Multi-layer quasi-zero-stiffness meta-structure for high-efficiency vibration isolation at low frequency

An easily stackable multi-layer quasi-zero-stiffness(ML-QZS)meta-structure is proposed to achieve highly efficient vibration isolation performance at low frequency.First,the distributed shape optimization method is used to design the unit cel,i.e.,the single-layer QZS(SL-QZS)meta-structure.Second,the stiffness feature of the unit cell is investigated and verified through static experiments.Third,the unit cells are stacked one by one along the direction of vibration isolation,and thus the ML-QZS meta-structure is constructed.Fourth,the dynamic modeling of the ML-QZS vibration isolation metastructure is conducted,and the dynamic responses are obtained from the equations of motion,and verified by finite element(FE)simulations.Finally,a prototype of the ML-QZS vibration isolation meta-structure is fabricated by additive manufacturing,and the vibration isolation performance is evaluated experimentally.The results show that the vibration isolation performance substantially enhances when the number of unit cells increases.More importantly,the ML-QZS meta-structure can be easily extended in the direction of vibration isolation when the unit cells are properly stacked.Hence,the ML-FQZS vibration isolation meta-structure should be a fascinating solution for highly efficient vibration isolation performance at low frequency.

Fractional-order visco-plastic constitutive model for uniaxial ratcheting behaviors

This paper proposes a novel unified visco-plastic constitutive model for uniaxial ratcheting behaviors. The cyclic deformation of the material presents remarkable time-dependence and history memory phenomena. The fractional(fractional-order)derivative is an efficient tool for modeling these phenomena. Therefore, we develop a cyclic fractional-order unified visco-plastic(FVP) constitutive model. Specifically, within the framework of the cyclic elasto-plastic theory, the fractional derivative is used to describe the accumulated plastic strain rate and nonlinear kinematic hardening rule based on the Ohno-Abdel-Karim model. Moreover, a new radial return method for the back stress is developed to describe the unclosed hysteresis loops of the stress-strain properly.The capacity of the FVP model used to predict the cyclic deformation of the SS304 stainless steel is verified through a comparison with the corresponding experimental data found in the literature(KANG, G. Z., KAN, Q. H., ZHANG, J., and SUN, Y. F. Timedependent ratcheting experiments of SS304 stainless steel. International Journal of Plasticity, 22(5), 858–894(2006)). The FVP model is shown to be successful in predicting the rate-dependent ratcheting behaviors of the SS304 stainless steel.

地方应用型高校材料化学专业人才培养体系的构建

论述了应用型地方高校材料化学专业构建稳定的人才培养体系的必要性和紧迫性。结合教学实践重点阐述了在课程体系设置、课堂教学、实验教学、校外实践等教学环节的改革,以及师资素质培养和转型、专业差异化发展、与地方经济融合等方面的有益探索。通过数据展现了在新的培养体系中取得的积极效果。最终通过政策引导,以产教融合为主线,构建稳定的适合地方应用型高校办学实际的专业人才培养体系。

Suppression of grazing-induced instability in single degree-of-freedom impact oscillators

As a typical non-smooth bifurcation, grazing bifurcation can induce instability of elementary near-grazing impact periodic motion in impact oscillators. In this paper,the stability for near-grazing period-one impact motion to suppress grazing-induced instabilities is analyzed, based on which, a control strategy is proposed. The commonly-used leading order zero time discontinuity mapping is extended to a higher order one to aid the perturbation analysis of the characteristic equation. It is shown that the degenerate grazing bifurcation can eliminate the singular term in the characteristic equation, leading to bounded eigenvalues. Based on such a precondition, the bounded eigenvalues are further restricted inside the unit circle, and a continuous transition between non-impact and controlled impact motion is observed. One discrete feedback controller that changes the velocity of the oscillator based on the selected Poincar′e sections is adopted to demonstrate the control procedure.

A quasi-zero-stiffness isolator with a shear-thinning viscous damper

Quasi-zero-stiffness(QZS)vibration isolators have been widely studied,because they show excellent high static and low dynamic stiffnesses and can effectively solve low-frequency and ultralow-frequency vibration.However,traditional QZS(T-QZS)vibration isolators usually adopt linear damping,owing to which achieving good isolation performance at both low and high frequencies is difficult.T-QZS isolators exhibit hardening stiffness characteristics,and their vibration isolation performance is even worse than that of linear vibration isolators under a large excitation amplitude.Therefore,this study proposes a QZS isolator with a shear-thinning viscous damper(SVD)to improve the vibration isolation performance of the T-QZS isolators.The force-velocity relation of the SVD is obtained,and a dynamic model is established for the isolator.The dynamic responses of the system are solved using the harmonic balance method(HBM)and the Runge-Kutta method.The vibration isolation performance of the system is evaluated using force transmissibility,and the isolator parameters are analyzed.The results show that compared with the T-QZS isolators,the proposed QZS-SVD isolator achieves the lower initial vibration isolation frequency and peak value,and exhibits better vibration isolation performance at medium and high frequencies.Moreover,the proposed isolator can withstand a large excitation amplitude in the effective vibration isolation range.

用于振动模态可操纵性的克拉尼图形拓扑优化设计

经典克拉尼图形(Chladni patterns,CP)是特征向量的视觉表示,在振动分析、乐器设计、粒子操纵和分离等领域中具有广泛的应用.均质结构通常会表现出高度规则化的CP,这限制它们的应用场景.因此,实现面向应用来逆向设计结构的CP是非常重要的,而这可以通过使用拓扑优化来实现CP的逆向设计.本研究提出了一种使用基于密度的拓扑优化算法来设计结构CP的方法.优化列式中,给出了描述图形的指标函数,并以最小化特征向量和目标CP之间的误差构建了优化目标.数值结果表明,所提出的方法可以为指定的单阶或多阶CP生成新颖的材料分布.这表明了本文所提出的方法能够实现CP的精确定制设计,进一步为面向应用的需求提供了更多选择.

Comprehensive study on fail-safe topological design method for 3D structures

Fail-safe topology optimization is valuable for ensuring that optimized structures remain operable even under damaged conditions.By selectively removing material stiffness in patches with a fixed shape,the complex phenomenon of local failure is modeled in fail-safe topology optimization.In this work,we first conduct a comprehensive study to explore the impact of patch size,shape,and distribution on the robustness of fail-safe designs.The findings suggest that larger sizes and finer distribution of material patches can yield more robust fail-safe structures.However,a finer patch distribution can significantly increase computational costs,particularly for 3D structures.To keep computational efforts tractable,an efficient fail-safe topology optimization approach is established based on the framework of multi-resolution topology optimization(MTOP).Within the MTOP framework,the extended finite element method is introduced to establish a decoupling connection between the analysis mesh and the topology description model.Numerical examples demonstrate that the developed methodology is 2 times faster for 2D problems and over 25 times faster for 3D problems than traditional fail-safe topology optimization while maintaining similar levels of robustness.

A Cyclic Constitutive Model Based on Crystal Plasticity for Body-Centered Cubic Cyclic Softening Metals

Under the framework of the small deformation crystal plasticity theory,a crystal plastic cyclic constitutive model for body-centered cubic(BCC)cyclic softening polycrystalline metals is established.The constitutive model introduces the isotropic softening rule that includes two different mechanisms:namely softening under monotonic deformation and softening under cyclic deformation on each slip system.Meanwhile,a modified Armstrong-Frederick nonlinear kinematic hardening rule is adopted.The appropriate explicit scale transition rule is selected to extend the single crystal constitutive model to the polycrystalline constitutive model.Then the model is used to predict the uniaxial and multiaxial ratcheting deformation of BCC axle steel EA4T to verify the rationality of the proposed model.The simulation results indicate that the newly established crystal plasticity model can not only describe the cyclic softening characteristics of BCC axle steel EA4T well,but also reasonably describe the evolution laws of uniaxial ratcheting and nonproportional multiaxial ratcheting deformation.Moreover,the established crystal plastic cyclic constitutive model can reasonably predict the ratcheting behavior of BCC single crystal as well.

非光滑准零刚度隔振器实验研究

理论上,与传统准零刚度隔振器(QZS)相比,含限位结构的准零刚度隔振器具有更低的位移传递率峰值和更宽的隔振频率范围.但是,目前还没有相关的实验研究证明这一理论结果的正确性.因此,为了研究含限位准零刚度隔振器在实际应用过程中的隔振特性,本文设计制作了相应的结构,并进行了动力学实验.首先,根据赫兹接触理论,建立了含限位结构的准零刚度隔振器(QZS-HCM)动力学模型.然后,对隔振器的结构进行设计和加工,制作了QZS-HCM系统,并对系统进行静力学实验来验证理论结果的正确性.接着,在简谐位移激励下,对QZS-HCM系统进行动力学测试,通过分析系统的位移传递率和时间历程来评价系统的隔振性能.同时,将QZS-HCM系统与传统QZS系统的动力学测试结果进行对比,表明QZS-HCM隔振器在大幅值位移激励下具有更低的位移响应峰值和更宽的隔振频带.

基于强化学习控制三连杆双足机器人实现节能周期步态

由于双足机器人有着强非线性和非光滑性的响应特性,为双足机器人设计高性能的步态控制器依旧是一个开放的研究领域.为了克服这些挑战,本文首先提出了一个包含躯干,并考虑碰撞与摩擦的三连杆双足机器人模型.随后,采用双延迟深度确定性策略梯度算法为该机器人设计了强化学习控制器.为了同时在下坡和上坡中实现节能周期步态,本文基于庞家莱截面法和幂函数构建了能为控制器提供学习指引的奖励函数.因此,所提出的控制器能够在不依赖于预先设计的参考轨迹或嵌入的不稳定周期步态的情况下,学会自适应输出精确的余弦力矩来实现控制目标.通过和神经网络比例微分控制器的对比研究表明,在不同的斜坡上,所提出的强化学习控制器均能实现精确、高效的周期步态,并具有较强的适应性和鲁棒性.

基于鸟类脖颈结构的仿生超冗余机器人

一些鸟类的脖颈具有出色的柔性弯曲能力,模仿其特性可以设计仿生机器人.该类仿生设计的主要挑战是如何处理鸟类脖颈固有的灵活性和冗余的自由度.本文以鸡的脖颈结构为例,提出了一种模仿其脖颈结构特征的仿生超冗余机器人的设计方法.在我们的设计中,首先研究鸡脖颈的多骨节连接与运动特性,并定义了一个具有弹簧和万向节组合的仿生椎骨单元(BVU)来模拟鸡脖颈结构.然后,由三根钢丝平行驱动的三个仿生椎骨单元形成单个颈椎节段.最后,连接四个相同的颈椎节段构成了所提出的仿生超冗余机器人.结合几何分析法和Denavit-Hartemberg(D-H)参数法,研究了仿生超冗余机器人的驱动空间、关节空间和任务空间的运动学关系.通过蒙特卡洛方法进一步计算了可达工作空间.此外,原型样机的单平面运动实验的最大位置偏差约为四个颈椎节段总长度的5.8%.同时,一系列空间形状的展示,包括S形仿生弯曲配置和对兴趣目标体的成功缠绕和提升,证明了所提出的设计方法的有效性、机器人具有优异的灵活性和应用潜力.

A limb-inspired bionic quasi-zero stiffness vibration isolator

Vibration reduction has always been one of hot and important topics in mechanical engineering,especially for the special measurement instrument.In this paper,a novel limb-inspired bionic structure is proposed to generate negative stiffness and design a new quasi-zero stiffness isolator via torsion springs,distinguishing from the existing tension spring structures in the literature.The nonlinear mathematical model of the proposed structure is developed and the corresponding dynamic properties are further investigated by using the Harmonic Balance method and ADAMS verification.To evaluate the vibration isolation performance,typical three-springs quasi-zero stiffness(TS QZS)system is selected to compare with the proposed bionic structure.And the graphical processing unit(GPU)parallel technology is applied to perform necessary two-parameter analyses,providing more insights into the effects of parameters on the transmissibility.It is shown that the proposed structure can show advantages over the typical TS QZS system in a wider vibration isolation range for harmonic excitation case and shorter decay time for the impact excitation case.

Numerical and Experimental Investigations on Tunable Low-frequency Locally Resonant Metamaterials

In this paper,a tunable locally resonant metamaterial is proposed for low-frequency band gaps.The local resonator composed of two pairs of folded slender beams and a proof mass is designed based on the theory of compliant mechanism.The design optimization on geometric parameters is carried out to fulfil the quasi-zero-stiffness property.The locally resonant metamaterial is formed by periodically arranged unit cells,and the transmittance of longitudinal wave is studied through three aspects:numerical predictions,finite element simulations and experimental tests.The variation trends revealed by these three methods match well with one another:the band gap moves to lower frequency and both its depth and width get smaller and smaller with the increase of pre-compression(Δ).The band gap overlays the frequency range of 73.10–92.38 Hz and 16.78–19.49 Hz atΔ=0mm andΔ=10mm,respectively,providing a wide range of tunability.Besides,the ultralow-frequency band gap can be achieved asΔapproaches 10 mm.This study may provide an avenue for achieving the tunable ultralow-frequency locally resonant band gap.

Efficient,high-resolution topology optimization method based on convolutional neural networks

Topology optimization is a pioneer design method that can provide various candidates with high mechanical properties.However,high resolution is desired for optimum structures,but it normally leads to a computationally intractable puzzle,especially for the solid isotropic material with penalization(SIMP)method.In this study,an efficient,high-resolution topology optimization method is developed based on the super-resolution convolutional neural network(SRCNN)technique in the framework of SIMP.SRCNN involves four processes,namely,refinement,path extraction and representation,nonlinear mapping,and image reconstruction.High computational efficiency is achieved with a pooling strategy that can balance the number of finite element analyses and the output mesh in the optimization process.A combined treatment method that uses 2D SRCNN is built as another speed-up strategy to reduce the high computational cost and memory requirements for 3D topology optimization problems.Typical examples show that the high-resolution topology optimization method using SRCNN demonstrates excellent applicability and high efficiency when used for 2D and 3D problems with arbitrary boundary conditions,any design domain shape,and varied load.

Multi-resolution nonlinear topology optimization with enhanced computational efficiency and convergence

Huge calculation burden and difficulty in convergence are the two central conundrums of nonlinear topology optimization(NTO).To this end,a multi-resolution nonlinear topology optimization(MR-NTO)method is proposed based on the multiresolution design strategy(MRDS)and the additive hyperelasticity technique(AHT),taking into account the geometric nonlinearity and material nonlinearity.The MR-NTO strategy is established in the framework of the solid isotropic material with penalization(SIMP)method,while the Neo-Hookean hyperelastic material model characterizes the material nonlinearity.The coarse analysis grid is employed for finite element(FE)calculation,and the fine material grid is applied to describe the material configuration.To alleviate the convergence problem and reduce sensitivity calculation complexity,the software ANSYS coupled with AHT is utilized to perform the nonlinear FE calculation.A strategy for redistributing strain energy is proposed during the sensitivity analysis,i.e.,transforming the strain energy of the analysis element into that of the material element,including Neo-Hooken and second-order Yeoh material.Numerical examples highlight three distinct advantages of the proposed method,i.e.,it can(1)significantly improve the computational efficiency,(2)make up for the shortcoming that NTO based on AHT may have difficulty in convergence when solving the NTO problem,especially for 3D problems,(3)successfully cope with high-resolution 3D complex NTO problems on a personal computer.

Influence analysis of sprinkler irrigation effectiveness using ANFIS

In order to improve sprinkler irrigation quality and promote actual irrigation efficiency,the influence analysis of sprinkler irrigation effectiveness(SIE)using ANFIS(Adaptive Neural Fuzzy Inference System)was implemented to balance moisture infiltration and water redistribution in soil field.Firstly,using a detailed description of governing equations proposed for sprinkler irrigation flow,the theoretical foundation and mathematical model of irrigation effectiveness can be established;Secondly,based on a complete preparation of experimental irrigation conditions,a series of calibration indexes quantifying SIE for sprinkler irrigation quality and infiltration efficiency were proposed;Then thirdly,a novel ANFIS system was designed and introduced to evaluate these key effectiveness indexes in actual working operations,so that a series of detailed influence analysis and comprehensive infiltration assessment focusing on sprinkler irrigation effectiveness could be achieved afterwards,which result to the realization of better infiltration equilibrium and higher water redistribution efficiency in actual irrigation test.Therefore,the qualification of sprinkler irrigation effectiveness was achieved,and in addition,the moisture infiltration improvement and soil moisture uniformity were facilitated also in return.

A Crystal-Plasticity Cyclic Constitutive Model for the Ratchetting of Polycrystalline Material Considering Dislocation Substructures

A crystal-plasticity cyclic constitutive model of polycrystalline material considering intra-granular heterogeneous dislocation substructures,in terms of three dislocation categories:mobile dislocations,immobile dislocations in the cell interiors and in the cell walls,is proposed based on the existing microscopic and macroscopic experimental results.The multiplication,annihilation,rearrangement and immobilization of dislocations on each slip system are taken as the basic evolutionary mechanism of the three dislocation categories,and the cross-slip of screw dislocations is viewed as the dynamic recovery mechanism at room temperature.The slip resistance associated with the isotropic hardening rule results from the interactions of dislocations on the slip systems.Meanwhile,a modified nonlinear kinematic hardening rule and a rate-dependent flow rule at the slip system level are employed to improve the predictive capability of the model for ratchetting deformation.The predictive ability of the developed model to uniaxial and mul-tiaxial ratchetting in macroscopic scale is verified by comparing with the experimental results of polycrystalline 316L stainless steel.The ratchetting in intra-granular scale which is obviously dependent on the crystallographic orientation and stress levels can be reasonably predicted by the proposed model.

Studies on the sound absorption and transmission loss performances of wood-based, natural and wastematerials

The sound absorption and sound transmission loss performances of the natural woods,hard and soft processed woods with attachment of the various natural orwastematerials were investigated in the present study using impedance tube with American Society for Testing Material(ASTM)standards.The sound absorption performances of all natural and all hard processed woods were very poor.It was found that filter mat made by the coconut fibre was the best material for sound absorption improvement of the hard processed woods.The sound absorption performance of the soft processed wood(cork)was better than all natural and all hard processed woods.Among all tested woods,it was found that the cork with attached tea bag made by corn fibre is the best selection for sound absorption application.The transmission loss performances of all natural woods were good.The effects of various materials on the transmission loss performances of all hard processed woods were not very significant.For cork with and without attachment of various materials,their transmission loss performances were not as good as the hard processed woods.Among all tested woods,it was found that Pterocarpus soyauxii and Quercus spp.(natural woods)are the best woods to be used in those applications when prevention of sound transmission is needed.It is recommended that cork is the best wood to be used in those applications where sound absorption and prevention of sound transmission are needed at the same time due to its good sound absorption performance while its ability on prevention of the sound transmission is also acceptable.