Computational studies on high-stiffness,high-damping SiC–InSn particulate reinforced composites

With the objective of achieving composite material systems that feature high stiffness and high mechanical damping, consideration is given here to unit cell analysis of particulate composites with high volume fraction of inclusions. Effective elastic properties of the composite are computed with computational homogenization based on unit cell analysis. The correspondence principle together with the viscoelastic properties of the indium–tin eutectic matrix are then used to compute the effective viscoelastic properties of the composite. Comparison is made with parallel experiments upon composites with an indium–tin eutectic matrix and high volume fractions of silicon-carbide reinforcement. The analytical techniques indicate that combinations of relatively high stiffness and high damping can be achieved in particulate composites with high SiC volume fractions. Based on analysis, the tradeoffs between stiffness and damping characteristics are assessed by changing the volume fraction, size, packing, and gradation of the particulate reinforcement phases. Practical considerations associated with realization of such composites based on the surface energy between the SiC and the InSn are discussed.     

Fracture of composite laminates by three-point bend

An experimental investigation has been carried out on the fracture behavior of T300/5208 graphite/epoxy laminates using three-point bend and tension tests. One laminate, a [0/±45/90] _ns orientation, was used withn=1 for tension specimens andn=4 and 8 for bend specimens. The crack-growth-resistance curve was constructed by compliance matching from crack-opening displacement and load data obtained from three-point bend tests. The value of effective crack length and corresponding crack resistance at onset of instability was nearly constant and independent of the initial crack length and laminate thickness. Experimental data from both bend and tension tests correlated reasonably well to the average-stress-failure criterion. Data were also compared with results found in the literature.     

Slip between the phases in two-phase water–oil flow in a horizontal pipe

This paper is devoted to slip phenomenon between the phases that occurs in unstable two-phase water–oil flow systems in a horizontal pipe. The emphasis is placed on the relation between the slip and the real (in situ) water fraction in a flowing mixture, as well as the substitute physical properties of the whole two-phase system. The experimental data collected throughout research served for the evaluation of the accuracy of the methods of real phase fraction in a water–oil flow system in horizontal pipes as they were referred to in the bibliography. Subsequently we have suggested the author indicate a method of determination of the fraction for two-phase liquid systems like O/W, W/O and W + O. In order to establish the specific equations, the drift-flux model has been used here.     

The influence of fiber length and fiber orientation on damping and stiffness of polymer composite materials

This paper describes the theoretical analysis, the experimental results and the curve-fitting of the analytical model to the experimental results on the influence of fiber length and fiber orientation on damping and stiffness of polymer-composite materials. The experimental results show that, as predicted, very low fiber aspect ratios are required to produce significant improvements in damping. Measurements and predictions also indicate that the control of lamina orientation in a continuous fiber-reinforced laminate may be a better approach to the improvement of damping than the control of the fiber aspect ratio. Paper was presented at 1985 SEM Spring Conference on Experimental Mechanics held in Las Vegas, NV on June 9–14, 1985.     

Overall electromechanical properties of a binary composite with 622 symmetry constituents.: Antiplane shear piezoelectric state

A binary composite is studied here, where the electroelastic properties of the constituent materials belong to the crystal class 622. A square arrangement of long continuous circular cylinders, the fiber phase, embedded in a homogeneous medium is consider here. The composite is in a state of antiplane shear piezoelectricity, that is, a coupled state of out-of-plane mechanical displacement and in-plane electric field, which is characterized by three electroelastic parameters: longitudinal shear modulus, shear stress piezoelectric coefficient and transverse dielectric constant. Our interest here lies in the determination of its effective properties. They are derived by means of the method of two spatial scales. Closed-form expressions are obtained for them. Only one of the four local (or canonical) problems that arise is needed. Two properties are thus found. The Milgrom–Shtrikman compatibility relation is used to fix the remaining one. The local problem is solved using potential methods of a complex variable. The solution involves doubly periodic Weierstrass elliptic and related functions. The final formulae for the overall properties show explicitly the dependence on (i) the properties of the phases, (ii) the radius of the cylindrical fiber and (iii) the lattice sums associated with the square array. The shear modulus is shown to depend explicitly not only on the rigidity of the phases but also on their piezoelectric and dielectric coefficients. Some natural organic substances have the symmetry 622 like collagen. Recently Silva et al. measured its electroelastic properties. Their data is used to show some numerical results of the derived formulae as a function of the fiber volumetric fraction.     

Material microstructure optimization for linear elastodynamic energy wave management

We describe a systematic approach to design material microstructures to achieve desired energy propagation in a two-phase composite plate. To generate a well-posed topology optimization problem we use the relaxation approach which requires homogenization theory to relate the macroscopic material properties to the microstructure, here a sequentially ranked laminate. We introduce an algorithm whereby the laminate layer volume fractions and orientations are optimized at each material point. To resolve numerical instabilities associated with the dynamic simulation and constrained optimization problem, we filter the laminate parameters. This also has the effect of generating smoothly varying microstructures which are easier to manufacture. To demonstrate our algorithm we design microstructure layouts for tailored energy propagation, i.e. energy focus, energy redirection, energy dispersion and energy spread.     

Hysteretic heat transfer study of liquid–liquid two-phase flow in a T-junction microchannel

Liquid–liquid two-phase flow in microchannels is capable of boosting the heat removal rate in cooling processes. Formation of different two-phase flow patterns which affect the heat transfer rate is numerically investigated here in a T-junction containing water-oil flow. For this purpose, the finite element method (FEM) is applied to solve the unsteady two-phase Navier–Stokes equations along with the level set (LS) equation in order to capture the interface between phases. It is shown that the two-phase flow pattern in microchannels depends on the flow initial condition which causes hysteresis effect in two-phase flow. In this study, the hysteresis is observed in flow pattern and consequently in the heat transfer rate. The effect of wall contact angle on the hydrodynamics and heat transfer in the microchannel is investigated to gain useful insight into the hysteresis phenomenon. It is observed that the hysteresis is significant in super-hydrophilic microchannels, while it disappears at the contact angle of 75^°. The effect of water to oil flow rate ratio (Q_wat/Q_oil) on the heat transfer is also studied. The flow rate ratio has a negligible effect on the Nusselt number (Nu) in the dripping regime, while the Nu decreases with an increase of Q_wat/Q_oil in the co-flow regime. The thickness of the oil film, velocity, and temperature distribution are studied in the co-flow regime. It is revealed that the normalized slip velocity reduces at higher values of Q_wat/Q_oil, which causes a reduction in the averaged Nu. In dripping regimes, higher flow rate ratios lead to a more frequent generation of droplet/slugs at a smaller size. The passage of the slugs or droplets increases the local Nu. Larger droplets generated at lower flow rate ratios cause a larger increase in the local Nu than smaller droplets. The temperature and velocity field around the droplets are also illustrated to investigate the heat transfer improvement. The generated vortex at the tip of the oil jet causes an increase in the velocity and Nu on the water side.     

An investigation of thermodynamic states during high-pressure fuel injection using equilibrium thermodynamics

A numerical investigation of mixing processes between an injected fuel (an n-alkane) and a chamber inert gas (nitrogen) was carried out for high-pressure fuel injection. The objective is to determine conditions for the coexistence of both liquid and gas phases under the typical ambient conditions encountered in diesel engines. A phenomenological investigation was built by coupling phase stability analysis with the energy conservation equation. Phase changes (including separation and combination) are predicted to occur so as to yield the lowest Gibbs free energy. It is also shown that predicted states without considering phase transitions can be very different from the corresponding thermodynamically correct states. By comparing four n-alkane/nitrogen mixtures it is shown that the lower limit of the two-phase region occurs at similar temperatures. However, heavy n-alkane/nitrogen mixtures have a larger upper limit, and phase separation occurs at higher temperatures. The present model predicts the existence of multiple phases locally in the dense spray jet under high temperature and pressure ambient conditions due to the significant reduction of the mixture temperature caused by vaporization and cooling.     

Effective properties of fiber and platelet systems and related phase arrangements in n-phase heterogenous media

For the structures which are neither aggregates nor inclusion-reinforced matrices, relevant property estimates are missing. We address composites with several, all or none co-continuous phases. Property estimates from various fibre systems and platelet systems that are likely to well approach the effective properties of such microstructures are examined. We provide arguments in favour of the relevancy of some of the investigated systems to represent phase co-continuity (or co-discontinuity), the reported results being supported by theoretical, topological and experimental considerations. The discussion is held in the linear elasticity framework with open extensions to other properties and behaviours.     

AN EXPERIMENTAL STUDY ON THE FLUIDELASTIC FORCES ACTING ON A SQUARE TUBE BUNDLE IN TWO-PHASE CROSS-FLOW

A tube in a square tube bundle of P/D=1·42 was oscillated in the lift direction in air–water two-phase cross-flow, and fluidelastic forces acting on the oscillated tube were measured. First, the tube amplitude was fixed to 3 mm (=0·136 D), and added mass, damping, and stiffness coefficients were obtained as a function of two-phase mixture characteristics such as nondimensional gap velocity and void fraction. When reference mixture density and velocity were estimated, the drift–flux model, in which the relative velocity between the gas and liquid phases was estimated, generated better results than the homogeneous model. The added mass coefficient was obtained from quiescent two-phase flow as a function of void fraction. Using the added mass coefficient, the added stiffness coefficient converged to zero with decreasing nondimensional gap velocity. This overcame the contradiction in the added stiffness estimation without added mass, in which the added stiffness coefficient did not converge to zero with decreasing nondimensional gap velocity. Next, the effects of the vibration amplitude on the fluidelastic force coefficients were considered. When the tube amplitude was 3 mm (=0·136 D) or less, the equivalent added stiffness and damping coefficients were almost constant and nonlinearity was small. This showed the validity of the fluidelastic force coefficients obtained based on the data of amplitude of 3 mm. The linearity did not exist when the tube displacement amplitude was 4·5 mm (=0·205 D) or more; a remarkable nonlinearity appeared in the equivalent added damping coefficient. A method to estimate the limit-cycle amplitude of the fluidelastic vibration was proposed when only one tube in the tube bundle was able to vibrate in the lift direction. The amplitude could be obtained from the amplitude at which the equivalent added damping coefficient changed from negative to positive with increase in the tube amplitude.     

An explicit formulation of a three-dimensional material damping model with transverse isotropy

A constitutive three-dimensional (3D) damping model is derived for transversely isotropic material symmetry, using the augmented Hooke's law [Intl. J. Solids Struct. 32 (1995) 2835] as a starting point. The proposed material model is tested numerically, via finite-element techniques, on a laminate structure built from stacked aluminium and Plexiglas plates. Effective 3D transversely isotropic material properties are given in terms of homogeneous material damping functions in connection with homogenised elastic laminate properties. Comparisons made between the results from the elastic (undamped) eigenvalue problem of the detailed (layerwise) model of the laminate and the effective 3D elastic model show that the homogenised model is reasonably accurate, in terms of predicted elastic eigenfrequencies for the first 20 modes. The dynamic homogenisation process, with damping included, is evaluated in terms of forced vibration response for the laminate structure, using effective transversely isotropic frequency dependent material properties. The dynamic 3D effective homogeneous material model is found to simulate very closely the detailed model in the studied frequency interval for the numerical test case.     

Onset of macroscopic instabilities in fiber-reinforced elastomers at finite strain

In this paper, we investigate theoretically the possible development of instabilities in fiber-reinforced elastomers (and other soft materials) when they are subjected to finite-strain loading conditions. We focus on the physically relevant class of “macroscopic” instabilities, i.e., instabilities with wavelengths that are much larger than the characteristic size of the underlying microstructure. To this end, we make use of recently developed homogenization estimates, together with a fundamental result of Geymonat, Müller and Triantafyllidis linking the development of these instabilities to the loss of strong ellipticity of the homogenized constitutive relations. For the important class of material systems with very stiff fibers and random microstructures, we derive a closed-form formula for the critical macroscopic deformation at which instabilities may develop under general loading conditions, and we show that this critical deformation is quite sensitive to the loading orientation relative to the fiber direction. The result is also confronted with classical estimates (including those of Rosen) for laminates, which have commonly been used as two-dimensional (2-D) approximations for actual fiber-reinforced composites. We find that while predictions based on laminate models are qualitatively correct for certain loadings, they can be significantly off for other more general 3-D loadings. Finally, we provide a parametric analysis of the effects of the matrix and fiber properties and of the fiber volume fraction on the onset of instabilities for various loading conditions.     

FRP片材在土建修复加固工程中应用的力学问题

纤维增强复合材料(FRP)片材由于其优异的力学性能和施工的便捷性而备受国内外土木建筑领域的高度关注,并得到了迅速的推广应用.本文结合国内外的研究现状及本课题组的研究工作,对FRP片材(纤维布、纤维板和纤维薄板)在土木建筑修复加固工程中应用的主要力学问题-FRP片材加固钢筋混凝土(RC)构件的静力学性能、疲劳性能、界面的力学性能、以及构件的耐久性等进行综述和讨论,并试图指出今后有关FRP片材修复加固RC构件力学性能的研究方向.     

Numerical simulation of fluid–particle hydrodynamics in a rectangular spouted vessel

A three-dimensional, Eulerian simulation was developed to describe isothermal, two-phase flow of the continuous (water) and dispersed (solid particles) phases in a rectangular spouted vessel. The mass and momentum conservation equations for each phase were solved using the finite volume technique, which treats each phase separately, while coupling them through drag, turbulence, and energy dissipation due to particle fluctuations. Particle–particle interactions via friction were also included.     

变角度纤维复合材料环扇形层合板的自由振动分析

与传统的直线纤维增强复合材料相比,变角度纤维复合材料具有更强的可设计性,为改善结构性能提供了更大的可能．鉴于此,本文将研究纤维的变角度铺设对复合材料环扇形层合板的自振频率及振动模态的影响．假设纤维的方向角沿环扇形板的径向线性变化,基于经典的层合板理论,采用微分求积法获得了环扇形层合板自由振动问题的数值解．通过与现有文献及ABAQUS有限元结果的比较验证了本文模型及方法的正确性和收敛性,并详细分析了纤维起始角和终止角的变化对层合板的自振频率及振动模态的影响．研究结果表明：与常刚度层合板相比,变角度纤维复合材料层合板的基频具有更大的调整空间,通过合理选择纤维起始角和终止角可有效提高层合板的基频．研究结果可为该种新型复合材料结构的优化设计提供一定的参考．     

Effect of transverse cracks on the mechanical properties of angle-ply composites laminates

A modified shear lag analysis, taking into account the notion of stress perturbation function, is employed to evaluate the effect of transverse cracks on the stiffness reduction in [±θ_n/90_m]_S angle-ply laminated composites. Effects of number of 90° layers and number of ±θ layers on the laminate stiffness have also been studied. The present results represent well the dependence of the degradation of mechanical properties on the fibre orientation angle of the outer layers, the number of cracked cross-ply layers and the number of uncracked outer ±θ layers in the laminate.     

Small-diameter parallel plate rheometry: a simple technique for measuring rheological properties of glass-forming liquids in shear

The rheological characterization of glass-forming liquids is challenging due to their extreme temperature dependence and high stiffness at low temperatures. This study focuses on the special precautions that need to be taken to accommodate high sample stiffness and torsional instrument compliance in shear rheological experiments. The measurement errors due to the instrument compliance can be avoided by employing small-diameter parallel plate (SDPP) rheometry in combination of numerical instrument compliance corrections. Measurements of that type demonstrate that accurate and reliable rheological data can be obtained by SDPP rheometry despite unusually small diameter-to-gap (d/h) ratios. Specimen preparation for SDPP requires special attention, but then experiments show excellent repeatability. Advantages and some current applications of SDPP rheometry are briefly reviewed. SDPP rheometry is seen as a simple and versatile way to measure rheological properties of glass-forming liquids especially near their glass transition temperature.     

大开口复合材料层合板强度破坏研究

复合材料层合板的各向异性及非均质,使得复合材料层合板内部的破坏形式非常复杂.在复合材料结构的设计中,为满足制造及使用功能上的需求,在复合材料层合板承力结构件上不可避免地需要设计各种开口.然而,含大开口复合材料层合板的强度破坏问题变得更为复杂,使得现有的强度理论面临新的挑战.针对碳纤维增强复合材料大开口层合板受单向拉伸载荷作用下的强度破坏问题进行了数值分析和实验研究.首先,根据Hashin准则和刚度退化模型,对含不同圆形开口尺寸的[0]_(10)单向铺层、[0/90]_5和[±45]_5正交铺层的层合板,进行了单向拉伸载荷作用下渐进失效的数值模拟分析,获得了对应结构的极限载荷和破坏模式.在此基础上,采用数字图像相关方法,进行复合材料大开口层合板强度破坏的实验研究.研究结果表明,大开口复合材料层合板在单向拉伸加载下主要呈现脆性破坏形式,破坏起始位置处于应力集中区.此外,破坏强度和失效模式与复合材料铺层方式和开口尺寸大小密切相关.其中[±45]_5铺层的开口层合板承载能力最弱,分层破坏最严重.开口尺寸越大,结构的极限载荷值越低.同实验测试结果相比,数值模拟对复合材料层合板的损伤失效分析略显不足,往往很难全面分析复合材料层合板破坏失效过程中的各种因素的影响.     

基于铺层参数的铺层方式与纤维分布协同优化的层合板最大刚度设计

纤维增强复合材料层合板的弹性性质依赖于单层板的纤维含量(体分比)以及铺层方式(总层数、各单层的厚度与铺设方向)。本文研究在给定材料用量条件下层合板的最大刚度设计问题,采用铺层参数作为铺层方式的描述参数、以铺层参数和单层板纤维含量体分比在层合板面内的分布的描述参数为设计变量,以层合板的柔顺性最小为目标,建立了铺层方式和纤维分布协同优化的层合板最大刚度设计问题的提法和求解方法,给出了具有最大刚度的层合板最优铺层方式和纤维含量的分布规律的设计实例。     

含椭圆孔的变角度复合材料层合板的屈曲性能研究

本文利用变角度复合材料的纤维方向角可沿平面位置任意连续变化的特点,提出在孔附近采用与孔同心的椭圆曲线作为纤维铺设路径的层合板铺层方案,以改善含椭圆孔的层合板的孔边应力集中,进而提高层合板的抗屈曲性能．主要研究内容有：利用ABAQUS软件分析本文提出的孔边特殊铺层方式下变角度复合材料层合板的面内应力分布及屈曲性能,通过与传统直线铺层方式以及线性变角度铺层方式进行比较,说明了本文提出的新铺层方式的优越性,并详细分析了椭圆孔的离心率、开孔尺寸及开孔方位对层合板的屈曲临界荷载的影响．研究结果可为含椭圆孔的变角度复合材料层合板的结构设计和优化提供一定的参考．