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2022 Vol. 43, No. 2
Published: 2022-04-28
121
Design and performance analysis of internal resonance based frequency up-converting piezoelectric oscillator applied in ultra-low-frequency vibration energy harvesting
Low frequency and ultra-low frequency vibration energy exists widely, but how to efficiently harvest it is still a technical problem. The paper presents an ultra-low-frequency vibration energy harvester based on the pendulum, the embedded double-cantilever beams,a pair of magnets and two mechanical stoppers by theoretical modeling, simulation analysis and experimental verification. The oscillating structure can convert the initial low-frequency excitation to 6-time-frequency voltage output based on 1:2:6 frequency-up-conversion mechanism, effectively improve the energy conversion efficiency of piezoelectric elements, and increase the output power of the oscillating structure. The prototype can reach the energy harvesting level of 1.4 mW at the excitation of 2 Hz and 0.2 m/s, presenting great potential for application.
2022 Vol. 43 (2): 121-130 [
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131
ACTIVE CONTROL ON INTERFACE STATE AND DEFECT PROTECTION OF ELASTIC WAVE METAMATERIALS
Wen-Jing Wen-JingSun Yi-Ze Yi-ZeWang
It is found that the hexagonal cell is a common repeating unit with dynamical topological properties in the wave periodic structure. In this work, a hexagonal cell is designed with active control properties. Piezoelectric patches are attached to unit cells and negative capacitance circuits are connected to regulate material parameters. Then, the active control on topological interface transmission and defect protection are achieved. Moreover, the defect protection depends on the structure direction. This investigation introduces the topological concepts of physics and acoustics into the elastic wave metamaterial plate, which can obtain the topological properties and actively controllable characteristics of flexural waves.
2022 Vol. 43 (2): 131-142 [
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143
Peridynamic modeling through micro-CT images for failure simulation of composite microstructure
Since the fields of fiber-reinforced composites expand continuously, the effect of microstructure damage on the mechanical properties of composites has become an essential yet unclarified issue. Therefore, it is necessary to develop a feasible and efficient method to model and simulate the microstructure failure of fiber-reinforced composites. In this paper, a peridynamic-based computational scheme is proposed to simulate the failure of composite microstructure, using discrete pixel points of micro-computed tomography (micro-CT) images taken from the micro-CT scanning technology. Peridynamics is a nonlocal theory based on integral equations. Therefore, it is more convenient to implement particle-based numerical methods, which makes it more effective to simulate the failure process from deformation to fracture. By introducing the gray threshold segmentation technology into the micro-CT image processing, a particle discretization containing in situ information of composite materials can be obtained, which is employed for peridynamic simulations. Consequently, the complex geometric reconstruction and finite element partition are unnecessary, whereas the in situ information of composite microstructure, such as fiber, matrix, and voids, is preserved as possible. The simulation results show that the peridynamic modeling based on micro-CT images can accurately capture the microstructure information of composites and successfully simulate the failure evolution of composite materials.
2022 Vol. 43 (2): 143-157 [
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158
Nonlinear Mechanical Behavior of Symmetrical S-FGM Circular Plate under Thermal Load
In this paper, the post-buckling and bending behavior of ceramic-metal-ceramic functionally graded circular plates (S-FGM) in a thermal environment are studied. The volume fraction of the material components of the circular plate complies with Sigmoid law and bears the effect of the temperature field that changes along the thickness of the circular plate. Based on the classical plate theory, the nonlinear equilibrium equation of the static problem of the symmetric S-FGM circular plate is derived by the energy method. The equation is solved numerically by shooting method, and the influence of different boundary conditions, material composition, thermal load and other factors on the mechanical behavior of the symmetrical S-FGM circular plate is studied by using the numerical results. The numerical results show that the mechanical behavior of the symmetric S-FGM circular plate is different from that of the ordinary FGM circular plate, and the temperature rise ratio of the upper and lower surfaces of the S-FGM circular plate has a significant influence on its mechanical behavior.
2022 Vol. 43 (2): 158-167 [
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168
Analytical model of micro/nano-cylinders considering Steigmann-Ogden surface effect
Due to a sharp increase of surface-to-volume ratio of nano-sized cylindrical subjects, the corresponding surface energy becomes significant. Therefore, it is essential to study the surface effect at the nano-scale. Herein we study the elastic response of nano-cylinders under diametral loading by taking into account of the Steigmann-Ogden (S-O) surface theory. The S-O surface is assumed as a zero-thickness film attached to a bulk material with bending stiffness and residual surface tension. Relative to the more popular Gurtin-Murdoch (G-M) interface model, S-O surface can resist not only tension but also bending. Based on continuum mechanics theory, the present work develops internal analytical expressions by solving the elastic governing equations through series expansion in cylindrical coordinate. The internal unknown coefficients are finally obtained by applying S-O surface model and extended diametral boundary conditions through mathematical orthogonality. When the surface bending stiffness parameters are ignored, the present results can be degenerated to the G-M model. Finite element simulations and experimental measurements in the literature are employed to verify the present theory with good agreement. Based on the credence of present solutions, the effects of surface bending stiffness parameters, surface residual stress and cylinder dimension are investigated and discussed on material properties of nano-cylinders. The results demonstrate that the S-O model generates different stress distributions relative to the G-M model, indicating that the surface bending stiffness parameters can not be ignored in the nano-scale. In addition, the surface residual stress plays a certain role in influencing the stress distributions of both solid and hollow nano-cylinders, especially on the surface of hollow ones. What’s more, it is found that the surface effect gradually increases with a decrease of the cylinder’s dimension. Finally, the analytical nature of the present solution offers attractive alternative to the numerical methods in studying elastic behavior and surface effects of nano-subjects.
2022 Vol. 43 (2): 168-176 [
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177
The Effect of Dislocation Stacking Under Uniaxial Tension on Microcrack Growth in Metallic Crystals
Abstract:TFor nanocrystalline materials, the interaction between cracks in nanocrystalline aluminum and slip surfaces formed by dislocations emitted from crack tips under uniaxial tensile loading is studied.By using the distributed dislocation method, the crack and slip surface are equivalent to the uniformly distributed continuous dislocation, and the stress field on the crack surface is obtained.The effect of dislocation free zone on the initiation and propagation of the microcrack is studied by introducing the dislocation free zone at the crack tip.The results show that the crack length is short without considering the dislocation free zone at the crack tip, and the microcracks will be formed at the grain boundary first, and the main crack will directly penetrate the crystal when the main crack is longer. When the angle between slip surface and crack tip is large, the number of dislocations emitted at the crack tip will be increased, thus inhibiting the propagation of main crack.When the dislocation free zone at the crack tip is considered, the microcracks appear in the dislocation free zone prior to the grain boundary. The mutual emission of dislocations between the main crack and the microcracks leads to the convergence of the crack and the microcracks at the crack tip, which effectively accelerates the propagation of the main crack.
2022 Vol. 43 (2): 177-185 [
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186
Research on the Normal Contact Stiffness of Rough Surfaces Based on the Ubiquitiform Theory
Based on the ubiquitiform theory and Hertz contact theory, the morphology of the rough surface is described by the ubiquitiformal islands to establish the elastic contact model of the joint surface, which characterizes the normal contact stiffness of the joint surface. Assuming that the height of the asperity of the rough contact surface satisfies the Gaussian distribution, by establishing the asperity micro-contact model, the ubiquitiformal complexity D of the rough surface and the minimum infimum of the measurement size δmin are introduced. Then the normal contact stiffness of the rough surface is derived on basis of ubiquitiformal model. Different with fractal model, the limited self-similar or self-affine structures with only integer dimensions. The ubiquitiformal complexity shows the complexity of the surface topography. the infimum of measuring size can reflect the details of the surface topography, and the surface topography is determined by the ubiquitiformal complexity and the minimum infimum of the area. After comparing and analyzing the results of Literature, it is found that the contact stiffness of the joint surface obtained by using the ubiquitiform island model is more consistent with the experimental results than fractal model. Numerical example results are carried out on the normal contact stiffness of rough surfaces with different ubiquitiformal complexity D, the minimum infimum of the measurement size δmin. It is shown that in the same area scale interval, the normal contact stiffness of rough surfaces increases with the increase of ubiquitiformal complexity, and vice versa; when other conditions remain unchanged, the normal contact stiffness of rough surface It increases with the decrease of the minimum infimum. When the ubiquitiformal complexity is larger, the change of the minimum infimum has more obvious influence on the contact stiffness. These results should provide theoretical guidance to evaluate the normal contact stiffness of rough surfaces and to improve accuracy of contact machine in engineering application.
2022 Vol. 43 (2): 186-194 [
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195
Effect of temperature gradient on the mechanical properties of thermotropic shape memory polymer plates
As a new type of intelligent material, shape memory polymer has been widely used in aerospace, biomedicine and other fields due to its superiorities in light weight, low cost, and large deformation recovery rate. Among them, the research on the mechanical behavior of thermotropic shape memory polymers is mostly focused on the overall temperature change, but the influence of temperature gradient is rarely considered. Existing studies have shown that the temperature gradient has a non-negligible effect on the mechanical properties of materials and the mechanical behavior of engineering structures. Based on the above, under the assumption of uniform stress, combined with heat transfer and thermally induced shape memory polymer phase transition constitutive theory, a new constitutive relationship considering temperature gradient is proposed. The rationality and accuracy of the model are verified by theoretical analysis and experimental test results ,In this work, The effect of continuously changing temperature gradient on the mechanical properties of shape memory polymer in one-dimensional transient heat conduction stage is discussed. The temperature field distribution varying with time and plate thickness position is determined by finite difference method, and the changes of storage strain, elastic modulus and stress at different thickness are analyzed.. In the one-dimensional steady-state heat conduction stage, the temperature gradient is a fixed value, the overall state under different boundary conditions is classified, and the changes in stored strain, elastic modulus and stress at different thicknesses in different states are simulated. The results show that when the temperature gradient changes continuously, the mechanical behavior of position changes significantly when its temperature is in the range where the phase transition occurs, so it should be specially monitored.When the temperature gradient is constant, the overall state and mechanical behavior will be affected by the boundary temperature conditions. The research in this work can not only provide ideas for monitoring the mechanical behavior of shape memory polymers and the realization of functional gradients under different heat conduction conditions, but also provide a theoretical basis for further engineering applications.
2022 Vol. 43 (2): 195-207 [
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208
Meshless simulation of multi-crack problems based on modified weight function
In computational fracture mechanics, dealing with discontinuous displacement field cross the cracks and the approximation of the singular stress fields around the crack tips are two never-ending difficulties. Especially for the case of multiple cracks, these two issues become more challenging. In this paper, a modified weight function for discontinuous field is developed for meshless method to deal with two-dimensional multi-crack problems. According to the geometric characteristics of the cracks, the local coordinate system of the modified weight function method is established. The derivation process of the formula for the modified weight function is presented. The correction strategies and schemes for different calculation points in the multi-crack calculation domain are proposed with an optimized calculation scheme. Comparing with the traditional methods used for dealing with discontinuity and singular fields in fracture problems, the modified weight function is much simpler to be implemented. Only the weight functions of nodes around each crack segment need to be modified and both the discontinuous displacement fields across multiple cracks and the singular fields at multiple crack tips can be captured simultaneously. In this paper, the element-free Galerkin method (EFGM) based on the modified discontinuous weight function is used to numerically analyze plates with Y-shaped crack, cross-shaped crack and double cracks emanating from a hole, respectively. In these numerical examples, the convergence analysis of different computing parameters is carried out in detail. The results of stress intensity factor and Von-mises stress are given and compared. Numerical results show that without introducing enriched basis function with singular terms, the modified weight function can obtain solutions of stress intensity factor with high accuracy, and can satisfactorily fit the singular fields at multiple crack tips. The meshless simulation strategy for the multi-crack problems based on the modified weight function established in this paper can be further applied to the analysis of multi-crack propagation problems.
2022 Vol. 43 (2): 208-219 [
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Fatigue Failure Prediction Method of Sintered NdFeB Material Based on Particle Filter
A novel method of predicting sintered NdFeB fatigue damage prediction based on particle filter and micro-magnetic monitoring against the urgency and uncertainty during the sintered NdFeB service is put forward. First, in order to accurately reflect the dynamic change relationship between system state variables and output variables, the force-magnetic coupling model of sintered NdFeB is introduced into the particle filtering theoretical framework. Furthermore, the particle filtering evolution equation based on relative permeability is designed. According to the changeable magnetic induction intensity signal during the fatigue damage of sintered NdFeB obtained from the micro-magnetic monitoring system, the particle filtering observation equation based on magnetic induction intensity is constructed. It means that the system status variable was characterized by input variables and output variables. Then, the sintered NdFeB fatigue damage prediction process based on particle filtering algorithm is realized by associating the evolution equation and the observation equation. By monitoring the magnetic induction intensity signal of the stress concentrated region, the characteristic relationship between stress and magnetic induction intensity in sintered NdFeB can be established. Therefore, it can dynamically detect and monitor the fatigue damage of NdFeB by observing the changes of magnetic induction intensity signals. Finally, in order to verify the feasibility and effectiveness of the fatigue failure process of sintered NdFeB by using particle filter method, fatigue experimental study on sintered NdFeB magnets were performed. The experimental results show that the estimation precision and accuracy rating of predicting the sintered NdFeB evolution of fatigue damage is high. Compared with the particle filter prediction and support vector machine, when the fatigue load is 120MPa, the RMSE (Root Mean Square Error) of the particle filter prediction result is only 76.69. And at this time the result of particle filter prediction is 1/4 of support vector machine. This research provides a new idea for predicting the fatigue failure of brittle sintered magnets.
2022 Vol. 43 (2): 220-233 [
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234
Stability Analysis of Functionally Graded Simply Supported Beams Considering Pre-buckling Coupling Deformation
The pre-buckling coupling deformation of functionally graded material (FGM) beams will be caused by stretching-bending coupling effect under some boundary conditions, and this deformation has an effect on the stability of FGM beams. It is assumed that the material properties of FGM beams only change along the thickness direction, the equilibrium equation and the buckling governing equation including the effect of the pre-buckling coupling deformation, of FGM beams are derived based on the classical nonlinear beam theory and the concept of physical neutral surface in this paper, which are solved numerically by the shooting method. The effects of pre-buckling coupling deformation, gradient index and temperature dependence of material properties on the nonlinear deformation and stability of FGM beams are discussed.
2022 Vol. 43 (2): 234-242 [
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