Mechanical spectra measurement to probe the magnetolattice coupling in cupric oxide

Mechanical spectra (complex Young's modulus Y*=Y′+iY″ versus frequency and temperature) of polycrystalline cupric oxide CuO were measured by vibrating reed method from liquid nitrogen temperature to room temperature at the kilohertz frequency. The abnormal behavior of internal friction and the change of slope of the real part of the complex Young's modulus versus temperature are observed around 213 and 230 K, where two successive magnetic transitions were established. At low temperatures, below 130 K, a higher internal friction platform with the decrease of temperature was clearly observed which might show a phase transition. Our study indicates that mechanical spectrum is an effective tool to detect subtle phase transitions and there is a strong magnetolattice coupling in CuO.     

有关力学谱和内耗工作的综合与回顾

本文广泛地收集了有关内耗，力学谱，超声衰减方面的专著及会议文集。反映了20世纪在此领域的英文，俄文出版的书籍。也列出了历次国际会议及前苏联，乌克兰，中国的国内会议。文中包括了点缺陷，电，声子，位错，晶界，电畴等诸方面在内的内耗与力学谱工作。     

The elastic anisotropy of nematic elastomers

We examine the robustness of order in nematic elastomers under mechanical strains imposed along and perpendicularly to the director when director rotation is prohibited. In contrast to electric and magnetic fields applied to conventional nematics, mechanical fields are shown theoretically and experimentally to greatly affect the degree of nematic order and related quantities. Unlike in liquid nematics, one can impose fields perpendicular to the director, thereby inducing biaxial order which should be susceptible to experimental detection. Nematic elastomers with unchanging director and degree of order should theoretically have the same elastic moduli for longitudinal and transverse extensions. This is violated when nematic order is permitted to relax in response to strains. Near the transition we predict the longitudinal modulus to be smaller than the transverse modulus; at lower temperatures the converse is true, with a crossover a few degrees below the transition. The differences are ascribed to the different temperature dependence of the stiffness of uniaxial and biaxial order. We synthesised side chain single-crystal nematic polymer networks, performed DSC, X-ray, birefringence, and thermo-mechanical characterisations, and then obtained linear moduli from stress-strain measurements. Received 29 September 2000     

Cellular solid behaviour of liquid crystal colloids 2. Mechanical properties

This paper presents the results of a rheological study of thermotropic nematic colloids aggregated into cellular structures. Small sterically stabilised PMMA particles dispersed in a liquid crystal matrix densely pack on cell interfaces, but reversibly mix with the matrix when the system is heated above . We obtain a remarkably high elastic modulus, , which is a nearly linear function of particle concentration. A characteristic yield stress is required to disrupt the continuity of cellular structure and liquify the response. The colloid aggregation in a “poor nematic” MBBA has the same cellular morphology as in the “good nematic” 5CB, but the elastic strength is at least an order of magnitude lower. These findings are supported by theoretical arguments based on the high surface tension interfaces of a foam-like cellular system, taking into account the local melting of nematic liquid and the depletion locking of packed particles on interfaces. Received 13 March 2000 and Received in final form 6 June 2000     

晶界弛豫研究50年

文章综述了我国科学工作者５０年来关于晶界弛豫研究的早期开拓和近期发展．前者包括扭摆内耗仪的发明、晶界内耗峰的发现和无序原子群晶界模型的提出．后者包括澄清了关于晶界内耗峰来源的争论,揭示了晶界弛豫具有一个临界温度,从而提出了一个适合于各种温度的综合的晶界模型．一个最重要的进展是关于竹节晶界内耗峰的发现与其机理的阐明,从而揭示了晶界附近的位错亚结构能够影响晶界本身的性质和结构．这对于研究多晶金属的力学性质提供了一个广阔的途径．另外,晶界与邻域位错的非线性交互作用的发现,为奠定非线性滞弹性这门新学科提供了实验基础     

饱和岩石滞弹性弛豫机理的实验研究

通过Metravib热机械分析仪,用正弦波加载方式,模拟地震波的传播。实验时固定静载为100N,正弦波动载荷恒为60N,将总载荷控制在屈服点以下。在温度为-50℃~90℃,升温速率保持在1℃/分,频率为5Hz~400Hz的条件下,对饱和泵油彭山砂岩和遂宁砂岩样品进行单轴循环加载实验,获得饱和泵油彭山砂岩和遂宁砂岩的衰减、虚模量、实模量、波速与温度和频率以及动载荷的关系。以此研究了饱和多孔岩石的衰减和虚模量、杨氏模量和弹性波波速随温度和频率的动态响应。取得了随频率增高,饱和多孔岩石的衰减峰和虚模量峰的峰位向高温方向移动的热激活弛豫机制。杨氏模量和弹性波波速随温度升高而下降,随频率增高而增大,具有频散效应;杨氏模量和弹性波波速随动载荷振幅的增大而降低。这些结果与低频共振的驻波实验取得了同样的热激活弛豫规律,说明热激活弛豫规律具有一定的普适性。     

Nonlinear elasticity of composite materials Landau coefficients in dispersions of spherical and cylindrical inclusions

We investigate the elastic properties of model composites, consisting in a dispersion of nonlinear (spherical or cylindrical) inhomogeneities into a linear solid matrix. Both phases are considered isotropic. Under the simplifying hypotheses of small deformation for the material body and of small volume fraction of the embedded phase, we develop a homogenization procedure based on the Eshelby theory, aimed at describing nonlinear features. We obtain the bulk and shear moduli and Landau coefficients of the overall material in terms of the elastic behavior of the constituents and of their volume fractions. The mixing laws for the nonlinear properties describe a complex scenario where possible strong amplifications of the nonlinearities may arise in some given conditions.     

Numerical study of the stress response of two-dimensional dense granular packings

We investigate the Green function of two-dimensional dense random packings of grains in order to discriminate between the different theories of stress transmission in granular materials. Our computer simulations allow for a detailed quantitative investigation of the dynamics which is difficult to obtain experimentally. We show that both hyperbolic and parabolic models of stress transmission fail to predict the correct stress distribution in the studied region of the parameters space. We demonstrate that the compressional and shear components of the stress compare very well with the predictions of isotropic elasticity for a wide range of pressures and porosities and for both frictional and frictionless packings. However, the states used in this study do not include the critical isostatic point for frictional particles, so that our results do not preclude the fact that corrections to elasticity may appear at the critical point of jamming, or for other sample preparation protocols, as discussed in the main text. We show that the agreement holds in the bulk of the packings as well as at the boundaries and we validate the linear dependence of the stress profile width with depth.     

Compression of a soft sphere packing

Mechanical properties of packings of deformable spheres of polyelectrolyte gel are studied experimentally. These particles are plunged into a brine. They have the property to swell and shrink when the concentration of salt of the solution is varied. An oedometric compression is performed imposing cycles of deformation at constant speed and constant salinity C_s. Under many different conditions, we study the laws of deformation relating the macroscopic compression force F, to the macroscopic strain . We find empirical non linear relations of the type . The values of this exponent m are discussed and compared to the results of measurements on a single sphere compressed on a plane as well as to the results of experiments and simulations on dry model granular assemblies. The swelling and deswelling properties of the spheres are used to perform isotropic compression tests. In this situation we determine the relation between the force at equilibrium and the macroscopic strain . The results are compared with those obtained in the oedometric compression tests. Received 27 January 1998