国际机械系统动力学学报(英文)最新文献

Environmental noise can lead to complex stochastic dynamical behaviors in nonlinear systems. In this paper, a Lorenz system with the parameter region with two stable fixed points and a chaotic saddle subject to white Gaussian noise is investigated as an example. Noise-induced phenomena, such as noise-induced quasi-cycle, three-state intermittency, and chaos, are observed. In the intermittency process, the optimal path used to describe the transition mechanism is calculated and confirmed to pass through an unstable periodic orbit, a chaotic saddle, a saddle point, and a heteroclinic trajectory in an orderly sequence using generalized cell mapping with a digraph method constructively. The corresponding optimal fluctuation forces are delineated to uncover the effects of noise during the transition process. Then the process will switch frequently between the attractors and the chaotic saddle as noise intensity increased further, that is, noise induced chaos emerging. A threshold noise intensity is defined by stochastic sensitivity analysis when a confidence ellipsoid is tangent to the stable manifold of the periodic orbit, which agrees with the simulation results. It is finally reported that these results and methods can be generalized to analyze the stochastic dynamics of other nonlinear mechanical systems with similar structures.

A localized Fourier collocation method is proposed for solving certain types of elliptic boundary value problems. The method first discretizes the entire domain into a set of overlapping small subdomains, and then in each of the subdomains, the unknown functions and their derivatives are approximated using the pseudo-spectral Fourier collocation method. The key idea of the present method is to combine the merits of the quick convergence of the pseudo-spectral method and the high sparsity of the localized discretization technique to yield a new framework that may be suitable for large-scale simulations. The present method can be viewed as a competitive alternative for solving numerically large-scale boundary value problems with complex-shape geometries. Preliminary numerical experiments involving Poisson, Helmholtz, and modified-Helmholtz equations in both two and three dimensions are presented to demonstrate the accuracy and efficiency of the proposed method.

Pyrotechnic devices are widely used in the aerospace and defense industries. However, these devices generate high-frequency and high-amplitude shock responses during their use, compromising safe operation of the system. In this paper, the application of a thin-walled circular tube as the energy absorber in pyrotechnic devices is investigated. To accurately predict the shock load and the buffer performance of the thin-walled circular tube, a coupled model connecting the energetic material combustion and finite element simulation is established. The validity of the coupled model is verified by comparing with experiments. Then, the collapse mechanism of the thin-walled circular tube is studied, and the influence of multiple structural parameters on its buffer performance is analyzed. The results show that the thin-walled circular tube effectively reduces the shock overload. The maximum shock overload reduced from 572 612g to 11 204g in the studied case. The structural parameters of the thin-walled circular tube mainly affect the deformation process and the maximum shock overload. The order of importance of structural parameters to the maximum shock overload is determined, among which the wall thickness has the most significant effect.

A crawler system provides much larger ground contact, leading to excellent terrain adaptability. Due to its structural characteristics, high-frequency vibration proportional to the vehicle speed is generated during the driving process. This is a result of the polygon and rolling effects between the track and the wheels. A field test of a tracked vehicle is performed to monitor movement signals of the chassis and a rocker arm. Their corresponding power spectral density distributions confirm the correctness of the frequency-calculation equation. Then, a novel elastic track tensioning device with a damper is designed as a cushion between the idler and the chassis. Depending on its geometry, the equivalent damping coefficient for a dynamic model is evaluated. Subsequently, the damping is altered in response to different operating conditions by a hybrid damping fuzzy semiactive control system. The controller accounts for both chassis and track vibration. Based on the transfer matrix method for multibody systems, a dynamical model of the track system is developed. Control performances are evaluated using two numerical simulations of obstacle crossing and off-road driving operations. Results indicate that the proposed semiactive tensioner is substantially better than the conventional one. This paper provides a novel feasible scheme for vibration reduction of tracked vehicles.

During a high-speed train operation, the train speed changes frequently, resulting in motion change as a function of time. A dynamic model of a double-row tapered roller bearing system of a high-speed train under variable speed conditions is developed. The model takes into consideration the structural characteristics of one outer ring and two inner rings of the train bearing. The angle iteration method is used to determine the rotation angle of the roller within any time period, solving the difficult problem of determining the location of the roller. The outer ring and inner ring faults are captured by the model, and the model response is obtained under variable speed conditions. Experiments are carried out under two fault conditions to validate the model results. The simulation results are found to be in good agreement with the results of the formula, and the errors between the simulation results and the experimental results when the bearing has outer and inner ring faults are found to be, respectively, 5.97% and 2.59%, which demonstrates the effectiveness of the model. The influence of outer ring and inner ring faults on system stability is analyzed quantitatively using the Lempel–Ziv complexity. The results show that for low train acceleration, the inner ring fault has a more significant effect on the system stability, while for high acceleration, the outer ring fault has a more significant effect. However, when the train acceleration changes, the outer ring has a greater influence. In practice, train acceleration is usually small and does not frequently change in one operation cycle. Therefore, the inner ring fault of the bearing deserves more attention.

Triboelectric nanogenerators (TENGs) represent a promising next-generation renewable energy technology. TENGs have become increasingly popular for harvesting vibration energy in the environment due to their advantages of lightweight, broad range of material choices, low cost, and no pollution. However, issues such as input force irregularity, working bandwidth, efficiency calculation, and dynamic modeling hinder the use of TENGs in industrial or practical applications. In this paper, the modeling process of the dynamical system of a TENG is reviewed from the perspective of energy flow. In addition, this paper reviews the main contributions made in recent years to achieve optimized output based on springs, magnetic forces, and pendulums, and introduces different ways to increase the bandwidth of TENGs. Finally, the main problems of TENGs in the process of harvesting vibration energy are discussed. This review may serve as a practical reference for methods to convert irregular mechanical input sources into optimized output performance toward the commercialization of TENGs.

Stereo-digital image correlation (Stereo-DIC) has been widely explored for modal analysis in plate-type structures due to its noncontact and full-field advantages. However, when the traditional stereo-DIC is adopted to capture the out-of-plane displacements, several challenging issues exist such as the development of surface speckles, asynchronous camera recording, and efficiency and accuracy degradation due to high computation costs. Moreover, with the captured out-of-plane displacements, effective and efficient evaluation of the high spatial resolution mode shapes and their application to damage localization are also critical problems. To tackle these issues, a speckle-projection DIC technique using a single high-speed camera is proposed to obtain the out-of-plane vibration displacements. Moreover, an enhanced peak-picking modal analysis method is adopted to enhance the estimation accuracy and efficiency of mode shapes. In addition, the low-rank property of mode shapes in an intact state and the spatial sparse property of damage locations are harnessed for the detection of damage positions without requiring reference data on the healthy state. Finally, the modal analysis and damage localization results based on the proposed speckle-projection DIC are compared with those of the traditional two-camera stereo-DIC technique to verify its feasibility and effectiveness. It is found that the differences in the identified resonant frequencies between these two methods are smaller than 1% for higher modes. Moreover, the proposed speckle-projection DIC has the same accuracy as the traditional two-camera stereo-DIC in terms of measurement accuracy, mode shape estimation, and damage localization.

Metastable β titanium alloys are promising materials for lightweight and energy-efficient applications due to their high strength and low density. Thermal–mechanical processing (TMP) is one of the most effective ways to improve the mechanical properties of such alloys. This paper describes a systematic TMP investigation on a new metastable β titanium alloy, including its dynamic mechanical behavior, and microstructure evolution, via isothermal compression tests and electron back-scattered diffraction characterizations. The results show that the compression stress increases with an increase in the strain rate and a decrease in the temperature. After yielding, the compression stress–strain pattern shows flow-softening behavior at a low temperature and a high strain rate, while sustaining a steady flow state at a high temperature and a low strain rate. The temperature-rise effect contributes to a large degree of flow softening at high strain rates. After the correction for temperature rise, the stress–strain constitutive relationships are established, showing that the compression behavior varies in different phase regions. Based on the microstructure characterizations, it is found that the dynamic recovery and dynamic recrystallization dominate the hot deformations in β phase region and at low strain rates, while the deformation band as an additional product is found in α + β phase region and at high strain rates. The results contribute to a better understanding of the TMP for the considered alloy and may also represent a useful database for β-Ti alloy applications in lightweight mechanical systems.

The dynamics of an ultra-precision machine tool determines the precision of the machined surface. This study aims to propose an effective method to model and analyze the dynamics of an ultra-precision fly-cutting machine tool. First, the dynamic model of the machine tool considering the deformations of the cutter head and the lathe head is developed. Then, the mechanical elements are classified into M subsystems and F subsystems according to their properties and connections. The M-subsystem equations are formulated using the transfer matrix method for multibody systems (MSTMM), and the F-subsystem equations are analyzed using the finite element method and the Craig–Bampton reduction method. Furthermore, all the subsystems are assembled by combining the restriction equations at connection points among the subsystems to obtain the overall transfer equation of the machine tool system. Finally, the vibration characteristics of the machine tool are evaluated numerically and are validated experimentally. The proposed modeling and analysis method preserves the advantages of the MSTMM, such as high computational efficiency, low computational load, systematic reduction of the overall transfer equation, and generalization of its computational capability to general flexible-body elements. In addition, this study provides theoretical insights and guidance for the design of ultra-precision machine tools.

To address the challenging task of effective sound absorption in the low and broad frequency band for underwater structures, we propose a novel grating-like anechoic layer by filling rubber blocks and an air backing layer into metallic grating. The metallic gratings are incorporated into the anechoic layer as a skeleton for enhanced viscoelastic dissipation by promoting shear deformation between rubber and metal plates. The introduction of an air backing layer releases the bottom constraint of the rubber, thus intensifying its deformation under acoustic excitation. Based on the homogenization method and the transfer matrix method, a theoretical model is developed to evaluate the sound absorption performance of the proposed anechoic layer, which is validated against finite element simulation results. It is demonstrated that a sound absorption coefficient of the grating-like anechoic layer of 0.8 can be achieved in the frequency range of 1294–10 000 Hz. Given the importance of sound absorption at varying frequencies, the weighted average method is subsequently used to comprehensively evaluate the performance of the anechoic layer. Then, with structural density taken into consideration, an integrated index is proposed to further evaluate the acoustic properties of the proposed anechoic layer. Finally, the backing conditions and the boundary conditions of finite-size structures are discussed. The results provide helpful theoretical guidance for designing novel acoustic metamaterials with broadband low-frequency underwater sound absorption.