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2015 Vol. 36, No. 6
Published: 2015-12-28
Review
461
Progress in Thermo-mechanical Cyclic Deformation of NiTi Shape Memory Alloy: Macroscopic/Microscopic Experiments and Theory
Progress in the studies on the thermo-mechanically coupled cyclic deformation of NiTi shape memory alloys (SMAs) is reviewed and commented. At first, the uniaxial and multiaxial cyclic deformation of NiTi SMAs and its strong thermo-mechanically coupled effect are summarized, and the physical mechanism of functional degeneration during the cyclic deformation is outlined; then, three types of cyclic constitutive models constructed in the macroscopic and microscopic senses, are addressed, and the capabilities of representative models to describe the thermo-mechanical cyclic deformation of NiTi SMAs are verified; finally, some suggestions are presented for the future work by summarizing the shortcomings of existing researches. In the content about the constitutive model, the crystal plasticity based thermo-mechanical coupling ones proposed by the authors and their co-workers are mainly introduced, and the effects of multiple inelastic mechanisms and strongly nonlinear thermo-mechanical coupling on the cyclic deformation of NiTi SMAs.
2015 Vol. 36 (6): 461-480 [
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481
Dielectric elastomers based soft machines actuated by ionic conductors: progress and perspectives
Dielectric elastomer-based soft machines have been widely applied to soft robots, energy harvesters, flexible electronics, etc. Conventional dielectric elastomer devices are actuated by non-transparent carbon paste electrodes or electronic conductors made of hard materials. Instead, the stretchable and transparent ionic conductors, with two soft materials combined, open the door to a unique way in the realization of soft machines. In this review, the recent progress on dielectric elastomer-based soft machines actuated by ionic conductors is summarized; several future research topics and perspectives with respect to this area are also suggested. Key words: ionic conductor, dielectric elastomer, hydrogel, ionogel, soft machine.
2015 Vol. 36 (6): 481-492 [
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582
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Contents
493
Elastic response of DNA molecules under stretching based on a 3-D elastic rod model
As a coarse-gained model, an elastic rod model subjected to the interfacial traction is established by the Kirchhoff’s theory. The interfacial traction between the rod and the solution environment is determined in term of the Young-Laplace equation, and the influences of the ionic concentration on the interfacial traction and elastic modulus are discussed. Based on this elastic rod model, the shooting method is used to simulate the equilibrium configurations of DNA under the actions of the interfacial traction and a longitudinal stretch applied at an end of segment. The shapes and sizes of the condensed DNA are discussed. The results show that the shapes of the DNA condensation are mainly determined by the competition between the interfacial energy and elastic strain energy, and the variations of end-to-end distance are consistent with the worm-like chain model.
2015 Vol. 36 (6): 493-498 [
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295
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499
Reverse non-conforming element NQ6
Element NQ6 drops its anti-distortion performance for additional linear items introduced into Q6’s non-conforming internal shape functions when forcing strong patch test. In order to resume and even improve the anti-distortion performance it is proposed to carry out reverse treatment on these linear modifications. The adjustment process is simple: taking the linear correction term as one-dimensional direction which influences element performance, doing reverse search to determine favorable step size. Some typical examples are tested, the results show that reverse adjustment is effective, with adjustment factor set to mirror value -1, and further extended to -2, the resulting elements are less sensitive to distortion than the original Q6 and NQ6, especially the value near -2 is efficacious for elimination of shear locking in MacNeal slender beam. Because of reverse treatment, new elements pass strong patch test only for parallelogram mesh, in general pass weak patch test. Except this, the overall performance and accuracy is close to the best of various 4-node quadrilateral elements. The proposed method, which obtains items to effect anti-distortion performance by strong patch test and then adjusts them reversely, suggests a new way to improve other Q6-type non-conforming element’s anti-distortion performance.
2015 Vol. 36 (6): 499-505 [
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329
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506
Analysis on meso-mechanisms of influence of rock block shape on mechanical properties of cemented soil-rock mixture
In order to explore the meso-mechanisms of influence of rock block shape on mechanical properties of cemented soil-rock mixture (SRM), Spherical, cubic and cuboid-shaped rock blocks are taken into consideration. SRM with cubic rock blocks are compared with that with spherical rock blocks to explore the influence of block angularity. SRM with cuboid-shaped rock blocks are compared with that with cubic rock blocks to explore the influence of block sphericity. Firstly, numerical models of cubic and cuboid-shaped rock blocks are constructed based on a 3-D DEM refined modeling technology for irregularly shaped particles. Then, 3-D DEM stochastic models of cemented SRM with 30% and 80% rock block proportion (RBP) are established, whose block shape is spherical, cubic and cuboid-shaped respectively. Then, a particle flow simulation of large scale triaxial tests of SRM is conducted to obtain strength and deformation characteristics of SRM with different rock block shape and RBP. Finally, Meso-mechanisms of influence of rock block shape on mechanical properties of cemented SRM with low and high RBP is intensively analyzed. The results show that RBP and rock block shape both significantly affect the mechanical behaviors of SRM and there is complicated interaction of the two factors. The evolution of micro-crack, mean rotation of rock blocks, strain energy and frictional work can well reflect the meso-mechanisms of influence of rock block shape.
2015 Vol. 36 (6): 506-516 [
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541
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517
Single particle crushing of rock in uniaxial compression tests
It is difficult to study particle crushing of rock using the existing classical theory because of the complex and irregular physical structure, but it is changed by fractal theory, which employed statistical concepts to explain the particle crushing strength. In this paper, the stress - strain distribution curve and the crushed particle size distributions is described based on a large number of single marble particle crushing test. Three types of particle crushing damage are summarized, fractal model of rock particles crushing is established by the distribution characteristics of the damaged particles, fractal dimension D of 2.48 is got. According to the fractal model, the theoretical formula of particle crushing strength is derived, the test data of marble particle crushing strength with the theoretical prediction equations are compared, the results are very consistent, and therefore, fractal theory is applicable for analysis of particle crushing strength.
2015 Vol. 36 (6): 517-523 [
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493
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Research Notes
524
D failure criterion for fiber reinforced ceramic matrix composites
Classical phenomenological strength theories are reasonable for orthotropic composites that present linear elastic mechanical behavior. For nonlinear/damageable fiber reinforced ceramic matrix composites (CMCs), there is few failure criterion developed in recent years to account for their rupture characteristics. In order to promote the engineering applications of CMCs, new failure criterion is of interest for structural strength design. In this paper, we deal with nonlinear and anisotropic problems and consider the influence of damage evolution process on failure strength. Firstly, virtual linearization of the inelastic behavior of CMCs was performed based on the phenomenon that a kind of approximately linear stress-strain relationship being observed in CMCs during the loading/unloading experiment. And then, a completely linearized stress-strain relation was formulated by a simple analytical method of damage mechanics. Subsequently, on the basis of damage equivalence assumption, the relationship between the principle damage variables and the associated stress components was deduced. Afterwards, the classical stress-based quadratic failure criteria were modified under both linear and nonlinear damage evolutionary cases by introducing damage parameters into the theoretical equations to replace those stress parameters. As a result, a new damage-based failure criterion named “D failure criterion” was established. This failure theory is capable to be the supplement and extension of the well-known phenomenological failure criteria. In addition, verification towards “D failure criterion” was conducted through a calculation example involving the tension-shear combined experiment of a plain woven C/SiC composite. The demanded damage evolution laws of the material were obtained by literature reserch. The simulated results show that: the failure envelope given by the newly proposed failure criterion is lower than Tsai-Hill criterion, and the failure curve is associated with the damage evolution laws. This also reveals the potential of “D failure criterion” for further applications in advanced ceramic composites within the framwork of continuum damage mechanics.
2015 Vol. 36 (6): 524-529 [
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323
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530
A meshless method for solving the 3D sound pressure distribution problem
The traditional FEM suffers from pollution and dispersion phenomena when it was used to compute sound pressure distribution problem. The meshless weighted least-square (MWLS) method is a meshless method based on the moving least- square (MLS) approximation, which has Low dispersion, high precision characteristics with solving the problem of sound pressure distribution problem. The MLS may form ill-conditioned system of equations, an accurate solution of which is difficult to obtain. In this paper, by using the weighted orthogonal basis function to construct the improved moving least-square (IMLS) approximation, we derive the formulas and perform the dispersion analysis for an improved meshless weighted least- square (IMWLS) method for three-dimensional (3D) sound pressure distribution problem. Results demonstrated that the pressure distribution and the sound pressure frequency response curve of the calculated reference values agreed very well. The error of peak and pollution are smaller than FEM. The computational cost compared with the element free Galerkin method significantly reduced Results demonstrated that the proposed method can easily reach higher frequency without losing accuracy.
2015 Vol. 36 (6): 530-536 [
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198
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537
Uncertainty Analysis of Structural Response Based on Convex Model and Pseudo-probability Distribution
Through the traditional response analysis method for uncertain structures based on non-probabilistic convex model, only the bounds can be obtained. So, in this paper a new uncertainty propagation method which combined volume ratio with first order second moment is proposed. Using this uncertainty propagation method, not only the bounds but also the pseudo-probability distribution of the structural response can be accurately evaluated. First, the uncertainty of structure parameter is modeled by multidimensional ellipsoid convex set. The ellipsoid is separated two parts by the state equation of the structural response with respect to uncertain parameters. So, the volume ratio of separated domain and the whole ellipsoid domain can be used to quantify the uncertainty of structural response. Second, in order to obtain the most probable expansion point and the approximate volume of the separated domain, the propagation equation of uncertain response is sequentially solved through First order second moment (FOSM) method. Finally, the uncertain response analysis of a typical six-bar truss is applied using the proposed method, Monte Carlo simulation and traditional method. The comparisons of the results demonstrate that the proposed method is more effective and accurate.
2015 Vol. 36 (6): 537-543 [
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279
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