FE-investigation of shear localization in granular bodies under high shear rate

Shear localization in granular materials under high shear rate is analysed with the finite element method and a micro-polar hypoplastic constitutive model enhanced by viscous terms. We consider plane strain shearing of an infinitely long and narrow granular strip of initially dense sand between two very rough walls under conditions of free dilatancy. The constitutive model can reproduce the essential features of granular materials during shear localization. The calculations are performed under quasi-static and dynamic conditions with different shear rates. In dynamic regime, the viscosity terms are formulated based on a modified Newtonian fluid and according to the formula by Stadler and Buggisch (Proceedings of the conference on Reliable flow of particulate solids, EFCE Pub. Series, vol 49. Chr. Michelsen Institute, Bergen, 1985). Emphasis is given to the influence of inertial and viscous forces on the shear zone thickness and mobilized wall friction angle.     

Continuous shear thickening and discontinuous shear thickening of concentrated monodispersed silica slurry

Concentrated slurry is known to exhibit shear thickening behavior, in which viscosity increases as the shear rate ascends. However, to identify the differences between this shear thickening behavior and rapid increases in viscosity (such as the dilatancy behavior of starch, sand, and other concentrated slurries) and the smooth increases in viscosity exhibited by concentrated slurries, this research investigated the rheological behavior of a polyethylene glycol suspension containing monodispersed silica particles with a size of 2.5 μm. The results found that the increases in viscosity as the shear rate ascends or the increases in elasticity as the strain ascends change consecutively from smooth and reversible behavior (i.e., continuous shear thickening (CST)) to rapid and irreversible behavior (i.e., discontinuous shear thickening (DST)) simply by increasing the concentration of the slurry, even in the case of concentrated slurries comprising of the same particles. DST is a manifestation of dilatancy in which a jamming transition occurs due to collisions between particles. Because CST transitions successively to DST, and the on-set shear stress of shear thickening of CST is almost the same with that of DST, shear thickening in the CST region may, therefore, also be regarded as the result of friction due to collisions between particles. This supports the research by Seto, Mari, Poon et al., which stated that shear thickening occurs due to friction from particle collisions.     

Interlaminar shear strength measurement of ceramic composites by asymmetric four point bend shear test

Asymmetric four-point bend test is used for the measurement of interlaminar shear strength of ceramic composite. It is shown by finite element analysis that the method provides a reasonably good approximation to a constant shear state in the sample. The use of very small samples makes this method very attractive, especially during the material development stage, when larger samples may be impractical or impossible to fabricate. This technique is also useful when bigger samples would be prohibitively expensive or impractical.     

Adiabatic shear banding under pure shear loading Part II: fractographic and metallographic observations

A metallographic and fractographic investigation was performed of dynamically loaded torsion specimens of VAR 4340 steel (HRC 40). Macroscopically, it was found that the shear bands propagated on several distinct planes around the circumference of the specimen. Microscopically, scanning electron microscope observations of the fracture surfaces reveal that microvoid nucleation and growth is the mechanism leading to shear band fracture. Patches of highly smeared and knobbly material, produced by sliding of the fracture surfaces against each other, were also found at several locations and suggest very high local temperatures. Metallographic cross sections revealed white-etching material, characteristic of transformed bands, only at limited locations, corresponding to regions of the fracture surface exhibiting a highly smeared or knobbly texture.These results indicate that microvoid nucleation and growth play a significant role in adiabatic shear banding and that one of the mechanisms for the formation of transformed bands is the extensive sliding and rubbing of the shear fracture surfaces against each other. Thus transformed bands may be, at least in some cases, a consequence rather than a cause of the final shear band fracture.     

Relation between shear strength at the fibre-matrix interphase and shear properties of resin matrix

The shear yield strength and the shear strength of a resin matrix increase almost linearly as the logarithm of the strain rate increases. This increasing tendency is almost the same at various temperatures. The strain rate temperature superposition held and an experimental equation was found to estimate the strain rate and temperature dependence of these shear properties. The strain rate and temperature dependence of the shear yield strength at the fibre-matrix interphase can be also estimated by the same equation. A strong quantitative relation was observed between the strain rate and temperature dependence of the shear properties of a resin matrix and that of the shear yield strength at the fibre-matrix interphase.     

DEM study of the shear behavior and formation of shear band in biaxial test

This paper aims to study the shear behavior of granular matter by DEM simulations. Granular samples are prepared by automatic clump generation algorithm to create particles of irregular shapes. Simulations of the biaxial test with membrane boundary condition are used to test the shear behavior of samples. A new method for computing sample volume in membrane boundary condition is proposed. Deviatoric stress and volumetric strain curves are plotted to describe contracting-dilatancy of granular materials during the shearing stage. Formation of the shear band is studied from particle rotation and particle displacement fields. The influence of confining pressure, initial porosity, and friction coefficient on the development of shear band are studied. Lower confining pressure, higher initial porosity can be resulted in later formation of shear bands.     

Quantitative imaging of nonlinear shear modulus by combining static elastography and shear wave elastography

The study of new tissue mechanical properties such as shear nonlinearity could lead to better tissue characterization and clinical diagnosis. This work proposes a method combining static elastography and shear wave elastography to derive the nonlinear shear modulus by applying the acoustoelasticity theory in quasi-incompressible soft solids. Results demonstrate that by applying a moderate static stress at the surface of the investigated medium, and by following the quantitative evolution of its shear modulus, it is possible to accurately and quantitatively recover the local Landau (A) coefficient characterizing the shear nonlinearity of soft tissues.     

Plasticity approach to shear design

The paper presents some plastic models for shear design of reinforced concrete beams. Distinction is made between two shear failure modes, namely web crushing and crack sliding. The first mentioned mode is met in beams with large shear reinforcement degrees. The mode of crack sliding is met in non-shear reinforced beams as well as in lightly shear reinforced beams. For such beams the shear strength is determined by the recently developed crack sliding model. This model is based upon the hypothesis that cracks can be transformed into yield lines, which have lower sliding resistance than yield lines formed in uncracked concrete. Good agreement between theory and tests has been found.     

Second order shear lag theory

The standard shear lag theory for elastic aligned short-fibre composites is extended to allow for a gradient of overall strain. The result is a one-dimensional strain gradient theory of the Toupin–Mindlin type. All parameters are the same as in the standard theory, and in the limits of weak strain gradients, large fibre aspect ratios or low elastic modulus ratios, the standard theory is recovered. The gradient effect is illustrated by a simple one-dimensional boundary value problem: a vertical composite rod fixed at both ends and loaded by gravity. The fibre length significantly affects the solution when the fibres are rigid and their length is near the rod length; but otherwise the effect is weak.     

Line crack subject to shear

Field equations of nonlocal elasticity are solved to determine the state of stress in the neighborhood of a line crack in an elastic plate subject to a uniform shear at the surface of the crack tip. A fracture criterion based on the maximum shear stress gives the critical value of the applied shear for which the crack becomes unstable. Cohesive stress necessary to break the atomic bonds is calculated for brittle materials.

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Résumé Les équations de champ d'élasticité non locale sont résolues afin de déterminer l'état de tension au voisinage d'une fissure linéaire dans une tôle élastique soumise à cisaillement uniforme à la surface de l'extrémité d'une fissure. Un critère de rupture basé sur la tension de cisaillement maximum fournit la valeur critique du cisaillement appliqué, pour lequel la fissure devient instable. La contrainte de cohésion nécessaire pour rompre les liaisons atomiques est calculée dans le cas des matériaux fragiles.

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The present work was supported by the Office of Naval Research.     

Stability of magnetogasdynamic shear flow

Summary Some general results on the stability of a compressible parallel shear flow permeated by an aligned magnetic field are derived. It is shown that the complex wave speed of an unstable wave lies in a semi-circle in the upper half plane with the range of the shear flow as diameter. Further both compressibility and magnetic field are found to be stabilizing. It is also shown that in a flow near a wall, two acoustic waves carry energy away from the wall. A semi-circle theorem for a non-planar compressible shear flow is also deduced.

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Stabilität magnetogasdynamischer Scherströmungen

Zusammenfassung Einige allgemeine Aussagen über die Stabilität einer kompressiblen, parallelen Scherströmung in einem Magnetfeld werden hergeleitet. Die komplexe Wellengeschwindigkeit der instabilen Welle liegt in einem Halbkreis in der oberen Halbebene, wobei der Radius durch die Breite der Scherströmung gegeben ist. Sowohl die Kompressibilität, wie das Magnetfeld üben einen stabilisierenden Einfluß aus. Was die Strömung in unmittelbarer Wandnähe angeht, so tragen zwei akustische Wellen Energie von der Wand fort. Abschließend wird ein Halbkreistheorem für nicht ebene kompressible Scherströmungen hergeleitet.

     

可恢复功能剪力墙结构研究进展

近年来,可恢复功能结构逐渐成为了抗震工程研究的热点。可恢复功能剪力墙,主要包括：配合框架结构同时使用的摇摆或自复位剪力墙,通过摇摆减轻地震作用的破坏,通过自重或施加预应力使结构复位;应用于剪力墙结构中的可更换剪力墙,通过对破坏部位的更换实现结构功能的快速恢复。介绍了自复位剪力墙结构,摇摆剪力墙结构及可更换剪力墙结构的相关研究进展,对研究领域的成果和不足进行了总结,并指出研究领域中存在的空白。