GRADUATE RESEARCH FELLOWSHIPS POSTDOCTORAL RESEARCH FELLOWSHIPS UNDERGRADUATE RESEARCH FELLOWSHIPS

A number of ( 1 ) Ph. D. graduate reseatrch fellowships, (2) postdoctoral fellowships, and (3) undergraduate research fellowships are available immediately in the area of neutron-diffraction materials research. Candidates with strong backgrounds in Materials Science, Metallurgy (including Processing, and Mechanical/Physical Behavior), Computational Materials Science,     

GRADUATE RESEARCH FELLOWSHIPS POSTDOCTORAL RESEARCH FELLOWSHIPS UNDERGRADUATE RESEARCH FELLOWSHIPS

A number of (1) Ph.D. graduate research fellowships, (2) postdoctoral fellowships, and (3) undergraduate research fellowships are available immediately in the area of neutron-diffraction materials research. Candidates with strong backgrounds in Materials Science, Metallurgy (including Processing, and Mechanical/Physical Behavior), Computational Materials Science,     

GRADUATE RESEARCH FELLOWSHIPS POSTDOCTORAL RESEARCH FELLOWSHIPS UNDERGRADUATE RESEARCH FELLOWSHIPS

A number of (1) Ph.D. graduate research fellowships, (2) postdoctoral fellowships, and (3)undergraduate research fellowships are available immediately in the area of neutron-diffraction materials research. Candidates with strong backgrounds in Materials Science, Metallurgy (including Processing, and Mechanical/Physical Behavior), Computational Materials Science,     

GRADUATE RESEARCH FELLOWSHIPS POSTDOCTORAL RESEARCH FELLOWSHIPS UNDERGRADUATE RESEARCH FELLOWSHIPS

~~GRADUATE RESEARCH FELLOWSHIPS POSTDOCTORAL RESEARCH FELLOWSHIPS UNDERGRADUATE RESEARCH FELLOWSHIPS$Materials Science and Engineering The University of Tennessee,Knoxvlle     

Water Vapor Sorption in Cementitious Materials—Measurement,Modeling and Interpretation

The rate and extent of uptake and release of moisture are critical in controlling the behavior of cementitious materials ranging from fluid transport to hygral deformations. While classically determined using an equilibrium (static) salt solution method (Baroghel-Bouny in Cem Concr Res 37:414–437, 2007), advanced capabilities offered by gravimetric dynamic vapor sorption (DVS) analyzers, are now permitting acquisition of sorption spectra at microgram ( $\upmu \hbox {g}$ ) resolution on the order of a few weeks. This work highlights new multicycle determinations of adsorption/desorption isotherms, acquired using a custom-built DVS analyzer for well-hydrated alite and ordinary portland cement pastes over a range of water-to-solid ratios ( $w/s$ , mass basis). Special focus is paid to describe measurement aspects relevant to acquiring reliable spectra, and their interpretation. Sorption isotherms are used to assess transport properties, and sorption hysteresis and its irreversibility following first drying. Based on an optimization-based criterion, the Young-Nelson model is selected to simulate sorption evolutions, including the effects of hysteresis. Sensitivity analyses carried out using this model are used to understand the role of parameters, including porosity and $w/s$ , on the hysteresis that develops from the first to subsequent sorption cycles.     

The automatic control of boundary-layer transition — Experiments and computation

In recent years there has been an increasing interest in the control of boundary-layer transition through the use of wall suction. In the current work suction is provided through one or more suction panels situated close to the leading edge of a plate. Experiments show that boundary-layer pressure fluctuation measurements can be used to identify the position of transition. Transition can be maintained at a desired location with minimum power consumption by employing an automatic adaptive feedback control loop which regulates the suction flow rates of two independent suction panels. This can be expressed as a constrained optimization problem. To allow the suction flow rates to be updated, a modified least mean squares algorithm is used within the control loop. Experimental measurements show that the control algorithm allows fast and stable convergence towards the optimum suction distribution for a double suction panel configuration. Numerical simulations have also been performed. The two-dimensional boundary layer was calculated allowing the viscous boundary layer to interact with the inviscid outer flow. Following linear stability theory the spatial growth rates are calculated by solving an Orr-Sommerfeld type eigenvalue problem, with the streamwise location of transition predicted via thee ^N -method. Applying the same optimization strategy as in the experiments, good qualitative agreement between computations and experiments was found. The optimization algorithm has been applied to computer models where the relation between suction flow rates and transition location is described by an empirical analytical function. This shows that the controller can in principle be applied to systems with more than two suction panels.Nomenclature b transition location with zero suction - d desired transition location - e(k) error signal - k iteration index - p rms pressure - p _ref reference rms pressure - r sum of the reference pressure - u streamwise velocity - u _e external velocity - inviscid external velocity - A wave amplitude - F( ) cost function - I identity matrix - N maximum amplification factor - P projection matrix - R Reynolds number - Re Reynolds number based on the boundary-layer thickness - R matrix of weights - Tu turbulence level - vector of suction flow rates - v normal velocity - v _wall suction velocity at the surface - x streamwise coordinates - x _m microphone location - x _T(k) measured transition location - y normal coordinate - y(k) sum of the measured pressures - w(k) noise - plate length - r +i _i - free stream velocity - * displacement thickness - gradient vector - Lagrange multiplier - controller gain - disturbance stream function - disturbance amplitude - wave frequency = complex wave number     

The Shallow Arch—General Buckling,Postbuckling, and Imperfection Analysis∗

A general discussion of the behavior of the shallow circular arch is presented. It is shown that, irrespective of specific loading or boundary conditions, it is possible to arrive at general conclusions regarding buckling, postbuckling, and imperfection sensitivity. General methods of analysis are established which lead to the determination of points of bifurcation and of postbuckling paths under symmetric loads. Modifications accounting for antisymmetric load components are introduced, with special emphasis on their asymptotic and limit load effect.

A typical numerical example is carried through in detail. The solution is “exact” in the sense of shallow arch theory. Its asymptotic behavior conforms to the asymptotic theory of Koiter.     

Ageing and collapse of bentonite gels—effects of Li,Na, K and Cs ions

The state of bentonite gels at the start of the ageing experiment must be well-defined, and this required the gels to be at a constant surface chemistry condition. This is achieved by allowing the freshly prepared gels to rest for a day. At this state, the yield stress is constant, provided that the gel is at an equilibrium breakdown state after stirring prior to each measurement. This point is also the yield stress at zero aged time. Ageing study then commenced, and the behaviour is generally characterised by an increasing yield stress with wait time. Alkali metal ion type and concentration affect the gel ageing and stability behaviour significantly. The ageing behaviour is most pronounced at low salt concentrations for the smallest and most strongly hydrated cations, Li ⁺and Na ⁺. The yield stress at any given aged time and its rate of increase are generally larger. Coarsening of these suspensions was observed. The opposite is true for the weakly hydrated K ⁺and Cs ⁺ions. At high concentrations of 0.5 and 1.0 M Cs, K and Na ions, the gels became unstable over time and phase-separated. The stability time of these weak gels was found to increase with decreasing cation size, Na > K > Cs. This stability time displayed a very strong quantitative correlation with the hydration bond length. Coarsening was also expected, but not observed due to the lack of integrity of these weak aggregates during particle size measurement. The recovery or ageing behaviour was fitted with both the Nguyen–Boger and Leong models.     

Reconciliation for Monoclinic Materials of the Linear,Elastically Isotropic Plate Theories of Reissner and Ladevèze

Reissner’s well-known sixth-order linear theory for the bending of elastically isotropic plates, here extended to elastically monoclinic materials, is shown to be a compatible precursor to a recent extension to monoclinic materials of Ladevèze’s exact fourth-order plate theory. With a solution of Reissner’s extended equations in hand, Ladevèze’s extended equations reduce to three equations for three unknowns, two additional, previously unknown functions now being determinant. The key link is the requirement that the stress resultants and couples of the two theories agree.     

The Relationship between Structural Model Design Competition and Innovation on Mechanics—Structure Course Groups in Applied Civil Engineering

介绍了结构模型设计制作的基本过程, 阐述了开展模型竞赛与土木工程专业应用型人才体系的关系. 在此基础上, 从强化实践力学$\!$-$\!$-$\!$结构课程体系、改革课程内容、设置开放性实践环节等方面阐述了构建创新性、应用型人才培养体系的思路, 并通过教学实例加以分析.     

On the cover of this issue: “Solid Cylinder Torsion for Large Shear Deformation and Failure of Engineering Materials” by W-Y. Lu et al.

Experimental Mechanics -     

The phenomena of shock wave reflection — a review of unsolved problems and future research needs

Although the phenomenon of shock wave reflection was discovered more than a hundred years ago, active research related to this phenomenon still goes on in many countries in the world (e.g., Australia, Canada, China, Germany, Israel, Japan, Poland, Russia and United States of America). As a matter of fact the research activity increased so drastically in the past decade and a half that a special scientific meeting dedicated to better understanding the reflection phenomena of shock waves, namely The International Mach Reflection Symposium was initiated in 1981 and was held since then in the major research centers actively involved in the research of shock wave reflections. In the present paper the status of the research of the phenomenon of shock wave reflection will be discussed in general, and unresolved problems and future research needs will be pointed out.This article was processed using Springer-Verlag T_EX Shock Waves macro package 1.0 and the AMS fonts, developed by the American Mathematical Society.     

The effect of microstructural morphology on the elastic,inelastic, and degradation behaviors of aluminum–alumina composites

Micromechanical models with idealized and simplified shapes of inhomogeneities have been widely used to obtain the average (macroscopic) mechanical response of different composite materials. The main purpose of this study is to examine whether the composites with irregular shapes of inhomogeneities, such as in the aluminum–alumina (Al–Al₂O₃) composites, can be approximated by considering idealized and simplified shapes of inhomogeneities in determining their overall macroscopic mechanical responses. We study the effects of microstructural characteristics, on mechanical behavior (elastic, inelastic, and degradation) of the constituents, and shapes and distributions of the pores and inclusions (inhomogeneities), and thermal stresses on the overall mechanical properties and response of the Al–Al₂O₃ composites. Microstructures of a composite with 20% alumina volume content are constructed from the microstructural images of the composite obtained by scanning electron microscopy (SEM). The SEM images of the composite are converted to finite element (FE) meshes, which are used to determine the overall mechanical response of the Al–Al₂O₃ composite. We also construct micromechanics model by considering circular shapes of the inhomogeneities, while maintaining the same volume contents and locations of the inhomogeneities as the ones in the micromechanics model with actual shapes of inhomogeneities. The macroscopic elastic and inelastic responses and stress fields in the constituents from the micromechanics models with actual and circular shapes of inhomogeneities are compared and discussed.     

The compressible Coanda wall jet—an experimental study of jet structure and breakaway

An underexpanded jet issuing from a convergent slot and blowing over a surface of convex streamwise curvature was studied experimentally. The jet was confined between side walls, with the slot aspect ratio varying between 40 and 6, but tests showed that in the area of interest close to the slot the flow was effectively two-dimensional. The ratio of slot width to the radius of curvature of the downstream surface varied between 0.05 and 0.33. The main techniques used were Schlieren and shadowgraph to show the jet structure, and surface flow visualization which revealed areas of separation and reattachment. Surface static pressures were also measured on the curved surface. The curved jet proved to have a shock cell structure similar to that of a plane jet. However, the cell structure disappeared more rapidly as the outer shear layer grew more quickly due to the destabilizing effect of the curvature on the turbulence in the shear layer. Even at modest upstream jet pressures, a separated region on the Coanda surface became evident. This region was characterized by a stagnant constant pressure part followed by a region of strongly reversed flow before reattachment took place. The separation was caused by the compression at the end of the first shock cell, with reattachment taking place where expansion in the second cell started. The separated region grew rapidly as the upstream pressure was increased, until, finally, reattachment failed to occur and the jet suddenly broke away from the surface. This work is related to studies of the Coanda flare, where the jet is axisymmetric. The high level of turbulence causes rapid entrainment of air and so gives us clean combustion. However there should be more general application to devices that use the Coanda effect, varying from fluidic devices to blown jet flaps on wings.     

ACTA MECHANICA SOLIDA SINICA——The Chinese Society of Theoretical and Applied Mechanics Vo1.16,No.1-No.4,2003 CONTENTS

Uneoliventional Gurtin一tyPe variational PrineiPles for firlite deformation elasto- dynalllics ........................................……Luo En,Zllu Huijianl一1A decagonalquasicl’ySta1 with an arc一ShaPed crack……Wdng Xu,Zhong Zhengl一8Propagation of、urfaee aeoustie waves in pre、tressed anisotropie layered Piezo- eleetrie strlle七ures ……Liu,Hua.Kuang Zhenbang,Cai Zhengnlillg认bng Tiejun,、Vang Zikunl一16Gurtin variational PI,121亡iPle and finite element 51仃lulation for dyna…