underground, this paper has discussed the elastic-plastic behaviors of coal and gangue during the impact process. The of relative position for the initial yield point was firstly determined combining the Hertz contact theory with the Drucker-Prager criterion. There are an approximate linear relationship between the relative position and the Poisson’s ratio. The mathematical models of the relative deformation, the contact force and the relative velocity were then established for the initial yield moment. The function relationship between the contact force and the deformation has been built based on the linear hardening model, and the theoretical model of the rebound distance has been derived by the law of conservation of energy and the relative velocity at the separation moment. The impact and rebound experiment for coal and gangue was carried out on the single particle impact experimental platform. The results show that the rebound distance increases with the initial velocity. The theoretical values of the rebound distance, calculated by the means of the material parameters, are in well agreement with the experimental means when there are few broken coal and gangue at a low initial velocity. The experimental means are less than the theoretical values because of the energy loss when coal and gangue are crushed.
2017 Vol. 38 (5): 442-450 [
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216
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451
Laboratory investigation on the mode-I fracture toughness and its correlation with strength characteristic of sandstone under the different chemical corrosion-rock interaction
The long-term immersion is adopted to explore the damage deterioration mechanism and mechanical prosperities of the sandstones under different chemical solutions. The variation regularities in the physical and mechanical prosperities were analyzed by using the sandstone sampled from the hydrofluctation belt of a typical bank slope in the Shanxi Tong chuan reservoir region as the objects of study. Experimental results show that the physical and mechanical prosperities of sandstone specimens have a significant degree of degradation under hydrochemical erosion, however, a difference exists in the deterioration degree of its physical and mechanical prosperities under different chemical solutions, i.e., acid solutions may aggravate the deterioration of chemical damage in sandstone, neutral and alkaline chemical solutions have a certain influence on its damage deterioration. At the early stage of the experiment, its damage deterioration is relatively small in strong alkaline solution. However, the deterioration of chemical damage in sandstones are increasing with the chemical corrosion time, that is, the deterioration degree in acid solution is the greatest followed by that in neutral and alkaline chemical solutions, and in the alkaline chemical solutions is relatively small. The fracture toughness KIC, compression strength and tensile strength of sandstone specimens have a significant damage deteriorating trend after chemical corrosion; however, there is a difference in the degree of damage deterioration in mechanical prosperities; that is, the damage deterioration of its fracture toughness KIC is the greatest, followed by that of tensile strength; that of compression strength is the smallest. There are obvious consistency among fracture toughness KIC, compression strength and tensile strength of sandstone specimens, those are obvious linear relationship. The crack propagation radius r can be used to indicate the degree of damage deterioration in mechanical prosperities under different chemical solutions.
2017 Vol. 38 (5): 451-464 [
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216
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465
STUDY ON LOW CYCLE FATIGUE TESTING TECHNOLOGY FOR THIN SHEET METAL
The specimen made of thin steel sheets is easy to buckle when it is subjected to tensile and compressive load in low cycle fatigue tests. Limited to the specimen length, typical mechanical extensometers are not suitable for strain measurement, which is a technical difficulty in engineering practice. In this paper, a non-contact optical extensometer based on digital image correlation combined with a microscopic imaging lens and an anti-buckling restraint device is developed to achieve real-time monitoring of the strain in material surface. The sampling frequency of the dynamic strain measurement system using a common CCD camera with optimized algorithm can reach up to 60 Hz. This technique provides a solution for thin steel sheets and can be potentially applied in low cycle fatigue tests.
2017 Vol. 38 (5): 465-472 [
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473
Probe to the Fracture Mechanism Caused by Supercritical CO2 Explosion Under the Impact of Initial stress
The exploration is used in underground engineering and initial stress has an obvious effect on the exploration. The homemade traxial loading explosion system is used to investigate concrete fractures under the coupled action. Supercritical CO2 explosion experiments were carried out on specimens to explore the process of cracks extension with initial stress. The experiments indicate that: during the explosion, the peak pressure is low and high pressure duration is long, which is different from the traditional explosion. Stress of the medium and mechanism of cracks extension are studied in the process with initial stress. The process is divided into two parts, the dynamic and the quasi-static. Firstly, the medium near a blasting hole is crushed forming a crushing zone, and there is a radial crack outside the crushing zone due to circumferential tensile stress. Then high pressure CO2 diffuses into fracture forming gas wedge, so cracks continue to extend. It is found that cracks form in the direction of maximum initial stress. The initial circumferential compresssive stress of the medium in that direction is least and therefore, the specimen is cracked because of tensile stress caused by supercritical CO2 .The initial circumferential compresssive stress of the medium in the vertical direction is highest, so the stress should be large enough to cause cracks and the medium cracks relatively late. The stress in vertical fracture direction prevents gas from diffusion. Therefore, a higher pressure is needed to form wedge. The result is that the wedge affects and has a positive effect on hindering crack propagation. Crack expansion velocity is in accordance with "S" curve with distance from the hole. The greater initial stress is, the greater velocity decline is in the same propagation distance, and crushing zone and crack area gradually reduce. Above all, it is suggested that the distance between holes are broaden and holes are reduced to improve exploration efficiency in the exploration with initial stress.
2017 Vol. 38 (5): 473-482 [
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228
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