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Quick Search     Adv Search 2023 Vol. 44, No. 5 Published: 2023-10-28 555 A new method for capillary bridge assembly based on flexible template curvature regulation The highly controllable assembly direction and rate of functional building blocks render capillary liquid bridge assembly methods have unique advantages in preparing multi-dimensional microstructures with controllable size and morphology, which have potential application in the production of micro and nano electronic devices with novel optical, electrical, and magnetic characteristics. However, in order to control the assembly direction of functional building blocks, this kind of method often relies on templates that are prepared by photolithography and require hydrophobic modification on the sides, making the process complicate and costly. In order to solve the above problem, through numerical simulation and theoretical analysis we firstly points out that the function of template is to regulate the apparent contact angle of the liquid bridge through pinning, thereby affecting the assembly direction of functional building blocks. Furthermore, according to the mechanism of the curvature to manipulate the apparent contact angle of droplet, we provide a new method to control the assembly direction by simple bending and deformation of the flexible template without chemical modification and processing. The feasibility of the provided method is firstly verified through two dimensional Lattice Boltzmann Method (LBM). The simulated results show that the assembly direction of functional building blocks depends on the positive or negative curvature of the flexible template. We also perform a series of experiments to verify the new method. Through simple control of the bending direction and degree of PDMS template, the assembly direction of functional building blocks is highly affected by the curvature of the template and an annular structure with adjustable width is successfully assembled. This work provides a simple and feasible new method for capillary bridge assembly, which can more effectively regulate the assembly of functional building blocks, and provides a new idea for its current applications in micro and nano electronic devices with novel characteristic. 2023 Vol. 44 (5): 555-564 [Abstract] ( 42 ) HTML (1 KB)  PDF   (0 KB)  ( 23 ) 565 Discrete dislocation dynamics modeling of the effect of lattice orientation on Mode I elastoplastic fracture in single crystals Metallic single crystals, which are widely used in industrial devices due to their excellent properties, make it a necessary task to study the mechanical behavior of metallic single crystals. Dislocation evolution affects the plastic deformation and fracture of crystals, while the nucleation and evolution of dislocations are related to the crystal orientation. Investigating the mechanism of the influence of crystal orientation on the elastoplastic cracking process, especially from the perspective of dislocation evolution and interaction between dislocations and cracks, has great significance in solving the challenging problems of elastoplastic fracture and brittle-ductile transition in metallic crystalline materials. However, the classical continuum mechanics framework has singularity problems when dealing with discontinuities such as cracks. In this paper, the Mode I elastoplastic fracture of a single crystal for four different orientations is investigated by using the superposition scheme for discrete dislocation dynamics (DDD) in the framework of peridynamics (PD). As a nonlocal alternative framework to classical continuum mechanics, PD models use spatial integration instead of spatial derivative, making it well-suited for problems where discontinuities may occur and develop. Moreover, PD models allow for integrating possible nonlocal effects induced by dislocations/microstructure/damage and their evolution. Therefore, the DDD-PD model can simulate elastoplastic fracture by considering the autonomous interactions between dislocations and crack growth. For obtaining the results, neither a preset cracking path nor a cohesive zone model is needed. The results show that the mechanical behavior of a single crystal under Mode I fracture exhibits evident orientation dependence, meaning that orientation change leads to differences in the crystal's toughness, critical applied strain, and fracture behavior. The simulation reproduces the brittle-ductile transition behavior in the facture of single crystals, which is related to the nucleation and motion of the dislocations near the crack tip. The numerical elastoplastic deformation and facture results also capture the Schmid factor dependence. Dislocations tend to nucleate and glide on the slip planes with a larger Schmid factor. We show that the distribution and evolution of dislocations on the slip system, affecting the crystal fracture pattern, vary with the crystal orientation. 2023 Vol. 44 (5): 565-577 [Abstract] ( 75 ) HTML (1 KB)  PDF   (0 KB)  ( 26 ) 578 Tensile Behavior of Pre-torsional Single Crystal Copper Nanorod：A Molecular Dynamics Simulation One-dimensional nanomaterials have different mechanical properties from macroscopic materials and have broad application prospects in micro and nano devices. The effects of pre-torsion degree, aspect ratio of nanorods and tensile strain rate on the tensile behavior of Pre-torsional single crystal copper nanorods were studied by molecular dynamics. The simulation results show that, at higher strain rates, pre-torsional deformation will reduce the yield strength of the axially <100> oriented nanorods, but will improve the tensile flow stress. By analyzing the evolution process of dislocation structure, it is found that the improvement of flow stress is due to the obstruction of dislocation movement by the initial dislocation network provided by pre-torsion deformation for the nanorods during the tensile process. With the increase of the pre-torsion degree, the initial dislocation density increases first and then decreases, which is reflected in the approximate change trend of the average plastic flow stress. Because the initial dislocation net will relieve the local stress concentration during the stretching process of the nanorods, the enhancement effect of pre-torsion on the tensile flow stress increases with the increase of the aspect ratio of the nanorods. At low strain rates, the dislocations produced by pre-torsion will alleviate dislocation starvation and reduce the flow stress of the nanorods compared with that of high strain rate. The tensile behavior of pre-torsional single crystal copper nanorod has been systematically studied, which is expected to provide theoretical support for understanding the plastic deformation mechanism of nanorods under complex loadings. 2023 Vol. 44 (5): 578-590 [Abstract] ( 40 ) HTML (1 KB)  PDF   (0 KB)  ( 20 ) 591 MD simulation and mechanical properties of GaN materials under low-dose irradiation According to the existing irradiation simulation and theoretical analysis results, molecular dynamics method is used to establish an effective irradiation approach to simulate the damage of wurtzite GaN materials in long-term low-dose neutron irradiation. First, the cascade collision process of GaN material subjected to different initial irradiation energies is simulated. The relationship between point defects and initial energy of PKA, and the evolution process of point defects are analyzed. By setting a number of recoil atoms with certain spatial distribution, whose energy is equivalent to the irradiation energy, a more realistically uniform distribution of irradiation damage is realized. The cascade process is then iterated to obtain the damage accumulation models of GaN materials under long-term low-dose irradiation. Using this computational approach, nano-indentation simulations of GaN materials under different irradiation doses are carried out, and the variations in the mechanical properties and the deformation mechanisms are investigated. Our study indicates that adopting five recoil-atoms with a proper spatial distribution can improve the computational efficiency and obtain an uniform irradiation damage distribution. It is obvious that the changes in mechanical properties are directly related to the defects caused by irradiation. The elastic modulus and indentation hardness of the c-plane of GaN before and after irradiation are higher than that of the m-plane. Under a relatively low irradiation dose, the elastic moduli of the c-plane and m-plane of GaN material increase slightly due to the hindering of dislocation nucleation and motion by the irradiation defects. However, with the increase of irradiation dose, the size of the amorphized regions increases which leads to the decreasing in the elastic modulus of GaN materials. A large number of disordered amorphous structures and the defect clusters are generated inside the GaN materials, which induces a decrease in indentation hardness of GaN after irradiation. 2023 Vol. 44 (5): 591-605 [Abstract] ( 97 ) HTML (1 KB)  PDF   (0 KB)  ( 22 ) 606 Reverse analysis for Plastic constitutive parameters based on nanoindentation test Nanoindentation technology is an important experimental method to study the properties of biomaterial, thin films, and coating systems. There’re reasonable and widely-used empirical formulae to obtain the hardness and Yong’s Modulus of the material through the indentation shape and the load-displacement curve. However, there’s still a strong demand for an effective methodology to derive the plastic properties of material. In this paper, we propose a simple and effective inversion analysis method to obtain the plastic parameters combining the FEM, neural network and genetic algorithm optimization technology based on the Johnson-cook constitutive model. The reliability of the method is verified by comparing the experimental results of the load-depression curve with the finite element results. 2023 Vol. 44 (5): 606-621 [Abstract] ( 83 ) HTML (1 KB)  PDF   (0 KB)  ( 28 ) 622 Identification of Pipeline Inner Wall Geometry Based on the PCA-BP Neural Network Pipelines, such as those used for petroleum and natural gas transportation, are subject to various factors such as temperature, pressure and corrosion during long-term operation. These factors can lead to structural defects within the pipelines, including collapse, deformation, rupture and corrosion. Thus, a pipeline inner wall geometric identification method based on principal component analysis (PCA) and back propagation (BP) neural network is proposed to tackle these problems. Firstly, an automated modeling technique for the adaptive inner wall of the pipeline is adopted using CAD secondary development technology, enabling the rapid acquisition of a large number of randomly generated geometric models. Subsequently, these models are subjected to two-dimensional constant magnetic field finite element analysis to obtain magnetic field response data of specific measurement points. This adaptive sample library generation system reduces the cost required for modeling and simulation while providing necessary data foundation for the identification of pipeline inner wall geometry. Meanwhile, the PCA method is utilized to reduce the dimensionality of the magnetic field response data. This dimensionality reduction technique effectively eliminates redundant information and improves the efficiency of the subsequent training process. Finally, a mapping relationship between the magnetic field responses of the measurement points and the geometric parameters of the pipeline wall is established using a BP neural network which optimized by the Levenberg-Marquardt algorithm. This mapping relationship enables the identification of the geometric shape of the pipe wall based on the magnetic field response data. The results of numerical examples demonstrate that the PCA-BP neural network-based pipeline inner wall geometric inversion analysis model can accurately and rapidly predict the geometric shape of the pipeline inner wall. Even for complex geometries with varying degrees of random errors, the proposed method exhibits strong identification performance. This work provides a new way to address structural defects within pipelines and offers an effective tool for pipeline safety assessment and health management. 2023 Vol. 44 (5): 622-636 [Abstract] ( 28 ) HTML (1 KB)  PDF   (0 KB)  ( 20 ) 637 Numerical Implementation and Applications of a Computational Multiscale Micromorphic Method for Porous Materials A direct numerical simulation (DNS) of porous materials with complex microstructures requires extremely detailed meshes, is thus very expensive in terms of modeling and computational costs. The multiscale method can greatly improve the computational efficiency for those problems by transforming a complex single-scale problem into simpler problems at multiple scales. In a conventional first-order multiscale framework, a macroscopic strain tensor characterizes the average deformation within the RVE. First-order frameworks can be used only for problems with a clear scale separation. However, in many porous materials (such as metal foams), the characteristic length of the scale of the heterogeneities is in the order of that of the macroscale. For those problems without scale separation, the micromorphic multiscale computational homogenization framework introduces an additional macroscopic kinematical field to characterize the average strain of inclusions within the RVE. This additional field, along with its higher-order terms, provides a more detailed description of the RVE. The displacement field is only required to be C0 continuous. This paper proposes a micromorphic multiscale approach for porous materials based on the multiscale micromorphic theory. The framework is numerically implemented in MATLAB. A kinematical field decomposition is introduced. The energy equivalence between microscopic and macroscopic scale is naturally guaranteed via the generalized Hill-Mandel condition. The macroscopic governing equations and boundary conditions are derived. The effectiveness of the resulting micromorphic multiscale analysis framework for porous materials is shown via two numerical examples. Compared to the conventional first-order multiscale method, the proposed method is able to accurately characterize macroscopic responses and microscopic field information of porous materials, and accounts for size effects in the considered material to a certain extent. In comparison to DNSs, the computational efficiency is significantly improved. The software developed in this paper has a good generality, and can provide a powerful tool for the design and practical engineering application of porous materials. 2023 Vol. 44 (5): 637-647 [Abstract] ( 60 ) HTML (1 KB)  PDF   (0 KB)  ( 23 ) 648 Cluster-based Nonuniform Transformation Field Analysis of Carbon Fiber Reinforced Composites Carbon fiber reinforced composites have attracted much attention in materials science due to their superior mechanical properties. It is difficult for conventional multiscale methods to provide substantial assistance to the research of such materials due to their huge computational costs. Nonuniform transformation field analysis is a very effective reduced-order homogenization method for elastoplastic multiscale analysis. However, the reduced-order model derived from this method has the shortcoming of low universality and high application threshold. Therefore, an improved reduced-order model is proposed by combining the nonuniform transformation field analysis with the k-means clustering algorithm. One can embed the required microscopic constitutive model into the reduced-order homogenization framework, without the need to derive a new reduced-order model. Based on the cluster-based nonuniform transformation field analysis, the influence of the microscopic plastic strain field evolution on the macroscopic response of the material is revealed, while the mechanical properties of carbon fiber reinforced composites are predicted. The numerical results show that the new reduced-order model can accurately predict the macroscopic mechanical properties of composite materials, and its acceleration rate reaches 103-104 compared to the traditional finite element computations. 2023 Vol. 44 (5): 648-658 [Abstract] ( 33 ) HTML (1 KB)  PDF   (0 KB)  ( 21 ) 659 Deformation and Electric Field Analysis of Piezoelectric Films Under the Effect of Local Temperature Difference Due to the trend of miniaturization of electronic devices, the size of many piezoelectric thin film devices has reached the nanometer scale. The electric field and displacement field in piezoelectric thin film devices can be controlled by using temperature field. At present, the influence of the coupling between the first and second order deformation on the internal mechanical and electrical behavior of piezoelectric thin films has not been considered in the two-dimensional plate theory research on thermal stress. In this paper, the traditional elastic Mindlin plate model is extended to multiple physical fields, and the thermal stress and pyroelectric effect are considered, and the coupling relationship between thickness stretch deformation, in-plane extensional deformation and second-order shear deformation is considered. A piezoelectric film model with local temperature field is established. As an application, the effects of local temperature changes on the deformation and electric field of piezoelectric thin films were studied. The displacement field and electric potential field of piezoelectric films during the thickness stretch deformation are calculated by using Navier solution theory. Then a numerical study was carried out. The results showed that: Local heating (cooling) would lead to thickness stretch (shrinkage) deformation in the loading area, and with in-plane extensional (shrinkage) deformation and second-order shear deformation, the in-plane deformation of the film reached the maximum in the temperature loading area of the upper surface and the lower surface, and the two areas produced potential wells (potential barriers) and potential barriers (potential wells) respectively. This shows that the local temperature field can be used to control the deformation and electric field of the piezoelectric film. This study is an extension of the structural analysis theory of piezoelectric thin films with thermoelastic and pyroelectric effects, and provides a reference for the structural design and optimization of micro and nano scale devices. 2023 Vol. 44 (5): 659-671 [Abstract] ( 82 ) HTML (1 KB)  PDF   (0 KB)  ( 21 ) 672 Local Steady-state wave field identification of delamination in composite laminates based on deep learning object detection algorithm This paper presents a layered damage detection method for carbon fiber reinforced plastic composite (CFRP) based on deep learning object detection algorithm. According to the hierarchical damage of CFRP under high frequency excitation, the local steady-state wave field will change. The object detection function of deep learning convolutional neural network is used to identify and locate the damage. In order to improve the generation efficiency of damage mode training samples, according to the local response characteristics of layered damage, this study adopted the transfer learning scheme and selected the isotropic aluminum plate structure with blind hole damage as the numerical model to replace the composite layered structure, which greatly improved the efficiency of numerical calculation. First, the YOLOv5s network is trained with the problem wave field diagram and corresponding label in the aluminum plate with damage. In the process of damage detection, the neural network will be trained by the input of the damaged CFRP steady state wave field excited by piezoelectric sheet. The recognition results of steady wave field show that the steady wave field recognition algorithm based on deep learning target detection proposed in this paper can quickly and accurately identify multiple spall damage in different positions in CFRP after migration learning training, providing a new detection method for rapid and intelligent composite damage detection. 2023 Vol. 44 (5): 672-678 [Abstract] ( 19 ) HTML (1 KB)  PDF   (0 KB)  ( 21 ) 679 Study on extra-DOF-free Generalized Finite Element Method in nearly incompressible analysis The generalized finite element method (GFEM) enriches the approximation space of the conventional finite element method (FEM). However, the extra degrees of freedom (DOFs) used in the traditional GFEM lead to the issue of linear dependence and the resulting stiffness matrix may be singular. The extra-dof-free GFEM which do not employ extra DOFs removes the issue of linear dependence completely and the conditioning of the stiffness matrix is restored. In this paper, the capability of the extra-dof-free GFEM for nearly incompressible analysis is investigated. Our numerical experiments clearly demonstrates that the extra-dof-free GFEM suffers from the volumetric locking, even though a quadratic approximation function is employed. Aiming at this issue, the techniques of assumed strain developed in FEM is introduced into the extra-dof-free GFEM. Two particular schemes, respectively, called selectively reduced integration (SRI) and mean dilatation (MD) are considered. In both schemes, the dilatational part of strains is assumed to be constant throughout the element. This constant is the dilatation at the center of the element in the first scheme, but in the second scheme it is the mean dilatation of the element. The modified strain matrices based on these two schemes are derived, i.e., the well-known B-bar matrix. Two benchmark examples are employed to investigate the performance of the proposed method in the nearly incompressible analysis. In the example of Cook's membrane, it is shown that volumetric locking of the extra-dof-free GFEM is increasingly severe when the Poisson ratio approaches 0.5. In contrast, the introduced assumed strain techniques can effectively handle volumetric locking and the membrane can always deform in a correct manner. In all the nearly incompressible tests, the accuracy and convergence of the extra-DOF-free GFEM are remarkably improved by the introduced SRI and MD techniques. But more than that, in such incompressible analysis, the extra-DOF-free GFEM also exhibits better robustness against mesh distortion than the conventional FEM. This is demonstrated by the investigation of the Cook's membrane example with distorted meshes. 2023 Vol. 44 (5): 679-686 [Abstract] ( 25 ) HTML (1 KB)  PDF   (0 KB)  ( 23 ) 687 Vibration Isolation Performance Analysis of Quasi-zero Stiffness Floating Slab Track System with Magnetorheological Damper The quasi-zero stiffness floating slab track system with magnetorheological damper was constructed, through introducing the horizontal spring negative stiffness mechanism and magnetorheological damper. Firstly, through the analysis of statics characteristics, the influence of structural parameters on the nonlinear stiffness and bearing capacity of the system are studied. Then, Bingham model was used to describe the magnetorheological damping force, and the nonlinear dynamic differential equation of the system was established. By using averaging method, the amplitude-frequency and phase-frequency characteristic equations were obtained, and the expression of the force transmissibility was derived. Finally, the effects of the nonlinear coefficient, vertical damping ratio, horizontal damping ratio, force excitation amplitude, coulomb damping force and viscous damping ratio on the force transmissibility are analyzed numerically. The vibration isolation performance of the quasi-zero stiffness floating slab track system with magnetorheological damper is compared with the traditional steel spring floating plate track system and the quasi-zero stiffness floating plate track system without magnetorheological damper. The results show that the proposed system with the matching parameters has low nonlinear stiffness within the displacement limit of the floating slab, and has the bearing capacity to meet the requirements of the actual working conditions. Increasing vertical damping ratio, horizontal damping ratio, coulomb damping force and viscous damping ratio can restrain the peak value of force transmissibility and improve the low frequency vibration isolation performance of the system. Increasing the nonlinear coefficient can reduce the initial vibration isolation frequency of the system and broaden the vibration isolation band. Large force excitation amplitude is beneficial to high frequency vibration isolation performance of the system. Compared with the traditional steel spring floating slab track and the quasi-zero-stiffness floating slab track without magnetorheological damper, the proposed system has better vibration isolation performance at low frequency. 2023 Vol. 44 (5): 687-698 [Abstract] ( 29 ) HTML (1 KB)  PDF   (0 KB)  ( 25 )

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