Structure, physical properties and dynamics of magnetorheological suspensions

The dynamic structure and physical properties (rheological, magnetic) of magnetorheological suspensions are described in terms of a stationary model, according to which a suspension in a magnetic field represents a system of ordered non-interacting ellipsoidal aggregates oriented at some angle to the fiow. The model is verified experimentally by measurements of steady-state effective rheological and magnetic characteristics.     

玄武岩纤维高延性水泥基复合材料的动态力学性能

利用玄武岩纤维和水泥基材料,通过一定配比融合制成了在静态拉伸试验中呈现多缝开裂、应变硬化、极限拉伸应变0.5%以上的玄武岩纤维高延性水泥基复合材料(basalt fiber engineered cementitious composites, BF-ECCs)。用分离式霍普金森压杆(split Hopkinson pressure bar, SHPB)装置对不同玄武岩纤维掺量的水泥基复合材料进行动态压缩和动态劈裂试验。结果表明:(1)在压、拉两种应力状态下,玄武岩纤维对水泥基复合材料的静态强度、动态强度均有增强,且高应变率下玄武岩纤维对抗压强度动态增幅较小,对劈裂强度动态增幅较大;(2) BF-ECC的抗压强度和劈裂强度均随应变率升高而显著提高,两者均可以采用动态增强因子(dynamic increase factor, DIF)反映动态强度的增幅,但劈裂强度的应变率敏感性强于抗压强度;(3)依据试验得到的普通水泥混凝土速率敏感性的CEB-FIP方程(2010)不适用于BF-ECCs。     

Isothermal extrusion of non-dilute fiber suspensions

The extrusion of a rod-like fiber suspension is a Newtonian solvent, as a first step to the fast and inexpensive production of composite materials, is investigated. The analysis is carried out by means of an integral constitutive equation for a non-dilute suspension, streamlined finite element for liquid with memory, and Newton iteration of nonlinear integro-differential equations. The predictions show substantial differences between dilute and nondilute fiber suspension regarding the processing conditions (pressure drop, velocity distribution, die-swell) and the resulting fiber orientation. Nondilute fiber suspensions exhibit substantial shear-thinning and negligible elasticity as evidenced by the small die-swell, and fiber concentration viscosity-thickening as evidenced by the large pressure drop. The fiber orientation is computed by solving the orientation distribution function along selected streamlines of the complex velocity field. It is shown that the fiber orientation far downstream can be made independent of the random fiber orientation at the inlet.     

Particle motions in sheared suspensions XXII: Interactions of rigid spheres (experimental)

Summary The behaviour of rigid spheres suspended in viscous liquids undergoing laminar shear flow was compared experimentally with the approximate theory developed earlier. Good agreement was obtained for a sphere near a wall and for equatorial interactions between two spheres of both equal and unequal diameters. In the region close to but not touching the wall, the theory byGoldman et al. accurately described the motion of a sphere. The prediction that a sphere touching the wall should stick to it could not be confirmed. A viewing device, designed to study motions in two planes simultaneously was used to make several interesting observations on permanent and separating doublet. Some similarities between collisions in the equatorial plane and two-body interactions of vertically oriented rigid circular cylinders were noted. The asymmetry and reversibility of triple collisions as well as the ease of formation of permanent doublets in flowing suspensions were established.

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Zusammenfassung Das Verhalten von starren, in viskosen Flüssigkeiten suspendierten Teilchen unter dem Einfluß von laminarem Scherfluß wurde experimentell mit der früher entwickelten, genäherten Theorie verglichen. Gute Übereinstimmung wurde erhalten für eine Kugel in Wandnähe und für äquatoriale Einflüsse zwischen zwei Kugeln von entweder gleichem oder verschiedenem Durchmesser. Die Theorie vonGoldmann et al. gab eine genaue Beschreibung der Bewegung einer Kugel in Wandnähe, aber ohne Wandberührung. Die Voraussage, daß eine Kugel an einer berührenden Wand kleben würde, konnte nicht bestätigt werden. Eine Beobachtungs-Einrichtung, die für das Studium der gleichzeitigen Bewegung in zwei Ebenen konstruiert war, wurde für verschiedene interessante Beobachtungen an permanenten und an sich trennenden Dubletts benutzt. Einige Änlichkeiten zwischen Zusammenstößen in der Äquatorial-Ebene und Zweikörperbeinflussungen von vertikal orientierten, starren kreisförmigen Zylindern wurde festgestellt. Die Asymmetrie und Reversibilität von dreifachen Stößen wie auch die leichte Bildung von permanenten Dubletts wurde im Fluß von Suspensionen festgestellt.

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This work was also supported by the National Heart Institute of the United States Public Health Service (Grant HE-05911).     

Shear rheometry and visualization of glass fiber suspensions

The nonlinear rheological behavior of short glass fiber suspensions has been investigated in this work by rotational rheometry and flow visualization. A Newtonian and a Boger fluid (BF) were used as suspending media. The suspensions exhibited shear thinning in the semidilute regime and weaker shear thinning in the transition to the concentrated one. Normal stresses and relative viscosity were higher for the BF suspensions than for the Newtonian ones presumably due to enhanced hydrodynamic interactions resulting from BF elasticity. In addition, relative viscosity of the suspensions increased rapidly with fiber content, suggesting that the rheological behavior in the concentrated regime is dominated by mechanical contacts between fibers. Visualization of individual fibers and their interactions under flow allowed the detection of aggregates, which arise from adhesive contacts. The orientation states of the fibers were quantified by a second order tensor and fast Fourier transforms of the flow field images. Fully oriented states occurred for shear rates around 20 s^− 1. Finally, the energy required to orient the fibers was higher in step forward than in reversal flow experiments due to a change in the spatial distribution of fibers, from isotropic to planar oriented, during the forward experiments.     

Particle motions in sheared suspensions XXI: Interactions of rigid spheres (theoretical)

Summary Based on a method developed byFaxén, approximate expressions were developed for the rotational and translational velocities of a single rigid sphere near a plane rigid wall bounding Couette flow, and near another sphere of either different or the same size in Couette flow. The solutions to the latter problem provide equations for the orbits of each sphere about the other, some of which are closed. In many respects the results are similar to the corresponding two-dimensional problems for cylinders.

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Zusammenfassung Angenäherte Gleichungen wurden aufgrund einer vonFaxén entwickelten Methode für die Rotations- und Translations-Geschwindigkeiten einer einzelnen, starren Kugel in der Nähe einer ebenen, starren Wand, die einenCouette- Fluß begrenzte, aufgestellt, und außerdem für eine zweite Kugel von gleicher oder verschiedener Größe imCouette-Fluß. Die Lösungen für dieses zweite Problem geben Gleichungen für die Bahnen jeder Kugel um die andere und einige davon stellen geschlossene Ausdrücke dar. In vieler Hinsicht sind die Resultate änlich wie für die entsprechenden zweidimensionalen Zylinder-Probleme.

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This work was supported in part by the National Heart Institute of the United States Public Health Service (Grant HE-05911).     

A study on validity of using average fiber aspect ratio for mechanical properties of aligned short fiber composites with different fiber aspect ratios

A closed-form solution using the actual distribution of the fiber aspect ratio is proposed for predicting the stiffness of aligned short fiber composite. The present model is the simplified form of Takao and Taya’s model and the extended version of Taya and Chou’s model, where Eshelby’s equivalent inclusion method modified for finite fiber volume fraction is employed. The validity of using average fiber aspect ratio for predicting the composite stiffness is justified in terms of the scatter of fiber aspect ratio, fiber volume fraction, and constituents‘ Young’s modulus ratio, comparing with the results by the present model. The guideline for selection of either the actual distribution or the average fiber aspect ratio is presented for the better prediction of the composite stiffness.     

高温后玄武岩纤维增强混凝土的动态力学特性

为研究温度、加载速率、纤维掺量对玄武岩纤维增强混凝土(BFRC)动态压缩强度和冲击韧度的影响,利用?100 mm分离式霍普金森压杆(SHPB)装置,对经历不同温度作用后的BFRC进行冲击加载实验。结果表明:高温后BFRC的动压强度及冲击韧度在同一温度下随平均应变率的上升近似线性增大;温度的升高总体上导致BFRC在同一加载速率下的动压强度及冲击韧度减小、应变率敏感性减弱;同一工况下,BFRC的动压强度和冲击韧度较素混凝土普遍提高,且当纤维体积掺量为0.2%时强韧化效果相对最佳。由此可见,高温后BFRC的冲击压缩特性受温度、加载速率、纤维掺量的综合作用影响,掺入玄武岩纤维可以有效降低高温后BFRC的损伤劣化程度。     

Simulation of fiber reinforced composite materials mold filling process and mechanical properties analysis

A dynamic simulation of fiber reinforced composite materials mold filling process with double inlets is presented based on the gas–solid–liquid model proposed by Yang et al. [B.X. Yang, J. Ouyang, J. Tao, C.T. Liu, Modeling and simulation of fiber reinforced polymer mold filling process by level set method, CMES – Computer Modeling in Engineering and Sciences 63 (3) (2010) 191–222]. Numerical results show that the fibers far away from the melt interface are in skin-core-skin structure, while those near the interface are almost parallel to the arc of the interface. When the two streams of melts meet, the weld line will be formed, where the orientation of fibers is perpendicular to the flow direction. The orientation of fibers of the numerical result shows well agreement with the experimental results. Finally, the mechanical properties of fiber reinforced composite materials are analyzed. The composite materials with skin-core-skin structure are regarded as laminated orthogonal plywood and the elastic modulus, the shear modulus and Poisson’s ratio are predicted under different slenderness ratios and fiber volume fractions.     

The flow of fiber suspensions in complex geometries

A continuum theory for dilute suspensions of large-aspect-ratio particles is applied to the flow of fiber suspensions through contractions. The theory, which incorporates the statistical orientation distribution function into the stress equation, predicts that the flow of dilute suspensions will differ qualitatively from the flow of the suspending fluid. The theory is in excellent agreement with experiments on the flow of suspensions of chopped-glass fibers through axisymmetric contractions, where substantial enlargement of the recirculating corner vortex is observed at volume fractions of 0.1% and less.     

Numerical prediction of fiber motion in a branching channel flow of fiber suspensions

Fiber orientation and dispersion in the dilute fiber suspension that flows through a T-shaped branching channel are simulated numerically based on the slender-body theory. The simulated results are consistent qualitatively with the experimental data available in the literature. The results show that the spatial distribution of fibers is dependent on the fiber aspect ratio, but has no relation with the volume fraction of fiber. The content ratio of fibers near the upper wall increases monotonically with an increasing Re number, and the situation is reverse for the region near the bottom wall. The orientation of fibers depends on Re number, however, the function of fiber volume fraction and aspect ratio is negligible. The fibers near the wall and in the central region of the channel align along the flow direction at all times, but the fibers in the other parts of the channel tend to align along the flow direction only in the downstream region.The project supported by the National Natural Science Foundation of China (10372090) and Doctoral Program of Higher Education in China (20030335001)The English text was polished by Ron Marshall     

Linear viscoelastic properties of emulsions and suspensions with thermodynamic and hydrodynamic interactions

We present a new approach to describe the rheological properties of dispersions with non-hydrodynamic interactions (steric, electrostatic and Van der Waals interactions) in the linear viscoelastic domain. Our model is based on the calculation of additional stresses resulting from interaction potentials between spheres and Brownian motion. We start from the statistical mechanical approaches which have been developed by Batchelor and Green and later Lionberger and Russel, to model the viscoelastic properties of emulsions and suspensions. We have extended their calculations to the more general case of viscoelastic deformable inclusions in a viscoelastic matrix. Our contribution lies in the computation of the hydrodynamic functions involved in the term describing interaction stresses. This computation is based on Palierne's results on the deformation field around a viscoelastic inclusion embedded in a viscoelastic matrix. We have also rewritten the conservation equation in the case of interest, over the whole frequency domain. We finally express the complex shear modulus of the dispersion as the sum of two terms : Palierne's complex shear modulus gives the purely hydrodynamic contribution; the interaction contribution depends on both the hydrodynamic properties and the interaction potential.     

碳纳米管纤维及其传感器力电性能实验研究

基于UTM T150微纳米力学测试系统搭建了纤维力电耦合测试平台,并对碳纳米管纤维进行了多组力电耦合作用下的拉伸性能实验研究。通过观察分析碳纳米管纤维的电阻-应变曲线,发现其电阻变化与应变大小密切相关,随着应变的不断变大,碳纳米管纤维的电阻也在逐渐增加。基于此,设计制作了碳纳米管纤维柔性应变传感器,并将其贴在标准试件上分别进行了弯曲变形测试、单次/循环加载性能测试以及标定实验。结果表明:当对试件进行加载时,碳纳米管纤维柔性应变传感器的电阻会随着载荷的增大而增大;当载荷逐渐消失时,其电阻会逐渐减小直至回归初始电阻值附近。这表明碳纳米管纤维具备优异的电学与力学特性,并且利用它所制作的柔性应变传感器也有着较好的灵敏度与稳定性。     

Dynamics of short fiber suspensions in bounded shear flow

We have studied the dynamics of non-colloidal short fiber suspensions in bounded shear flow using the Stokesian dynamics simulation. Such particles make up the microstructure of many suspensions for which the macroscopic dynamics are not well understood. The effect of wall on the fiber dynamics is the main focus of this work. For a single fiber undergoing simple shear flow between plane parallel walls the period of rotation was compared with the Jeffrey’s orbit. A fiber placed close to the wall shows significant deviation from Jeffrey’s orbit. The fiber moving near a solid wall in bounded shear flow follows a pole-vaulting motion, and its centroid location from the wall is also periodic. Simulations were also carried out to study the effect of fiber–fiber interactions on the viscosity of concentrated suspensions.     

The effects of micro-defects and crack on the mechanical properties of metal fiber sintered sheets

The effects of three types of defect (i.e., two micro defects—broken fibers and separation of fiber joints and one macro defect—crack) on the mechanical properties of porous metal fiber sintered sheets (MFSSs) are investigated by a combination of numerical simulation, analytical modeling, and experimental test. All simulations are based upon the previously developed micromechanics random beam model (Jin et al., 2013). Broken fibers are realized by removing cell edges (i.e., fibers between two joints) in an otherwise perfect model. Their induced decreases in the elastic moduli and strengths are found to be much lower than those of two dimensional (2D) foams and Kagome grids. For the defect in the form of separation of fiber joints, both analytical and numerical models are developed. The predicted linear decreases in the moduli and strengths (except for the compressive strength) with increasing number of separated fiber joints indicate that MFSSs be insensitive to the defect of joint separation. To explore the effect of crack, fracture toughness of MFSSs is measured and is found to be significantly higher than that of metal foams of the same relative density (i.e., volume fraction of the constituent solid material). The underlying ductile mechanism of MFSSs is further investigated by numerical simulations, showing that plastic deformation spreads all over the fibers in ligament rather than concentrates around crack tip. This study shows that MFSSs are superior in view of their resistance to the considered micro-defects and crack.     

A new Monte Carlo model for predicting the mechanical properties of fiber yarns

Understanding the complicated failure mechanisms of hierarchical composites such as fiber yarns is essential for advanced materials design. In this study, we developed a new Monte Carlo model for predicting the mechanical properties of fiber yarns that includes statistical variation in fiber strength. Furthermore, a statistical shear load transfer law based on the shear lag analysis was derived and implemented to simulate the interactions between adjacent fibers and provide a more accurate tensile stress distribution along the overlap distance. Simulations on two types of yarns, made from different raw materials and based on distinct processing approaches, predict yarn strength values that compare favorably with experimental measurements. Furthermore, the model identified very distinct dominant failure mechanisms for the two materials, providing important insights into design features that can improve yarn strength.     

Influence of processing parameters on fiber damage and mechanical properties of extruded composite with aluminum matrix and short carbon fibers

Undirectionally oriented short fiber composites have been prepared by hot extrusion from aluminum powder and carbon fiber mixtures. Effects of processing parameters on fiber damage and interfacial reaction have been studied. The quality of interfacial bonding is shown to strong influence the critical fiber length and mechanical properties of short fiber composite.     

Analysis of rheological properties of fiber suspensions in a Newtonian fluid by direct fiber simulation. Part 3: Behavior in uniaxial extensional flows

The present work presents a predictive direct fiber simulation of fiber suspensions under uniaxial extensional flows by Stokesian dynamics, considering both near- and farfield hydrodynamic intrafiber interactions and lubrication approximation for interfibers. Initially, code validation is performed and simulation predictions compared with experimental data. Afterwards, the orientation tensor under flow is discussed as a function of volume fraction, aspect ratio and flexibility. Based on the results, a general constitutive equation with a proposed parameter is proposed and verified by comparing experimental data of high volume fraction and high aspect ratio systems.     

A micro-polar theory for binary media with application to phase-transitional flow of fiber suspensions

A phase-transitional flow takes place during the filling stage by injection molding of short-fiber reinforced thermoplastics. The mechanical properties of the final product are highly dependent on the flow-induced distribution and orientation of particles. Therefore, modelling of the flow which allows to predict the formation of fiber microstructure is of particular importance for analysis and design of load bearing components. The aim of this paper is a discussion of existing models which characterize the behavior of fiber suspensions as well as the derivation of a model which treats the filling process as a phase-transitional flow of a binary medium consisting of fluid particles (liquid constituent) and immersed particles-fibers (solid-liquid constituent). The particle density and the mass density are considered as independent functions in order to account for the phenomenon of sticking of fluid particles to fibers. The liquid constituent is treated as a non-polar viscous fluid, but with a non-symmetric stress tensor. The state of the solid-liquid constituent is described by the antisymmetric stress tensor and the antisymmetric moment stress tensor. The forces of viscous friction between the constituents are taken into account. The equations of motion are formulated for open physical systems in order to consider the phenomenon of sticking. The chemical potential is introduced based on the reduced energy balance equation. The second law of thermodynamics is formulated by means of two inequalities under the assumption that the constituents may have different temperatures. In order to take into account the phase transitions of the liquid-solid type which take place during the flow process a model of compressible fluid and a constitutive equation for the pressure are proposed. Finally, the set of governing equations which should be solved numerically in order to simulate the filling process are summarized. The special cases of these equations are discussed by introduction of restricting assumptions.Received: 6 May 2002, Accepted: 16 December 2002, Published online: 29 July 2003PACS: 83.10.Ff, 83.70.Hg, 83.50.Cz Correspondence to: H. Altenbach