光声效应在研究铁电陶瓷相变中的应用

本文报导了用光声技术研究铁电陶瓷 Pb(Zr,Ti,Sn)O_3(PSZT),透明 PLZT 以及 Li_(1-x)Na_xNbO_3(LNN)的相变,清楚地揭示了这些陶瓷所呈现的不同相变特性以及相变点随组分的变化,与某些常规研究方法相比较,显示了在相变点附近,光声信号的幅度和相位的变化非常灵敏。并用热力学解释了实验结果。     

Microtomographic characterization of columnar Al–Cu dendrites for fluid flow and flow stress determination

During the twin roll casting of Al alloys, the interdendritic liquid may flow as the two solidification fronts are compressed together between the rolls. This can lead to defects such as centerline segregation. To understand the flow properties of the interdendritic liquid, samples of Al–12 wt.% Cu were solidified directionally in a Bridgman furnace and quenched to capture the growing columnar dendritic structures. The quenched samples were scanned using a laboratory X-ray microtomography (XMT) unit to obtain the 3D structure with a voxel resolution of 7.2 μm. Image analysis was used to separate the Al dendrite from the interdendritic Al–Al₂Cu eutectic. Flow between the dendrites was simulated by solving the Stokes equation to calculate the permeability tensor as a function of the fraction solid. The results were compared to prior experimental measurements and calculations using synchrotron tomography observations of equiaxed structures. Elasto–plastic finite element (FE) simulations were performed on the dendritic structures to determine flow stress behavior as a function of fraction solid. It was found that the standard approximations for the reduction in flow stress in the semi-solid have a variation in excess of 100% from that calculated using the true structure. Therefore, it is critical to simulate the actual dendrite for effective flow stress determination.     

The dynamic response of end-clamped sandwich beams with a Y-frame or corrugated core

The dynamic response of end-clamped monolithic beams and sandwich beams has been measured by loading the beams at mid-span using metal foam projectiles. The AISI 304 stainless-steel sandwich beams comprise two identical face sheets and either prismatic Y-frame or corrugated cores. The resistance to shock loading is quantified by the permanent transverse deflection at mid-span of the beams as a function of projectile momentum. The prismatic cores are aligned either longitudinally along the beam length or transversely. It is found that the sandwich beams with a longitudinal core orientation have a higher shock resistance than the monolithic beams of equal mass. In contrast, the performance of the sandwich beams with a transverse core orientation is very similar to that of the monolithic beams. Three-dimensional finite element (FE) simulations are in good agreement with the measured responses. The FE calculations indicate that strain concentrations in the sandwich beams occur at joints within the cores and between the core and face sheets; the level of maximum strain is similar for the Y-frame and corrugated core beams for a given value of projectile momentum. The experimental and FE results taken together reveal that Y-frame and corrugated core sandwich beams of equal mass have similar dynamic performances in terms of rear-face deflection, degree of core compression and level of strain within the beam.     

预应力混凝土连续箱梁城市特宽桥试验分析

简要介绍了城市特宽桥的研究现状,通过对一座典型的三跨（30＋45＋30）变截面预应力混凝土连续箱梁城市特宽桥的空间有限元受力分析,并结合荷载试验研究,探讨了城市特宽桥的受力特点和应用前景。     

摩擦对高温合金复杂截面圆环多道次滚压成形不均匀变形作用

在高温合金复杂截面薄壁圆环多道次滚压成形过程中,板料要经历多场多因素偶合作用下复杂的不均匀塑性变形和组织演化历程,特别是不均匀塑性变形通常是导致薄壁件成形起皱、椭圆和开裂等缺陷产生的主要因素。摩擦是描述环件与辊轮接触作用的主要参数,很大程度上决定着该环件多道次滚压成形过程中塑性变形的不均匀性,严重的影响着其滚压成形质量。为此,本文以有限元仿真为主并结合实验和理论方法,对高温合金GH4169薄壁w形圆环多道次滚压成形进行了系统的分析;进而提出了不均匀变形度的表征方法,研究获得了摩擦对高温合金复杂截面薄壁圆环多道次滚压成形不均匀变形的影响规律。结果表明：随着滚压成形的进行等效应力极值逐渐增大,且从动辊进给时环件弯曲部分材料变形量较大;随着驱动、从动辊与环件之间摩擦系数的增加,不均匀变形程度先减小,后逐渐增大;导向辊与环件之间的摩擦对不均匀变形度的影响不大。     

Verbundverhalten von Vollgewindeschrauben in Brettschichtholzbauteilen

Bond behaviour of self‐tapping screws being used as reinforcement in glue‐laminated timber elements – Part 2: Analytical and numerical investigations as well bond model derivation for the calculation of anchorage length In favor of the investigation of bond behaviour, force transfer and anchorage length of self‐tapping screws, several tests have been realized at the Chair of Structures and Structural Design in cooperation with the Institute for Building Material Research of the RWTH Aachen University. The experimental investigations comprise more than 160 pull‐out tests of screws with long embedment length and over 84 load distributions tests. Additionally, several tests displaying the effect of longitudinal cracks in the surrounding wood as well as the effect of the screw tip have been conducted. Through various analyses of the bond behaviour, the experimental investigations form the basis for the calibration and evaluation of the numerical models and allow a prediction of the force transfer of the screws in glue‐laminated elements. Design rules that enable the application of the self‐tapping screws as reinforcement in timber elements have been derived from the knowledge obtained in the experimental and numerical investigations. This paper, which results from a research project funded by the German Research Foundation [1], presents the results of investigations on the bond behavior of self‐tapping screws in glue‐laminated timber elements. Part 1 elaborates on the experimental investigations [2] whereas part 2 illustrates the numerical analyses and presents a bond model, which enables the design of the anchorage length and the safe application of the screws as reinforcement in timber elements.     

WCDMA基站传输技术的研究

随着国内3G网络大规模建设和应用,以及3G业务尤其是数据业务的急剧增长,对基站传输的要求也越来越高,现有基站的传输方式将成为3G业务发展的瓶颈。对E1、FE等基站传输组网方案进行了分析、研究,并对WCDMA基站的传输规划提出了建议。     

基于目标索力的“位移-荷载双控”张拉算法研究及应用

针对深圳世界大学生运动会体育中心体育场单层折面空间网格钢屋盖结构的整体模型承载力试验,进行了加载控制方案的研究。对于试验中手拉葫芦分级分组张拉加载过程模拟分析、张拉误差现场控制等关键问题,提出了"位移-荷载双控"的张拉算法,为试验加载的实施奠定了理论基础。研究表明,一体化有限元模型和上述张拉算法结合能够很好地对张拉过程进行模拟分析。相比通常的迭代算法,本文提出的"位移-荷载双控"的补偿张拉算法具有较高的求解效率和精度,可以有效地解决类似工程实践中的张拉问题。