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2019 Vol. 40, No. 1
Published: 2019-02-28
1
Strength and Toughness Properties in Nanotwinned Metals and Gradient-nanostructured Metals: A Review
Metallic materials have been widely used in aviation, aerospace and civil industries. To achieve a metallic material with high yield strength and good ductility has become an important issue in the disciplines of materials, physics and mechanics. The traditional strengthening methods, such as strain hardening, solid-solution alloying, phase transformation, grain refinement, and second-phase dispersion strengthening, although can improve the strength, would greatly weaken the plastic properties. In recent years, experimental studies have demonstrated that interface design and microstructural control enable the metallic materials to be prepared with a good combination of high yield strength and high ductility. It has been proved that the interactions between the dislocations and various interfaces and the weakened stress concentration through the optimization of microstructures are the primary mechanisms of strengthening and toughening. On the basis of experimental observations, researchers applied the atomic methods to analyze the plastic deformation in the metallic materials with high strength and high ductility quantitatively, and gave insights into the strengthening mechanisms and failure behaviors. On the other hand, the mechanism-based theoretical model and the finite element approach were developed to describe the mechanical behaviors of novel metallic materials with excellent mechanical properties. In this work, we review the experimental and theoretical studies on the mechanical properties such as strength and ductility, and plastic deformations of nanotwinned metals and gradient-nanostructured metals; and put forward the prospect of optimization of high yield strength and high ductility for novel nanostructured metals.
2019 Vol. 40 (1): 1-20 [
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435
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21
The flexoelectric response of nanobeam based on the general strain gradient theory
Flexoelectricity, the special electromechanical coupling between strain gradient and polarization, exists in all dielectric materials. It has received wide attention in multiple fields including energy harvesting, sensing and actuation. However, the effect of elastic strain gradient on flexoelectric response has typically been ignored or underestimated in the studies of flexoelectricity of nano-dielectric structures, which is solved in this paper. According to the general strain gradient elasticity theory, it is strictly proved that only three length-scale parameters are independent, and the applications of strain gradient theory with one or two scale parameters in the literature are only in its simplified forms. Based on this theory, a theoretical model of three-dimensional dielectric structure considering the generalized strain gradient elasticity is established. Then, using this model, the governing equations and boundary conditions of a bending nanobeam are obtained by Hamilton’s variational principle. The one-dimensional cantilever nano-beam is taken as an example to study the flexoelectric response of its bending and energy harvesting characteristics. The results show that the flexoelectric response of structure exhibits size effect, and the elastic strain gradient influences this effect to some extent, especially when the structural scale is smaller than the length-scale parameters. On the other hand, the results show that extreme values of displacement and energy efficiency exist with the increase in structural scale, when the elastic strain gradient theory is considered. Furthermore, it is found that flexoelectricity coupling with external voltage will lead to the beam’s inhomogeneous boundary conditions. In short, the elastic strain gradient significantly impacts the displacement, polarization, electric potential, and energy efficiency of a dielectric nanobeam with incorporation of flexoelectricity. This work provides a theoretical basis for further understanding of the mechanism of flexoelectricity at nanoscale and the effect of elastic strain gradient theory on flexoelectricity. It can be helpful for the design of nanoscale flexoelectric energy harvesters.
2019 Vol. 40 (1): 21-29 [
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363
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30
Stress concentration in octagonal honeycombs due to missing cell walls subjected to the biaxial loading
Stress concentration due to missing cell walls is an urgent problem to be solved in the additive manufactured process of cellular structures. Approximated the defect region with an elliptical hole and based on the complex function method, analytic formulas of the tensile stress and analytical method of the bending stress are proposed. Stress of the octagonal honeycombs with missing cell walls along one row and more rows under the biaxial loading are calculated and compared with the results of FEM, verifies the effectiveness of the proposed method. Results indicate that the analytical solutions of the tensile stress are fitted well to the numerical simulation results and there is an obvious linear relationship between the bending moment and the stress gradient. Moreover, comparison with the hexagonal honeycomb, stress concentration effects of the octagonal honeycomb with more rows defects in the case of and are found lower, which provide some theoretical guidance for the design of additive manufactured honeycombs.
2019 Vol. 40 (1): 30-38 [
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222
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39
Landing Process of Lunar Lander Simulated with Coupled DEM-FEM Model
The safe landing of the lunar lander is an important guarantee for the safety of astronaut, carrying equipment and instrument, and follow-up work of the lander. In this study, a discrete element-finite element coupled model is established to analyze the interaction between the lunar soil and the lander. In this coupled model, the lunar soil is represented by the spherical discrete elements with a bonding function. The lander is constructed with the combined finite element model of shell elements and beam elements. Meanwhile, a compressible spring model is used for the supporting leg connected with the cushion to realize the buffer capacity. In order to analyze the process of landing on different lunar surfaces, different flat and sloped lunar surfaces are simulated and the interaction between the lander and the lunar soil in difference landing modes are studied. In addition, the relationship between impact of the peak size and the role of time is analyzed and the reasons for the small impact under high slope are discussed in terms of energy. This work provides a useful reference for a safe landing of the lunar lander.
2019 Vol. 40 (1): 39-50 [
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309
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51
Vibration response method for liquid-filled thin cylindrical shell with crack damage
Thin cylindrical shell fluid shock vibration response is a complex fluid-structure interaction (FSI) issue, and with importance significance to the state monitoring and damage recognition of thin shell. Based on the Flügge shell stress theory, the high-order partial differential equations (PDE) of thin shell motion are set up, and the system vibration response is obtained by the wave propagation method. The peripheral fluid is defined as the ideal sound media, and the sound pressure field is described by the Helmholtz equation. According to the above hypothesis, the forced vibration response and evolution regularities of thin cylindrical shell considering fluid-structure interaction are acquired. To conduct the recognition for crack damage of thin shell, the partial flexibility matrix is constructed by the facture mechanical principles, in combination with the breathing linear spring model (LSM), the adjacent stress and displacement conditions are built up. Consequently, the forced vibration response of liquid-filled thin cylindrical shell with crack is obtained, and a vibration power flow based crack damage recognition method is presented. The results showed that the displacement responses on the radial, axial and circumferential directions of liquid-filled thin cylindrical shell are with apparent differences by the nonlinear excitations; the peak values of radial and axial displacements take on distance delay phenomenon; the radial displacement contains more peak values, and has more close relation with fluid pulse excitation; the crack can decrease the partial flexibility and system natural frequency, and can change the power flow characteristics of the FSI system; by the increasing of crack depth, the transmission coefficient of vibration wave will be decreased; the normalized input power flow can recognize the crack damage of liquid-filled thin cylindrical shell. This research not can provide useful references to the modeling-solving of liquid-filled thin shell vibration response, but can offer potential solution scheme for the structure crack recognition under the condition of fluid-solid coupling.
2019 Vol. 40 (1): 51-73 [
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185
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74
The ICVEFG method for steady-state heat conduction problems in orthotropic media
In this paper, the improved complex variable element-free Galerkin (ICVEFG) method is applied to analyze steady-state heat conduction problems in orthogonal media. The improved complex variable moving least-squares (ICVMLS) method is used to establish the approximation function of two dimensional temperature fields for orthogonal media. By adopting same vector form approximation of field variables as that in the complex variable moving least squares (CVMLS) method, the ICVMLS approximation retains the advantages in improving the computing efficiency of the shape functions for two-dimensional problems. Meanwhile, the precisely defined norm error in the ICVMLS approximation ensures the high accuracy of the approximate solutions. In our numerical implementation, the penalty method is used to apply the essential boundary conditions. The Galerkin integral weak form of steady-state heat conduction in orthotropic medium is derived and the corresponding calculation formula is presented. The compute program is compiled to analyze three example problems of heat conduction in orthogonal media. And the validity of the proposed method is illustrated by comparing with the analytical solutions. The numerical results show that the proposed ICVEFG method can obtain highly accurate temperature fields for orthotropic heat conduction problems. The numerical accuracy and efficiency of the proposed ICVEFG method are also compared with the complex variable element-free Galerkin (CVEFG) method in which the CVMLS approximation is used. For all three examples, it is found that the accuracy of the solutions using the ICVEFG method is much higher than the accuracy of those obtained by the CVEFG method. But the CPU time used for the ICVMLS shape functions is almost the same as that for the CVMLS shape functions. That means, compared with the CVEFG method, the ICVEFG method can greatly improve numerical accuracy and robustness without reducing numerical efficiency. The proposed ICVEFG method for orthotropic steady-state heat conduction problems is proved to be computationally efficient, robust and accurate.
2019 Vol. 40 (1): 74-81 [
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281
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82
Fracture mechanics of cracked elliptical hole based on surface elasticity theory
Mode III fracture characteristics of a nano-sized cracked elliptical hole is investigated. Based on the surface elasticity theory and the conformal mapping technique, closed form solutions of the stress fields around the defects (crack and elliptical hole) and the stress intensity factor at crack tip are presented by using the complex potential function. The present solution is fairly general such that many existing and new results can be regarded as special degenerated cases. By applying the analytical solutions to typical examples, effects of the size of defects, the shape ratio of elliptical hole and the relative size of crack on the stress intensity factor are discussed. The results show that the stress intensity factor is size-dependent significantly when considering the surface effect of the defects at the nanometer scale. The size effect of the defects decreases with the increase of the defects size, and gradually approaches the classical elastic theory. The variation of the stress intensity factor with shape ratio of the elliptical hole is related to the surface constant of the defects. With the increase of the elliptical hole shape ratio, the dimensionless stress intensity factor increases slightly and then decreases for the case of classical elastic theory and positive surface constant. When the surface constant is negative, the dimensionless stress intensity factor monotonously decreases to a stable value. The effect of the surface effect of defects depends on the shape ratio of the elliptical hole, and the very high shape ratio shields the contribution of the surface effect. With the relative size of the crack increases, the dimensionless stress intensity factor increases first to the maximum and then decreases. The surface effect is weak when the relative size of the crack is very small, whereas the surface effect is obvious when the relative size of the crack is large.
2019 Vol. 40 (1): 82-89 [
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351
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90
Quasi-static Spherical Cavity Expansion Model of Cellular Steel-tube-confined-concrete Targets and Its Application
Cavity expansion theory is a common method to establish engineering models of penetration problems. For the penetration problem of cellular steel-tube-confined-concrete (STCC) targets, a quasi-static spherical cavity expansion model is firstly developed on the basis of inclusion of the comprehensive confinement of steel tube and the peripheral concrete on the impacted cell and the assumption that failure behavior of concrete obeys the Hoek-Brown (H-B) criterion in the comminuted region; and then influences of the comprehensive confinement stiffness on the penetration process are analyzed. The numerical results show that the radial stress of STCC targets on the cavity wall does not remain constant during the expansion process, which is different with those of infinite concrete targets; and the pressure on cavity wall increases with the increase of the comprehensive confinement stiffness; and the predicted results of depth of penetration are in good agreements with those of experiments in the existing literatures.
2019 Vol. 40 (1): 90-98 [
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195
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