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Quick Search     Adv Search 2021 Vol. 42, No. 1 Published: 2021-02-28 1 Flexoelectric Effect in Solid Dielectrics DOI: 10.19636/j.cnki.cjsm42-1250/o3.2020.053 The flexoelectric effect in solid dielectrics refers to the elastic strain gradient generated electric polarization, or the elastic deformation induced by an electric field strain. Since the strain gradient can locally break the inversion symmetry of materials, the flexoelectric effect is a universal electromechanical coupling effect in solid dielectrics. The flexoelectric effect is inversely proportional to the scale of materials due to the strain gradient and electric field gradient, therefore, it becomes significant or even dominant the overall physical properties of materials at the nanoscale. Compared with piezoelectric effect and electrostrictive effect, the flexoelectric effect has the unique size effect characteristics. Moreover, the flexoelectric effect is not limited by the symmetry of materials and Curie phase transition temperature of ferroelectric materials. This work reviewed the flexoelectric effect in solid dielectrics, especially on the theoretical and experimental understanding of it in solid dielectrics. In addition, a perspective on the future directions of the flexoelectric effect in solid dielectrics is provided to discuss the flexoelectric effect in materials. 2021 Vol. 42 (1): 1-32 [Abstract] ( 468 ) HTML (1 KB)  PDF  (0 KB)  ( 168 ) 33 Deep learning-assisted accurate defect reconstruction using ultrasonic guided waves DOI: 10.19636/j.cnki.cjsm42-1250/o3.2020.041 Ultrasonic guided wave technology has played a significant role in the field of nondestructive testing due to its advantages of high propagation efficiency and low energy consumption. At present, the existing methods for structural defect detection and quantitative reconstruction of defects by ultrasonic guided waves are mainly derived from the guided wave scattering theory. However, taking into account the high complexity in guided wave scattering problems, assumptions such as Born approximation used to derive theoretical solutions lead to poor quality of the reconstructed results. Other methods, for example, optimizing iteration, improve the accuracy of reconstruction, but the time cost in the process of detection has remarkably increased. To address these issues, a novel approach to quantitative reconstruction of defects based on the integration of convolutional neural network with guided wave scattering theory has been proposed in this paper. The neural network developed by this deep learning-assisted method has the ability to quantitatively predict the reconstruction of defects, reduce the theoretical model error and eliminate the impact of noise pollution in the process of inspection on the accuracy of results. To demonstrate the advantage of the developed method for defect reconstruction, the thinning defect reconstructions in plate have been examined. Results show that this approach has high levels of efficiency and accuracy for reconstruction of defects in structures. Especially, for the reconstruction of the rectangle defect, the result by the proposed method is nearly 200% more accurate than the solution by the method of wavenumber-space transform. For the signals polluted with Gaussian noise, i.e., 15 db, the proposed method can improve the accuracy of reconstruction of defects by 71% as compared with the quality of results by the tradional method of wavenumber-space transform. In practical applications, the integration of theoretical reconstruction models with the neural network technique can provide a useful insight into the high-precision reconstruction of defects in the field of non-destruction testing. 2021 Vol. 42 (1): 33-44 [Abstract] ( 360 ) HTML (1 KB)  PDF  (0 KB)  ( 157 ) 45 Study on Friction and Energy Dissipation Mechanism of Mineral Asperities in the Interface of Nacre DOI: 10.19636/j.cnki.cjsm42-1250/o3.2020.043 Although composed of relatively weak constituent materials (stiff but brittle minerals and tough but soft biopolymers), nacre can achieve excellent mechanical properties through fabulous designs of multiple structural hierarchies. At microscale, nacre exhibits the well-known “brick-and-mortar” structure, which has attracted considerable research attention. At nanoscale, the mineral platelet (i.e., “brick”) surfaces are featured with distributed nanoscale asperities, which is believed to play an important role in the interface strengthening and toughening. Based on the kinetic and contact analysis, a theoretical model was established herein to characterize the friction and energy dissipation behaviors between the neighboring mineral platelet surfaces in nacre. In the model, the equivalent interface friction coefficient, shear strength, and energy dissipation can be expressed as functions of two dimensionless geometrical parameters: the asperity aspect ratio α=A/l, and the ratio of asperity height to platelet thickness β=A/D. The theoretical predictions were compared with the finite element simulation results, and the good agreement validated the theoretical model. Further studies via the theoretical model led to the following three major findings: (1) the presence of asperities on the mineral platelet surfaces significantly enhances the interface friction, strength and energy dissipation during their relative sliding; (2) the equivalent interface friction coefficient increases with α, but is independent of β; (3) the increases of α and β can both improve the equivalent shear strength and frictional energy dissipation. These results and conclusions are of great significance not only for understanding the interface strengthening and toughening mechanism in nacre but also for guiding the design of related biomimetic composite materials. It is worth noting that the current work mainly focused on the role of mineral asperity, and did not take into consideration the influence of mineral platelet deformation along the interface, biopolymer matrix, mineral bridge, etc. To better understand the interface strengthening and toughening mechanism of nacre, the individual and synergistic roles of these possible factors should be further clarified, which correspondingly asks for more complicated theoretical and numerical models. 2021 Vol. 42 (1): 45-52 [Abstract] ( 217 ) HTML (1 KB)  PDF  (0 KB)  ( 157 ) 53 Influence of a Functionally Graded Layer on the Anti-plane Shear Behavior of a Periodic Fibrous Composite DOI: 10.19636/j.cnki.cjsm42-1250/o3.2020.030 Based on the complex variable techniques combined with the boundary collocation method, a semi-analytical procedure is proposed to explore the influence of a functionally graded layer on the anti-plane shear behavior of a periodic fibrous composite. The distribution of the inclusions in the matrix is assumed to be periodically uniform so that the composite is represented by a square unit cell with a single inclusion coated with a functionally graded layer. The elastic properties of the functionally graded layer are assumed to vary continuously in the normal direction of the interface so that we may use a group of homogeneous perfectly-bonded sublayers (each having individual elastic constants) to describe approximately the mechanical response of the functionally graded layer to the interaction between it and the surrounding bulk (inclusion and matrix). Specific series with unknown coefficients are introduced to describe the complex potential functions of the representative unit cell of the composite. The unknown coefficients are determined from the continuity conditions on the interface and the periodic boundary conditions imposed on the edge of the unit cell. Once the complex potential functions are determined, the effective moduli of the composite are obtained according to the average-field theory. The effects of the fiber volume fractions, modulus of each component and material gradient parameter of the functionally graded layer on the properties of the composite are discussed via several numerical examples. The results show that whether the modulus of the matrix is larger or smaller than that of the inclusion, one may always design an appropriate material gradient parameter of the functionally graded layer to reduce the equivalent stress concentration around the inclusion. In addition, it is found that the material gradient parameter of the functionally graded layer may have a non-negligible influence on the effective moduli of the composite only when the inclusion is stiffer than the matrix. 2021 Vol. 42 (1): 53-62 [Abstract] ( 180 ) HTML (1 KB)  PDF  (0 KB)  ( 166 ) 63 Modeling and Simulation Analysis of Elliptic-Plastic Normal Contact Stiffness of Joint Surface Based on Anisotropic Fractal Theory DOI: 10.19636/j.cnki.cjsm42-1250/o3.2020.039 Establishing a more realistic contact model of joint surfaces is one of the key way to explore the variation law of stiffness characteristics. Firstly, according to the anisotropic fractal geometry theory, this paper equates the micro-convex body on the joint surface to an ellipsoid; Secondly, the two-dimensional joint distribution density function of the micro-contact area and eccentricity of the elliptical contact point was obtained by combining the micro-contact area distribution function and the related theory of probability theory; Finally, based on the Hertz theory, the elliptic elastoplastic normal contact stiffness model of the joint surface was established, and the related factors affecting the normal stiffness of the joint surface were numerically simulated and analyzed using MATLAB software. The results show that the eccentricity distribution of the elliptical micro-contact points on the joint surface has a significant effect on the total rigidity of the joint surface. The total stiffness of the joint surface increases with the increase of the shape parameter , but decreases with the increase of the shape parameter ; The normal stiffness of the joint surface increases with the increase of the normal load. When the load is constant, the normal contact stiffness of the joint surface increases with the increase of the plastic index and decreases with the increase of the fractal roughness, but increases first and then decreases with the increase of the fractal dimension. This model provides a certain theoretical basis for model optimization to improve calculation accuracy. 2021 Vol. 42 (1): 63-76 [Abstract] ( 229 ) HTML (1 KB)  PDF  (0 KB)  ( 166 ) 77 Wrinkling Analysis of Film Embedded in Compliant Layers Based on Different Plate Theories DOI: 10.19636/j.cnki.cjsm42-1250/o3.2020.044 In this paper, the governing equations for the wrinkling of a film sandwiched between two compliant layers are derived based on classical plate theory, first-order shear deformation theory and high-order shear deformation theory, respectively. The two compliant layers are treated as elastic bodies with finite thickness under plane strain condition along one inplane direction. The Airy stress function is then employed to solve the stress field in the two compliant layers subject to clamped and free boundary conditions. With the calculated pressure difference between the upper and lower layers, the governing equations are solved by the linear perturbation method and the critical load expression is thus obtained, which determines the periodic sinusoidal wrinkling of the film. As a validation, the finite element method is also employed to solve the wrinkling of the sandwich structure and the results are compared with our analytical solutions. Good agreement is achieved between the finite element and analytical results. It is shown that, albeit small differences exist, the results obtained based on the classical plate theory can provide sufficient accuracy compared with the shear deformation theories of different orders. Finally, the parameter analysis is conducted to illustrate the influences of boundary conditions, material properties and thicknesses of the film and the compliant layers on the critical loading. The limiting cases, where the thickness of the compliant layers is set to be zero or infinite, are also discussed. The results can serve as guidelines for the optimal designs of various substrate/film structures such as stretchable electronics. 2021 Vol. 42 (1): 77-86 [Abstract] ( 255 ) HTML (1 KB)  PDF  (0 KB)  ( 158 ) 87 Magnetic Field Effect on Flutter Stability of a Fluid-conveying Cantilevered Carbon Nanotube under Different Temperature Fields DOI: 10.19636/j.cnki.cjsm42-1250/o3.2020.027 CNT (Carbon nanotube)-based fluidic systems hold a great potential for emerging medical applications and nano-electromechanical systems (NEMS). One of the critical issues in designing such fluid structure interaction (FSI) systems is how to avoid the vibration induced by the fluid flow, which is undesirable and may even promote the dynamic structural instability. The main objective of the present research is to investigate the flutter instability of a cantilevered single-walled carbon nanotube (SWCNT) induced by fluid flow under a longitudinal magnetic field and different temperature fields. To obtain a dynamical model for the system, the CNTs are modeled as nonlocal Euler–Bernoulli beams. The governing partial differential equations of the transverse vibration and associated boundary conditions are derived by Hamilton’s principle. Then, the differential transformation method (DTM) is applied to solve the governing equations of the FSI systems, and some numerical examples are presented to investigate the effects of nonlocal parameters, temperature and longitudinal magnetic field on the critical flow velocity at which flutter may occur. Numerical results show that the nonlocal small-scale parameter makes the fluid-conveying CNT more flexible, and the addition of a temperature field leads to much richer dynamic behaviors of the CNT system. The above analytical results obtained are found to be in good agreement with those presented in the literature. More importantly, it can be concluded that no matter what the temperature field is, the critical flutter velocity will be improved significantly by applying a longitudinal magnetic field, although there exists an upper limit for this enhancement, which is dependent on temperature variation. The numerical results demonstrate that to improve the dynamic stability of the nanoscale FSI systems, it is not reasonable to just increase the intensity of the axial magnetic field. Thus, the results of the present study may facilitate further analysis of nonlocal vibration, and the design of nanotubes in the presence of multi-physics fields. 2021 Vol. 42 (1): 87-93 [Abstract] ( 221 ) HTML (1 KB)  PDF  (0 KB)  ( 158 ) 94 Elastoplastic Behaviors of DP Steel Sheet under Multi-step Strain Path Evolution DOI: 10.19636/j.cnki.cjsm42-1250/o3.2020.038 This study aims to analyze the elastoplastic characteristics of DP steels under complex loading conditions. The three-step tensile tests associated with stain path change were proposed and conducted. The comparison of mechanical behaviors such as degradation of elastic modulus, anisotropic hardening and permanent softening between monotonic cyclic loading and non-coaxial loadings were performed and analyzed. The results show that the transient anisotropy hardening behavior of the materials at the initial stage of reloading is related to the strain path. The increase of reloading angle, pre-strain magnitude, or martensite content can exacerbate permanent softening of the materials. Meanwhile, the evolutions of elastic modulus of DP steels under different loadings were compared. It is found that the degree of elastic modulus degradation also rises with the increase of reloading angle, pre-strain magnitude, or martensite content. In addition, the results indicate that the evolution of elastic modulus degradation under complex loading path changes is more significant than that under monotonic cyclic loading. 2021 Vol. 42 (1): 94-106 [Abstract] ( 190 ) HTML (1 KB)  PDF  (0 KB)  ( 170 )

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