Acta Mechanica Sinica最新文献

Mesh reflector antennas are the mainstream of large space-borne antennas, and the stretching of the truss achieves their deployment. Currently, the truss is commonly designed to be a single degree of freedom (DOF) deployable mechanism with synchronization constraints. However, each deployable unit’s drive distribution and resistance load are uneven, and the forced synchronization constraints lead to the flexible deformation of rods and difficulties in the deployment scheme design. This paper introduces an asynchronous deployment scheme with a multi-DOF closed-chain deployable truss. The DOF of the truss is calculated, and the kinematic and dynamic models are established, considering the truss’s and cable net’s real-time coupling. An integrated solving algorithm for implicit differential-algebraic equations is proposed to solve the dynamic models. A prototype of a six-unit antenna was fabricated, and the experiment was carried out. The dynamic performances in synchronous and asynchronous deployment schemes are analyzed, and the results show that the cable resistance and truss kinetic energy impact under the asynchronous deployment scheme are minor, and the antenna is more straightforward to deploy. The work provides a new asynchronous deployment scheme and a universal antenna modeling method for dynamic design and performance improvement.

The gear transmission system directly affects the operational performance of high-speed trains (HST). However, current research on gear transmission systems of HST often overlooks the effects of gear eccentricity and running resistance, and the dynamic models of gear transmission system are not sufficiently comprehensive. This paper aims to establish an electromechanical coupling dynamic model of HST traction transmission system and study its electromechanical coupling vibration characteristics, in which the internal excitation factors such as gear eccentricity, time-varying meshing stiffness, backlash, meshing error, and external excitation factors such as electromagnetic torque and running resistance are stressed. The research results indicate that gear eccentricity and running resistance have a significant impact on the stability of the system, and gear eccentricity leads to intensified system vibration and decreased anti-interference ability. In addition, the characteristic frequency of gear eccentricity can be extracted from mechanical signals and current signals as a preliminary basis for eccentricity detection, and electrical signals can also be used to monitor changes in train running resistance in real time. The results of this study provide some useful insights into designing dynamic performance parameters for HST transmission systems and monitoring train operational states.

T-carbon is a new allotrope of carbon materials, and it displays high hardness and low density. Nevertheless, the hardening mechanisms of T-carbon thin films under nanoindentation remain elusive. This work utilizes molecular dynamics simulation to explore the hardening mechanisms of T-carbon thin films under nanoindentation with variations of loading velocities and temperatures. The results reveal that a loading velocity increase at a given temperature raises the nanoindentation force. The increase in nanoindentation force is due to graphitization, which is related to the fracture of tetrahedral structures in T-carbon thin films. However, increased graphitization caused by an increased temperature lowers the nanoindentation force at a given loading velocity. The increased graphitization is influenced by both the fractured tetrahedrons and the deformation of inter-tetrahedron bond angles. This is attributed to the loss of thermal stability and the lower density of T-carbon thin films as the temperature increases. These findings have significant implications for the design of nanodevices for specific application requirements.

A broadband tunable acoustic metasurface (BTAM) is conceived with Helmholtz resonators (HRs). The tunability of HRs’ neck enables precise control over the phase shift of the unit cell. Through careful arrangement of unit cells, the BTAMs are engineered to exhibit various phase differences, thereby inducing anomalous reflections and acoustic focusing. Numerical simulations demonstrate the BTAM’s remarkable efficacy in manipulating the angle of reflection wave and achieving wave focusing across a broadband frequency range. Experimental investigations of the phase shift and anomalous reflection further validate the design of metasurface. This work contributes to the fields of broadband and tunable acoustic wave manipulation and provides a flexible and efficient approach for acoustic control devices.

Failure of thermal barrier coatings (TBCs) can reduce the safety of aero-engines. Predicting the lifetime of TBCs on turbine blades under real service conditions is challenging due to the complex multiscale computation required and the chemo-thermo-mechanically coupled mechanisms involved. This paper proposes a multiscale deep-learning method for TBC failure prediction under typical thermal shock conditions involving calcium-magnesium-alumina-silicate (CMAS) corrosion. A micro-scale model is used to describe local stress and damage with consideration of the TBC microstructure and CMAS infiltration and corrosion mechanisms. A deep learning network is developed to reveal the effect of microscale corrosion on TBC lifetime. The modeled spalling mechanism and area are consistent with the experimental results, with the predicted lifetime being within 20% of that observed. This work provides an effective method for predicting the lifetime of TBCs under real service conditions.

The work comparing the Yamada-Ota and Xue models for nanoparticle flow across a stretching surface has benefits in nanotechnology, medicinal treatments, environmental engineering, renewable energy, and heat exchangers. Most published nanofluid flow models assumed constant thermal conductivity and viscosity. With such great physiognomies in mind, the novelty of this work focuses on comparing the performance of the nanofluid models, Xue, and Yamada-Ota models on a stretched sheet with variable thickness under the influence of a magnetic field and quadratic thermal radiation. The altered boundary layer equations for momentum and temperature, subject to adequate boundary conditions, are numerically solved using an optimized, efficient, and extensive bvp-4c approach. The effects of non-dimensional constraints such as magnetic field, power index of velocity, wall thickness parameter, and quadratic radiation parameter on momentum and temperature profile in the boundary layer area are analyzed thoroughly and outcomes were illustrated graphically. Additionally, the consequences of certain distinctive parameters over engineering factors are also examined and results were presented in tabular form. From the outcomes, it is seen that fluid velocity slows down in the presence of a magnetic field but the opposite nature is observed in the case of temperature profile. With a higher index of velocity, the velocity profile decreases and the temperature field elevates. It has been found that the presence of quadratic convection improves the temperature field. The outcomes of the two models are compared. The Yamada-Ota model performed far better than the Xue model in the heat transfer analysis.

In daily life, human need various senses to obtain information about their surroundings, and touch is one of the five major human sensing signals. Similarly, it is extremely important for robots to be endowed with tactile sensing ability. In recent years, vision-based tactile sensing technology has been the research hotspot and frontier in the field of tactile perception. Compared to conventional tactile sensing technologies, vision-based tactile sensing technologies are capable of obtaining high-quality and high-resolution tactile information at a lower cost, while not being limited by the size and shape of sensors. Several previous articles have reviewed the sensing mechanism and electrical components of vision-based sensors, greatly promoting the innovation of tactile sensing. Different from existing reviews, this article concentrates on the underlying tracking method which converts real-time images into deformation information, including contact, sliding and friction. We will show the history and development of both model-based and model-free tracking methods, among which model-based approaches rely on schematic mechanical theories, and model-free approaches mainly involve machine learning algorithms. Comparing the efficiency and accuracy of existing deformation tracking methods, future research directions of vision-based tactile sensors for smart manipulations and robots are also discussed.

Flexoelectricity, an electromechanical coupling between strain gradient and electrical polarization in dielectrics or semiconductors, has attracted significant scientific interest. It is reported that large flexoelectric behaviors can be obtained at the nanoscale because of the size effect. However, the flexoelectric responses of centrosymmetric semiconductors (CSs) are extremely weak under a conventional beam-bending approach, owing to weak flexoelectric coefficients and small strain gradients. The flexoelectric-like effect is an enhanced electromechanical effect coupling the flexoelectricity and piezoelectricity. In this paper, a composite structure consisting of piezoelectric dielectric layers and a CS layer is proposed. The electromechanical response of the CS is significantly enhanced via antisymmetric piezoelectric polarization. Consequently, the cross-scale mechanically tuned carrier distribution in the semiconductor is realized. Meanwhile, the significant size dependence of the electromechanical fields in the semiconductor is demonstrated. The flexoelectronics suppression is found when the semiconductor thickness reaches a critical size (0.8 µm). In addition, the first-order carrier density of the composite structure under local loads is illustrated. Our results can suggest the structural design for flexoelectric semiconductor devices.

Unlike the post-buckling behaviors of classical piezoelectric cylindrical shell, the size-dependent effect of flexoelectric material and high strain gradient in the post-buckling process play an important role in the stability analysis of the micro/nano cylindrical shells. To reveal the impacts on the post-buckling of flexoelectric cylindrical shells, an accurate post-buckling model for the flexoelectric cylindrical shells under axial compression is proposed based on the higher-order shear deformation shell theory and von Karman geometrical nonlinearity. The size-dependent post-buckling equilibrium path with mode-jumping phenomena is obtained by using Galerkin’s method and Newton-Raphson method. The predicted results are in agreement with those reported in the open literature. A detailed parametric study is also carried out to investigate the influence of geometrical parameters, flexoelectric coefficients, and electric voltage on the size-dependent post-buckling behaviors of flexoelectric cylindrical shells.

Gradient structures (GS) play a crucial role in achieving a balance between strength and plasticity in metals and alloys. However, there is still a lack of understanding of the mechanisms that maintain a plasticity gradient to prevent the premature failure of fine grains in GS materials. In this work, by incorporating experimental data and the Hall-Petch relationship, we develop a size-dependent crystal plasticity model to investigate the deformation mechanisms for enhancing the strength and plasticity in polycrystalline high entropy alloys. The simulations of the GS model align well with the experimental results, exhibiting strong strain and stress gradients to improve the mechanical properties. Under the conditions of significant deformation incompatibility, the strain gradient predominantly drives the enhancement of plasticity mechanisms. As the deformation incompatibility decreases, the stress gradient begins to play a significant role in comparison with the strain gradient. This shift is attributed to the regular variations in dislocation density within different domains. As the grain size gradients and loads decrease, the dislocation density becomes more uniform across the domains, hindering the formation of strong domain boundaries. While this may impede the activation of strain gradients, it facilitates the activation of stress gradients as a supplementary measure. By designing multilayered GS structures to alter the distribution of dislocation density, we can control the activation levels of stress and strain gradients, thereby influencing the plasticity mechanisms and mechanical properties of the material.