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2016 Vol. 37, No. 3
Published: 2016-06-21
193
Research progress in mechanical experiments and theory of magnetoelectric composite materials
Yongmao Pei
Magnetoelectric composite materials possess the properties of both ferromagnetism and ferroelectricity, and more importantly, the magnetoelectric coupling effect. This kind of functional materials have great potential applications in the fields of novel magnetic-electric devices, spintronics devices, and high performance information storage, and have become one new hot research topic in the field of functional materials. In this paper, we reviewed the experimental studies of the magnetoelectric coupling effect, as well as the development of the multi-field instrument for micro/nano-mechanical tests and the corresponding characterization methods. In addition, studies of the multi-field coupling constitutive theory, fracture mechanics and multi-scale simulation methods were also reviewed. Furthermore, some suggestions were presented for the future work by summarizing the shortcomings of the existing research.
2016 Vol. 37 (3): 193-207 [
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208
A review of numerical integration approaches for Galerkin meshfree methods
Meshfree methods are capable of constructing arbitrary order smoothing and compatible shape functions only through unstructured nodes and do not reply on elements with specific connectivity. Compared with the conventional finite element methods, meshfree methods show obvious advantages in modeling large deformation, moving boundary and higher order problems. Galerkin meshfree methods are one class of most widely used meshfree methods. Although no elements are required for the shape function construction, Galerkin meshfree methods do need some kind of background cells to perform the weak form integration. Due to the rational nature and overlapping characteristics of meshfree shape functions, commonly higher order Gauss quadrature is necessary for the numerical integration of Galerkin meshfree methods, which leads to low computational efficiency and considerable difficulty to model large scale practical problems. Consequently, the development of efficient and robust integration algorithms has been an important topic for Galerkin meshfree methods. This paper first briefly discusses the imposition of essential boundary conditions, and then presents a detailed review on some typical numerical integration approaches for Galerkin meshfree methods, where the characteristics for various integration algorithms are outlined.
2016 Vol. 37 (3): 208-233 [
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234
Tensile mechanical properties and influence factors sensitivity analysis of polycrystalline graphene
Grain size, temperature and strain rate have significant effect on the mechanical properties of nanomaterials. By molecular dynamics (MD) simulation, the present paper analyzes the Young's elastic modulus, ultimate stress and ultimate strain of different grain size of polycrystalline graphene under different temperature and tensile strain rate. The results show that the grain size, temperature, tensile strain rate have significant influence on tensile mechanical properties. Using the theory of orthogonal experimental method, influence factors sensitivities of grain size, temperature, tensile strain rate on Young's elastic modulus, ultimate stress and ultimate strain are analyzed. The results show that, for the Young's elastic modulus and ultimate stress, the sensitivity order from large to small are grain size, temperature and tensile strain rate, while that is grain size, tensile strain rate and temperature for ultimate strain. The research results can offer reference to theory research and engineering application of polycrystalline graphene.
2016 Vol. 37 (3): 234-246 [
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247
Study on Size Dependence and Pull-in Instability of Electrostatically Actuated NEMS with Casimir Force
Based on the strain gradient elasticity theory and Hamilton principle, a size-dependent model including Casimir force for the electrostatically actuated Nano-Electro-Mechanical Systems(NEMS) is presented, and the governing equation and boundary conditions are derived. The problem is solved numerically with the help of generalized differential quadrature method and pesudo-arclength algorithm. The results reveal that Casimir force can reduce the pull-in voltage of system. And once scale of system reach to the critical value (the gap between two electrodes is less than minimum gap, or the length of movable beam is larger than the detachment length), the pull-in instability would occur without voltage applied. The study may be helpful for the design and theoretical modeling of NEMS incorporating Casimir force.
2016 Vol. 37 (3): 247-253 [
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363
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254
Effect of temperature dependency on thermoelastic response during transient thermal shock
The thermoelastic behavior, induced by transient thermal shock, has significant difference once considering the temperature dependency of material properties. The research on the thermoelastic response of materials with variable material properties, especially the prediction of thermal stresses, is very important to evaluate their lives in some circumstance with extreme heat supply. The thermoelastic response of materials with temperature-dependent properties, suffered the transient thermal shock, are studied in this paper. The equations with variable material properties are proposed in the context of generalized theory of thermoelasticity without energy dissipation (Green-Naghdi II model). The analytical solutions of the problem for a semi-infinite body, subjected to a sudden thermal shock in its boundary, are derived by means of the Laplace transform technique and the Krichhoff transform, where the variable thermal material properties are assumed to be a linear function of real temperature. The explicit s of two waves, named as thermal wave and thermoelastic wave, respectively, are obtained from these analytical solutions, which can clearly reveal the propagation of these two waves and their connection with characteristic parameters, such as thermo-mechanical coupling coefficient and temperature factors. The distributions of the displacement, temperature and stresses, associated with finite propagation velocity of heat signal, are obtained and plotted, meanwhile, the comparison with the results obtained from the case with constant material properties are also conducted to reveal the effect of the temperature dependency of material properties. The results show that the propagation of two waves, as well as the distribution of each physical field, are changed when involving the variation of thermal material properties, and these changes are dependent on the value of thermo-mechanical coupling coefficient.
2016 Vol. 37 (3): 254-263 [
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267
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264
Theoretical and applied research on natural frequencies of cracked cantilever beams with a auxiliary mass
In this paper, theoretical research on natural frequencies of cracked cantilever beams with a auxiliary mass are carried out. The transverse deflection of the cracked beam is constructed by adding a polynomial function, which represents the effects of a crack, to the polynomial function which represents the response of the intact beam. Then, the analytical formulation of the distribution of natural frequencies due to auxiliary mass over the beam is obtained through dynamics method. Through the analytical formulation the natural frequencies of the system change due to the roving of the mass along the cracked beam, which was called the natural frequencies curve(NFC). The NFC is shown to compare very well with those obtained using finite element method (FEM). In addition, due to the NFC contains the defect information of the beam, based on the NFC the damage on the beam can easily identify by using stationary wavelet transform (SWT). Finally, the influence to the natural frequency of the system, which caused by the quality block size, defect depth, defect location and other factors, is considered by using the analytical formulation in this paper. The results show the reliability of the theoretical derivation and the accuracy of damage identification.
2016 Vol. 37 (3): 264-272 [
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324
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273
Measuring plastic yield stress of magnetron sputtered aluminum thin film by nanoindentation
A simple experimental technique is presented for measuring yield stress of magnetron sputtered aluminum thin film by combining the curvature and sphere-tip nanoindentation technology. The mechanical model of spherical indentation is established，finite element simulations are conducted，Aluminum film with a thickness of approximately 1 μm is deposited on silicon wafer substrate by a DC magnetron sputtering system，both curvature and nanoindentation tests are carried out on the Al film/Si substrate system，and the yield stress of Al film is measured to be 371 MPa by least square curve fitting of the indentation data. This method can also be used to study the mechanical properties of other thin films and small volume materials.
2016 Vol. 37 (3): 273-279 [
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280
The analysis of error and modification of the virtual crack closure technique for crack of mode I
The origin of the error of the virtual crack closure technique (VCCT) has been analyzed. According to Griffith’s theory of ellipse crack, a modified method has been given out for the assumption of equal displacement of VCCT. The problem of two dimension mode I crack spread has been simulated by the finite element method. And the result has been compared with theory result. The result indicated that the modified method can get the better precision especially when the crack is short.
2016 Vol. 37 (3): 280-283 [
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258
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284
Simulation of damage evolution for reinforced concrete beams based on single spring joint element method
Due to the two nonlinear factors involved the materials of the concrete and reinforced bar and the interaction of two materials, the whole damage process of the RC structures could hardly be simulated based on the conversional methods. In the present work, the four-parameter damage model which considers the material nonlinearities, combined with the single spring joint element method based on mixed coordinate system which is used to simulate the interaction of the two materials is introduced to depict the damage process of RC strctures. Numerical examples not only demonstrate the validity and accuracy of the proposed approach but also show that this method could be applied to engineering application.
2016 Vol. 37 (3): 284-289 [
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304
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