Shear banding during nonlinear creep with a solution of monodisperse polystyrene

Creep experiments with a solution of polystyrene (M _w = 2.6 MDa, 16 vol.%, 25 °C) in diethyl phthalate are reported for stresses between 100 and 2,500 Pa (≈ 3G _N ⁰/4). The aim was to look for a flow transition as reported for strongly entangled poly(isobutylene) solutions. The experiments with the polystyrene solution were repeated for cone angles of 2, 4, and 6° (radius 15 mm) and showed no dependence on cone angle. The Cox–Merz rule was not fulfilled for stresses beyond about 800 Pa. The tangential observation with a CCD camera showed that the edge took a concave shape because of the second normal stress difference. Beyond 1,000 Pa, the concave edge develops into a crevice, thus substantially reducing the effective cross-section. This leads to runaway in a constant torque experiment. At p ₂₁ = 800 Pa, head-on particle tracking confirms that the originally linear velocity profile takes a gooseneck shape, thus revealing shear banding. When the creep stress is stepped down to 100 Pa, this velocity profile evolves back to a linear one. The conclusion from this work is that even if nonlinear creep experiments are reproducible and a steady state is reached, this does not mean that the flow field is homogeneous. This paper was presented at Annual European Rheology Conference (AERC) held in Hersonisos, Crete, Greece, April 27–29, 2006.     

同时发射和接收纵波和横波的压电复合材料换能器

通过控制压电复合材料中压电相的分布形式，本文具有在百度模振动的１－３型压电复合材料和具有扭转模振动的拼接２－２型压电复合材料组合在一起，构成一种混俣模式压电材料，并测试了这种压电材料晶片的性能参数，使用这种晶片作的换能器可以根据使用者的要求，单独产生纵波或横波，也可同时产生这两种波。     

Dislocation structure of shear bands in single crystals

A mathematical model is proposed for the development of a shear band in crystals. The model is based on the mechanism of double cross-slips of screw-dislocation segments. Equations are derived to study instability of the uniform distribution of dislocations. A solution is found in the form of a traveling wave, which describes the shear-band structure. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 6, pp. 102–113, November–December, 2006.     

Changes in electrical resistance of carbon‐black‐filled silicone rubber composite during compression

We studied the changes in the electrical resistance of carbon black filled silicone rubber composite, which is the sensitive element of the flexible force sensor, as a function of time during compression. The experimental results show that there is a sudden increase of the electrical resistance along with the sudden increase of the stress immediately after the compression. When the sample strain is kept constant, the electrical resistance and the stress both decay with time. The data of the stress relaxation and the resistance relaxation both can be fitted by the linear combination of two exponential functions. Based on the shell structure theory, the experimental phenomena are explained from the view that the uniaxial pressure induces the changes in the effective conductive paths. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2700–2706, 2007     

用横场伊辛模型研究应力对铁电薄膜的热力学性质的影响

用横场伊辛模型研究了铁电薄膜的热力学性质.在体系的哈密顿量中引入一个两维的在平面内的应力，并假设应力从基底材料和薄膜材料之间的界面层到薄膜的表面层是呈指数形式衰减的.结果显示：压应力有利于极化，使居里温度向高温区移动，而张应力对极化和居里温度的影响正好相反.扩散长度对铁电薄膜的热力学性质有很大的影响. 关键词： 铁电薄膜 应力 横场伊辛模型     

Stability of Tokamak Equilibrium with Internal Transport Barrier against High n MHD Ballooning Mode

Internal transport barriers （ITBs） are phenomena associated with improved confinement mode of tokamak plasmas. Within the region where the ITB locates, the plasma pressure has a large gradient while the magnetic shear s has a minimum so that within and near the ITB, the absolute value of the shear is very low. Physics involved is plentiful, from the macroscopic （ MHD ） stability, to the suppression of microscopic instabilities thought to be responsible for anomalous transport. The treatment of very low shear also poses some theoretical difficulties.     

高地应力下大型地下洞室拱形优化研究

高地应力区的大型洞室开挖会引起洞室拱周附近围岩应力集中和塑性区加大以及塑性应变的增加，但过高的应力集中会增加岩爆发生的几率，塑性区的加深会促使围岩失稳；洞室拱形形状会影响围岩应力集中和塑性区大小的分布，因此，以某高地应力下的大型发电洞岩体地质资料为依托，用弹塑性有限元分析高地应力地下大型发电洞室常采用的3种洞室拱形（即单心圆拱、三心圆拱和椭圆拱）对拱部围岩应力的影响，计算结果表明这种影响是非常明显的，有的应力集中系数达到3．33，无论采用哪种拱形，均避不开拱座附近应力集中现象。根据开挖后拱部围岩的塑性耗散能可以判断，无论初始地应力侧压系数是多少，椭圆拱是最优的，其次是三心圆拱。     

Analysis of anisotropy and plastic spin effects on localization phenomena

Summary The main objective of the paper is the investigation of the influence of the anisotrophy and plastic spin effects on criteria for adiabatic shear band localization of plastic deformation. A theory of thermoplasticity is formulated within a framework of the rate-type covariance material structure with a finite set of internal state variables. The theory takes into consideration such effects as plastic non-normality, plastic-induced anisotropy (kinematic hardening), micro-damage mechanism, thermomechanical coupling and plastic spin. The next objective of the paper is to focus attention on cooperative phenomena in presence of the plastic spin, and the discussion on the influence of synergetic effects on localization criteria. A particular constitutive law for the plastic spin is assumed. The necessary condition for a localized plastic deformation region to be formed is obtained. This condition is accomplished by the assumption that some eigenvalues of the instantaneous adiabatic acoustic tensor vanish. A procedure has been developed which allows us to discuss two separate groups of effects on the localization phenomenon along a shear band. Plastic spin, spatial covariance and kinematic hardening effects are investigated at an isothermal process in an undamaged solid. In the second case, an adiabatic process in a damaged solid is discussed when the spatial covariance terms and the plastic spin are neglected. Here the thermomechanical coupling, micro-damage mechanism and kinematic hardening effects are examined. For both cases, the criteria for adiabatic shear band localization are obtained in an exact analytical form. Particular attention is focused on the analysis of the following effects: (i) plastic non-normality; (ii) plastic spin; (iii) covariant terms; (iv) plastic strain-induced anisotropy; (v) micro-damage mechanism; (vi) thermomechanical couplings. Cooperative phenomena are considered, and synergetic effects are investigated. A discussion of the influence of the plastic spin, kinematic hardening and covariant terms on the shear band localization conditions is presented. A numerical estimation of the effects discussed is given. Received 24 April 1997; accepted for publication 23 December 1997     

Digital shifting photoelasticity with optical enlarged unwrapping technology for local stress measurement

In this paper, photoelasticity combined with phase shifting technology is used to obtain stress distribution within the stress concentration zone. Both the optical enlarged unwrapping technology and the combined path shear difference technology are provided for evaluating the local stress information. By means of a phase shifting photoelastic experiment of a diametric-compressed disc, the stress components surrounding the local loading zone are determined. The results show not only a good improvement compared with conventional photoelastic analysis but also a good agreement with theoretical data.     

Numerical validation of the shear compression specimen. Part I: Quasi-static large strain testing

The shear compression specimen (SCS), which is used for large strain testing, is thoroughly investigated numerically using three-dimensional elastoplastic finite element simulations. In this first part of the study we address quasi-static loading. A bi-linear material model is assumed. We investigate the effect of geometrical parameters, such as gage height and root radius, on the stress and strain distribution and concentration. The analyses show that the stresses and strains are reasonably uniform on a typical gage mid-section, and their average values reflect accurately the prescribed material model. We derive accurate correlations between the averaged von Mises stress and strain and the applied experimental load and displacement. These relations depend on the specimen geometry and the material properties. Numerical results are compared to experimental data, and an excellent agreement is observed. This study confirms the potential of the SCS for large strain testing of material.