Material microstructure effects on ultrasonic waves under simple shear and pure shear

In this paper, the longitudinal and transverse wave velocities propagating into aluminum alloy A6061 under simple shear and pure shear were studied experimentally. Compared with the same velocity change tendencies of transverse wave under simple shear and pure shear, longitudinal wave velocity showed different change tendencies under both shear states, regardless of the same shear strain states in two cases. Furthermore, the measured longitudinal wave velocity changes under pure shear state were apparently categorized into two types according to different material surface states, i.e., simply decreasing change tendency at polished surface state, and chaotic change tendencies at unpolished material surface state. Consequently, the finite element analysis was carried out to investigate the deformation-induced microstructure effects on ultrasonic waves under plastic deformation. The analyzed results indicate that the transverse wave velocity changes depend on textures mainly, whereas the longitudinal wave velocity shows a sensitive response to the point defects.     

Characterization of Layered Cylindrical Structures Using Axially Symmetric Waves

For the characterization of the unknown material properties of a layered cylindrical structure, axially symmetric wave signals transmitted and reflected by the structure have been used. Since only a single wave mode propagates in the structure, the measurement and analysis of the transmitted and reflected signals can be simplified significantly. The evaluation of the material properties of the layers can be achieved with great accuracy. In this paper, we first derive the transmission and reflection coefficients for the layered cylindrical structure sonified axisymmetrically by an incident cylindrical wave. We then relate the spectra of the transmitted and reflected wave signals to the transmission and reflection coefficients as ratio functions. The time-domain signals transmitted and reflected by the structure can then be reconstructed from a routine application of the Fourier integrals. A three-layered aluminum/epoxy/aluminum tube is used to illustrate the application of the expressions for both the forward and inverse problems. The results show that the technique developed in this study can be used very effectively for the characterization of layered cylindrical structures.     

Free surfaces: shape sensitivity analysis and numerical methods

In this paper we consider numerical methods for stationary free boundary problems. We start by analysing systematically different shape optimization formulations of a model problem and show how the optimality conditions relate to construction of trial type methods. Shape sensitivity analysis of the free boundary leads also to the so‐called total linearization method which combines the good properties of Newton method and trial methods, i.e. fast convergence and relative simplicity of implementation. Detailed implementation for a model problem together with numerical tests is presented. Copyright © 1999 John Wiley & Sons, Ltd.     

海上移动视频监控通信系统的设计和应用

介绍了用超短波作为传输手段的海上移动视频监控通信系统.该系统利用最新的超短波COFDM无缝漫游技术,使海上移动监控点的图像、数据、语音通过中继站实时传输到监控中心,指挥决策人员可以通过现场图像和监控中心的数据平台及专家系统生成的各种预案,快速做出正确决策处理突发事件.     

Transonic instability in entrance part of mixing chamber of high-speed ejector

Introduction The research of flow structure in the entrance part of the mixing chamber of two-dimensional supersonic ejector[1,2] shows, how this structure depends both on stagnation pressure ratio of streams p01/p02[3] and on back pressure ratio pb/p02 [4]. It was found out that the structure of shock waves is not stable, but it oscillates less or more. For the high back pressure ratio a terminal shock wave is in the mixing chamber and due to this shock wave the mixing processes change quali…     

海市蜃楼现象的真实感绘制

虚无飘渺的海市蜃楼是一种非常奇妙的自然现象,其是由特定条件下大气的折射和全反射引起的．文中提出了一种对海市蜃楼现象进行建模与绘制的新方法：通过分析不同类型海市蜃楼的成因,建立了对应的不同大气温度变化模型;提出了基于大气重力波模型的海市蜃楼动态变化模型;考虑环境对海市蜃楼的影响,对海市蜃楼的整体进行绘制;在此基础上,绘制出不同条件下不同海市蜃楼的真实感场景,包括海洋、沙漠、公路上的蜃景,并模拟了蜃景的动态变化．     

基桩成孔质量的超声波检测对策探讨

论述利用超声波对钢筋混凝土钻孔桩成孔质量检测的工作原理及其在大直径钻孔灌注桩中的应用.通过工程实例,介绍成孔质量状况的判读方法和成孔参数的计算方法,分析成孔质量对钻孔灌注桩基础的成桩质量的影响因素.     

混沌光的杨氏干涉条纹

提出了混沌光的一种理想的微观模型,在此基础上运用统计方法推出了混沌光束杨氏干涉条纹的强度空间分布及可见度,给出了一幅从微观到宏观的清晰物理图像.     

IMPROVEMENT OF MATERIALS WELDABILITY BY SHOCK WAVES

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Ultrasonic measurements of stiffness in thermal-mechanically fatigued IM7/5260 composites

In recent years, ultrasonic methods have been developed that can measure the mechanical stiffness of composites. The Lamb wave velocity is directly related to the material parameters, so an effective method exists to ascertain the stiffness of composites by measuring the velocity of these waves. In this study, a Lamb wave measurement system was used to measure the bending and out-of-plane stiffness coefficients of thermoset composite laminates undergoing thermal-mechanical loading. A series of 16 ply and 32 ply composite laminates were subjected to thermal-mechanical fatigue (TMF) in load frames equipped with special environmental chambers. The composite system studied was a graphite fiber-reinforced bismaleimide thermoset, IM7/5260. The samples were subjected to both high and low temperature profiles as well as high-strain and low-strain profiles. The bending and out-of-plane stiffnesses for composite samples that have undergone over 6000 cycles of combined thermal and mechanical fatigue are reported. The Lamb wave generated elastic stiffness results have shown decreases of up to 64% at 4706 cycles for samples subjected to TMF at high temperatures and less than a 10% decrease at over 6000 cycles for samples subjected to TMF at low temperatures.