多环形槽结构磁流变阻尼器的实验建模

对自行设计的、多环形槽结构磁流变阻尼器进行了理论分析与实验建模.该阻尼器的主要特点是在阻尼活塞周向表面上开有若干个矩形齿状环形槽,并且通过磁路设计,使流经阻尼通道处的磁流变液流动方向与其作用的磁力线方向垂直,用以增大阻尼力和阻尼力变化范围.然后从磁流变液的流变特性、电磁学的角度出发,利用修正了的非牛顿流体宾汉模型、结合实验数据,建立了该阻尼器的力学模型.利用该模型绘制和分析了外加磁场(通过施加电流实现)和阻尼力之间关系曲线,与实验结果较好吻合,从而证明了模型的正确性,为磁流变液阻尼器设计和性能预测提供了参考.     

Experimental and theoretical investigation on mechanical behaviors of TB991 weld sealant under cyclic loading

The mechanical behaviors of TB991 weld sealant under cyclic loading conditions were experimentally investigated. The evolution of relaxation stress, cyclic softening, and dissipated energy was evaluated with the effect of strain amplitude and mean strain. The experimental results showed that the stress–strain response curves of the first loading-unloading and cyclic loading-unloading were significantly different. The phenomenon of stress relaxation and cyclic softening occurred under cyclic strain loading conditions. Furthermore, the relaxation stress and dissipated energy decreased rapidly during the initial cyclic loading and then steadily decreased with the increase of cycle number, while the cyclic softening increased rapidly at first and then steadily. Besides, a viscoelastic constitutive model was proposed which can describe the different shapes of stress–strain curve between the initial loading-unloading and the cyclic loading-unloading and also considers the cyclic stress relaxation and cyclic softening of the materials under cyclic loading condition. Comparisons between the numerical results and the experimental data demonstrated that the proposed model can better describe the mechanical behavior of TB991 weld sealant under cyclic loading conditions.     

Single and double-step stress relaxation and constitutive modeling of viscoelastic behavior of swelled and un-swelled natural rubber loaded with carbon black

A phenomenological one-dimensional constitutive model, characterizing the mechanical behavior of viscoelastic natural rubber filled with the percolation concentration of HAF carbon black is developed in this investigation. This simple differential form model is based on a combination of linear and non-linear springs with dashpots, incorporating typical polymeric behavior such as shear thinning, thermal softening and non-linear dependence on deformation. The material parameters for this model are determined for the investigated vulcanizates. The model was also developed on same samples after immersion in kerosene for different intervals of times. One step mechanism of relaxation was appeared for straining the samples to different strain levels with constant strain rate. On the other hand, two step mechanisms of relaxation were appeared on straining specimens to same strain level but with different strain rates.     

High strain rate mechanical behavior of epoxy under compressive loading: Experimental and modeling studies

Investigations on high strain rate behavior of epoxy LY 556 under compressive loading are presented. Compressive Split Hopkinson Pressure Bar (SHPB) apparatus was used for the experimental investigations. The studies are presented in the strain rate range of 683-1890 per second. It was generally observed that the compressive strength is enhanced at high strain rate loading compared with that at quasi-static loading. During SHPB testing of the specimens, it was observed that the peak force obtained from the strain gauge mounted on the transmitter bar is lower than the peak force obtained from the strain gauge mounted on the incident bar. Further, an analytical method is presented based on variable rate power law for the prediction of compressive strength at high strain rate loading for epoxy LY 556. Using the analytical method, high strain rate compressive stress-strain behavior is presented up to strain rate of 10,000 per second.     

PDMS在阶跃应变中松弛模量与动态剪切中广义弹性模量的比较

本文采用旋转流变仪对比研究了聚二甲基硅氧烷(PDMS)在两平行板间的动态剪切流动和阶跃应变的关系,应用Cho等人提出的应力分解方法考察了大振幅剪切流中的广义弹性模量,发现在线性区及非线性区的一部分,随着角频率ω的增加,在时间尺度为t=1/ω时,大振幅剪切流中的广义弹性模量逐渐接近阶跃应变中的松弛模量。     

隔离开关壳体有限元分析及力学性能测试

为了验证金属绝缘开关壳体的力学性能, 通过ANSYS有限元分析法和应变电测法分析了壳体在标准压力 (0. 95Mpa) 及以下应力分布情况, 并对最大应力点进行应力线性分析。最大应力点位于拔口右侧相贯线处, 约为117Mpa, 与电测法实验值112. 9Mpa相近。在标准压力下壳体升压降压时应变均在弹性形变之内, 没有发生塑性变形。应力线性分析结果显示筒体设计合理可靠。实验结果与有限元分析值相符。     

电场磁场对电流变液体减振器性能影响的研究

由于电流变液和磁流变液具有良好的可控性能和力学性能而在工程上具有广阔的应用前景.本文将其用于环形间隙通道外置的双缸电流变液体减振器.同时,对电流变液减振器在同时外加电场和垂直磁场作用下阻尼力的变化进行了理论研究和台架实验研究.结果表明,在外加电场和垂直磁场作用下,电流变效应得到加强,改善了减振器的示功特性.     

A procedure for determining the true stress-strain curve over a large range of strains using digital image correlation and finite element analysis

A procedure for determining the stress-strain curve including post-necking strain is proposed. Hourglass type specimens were used for tensile tests, and the stress-strain curves were identified through an iteration process using finite element analysis. The strain at the position of minimum diameter was measured by digital image correlation. This procedure was applied to carbon steel of various degrees of cold work. The radius of the minimum section of the hourglass type specimen was changed in order to investigate the effect of stress triaxiality on the failure strain. The procedure could derive the stress-strain curve including the post-necking strain. From the obtained curve, it was shown that the stress-strain curves for different degrees of cold work were almost identical when the plastic strain by the cold working was added to the strain. Furthermore, it was revealed that the true stress-strain curve could be approximated well by the power law equation and the curve could be estimated by using the stress-strain relation for before-necking strain.     

Experimental and finite element analysis of a very large pultruded FRP structure subjected to free vibration

This paper reports field test results to determine the dynamic behavior of a large all-GFRP space-reticular structure. The first experimental results are illustrated in the frame of a wider research aimed to study the dynamic and static performance of an all-GFRP structure built in L’Aquila (Italy), i.e. a temporary shelter to an ancient church collapsed after the earthquake of 6th of April 2009. Free vibrations were given to the structure with an instrumented hammer and recorded by a laser vibrometer. This kind of test gives the possibility to deduce some strategic information upon structural performance in the field of dynamic identification without invasive tests. In the detail the experimental data of the fundamental frequency and period and damping coefficient are here presented, together with displacements deduced. A comparison to a finite element analysis and closed form solutions suitable to structural vibration analysis allows first final considerations in the field of all-GFRP structure to be done.     

永磁调节式MR阻尼器试验研究及工程应用

设计制作了永磁调节式MR阻尼器，在专用加载试验台上对该阻尼器的力学性能进行了试验研究，分析了影响阻尼力的各种因素，对永磁式MR阻尼器和油阻尼器、橡胶阻尼器进行了拉索减振的现场对比试验研究。结果表明，所设计制作的永磁式MR阻尼器阻尼力可调范围大，比油阻尼器具有更好的温度稳定性和更大的耗能减振能力，能保证每根拉索取得最优的减振效果。该阻尼器巳成功应用于长沙洪山大桥，解决了该桥严重的风致振动问题。     

磁流变液阻尼器-转子-滑动轴承系统稳定性实验研究

通过实验研究了支承在磁流变液阻尼器和滑动轴承上的转子系统在振动主动控制过程中的运动稳定性问题，实验发现，当转子升速，控制电流稳定时，随着控制电流的增大，在一定转速范围内会出现由滑动轴承引起的油膜涡动和油膜振荡，而当转速稳定，突然施加或撤除控制电流时，转子的振动可在短时间内达到新的稳态，不会发生失稳，此后，在一定转速和控制电流条件下转子系统仍会发生失稳，但采用开关控制抑制转子临界振动时系统能稳定运转，研究表明，由控制电流决定的阻尼器支承刚度是影响转子系统稳定性的关键因素。     

磁流变阻尼器的性能试验与神经网络建模

磁流变阻尼器是一种新型的智能振动控制装置.通过磁流变阻尼器的性能试验,研究了在不同电流输入下阻尼力-位移、阻尼力-速度之间的关系,分析了摩擦型磁流变阻尼器的主要特点.采用BP神经网络,建立了磁流变阻尼器的正向模型和逆向模型.仿真结果显示,神经网络模型能准确地预测磁流变阻尼器的阻尼力和控制电流,证明该方法的有效性.与已有的模型相比,具有精度高,计算简便等特点.     

Experimental and theoretical investigation of a polymer subjected to cyclic loading conditions

There are many machine components made of polymeric materials, such as gears, which are subjected to cyclic loading conditions. To design such components, it is necessary to arrive at a suitable mathematical model that can describe the mechanical response of polymeric materials. In this paper, we derive a mathematical model for rate-type solids using thermodynamical framework developed by Rajagopal and Srinivasa (K.R. Rajagopal, A.R. Srinivasa, A thermodynamic frame work for rate type fluid model, Journal of Non-Newtonian Fluid Mechanics 88 (2000) 207-227) (also see Section 5 of Kannan and Rajagopal (K. Kannan, K.R. Rajagopal, A thermomechanical framework for the transition of a viscoelastic liquid to a viscoelastic solid, Mathematics and Mechanics of Solids 9 (2004) 37-59)), which was used by Rajagopal and Srinivasa to derive a mathematical model for isotropic, rate-type liquids. Uniaxial cyclic loading and stress relaxation experiments were conducted. The predictions of the model agreed well with the experimental data.     

Mechanical behavior of glass/epoxy tubes under combined static loading. Part I: Experimental

A series of biaxial static tests of E-glass/epoxy tubular specimens [±45]₂, subjected to combined torsion and tension/compression were performed to simulate complex stress states encountered in a wind turbine rotor blade. The failure locus in the effective axial-shear stress plane was derived experimentally while in-plane strain tensor components were measured in the tube outer surface. By means of shell theory and strain measurements in the surface of the specimen, the in-plane shear response of the outer ply was obtained, revealing dependence each time to the combined tube loading. The correlation established between the ratio of transverse normal and in-plane shear stress in the principal coordinate ply system and the elastic shear modulus, suggested a strong dependence, warning on the implications for design and certification procedures.     

Experimental observations and modelling of cyclic and relaxation behaviour of the Ni-based superalloy Haynes 282

In this paper, the high temperature cyclic and relaxation behaviour of Ni-based superalloy Haynes 282 is investigated. Low-cycle fatigue (LCF) tests with and without hold time have been performed at two elevated temperatures, 650 and 730 °C. The test results are presented and analysed with respect to the cyclic behaviour and the stress relaxation behaviour. Based on this analysis, a Chaboche type of elasto-viscoplastic material model is formulated and calibrated with respect to the cyclic experimental data. Furthermore, the effect of the scatter, observed in the initial yield stresses of the LCF tests, is considered in the calibration of the material model. Finally, a Golos–Ellyin strain energy density fatigue criterion is used to predict the fatigue life and how the scatter in the LCF tests influences the predicted fatigue lives is studied. Moreover, to account for the reduction in fatigue life due to stress relaxation, a frequency modification of the Golos–Ellyin fatigue criterion is proposed and evaluated.     

Comparison of creep strength and intrinsic ductility for serviced and reheat treated T91 steel based on stress relaxation testing

Abstract

High precision stress relaxation tests (SRT) at four temperatures were conducted on T91 (9%Cr) steel after extended boiler service, and also after re-heat treatment. Relative differences in creep strength, measured over five decades in strain rate were dependent on test temperature. Using an established correlation between strain rate sensitivity and elongation at failure, intrinsic ductility values as a function of stress and test temperature were determined. The general trend of a minimum in ductility in terms of stress or strain rate was consistent with long term creep rupture data on T91, and with literature data on alloy steels. However, the precision and repeatability of the SRT analysis contrasted with the appreciable scatter and heat to heat variation in traditional testing. It is argued that the current creep strength evaluation based on the nearly constant state measurement from the SRT test is superior to the measurement of stress dependence of minimum creep rate in traditional creep rupture testing. The complexity of primary creep in laboratory testing, which may not be significant at operating stresses where loading strains may be fully recoverable (anelastic), does not apply to the SRT. Since very low strain rates are achieved in a one day test, the procedures for setting of design allowables and design analysis based on the SRT data should not be significantly different from current practice. This technique offers accelerated alloy development and optimisation for creep strength and also ductility, and hence resistance to notch sensitivity.     

Mechanical and electrical behaviors of polymer particles. Experimental study of the contact area between two particles. Experimental validation of a numerical model

The present paper is devoted to the analysis of mechanical and electrical behaviors observed on particulate polymer granular materials. The constituting particles obtained these physical properties by coating the polymer spherical substrate with a conducting polymer: polypyrrole (PPy) which confers electrical conducting properties to the particle, while preserving its mechanical properties. Particle contacts dominate the behavior of the granular media and, consequently, size, morphology, roughness and plasticity of the particles play a crucial role in this behavior. Scanning electron microscope (SEM) and atomic force microscope (AFM) were used to study the surface state and the contact area between neighbors. An experimental set up, based on the measurement of the displacement of contacting particles subjected to a normal force and of the variation of the electrical resistance of the packing, allowed the study of both the mechanical and electrical behaviors of the particle system. The experimental results took into account the plastic deformation under varying loading and unloading conditions; they were consistent with theories of contact mechanics, thus validating the existing models.     

Development of viscoelastic/rate-sensitive-plastic constitutive law for fiber-reinforced composites and its applications. Part I: Theory and material characterization

The viscoelastic/rate-sensitive plastic constitutive law to describe the nonlinear, anisotropic/asymmetric and time/rate-dependent mechanical behavior of fiber-reinforced (sheet) composites was developed under the plane stress condition. In addition to the theoretical aspect of the developed constitutive law, experiments to obtain the material parameters were also carried out for the woven fabric composite based on uni-axial tension and compression tests as well as stress relaxation tests, while the numerical formulation and verifications with experiments are discussed in Part II.     

Characterizing and modeling the non-linear viscoelastic tensile deformation of a glass fiber reinforced polypropylene

On the basis of comprehensive experimental investigations on a long glass fiber reinforced polypropylene (PP-LGF) a novel rheological material model is developed. It features a decomposition of the stress into a time independent quasi-static and a time and strain dependent viscous contribution. Furthermore it allows for plastic deformations starting from the very beginning of straining and is thereby able to reproduce the absence of a purely linear elastic domain going along with the nonexistence of a defined yield point, characteristic for many fiber reinforced thermoplastic polymers. In order to approach the true quasi-static material behavior, various tensile tests were carried out. The viscous material behavior was deduced from a series of stress relaxation experiments and is described by Eyring’s equation with strain dependent viscosity parameters.     

Evaluation of compressive behaviour of porous structures under large deformation using micro-CT,DVC and micro-FE

Compressive behaviour of open and closed cell polyurethane foam samples under large deformation is studied using micro-Computed Tomography (micro-CT), Digital Volume Correlation (DVC) technique and micro-Finite Element (micro-FE) modelling. The micro-CT images of the foam samples at different compression strains are used to determine anisotropy in the foams, to obtain qualitative information on deformation mechanisms, to quantify the deformation and strains using a local DVC approach and to generate images for micro-FE modelling of the foam samples. Micro-FE modelling predicts the deformation using an elastoplastic material model coupled with continuum damage mechanics. Two different types of boundary conditions, experimentally derived (ExBC) and interpolated from DVC (IPBC), were implemented to evaluate the displacements in the micro-FE models. A reduced integration scheme in micro-FE analysis resulted in high artificial energy and was discarded in favour of full integration. The displacement predicted by IPBC matched with DVC displacement contours for closed cell foam. The ExBC-predicted axial displacement (W) showed a better agreement with DVC than transverse displacements (U, V) contours. However, a significant statistical comparison (R² > 0.70) of all displacements was obtained for both IPBC and ExBC. For open cell foam, both boundary conditions predicted a significant difference in the displacement contours with respect to DVC measurements. Still, the axial displacements of ExBC and IPBC showed a better statistical significance (R² > 0.70).