Acta Mechanica Solida Sinica最新文献

Surficial water adsorption and interfacial water condensation as natural phenomena play an essential role in the contact adhesion and friction performances of the solid interface. As the characteristic dimensions downscale to nanometers, the structure and dynamics of the water film at an interface differ significantly from those of its bulk counterpart. In particular, a specific wetting condition termed as the tacky regime has recently sparked great interest in the community, where transient high friction and contact instabilities are observed at the interface that is subjected to the wet-to-dry transition. Unveiling the influence of nanoscale water film on the friction enhancement in the tacky regime will provide theoretical guidance for the friction regulation in the wetting condition. In this article, special emphasis is placed on the development of experimental techniques which allow the visualization of the contact interface (e.g., contact surface deformation, real contact area) and characterization of water film structures (e.g., film thickness, molecular configuration). Building upon the accumulation of recent research activities, we provide an overview of significant advances in understanding the critical mechanisms for friction enhancement, such as vertical capillary force, interfacial shear strength, and ice-like water. Some common design strategies are further given to regulate the friction behavior by tuning the distribution of the water film, surface roughness, and elastic modulus. Finally, we end this review article with a summary of the research status and outlook on areas for future research directions.

Star-shaped lattice structures with a negative Poisson’s ratio (NPR) effect exhibit excellent energy absorption capacity, making them highly promising for applications in aerospace, vehicles, and civil protection. While previous research has primarily focused on single-walled cells, there is limited investigation into negative Poisson’s ratio structures with nested multi-walled cells. This study designed three star-shaped cell structures and three lattice configurations, analyzing the Poisson’s ratio, stress–strain relationship, and energy absorption capacity through tensile experiments and finite element simulations. Among the single structures, the star-shaped configuration r3 demonstrated the best elastic modulus, NPR effect, and energy absorption effect. In contrast, the uniform lattice structure R3 exhibited the highest tensile strength and energy absorption capacity. Additionally, the stress intensity and energy absorption of gradient structures increased with the number of layers. This study aims to provide a theoretical reference for the application of NPR materials in safety protection across civil and vehicle engineering, as well as other fields.

This paper presents an improved level set method for topology optimization of geometrically nonlinear structures accounting for the effect of thermo-mechanical couplings. It derives a new expression for element coupling stress resulting from the combination of mechanical and thermal loading, using geometric nonlinear finite element analysis. A topological model is then developed to minimize compliance while meeting displacement and frequency constraints to fulfill design requirements of structural members. Since the conventional Lagrange multiplier search method is unable to handle convergence instability arising from large deformation, a novel Lagrange multiplier search method is proposed. Additionally, the proposed method can be extended to multi-constrained geometrically nonlinear topology optimization, accommodating multiple physical field couplings.

This paper studies the vibration responses of porous functionally graded (FG) thin plates with four various types of porous distribution based on the physical neutral plane by employing the peridynamic differential operator (PDDO). It is assumed that density and elastic modulus continuously vary along the transverse direction following the power law distribution for porous FG plates. The governing differential equation of free vibration for a porous rectangular FG plate and its associated boundary conditions are expressed by a Lévy-type solution based on nonlinear von Karman plate theory. Dimensionless frequencies and mode shapes are obtained after solving the characteristic equations established by PDDO. The results of the current method are validated through comparison with existing literature. The effects of geometric parameters, material properties, elastic foundation, porosity distribution, and boundary conditions on the frequency are investigated and discussed in detail. The highest fundamental dimensionless frequency occurs under SCSC boundary conditions, while the lowest is under SFSF boundary conditions. The porous FG plate with the fourth pore type, featuring high density of porosity at the top and low at the bottom, exhibits the highest fundamental frequency under SSSS, SFSF, and SCSC boundary conditions. The dimensionless frequency increases with an increase in the elastic foundation stiffness coefficient.

The presence of residual stresses in materials or engineering structures can significantly influence their mechanical performance. Accurate measurement of residual stresses is of great importance to ensure their in-service reliability. Although numerous instrumented indentation methods have been proposed to evaluate residual stresses, the majority of them require a stress-free reference sample as a comparison benchmark, thereby limiting their applicability in scenarios where obtaining stress-free reference samples is challenging. In this work, through a number of finite element simulations, it was found that the loading exponent of the loading load-depth curve and the recovered depth during unloading are insensitive to residual stresses. The loading curve of the stress-free specimen was virtually reconstructed using such stress-insensitive parameters extracted from the load-depth curves of the stressed state, thus eliminating the requirement for stress-free reference samples. The residual stress was then correlated with the fractional change in loading work between stressed and stress-free loading curves through dimensional analysis and finite element simulations. Based on this correlation, an instrumented sharp indentation method for measuring equibiaxial residual stress without requiring a stress-free specimen was established. Both numerical and experimental verifications were carried out to demonstrate the accuracy and reliability of the newly proposed method. The maximum relative error and absolute error in measured residual stresses are typically within ± 20% and ± 20 MPa, respectively.

The mechanical properties of solid oxide fuel cells (SOFCs) can limit their mechanical stability and lifespan. Understanding the correlation between the microstructure and mechanical properties of porous electrode is essential for enhancing the performance and durability of SOFCs. Accurate prediction of mechanical properties of porous electrode can be achieved by microscale finite element modeling based on three-dimensional (3D) microstructures, which requires expensive 3D tomography techniques and massive computational resources. In this study, we proposed a cost-effective alternative approach to access the mechanical properties of porous electrodes, with the elastic properties of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ cathode serving as a case study. Firstly, a stochastic modeling was used to reconstruct 3D microstructures from two-dimensional (2D) cross-sections as an alternative to expensive tomography. Then, the discrete element method (DEM) was used to predict the elastic properties of porous ceramics based on the discretized 3D microstructures reconstructed by stochastic modeling. Based on 2D microstructure and the elastic properties calculated by the DEM modeling of the 3D reconstructed porous microstructures, a convolutional neural network (CNN) based deep learning model was built to predict the elastic properties rapidly from 2D microstructures. The proposed combined framework can be implemented with limited computational resources and provide a basis for rapid prediction of mechanical properties and parameter estimation for multiscale modeling of SOFCs.

Graphical Abstract

Damped acoustic systems have a limited quality factor due to intrinsic loss. By introducing gain elements, a method to enhance the quality factor of damped systems is proposed based on the concept of bound states in the continuum (BICs). The acoustic model under study is a two-port waveguide system installed with two side Helmholtz resonators connected by a coupling tube. Based on the temporal coupled-mode theory, a Hamiltonian matrix with both intrinsic and radiation losses is used to characterize the resonance behavior of the coupled resonators. To achieve a high quality factor, acoustic gain is introduced to compensate the intrinsic loss, leading the Hamiltonian parameters toward a quasi-BIC condition. Numerical simulation demonstrates a gain-assisted and quasi-BIC-supported extremely high quality factor in damped acoustic systems. The concept is further utilized to design a sensor model for particle size detection. The enhanced sensing performance due to high quality factors is numerically demonstrated. The findings suggest potential applications in acoustic sensing and detection devices.

Lithium ion batteries are important for new energy technologies and manufacturing systems. However, enhancing their capacity and cycling stability poses a significant challenge. This study proposes a novel method, i.e., modifying current collectors with perforations, to address these issues. Lithium ion batteries with mechanically perforated current collectors are prepared and tested with charge/discharge cycles, revealing superior capacity as well as enhanced electrochemical stability over cycles. Impedance spectroscopy, scanning electron microscopy, and peeling tests are conducted to investigate the underlying mechanisms. Higher peel resistance, minimized interface cracking, and reduced electrical impedance are found in the perforated electrodes after cycles. Investigations indicate that the perforation holes on current collectors allow the active materials coating on the two sides of the current collector to bind together and, thus, lead to enhanced adhesion between the current collector and active layer. Mechanical simulation illustrates the role of perforated current collectors in curbing interface cracking during lithiation, while electrochemical simulation shows that the interfacial cracking hinders the diffusion of lithium ions, thereby increasing battery impedance and reducing the cyclic performance. This investigation reveals the potential of designing non-active battery components to enhance battery performance, advocating a nuanced approach to battery design emphasizing structural integrity and interface optimization.

The microstructure of positive electrode polycrystalline particles (secondary particles) directly affects their diffusion and mechanical properties. In this study, a quantitative evaluation of the effects of grain anisotropy on the overall diffusion and mechanical properties of secondary particles is conducted, which is based on a simplified 2D polycrystalline model with hexagonal anisotropic grains (primary particles) with different distributed orientations. The research results indicate that consistent grain orientation can promote the uniform distribution of lithium ions, while lower diffusion anisotropy can promote the diffusion of lithium ions along shorter paths, thereby improving the diffusion properties of secondary particles. Lower elastic anisotropy of grains and a grain orientation distribution with a 60° angle favor maintaining the macroscopic elastic isotropy of secondary particles. The study also found that when the number of grains is large enough and the orientation distribution is sufficiently random, secondary particles exhibit macroscopic diffusion isotropy and elastic isotropy.

This paper investigates the effect of hydrogen on the transformation ratcheting of NiTi shape memory alloy (SMA) wires in the experimental and theoretical aspects. In the aspect of experiments, the NiTi SMA orthodontic wires are hydrogen charged by the electrochemical charging method at room temperature with varying charging durations and charging lengths. After that, the ex-situ cyclic tension-unloading experiments are performed for the charged and non-charged wires. Experimental results reveal that the two transformation platforms (two-step MT) occur during the forward MT at the beginning and end of cyclic deformation for hydrogen-charged wires, which can be regarded as a global response of the non-charged and charged regions. Furthermore, this two-step MT and transformation ratcheting aggravate with the increase of the charging duration. In the aspect of the theoretical model, a diffusional-mechanically coupled constitutive model is developed. In this constitutive model, the strain is considered as four components: elasticity, transformation (MT), hydrogen expansion and transformation-induced plasticity (TRIP). Combining Helmholtz free energy and Clausius–Duhem inequality, the thermodynamic driving forces of MT and TRIP are obtained. Fick’s law and the mass conservation equation are incorporated to derive the evolution of hydrogen concentration. A transition from material points to the whole wire is employed to extend the model from a material point to the entire wire, and the overall response with a heterogeneous hydrogen concentration field is obtained. The proposed model's ability to predict the transformation ratcheting of the non-charged and charged NiTi SMA wires is verified by contrasting predictions and experimental results.