ORIGIN OF HEAT EFFECTS AND SHEAR MODULUS CHANGES OF A Cu-BASED AMORPHOUS ALLOY 1)

剪切模量在非晶合金黏性流动、扩散及结构弛豫等行为中起着重要作用. 宏观剪切弹性决定非晶合金热流变化.探索非晶合金在结构弛豫和玻璃转变过程中宏观力学性能与热流的关联有助于理解其力学行为起源. 本研究基于自间隙理论对Cu$_{49}$Hf$_{42}$Al$_{9}$非晶合金热流、剪切模量及黏度进行研究,建立剪切模量与热流之间的关联. 通过测量剪切模量精确测定自间隙缺陷浓度演化规律.从能量角度出发,通过激活能图谱探索自间隙缺陷浓度对非晶合金热力学性能的影响. 借助于动态力学分析仪研究非晶合金从室温到过冷液相区的动态弛豫行为,探索物理时效引起的结构弛豫以及内耗演化规律. 研究结果表明,自间隙理论可准确描述非晶合金的弛豫动力学、剪切软化及结构弛豫诱导的力学行为. 结合热流数据可以很好描述铸态和弛豫态非晶合金剪切模量随温度演化过程,激活能图谱直观表述了单位激活能可激活的自间隙缺陷浓度. 自间隙缺陷在结构弛豫中湮灭,表现为玻璃体系结构向更稳定状态迁移.在玻璃化转变过程中,缺陷浓度显著升高伴随热吸收,表现为原子大规模协同运动和剪切软化. 物理时效诱导非晶合金内耗和原子移动性降低. 过冷液相区内原子移动性高至消除了结构弛豫影响.     

Bifurcation and Chaos Analysis of Stochastic Duffing System Under Harmonic Excitations

The Chebyshev polynomial approximation is applied to the dynamic response problem of a stochastic Duffing system with bounded random parameters, subject to harmonic excitations. The stochastic Duffing system is first reduced into an equivalent deterministic non-linear one for substitution. Then basic non-linear phenomena, such as stochastic saddle-node bifurcation, stochastic symmetry-breaking bifurcation, stochastic period-doubling bifurcation, coexistence of different kinds of steady-state stochastic responses, and stochastic chaos, are studied by numerical simulations. The main feature of stochastic chaos is explored. The suggested method provides a new approach to stochastic dynamic response problems of some dissipative stochastic systems with polynomial non-linearity.     

Force reconstruction based on the union of singular spectral analysis and regularization method

Abstract

To overcome the problem of some ill-posed inverse problem of force reconstruction, which is caused by the noise in the measured responses and small singular values of the structure, a technology of force reconstruction based on a hybrid method of singular spectral analysis (SSA) and the Landweber regularization method is proposed in this study for the first time. The SSA is used to filter the structural response before using Landweber regularization. A new choice method of phase space reconstruction dimension is theoretically proposed, and the minimum embedding dimension is determined by the concept of optimizing difference spectrum theory. The feasibility of this method was demonstrated through three kinds of force reconstructions. The numerical simulation results and an acoustic vibration experiment demonstrated that the proposed method is more effective than the traditional method.

Communicated by Wei-Chau Xie.     

Damage simulation of repaired composite laminate with rectangular cut-out

Repaired composite laminate with rectangular cut-out is studied by tensile experiments and FEM simulation in this paper. A user damage subroutine is implemented to ABAQUS to simulate the damage of laminate. Good agreements between experimental data and numerical results are reached. From the experiment and FEM simulation results, the integrality of composite structure is destroyed due to the existence of cut-out. However, the asymmetry of repair gets this effect farther. From damage simulation, the initiation and development of three types of damages are predicted. The matrix cracking occurs at first, fiber–matrix shear and fiber break are subsequent. The effect of matrix cracking to the carrying capacity of structure is very small, but when fiber–matrix shear and fiber break occur, dangerous to structure, the carrying capacity of structure loses rapidly.     

Fatigue failure of polyethylene methacrylate in adhesive assembly under unsymmetrical bending

In this paper, a special bending fatigue experiment was firstly performed to investigate the fatigue behavior of polyethylene methacrylate in adhesive assembly. Fatigue lifetime property (S–N curve) was obtained. Finite element calculations on the whole structures also gave the same results with the testing. Based on the experimental data and finite element analysis, a local stress law of predicting bending fatigue lifetime was put forward. The predication lifetime for the polyethylene methacrylate agreed well with the experimental results. Following the strain energy density (SED) criterion was applied to predict the crack initiation and growth path of the adhesive assembly. The predicted results were in good agreement with the optical microscopy (OM) failure image of the failure specimen. SEM image of fracture further showed that there were lots of parallel fatigue lines with perpendicularity to the direction of crack, and an obvious boundary from the crack propagation failure to final brittle fracture.     

On the modified Green operator integral for polygonal,polyhedral and other non-ellipsoidal inclusions

A “strange” particularity of polyhedral inclusions and of fibres of regular polygonal cross sections has been recently stressed in the literature. For respectively fully or transversally isotropic elasticity of the embedding material, they have a mean and a central Green operator integral both equal to the uniform one of respectively the sphere or the cylindrical fibre. In using the Radon transform (RT) method, this particularity is here shown to be shared by much larger shape types in the same limits of material elasticity symmetry. As a subcase, even more shape types fulfill the similar particularity for material linear properties of second-rank characteristic tensor, such as thermal conductivity, magnetic or dielectric properties. When calculated using the RT method, the modified Green operator integral at any interior point of a bounded domain (inclusion) takes the form of a weighted average over an angular distribution of a single elementary operator. The weight function is geometrically defined from the characteristic function of the domain, and the four-rank or second-rank elementary operator depends on the material linear property of concern. The RT method simply shows that the noticed particularity is due to matching symmetry between the inclusion shape (through the weight function) and the material property (through the elementary operator). The general geometrical characteristics of the inclusion shapes belonging to these sphere-class and cylindrical-fibre-class are specified, and some remarkable shapes of these classes are commented.     

Bivariate empirical mode decomposition and its contribution to wind turbine condition monitoring

Accessing difficulties and harsh environments require more advanced condition monitoring techniques to ensure the high availability of offshore wind turbines. Empirical mode decomposition (EMD) has been shown to be a promising technique for meeting this need. However, EMD was developed for one-dimensional signals, unable to carry out an information fusion function which is of importance to reach a reliable condition monitoring conclusion. Therefore, bivariate empirical mode decomposition (BEMD) is investigated in this paper to assess whether it could be a better solution for wind turbine condition monitoring. The effectiveness of the proposed technique in detecting machine incipient fault is compared with EMD and a recently developed wavelet-based ‘energy tracking’ technique. Experiments have shown that the proposed BEMD-based technique is more convenient than EMD for processing shaft vibration signals, and more powerful than EMD and wavelet-based techniques in terms of processing the non-stationary and nonlinear wind turbine condition monitoring signals and detecting incipient mechanical and electrical faults.     

Dynamic symmetry breaking and structure-preserving analysis on the longitudinal wave in an elastic rod with a variable cross-section

The longitudinal wave propagating in an elastic rod with a variable cross-section owns wide engineering background, in which the longitudinal wave dissipation determines some important performances of the slender structure. To reproduce the longitudinal wave dissipation effects on an elastic rod with a variable cross-section, a structure-preserving approach is developed based on the dynamic symmetry breaking theory. For the dynamic model controlling the longitudinal wave propagating in the elast...     

Noise-induced vegetation transitions in the Grazing Ecosystem

The Grazing Ecosystem is a special case of predator-prey systems, which has attracted widespread attention since a long time ago. Because of the ubiquity of noise, there is a growing need to research the influences of noise on the Grazing Ecosystem. This paper is devoted to investigating the transition behaviors of the high vegetation biomass in the Grazing Ecosystem subjected to Gaussian noise and Lévy noise, respectively. Firstly, the original system is translated into the Itô stochastic differential equation, which is utilized to derive the analytical expression of the escape probability through the Dirichlet boundary value problems. Then the transitions between the two vegetation potential wells are explored by calculating the size of the stochastic basin of attraction based on the escape probability. The comparison between the analytical results and the ones through Monte Carlo simulations shows that the proposed method works very well. It turns out that the Gaussian white noise intensity, Lévy noise stability parameter and herbivore density have different impact mechanisms on the basin stability of high density vegetation in the stochastic Grazing Ecosystem.     

Calculation of the Temperature Dependence of the Unrelaxed Shear Modulus of High-Entropy Bulk Metallic Glasses from Calorimetric Data

JETP Letters - A method is proposed to calculate the temperature dependence of the unrelaxed shear modulus of high-entropy bulk metallic glasses from differential scanning calorimetry data. The...