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2021年 第42卷 第2期    刊出日期：2021-02-01

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论文

Soft pneumatic actuators by digital light processing combined with injection-assisted post-curing

Qiang ZHANG, Shayuan WENG, Zeang ZHAO, H. J. QI, Daining FANG

2021, 42(2):  159-172.  doi:10.1007/s10483-021-2705-7

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The soft robotics display huge advantages over their rigid counterparts when interacting with living organisms and fragile objects. As one of the most efficient actuators toward soft robotics, the soft pneumatic actuator (SPA) can produce large, complex responses with utilizing pressure as the only input source. In this work, a new approach that combines digital light processing (DLP) and injection-assisted post-curing is proposed to create SPAs that can realize different functionalities. To enable this, we develop a new class of photo-cross linked elastomers with tunable mechanical properties, good stretchability, and rapid curing speed. By carefully designing the geometry of the cavities embedded in the actuators, the resulting actuators can realize contracting, expanding, flapping, and twisting motions. In addition, we successfully fabricate a soft self-sensing bending actuator by injecting conductive liquids into the three-dimensional (3D) printed actuator, demonstrating that the present method has the potential to be used to manufacture intelligent soft robotic systems.

A screw dislocation near one open inhomogeneity and another closed inhomogeneity both permitting constant interior stresses

Xu WANG, Ping YANG, P. SCHIAVONE

2021, 42(2):  173-182.  doi:10.1007/s10483-021-2702-8

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We prove that the interior stresses within both a non-parabolic open in-homogeneity and another interacting non-elliptical closed inhomogeneity can still remain constant when the matrix is simultaneously under the action of a screw dislocation and uniform remote anti-plane stresses. The constancy of interior stresses is realized through the construction of a conformal mapping function for the doubly connected domain occupied by the surrounding matrix. The mapping function is endowed with the information describing the screw dislocation via the incorporation of two specifically defined loga-rithmic terms. The constant interior stress fields are observed to be independent of the specific open and closed shapes of the two inhomogeneities and the existence of the screw dislocation. In contrast, the existence of the neighboring screw dislocation significantly affects the open and closed shapes of the two inhomogeneities.

Analytical solution for the stress field of hierarchical defects: multiscale framework and applications

Baijian WU, Sheng ZHOU, Zhaoxia LI

2021, 42(2):  183-208.  doi:10.1007/s10483-021-2673-9

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Hierarchical defects are defined as adjacent defects at different length scales. Involved are the two scales where the stress field distribution is interrelated. Based on the complex variable method and conformal mapping, a multiscale framework for solving the problems of hierarchical defects is formulated. The separated representations of mapping function, the governing equations of potentials, and the stress field are subsequently obtained. The proposed multiscale framework can be used to solve a variety of simplified engineering problems. The case in point is the analytical solution of a macroscopic elliptic hole with a microscopic circular edge defect. The results indicate that the microscopic defect aggregates the stress concentration on the macroscopic defect and likely leads to global propagation and rupture. Multiple micro-defects have interactive effects on the distribution of the stress field. The level of stress concentration may be reduced by the coalescence of micro-defects. This work provides a unified method to analytically investigate the influence of edge micro-defects within the scope of multiscale hierarchy. The formulated multiscale approach can also be potentially applied to materials with hierarchical defects, such as additive manufacturing and bio-inspired materials.

Couple stress-based nonlinear primary resonant dynamics of FGM composite truncated conical microshells integrated with magnetostrictive layers

Yong YANG, S. SAHMANI, B. SAFAEI

2021, 42(2):  209-222.  doi:10.1007/s10483-021-2704-6

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The size-dependent geometrically nonlinear harmonically soft excited oscillation of composite truncated conical microshells (CTCMs) made of functionally graded materials (FGMs) integrated with magnetostrictive layers is analyzed. It is supposed that the FGM CTCMs are subjected to mechanical soft excitations together with external magnetic fields. An analytical framework is created by a microstructure-dependent shell model having the 3rd-order distribution of shear deformation based on the modified couple stress (MCS) continuum elasticity. With the aid of the discretized form of differential operators developed via the generalized differential quadrature technique, a numerical solution methodology is introduced for obtaining the couple stress-based amplitude and frequency responses related to the primary resonant dynamics of the FGM CTCMs. Jump phenomena due to the loss of the first stability branch and falling down to the lower stable branch can be seen in the nonlinear primary resonance of the FGM CTCMs. It is demonstrated that the hardening type of nonlinearity results in bending the frequency response to the right side, and the MCS type of size effect weakens this pattern. Moreover, for higher material gradient indexes, the hardening type of nonlinearity is enhanced, and the MCS-based frequency response bends more considerably to the right side.

Interaction effects on heat conduction and thermal stress in an infinite elastic plane with two circular holes

Kun SONG, Deshun YIN, Haibing YANG, P. SCHIAVONE

2021, 42(2):  223-234.  doi:10.1007/s10483-021-2695-7

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Numerous researches have focused on the physical behavior of an elastic material in the vicinity of a single hole under the assumption that the interaction effects arising from the introduction of multiple holes remain negligible if the holes are placed sufficiently far from each other. In an effort to understand hole interaction effects on heat conduction and thermal stress, we consider the case when two circular holes are embedded in an infinite elastic material and use complex variable methods together with numerical analysis to obtain solutions describing temperature and elastic fields in the vicinity of the two circular holes. The results indicate that the interaction effects on temperature distribution and stress strongly depend on the relative size of the two holes and the distance placed between them but not on the actual size of the holes.

Quench characteristics and mechanical responses during quench propagation in rare earth barium copper oxide pancake coils

Mengdie NIU, Jing XIA, Huadong YONG, Youhe ZHOU

2021, 42(2):  235-250.  doi:10.1007/s10483-021-2699-6

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Quench and mechanical behaviors are critical issues in high temperature superconducting (HTS) coils. In this paper, the quench characteristics in the rare earth barium copper oxide (REBCO) pancake coil at 4.2 K are analyzed, and a two-dimensional (2D) axisymmetric electro-magneto-thermal model is presented. The effects of the constituent materials, background field, and coil size are analyzed. An elastoplastic mechanical model is used to study the corresponding mechanical responses during the quench propagation. The variations of the temperature and strain in superconducting layers are compared. The results indicate that the radial strain evolutions can reflect the transverse quench propagation and the tensile hoop and radial stresses in superconducting layers increase with the quench propagation. The possible damages are discussed with the consideration of the effects of the background field and coil size. It is concluded that the high background field significantly increases the maximum tensile hoop and radial stresses in quenching coils and local damage may be caused.

Bending analysis of five-layer curved functionally graded sandwich panel in magnetic field: closed-form solution

M. SHABAN, H. MAZAHERI

2021, 42(2):  251-274.  doi:10.1007/s10483-021-2675-7

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In this paper, an exact closed-form solution for a curved sandwich panel with two piezoelectric layers as actuator and sensor that are inserted in the top and bottom facings is presented. The core is made from functionally graded (FG) material that has heterogeneous power-law distribution through the radial coordinate. It is assumed that the core is subjected to a magnetic field whereas the core is covered by two insulated composite layers. To determine the exact solution, first characteristic equations are derived for different material types in a polar coordinate system, namely, magneto-elastic, elastic, and electro-elastic for the FG, orthotropic, and piezoelectric materials, respectively. The displacement-based method is used instead of the stress-based method to derive a set of closed-form real-valued solutions for both real and complex roots. Based on the elasticity theory, exact solutions for the governing equations are determined layer-by-layer that are considerably more accurate than typical simplified theories. The accuracy of the presented method is compared and validated with the available literature and the finite element simulation. The effects of geometrical and material parameters such as FG index, angular span along with external conditions such as magnetic field, mechanical pressure, and electrical difference are investigated in detail through numerical examples.

Nonlinear energy harvesting with dual resonant zones based on rotating system

Dan WANG, Zhifeng HAO, Fangqi CHEN, Yushu CHEN

2021, 42(2):  275-290.  doi:10.1007/s10483-021-2698-8

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An electromagnetic nonlinear energy harvester (NEH) based on a rotating system is proposed, of which the host system rotates at a constant speed and vibrates harmonically in the vertical direction. This kind of device exhibits several resonant phenomena due to the combinations of the rotating and the vibration frequencies of the host system as well as the cubic nonlinearity of the NEH. The governing equation of motion for the NEH is derived, and the dynamic responses and output power are investigated with the multiple scale method under the 1:1 primary and 2:1 superharmonic resonant conditions. The effects of system parameters including the nondimensional external frequency, the rotating speed, and the nonlinear stiffness on the responses of free vibration for the system are studied. The results of the primary resonance show that the responses exhibit not only the resonant characteristics but also the nonlinear dynamic characteristics such as the saddle-node (SN) bifurcation. The coexistence of multiple solutions and the varying trends of responses are verified with the direct numerical simulation. Moreover, the effects of system parameters on the average output power are investigated. The results of the analyses on the two resonant conditions indicate that the large power can be harvested in two resonant frequency bands. The effect of resonance on the output power is dominant for the 2:1 superharmonic resonance. Moreover, the results also show that introducing the nonlinearity can increase the value of the output power in large frequency bands and induce the occurence of new frequency bands to harvest the large power. The efficiency of the harvested power could be improved by the combined effects of the resonance as well as the nonlinearity of the NEH device. Suitable parameter conditions could help optimize the power harvesting in design.

An efficient method for vibration and stability analysis of rectangular plates axially moving in fluid

Yanqing WANG, Han WU, Fengliu YANG, Quan WANG

2021, 42(2):  291-308.  doi:10.1007/s10483-021-2701-5

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An efficient method is developed to investigate the vibration and stability of moving plates immersed in fluid by applying the Kirchhoff plate theory and finite element method. The fluid is considered as an ideal fluid and is described with Bernoulli's equation and the linear potential flow theory. Hamilton's principle is used to acquire the dynamic equations of the immersed moving plate. The mass matrix, stiffness matrix, and gyroscopic inertia matrix are determined by the exact analytical integration. The numerical results show that the fundamental natural frequency of the submersed moving plates gradually decreases to zero with an increase in the axial speed, and consequently, the coupling phenomenon occurs between the first- and second-order modes. It is also found that the natural frequency of the submersed moving plates reduces with an increase in the fluid density or the immersion level. Moreover, the natural frequency will drop obviously if the plate is located near the rigid wall. In addition, the developed method has been verified in comparison with available results for special cases.

Moore-Gibson-Thompson theory for thermoelastic dielectrics

J. R. FERNÁNDEZ, R. QUINTANILLA

2021, 42(2):  309-316.  doi:10.1007/s10483-021-2703-9

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We consider the system of equations determining the linear thermoelastic deformations of dielectrics within the recently called Moore-Gibson-Thompson (MGT) theory. First, we obtain the system of equations for such a case. Second, we consider the case of a rigid solid and show the existence and the exponential decay of solutions. Third, we consider the thermoelastic case and obtain the existence and the stability of the solutions. Exponential decay of solutions in the one-dimensional case is also recalled.