Acta Mechanica Sinica最新文献

This paper proposes a novel idea by integrating a torsion dynamic vibration absorber with a triboelectric energy harvester to realize synchronous torsional vibration suppression and energy harvesting in a rotor system. The most fantastic feature of the proposed torsion harvester-absorber system (HAS) is the quasi-zero-stiffness (QZS) characteristic for suppressing and harvesting low-frequency vibration energy. The QZS characteristic is realized by combining negative stiffness magnet couplings (NSMC) in parallel connection with a pair torsion coil spring. A theoretical model of the NSMC is established based on the equivalent magnetic charge method, and parametric studies are conducted to provide a guideline for the design of the NSMC. Furthermore, the dynamic model of the host oscillator with a torsion QZS HAS is established based on Lagrange’s equation, and then the dynamic amplification factor is obtained using the harmonic balance method. The effects of geometric parameters on both the performances of vibration mitigation and energy harvesting are investigated. Finally, the parameters of the torsion QZS HAS are optimized using H_∞ optimization method and genetic algorithm, respectively. This enables the torsion QZS HAS to effectively suppress low-frequency vibrations of the rotor system while simultaneously harvesting energy over a wide frequency band.

Back stress has been proven to be the primary mechanism for superior mechanical properties of heterogeneous structures, but its quantitative contribution remains vague. The main purpose of this work is to clarify the contributions of back stress components, i.e., intergranular residual stress and intragranular back stress, to the mechanical properties of heterogeneous structures based on the crystal plasticity theory. The results show that the intragranular back stress is smaller than the intergranular residual stress but contributes significantly to the strain hardening of the heterogeneous bimodal structures. In addition, the contributions of misorientation and grain size to back stress are quantitatively analyzed, and the near-linear relationship between hetero-deformation induced hardening and strain gradient is found. These findings emphasize the essential role of the intragranular back stress induced by strain gradient and provide an in-depth understanding of the elaborate roles of back stress in heterogeneous structures.

This paper proposes a non-intrusive computational method for mechanical dynamic systems involving a large-scale of interval uncertain parameters, aiming to reduce the computational costs and improve accuracy in determining bounds of system response. The screening method is firstly used to reduce the scale of active uncertain parameters. The sequential high-order polynomials surrogate models are then used to approximate the dynamic system’s response at each time step. To reduce the sampling cost of constructing surrogate model, the interaction effect among uncertain parameters is gradually added to the surrogate model by sequentially incorporating samples from a candidate set, which is composed of vertices and inner grid points. Finally, the points that may produce the bounds of the system response at each time step are searched using the surrogate models. The optimization algorithm is used to locate extreme points, which contribute to determining the inner points producing system response bounds. Additionally, all vertices are also checked using the surrogate models. A vehicle nonlinear dynamic model with 72 uncertain parameters is presented to demonstrate the accuracy and efficiency of the proposed uncertain computational method.

A comprehensive dynamic model for thermal buckling, elastic vibration and transient response analysis of rotating nano-composite porous metal-matrix microbeams reinforced with graphene nanoplatelets (GNPs) under a uniform thermal gradient is proposed. Various pore distribution patterns are considered together with different GNPs dispersion rules according to the specific functions. The extended rule of mixture and Halpin-Tsai micromechanics model are employed to evaluate the effective material properties of the nanocomposites. Based on the modified couple stress theory and the improved third-order shear deformation theory, the dynamic equations of the rotating microbeam are established by the Lagrange’s equation. The Chebyshev-based Galerkin method is adopted to discretize these equations, which are then solved by the complex modal analysis and Runge-Kutta-Merson method. Convergence study and comparisons with previous literature are conducted for validation of the present method. A parametric study performed analyzes the effects of angular velocity, thickness-to-length scale parameter ratio, porosity coefficient, weight fraction and geometry of GNPs together with distribution patterns of GNPs and pore on the critical buckling temperature rise, fundamental frequency and time-dependent response of the rotating nanocomposite microbeams. The results reveal significant effects of these parameters on the relevant mechanical behaviors, some of which are even contrary to expectations. Therefore, it is necessary to further study this kind of rotating nanocomposite structures for the optimal design.

This article investigates the second overtone thickness-extensional (TE2) vibrations and associated mode-coupling behaviors in ZnO piezoelectric film bulk acoustic resonator (FBAR), utilizing its wave dispersion relation and the higher-order stress balance principle. By superimposing the general wave solutions of multiple eigenmodes within the frequency range of the TE2 mode, mode-coupling solutions for ZnO FBAR are constructed. The substitution of these mode-coupling solutions into the higher-order stress balance principle, as laterally weak boundary conditions, leads to the frequency spectrogram equation, determining the relationship between resonance frequency and plate length-to-thickness ratio. A modified algorithm that combines the bisection method and the complex modulus ratio method is developed to solve the dispersion equation and frequency spectrogram equation (namely a kind of 2D complex transcendental equations) accurately and efficiently. The obtained results indicate that the operational TE2 mode may couple to unwanted 3^rd thickness-shear, fundamental thickness-shear, and flexural modes. Moreover, the mode-coupling behaviors depend strongly on resonance frequencies and plate length-to-thickness ratio. The displacement distributions of total displacement components, alongside the main displacement components of all considered eigenmodes, clearly demonstrate the variety of coupling behaviors. According to the obtained frequency spectrograms, the desirable values of plate length-to-thickness ratio for a clean operating mode with very weak coupling intensity are determined. These findings are of vital importance for the understanding of the mode-coupling mechanism in overtone thickness-extensional FBARs, which will facilitate the structural design and optimization of FBAR devices.

While liquid-filled porous materials widely exist in both natural and engineering fields, their overall thermo-mechanical behaviors are influenced by the combined effects of solid skeleton, pore-filling liquid, and pore structure. When the pores are sufficiently small (e.g., micro/nano-scale pores), surface effects also play a significant role. Accounting for surface effects and liquid compressibility, we develop a theoretical model to predict the effective thermo-mechanical properties of liquid-filled porous materials. Idealized spherical compressible liquid inclusions distributed randomly in an elastic solid matrix are considered, with two scenarios separately considered. In the first scenario, the liquid inclusions are isolated so that the liquid does not flow freely. The effective coefficient of thermal expansion (CTE) and effective bulk modulus of the two-phase material are obtained via the generalized self-consistent method. In the second scenario, the liquid inclusions are connected by microchannels. We adopt a top-down approach (the mixture theory) to establish general thermo-mechanical constitutive relations for liquid-filled porous materials with surface effects, and then use a bottom-up (micromechanics) approach to determine the coupling coefficients (effective thermo-mechanical parameters) in these constitutive relations. Results show that the presence of surface stress at the solid-liquid interface increases the effective CTE and decreases the effective bulk modulus, especially when liquid compressibility is relatively large; however, the decrease in surface stress caused by increasing temperature weakens such effect. This research not only reveals the mechanism of thermo-mechanical coupling in liquid-filled porous materials having small pores but also provides a theoretical basis for accurate prediction of their thermo-mechanical responses in complex load environments.

Self-propulsion of a deformable ellipse immersed in an unbounded inviscid fluid is discussed in order to explore the effect of the deformation and controlled rotation of the body coupled with the shift of its internal mass on the self-motion. The ellipse is capable of symmetric deformation along the two orthogonal axes and endowed with some self-regulation ability via the shift and rotation of its internal mass. From the model, the appropriate velocity potential induced by the motion of the ellipse with the deformation in an otherwise undisturbed fluid is derived, and then the equations of motion are obtained by means of integrals of the unsteady fluid pressure. The equations are utilized to explore self-translational behaviors of the ellipse through the cyclic shift of its internal mass and deformation coupled with its own controllable rotation. Analysis and numerical results show that the ellipse can break the kinematic time-reversal symmetry by properly adjusting its own rotation to coordinate with the deformation and the cyclic shift of the inner mass to meet a forward criterion, and push itself to move persistently forward without a regression at zero system momentum, exhibiting some basic serpentine movements according as the ellipse performs complete revolutions or oscillates between two extreme yaw angles during its self-motion.

Soft biological tissues are challenging materials for both testing and modeling. Despite the development of many constitutive models, the processing of choosing the most suitable model remains heuristic, relying significantly on personal experience and preference. Another issue is that the amount of collected experimental data is always finite. In this study, we trained a constitutive artificial neural network based on experimental data of cattle skeletal muscle tissue for the self-directed auto-discovery of constitutive models. The discovered models inherently satisfy thermodynamic consistency, material objectivity, polyconvexity, and necessary physical restrictions. Two constitutive models have been discovered by the trained neural network. Considering the constraints of finite experimental data, the generality and reliability of the auto-discovered constitutive models remain to be analyzed. Through experimental data of pig skeletal muscle tissue, we assess the goodness-of-fit and parameter identifiability of the automatically discovered constitutive models. At first glance, both auto-discovered models have excellent prediction accuracy. Further exploration from the perspective of information geometry suggests that one of the auto-discovered models is superior to the other in terms of parameter identifiability. The findings of the current work are expected to extend our understanding of auto-discovered constitutive models and offer a new perspective to advance machine learning-driven mechanics.

The wetting phenomenon of composite substrates in hypergravitational environment has a huge application in electronic devices and astronaut healthcare in aerospace missions. In the present contribution, the governing equation of high-G droplets on the composite substrate is firstly established in the hypergravitational environment. Meanwhile, the apparent contact angles at the contact line between droplets and substrates with different stiffness gradients are achieved. Then, we analyze the effects of hypergravity factor and the substrate stiffness on the wetting profile of high-G droplets. By introducing the droplet volume and contact angle into the Bond number, the scaling law of the high-G droplet profile is established, and we find that the contact radius of the droplet R / S^0.5 has a linear relationship with ρω²rl²S / (γ_LVθ), while the droplet height H / S^0.5 has a power-law relationship with ρω²rl²S / (γ_LVθ). Finally, we explain the profiles of high-G droplets during the wetting process by illustrating energy components of the entire system and find that the substrate with positive triangular stiffness and inverted triangular stiffness show opposite evolution laws. On a substrate with inverted triangular stiffness, the gravitational potential energy is more dominant.

The study investigates the partial slip contact problem between thermoelectric material and periodic punches including flat and cylindrical punches under various loading conditions, including normal force, energy flux, and electric current density. When developing and enhancing thermoelectric devices intended for energy harvesting and temperature regulation, the relationship between punch geometry and thermoelectric material plays a crucial role. The periodic contact problem leads to the formation of a singular integral equation with a Hilbert kernel, distinguishing it from the traditional Cauchy kernel. It delves into the impact of the thermo-electric-mechanical coupling effect on the evolution of stick-slip zone length and contact stress during partial slip. By formulating the current nonlinear problem into a set of singular integral equations, the study identifies primary variables, including normal and tangential contact stresses, as well as slip and stick zones. The partial contact problem is further simplified by applying the Goodman approximation, enabling an iterative approach to determine the stick-slip zone, contact zone size, and stress distribution. Notably, the research reveals that factors such as the shape of the punch, the coefficient of friction, and TE parameters significantly influence stress strength and the characteristics of the stick-slip zone.