Dynamic crack propagation with cohesive elements: a methodology to address mesh dependency

In this paper, two brittle fracture problems are numerically simulated: the failure of a ceramic ring under centrifugal loading and crack branching in a PMMA strip. A three‐dimensional finite element package in which cohesive elements are dynamically inserted has been developed. The cohesive elements' strength is chosen to follow a modified weakest link Weibull distribution. The probability of introducing a weak cohesive element is set to increase with the cohesive element size. This reflects the physically based effect according to which larger elements are more likely to contain defects. The calculations illustrate how the area dependence of the Weibull model can be used to effectively address mesh dependency. On the other hand, regular Weibull distributions have failed to reduce mesh dependency for the examples shown in this paper. The ceramic ring calculations revealed that two distinct phenomena appear depending on the magnitude of the Weibull modulus. For low Weibull modulus, the fragmentation of the ring is dominated by heterogeneities. Whereas many cracks were generated, few of them could propagate to the outer surface. Monte Carlo simulations revealed that for highly heterogeneous rings, the number of small fragments was large and that few large fragments were generated. For high Weibull modulus, signifying that the ring is close to being homogeneous, the fragmentation process was very different. Monte Carlo simulations highlighted that a larger number of large fragments are generated due to crack branching. Copyright © 2003 John Wiley & Sons, Ltd.     

美军密码装备建设新进展

本文介绍了近期美军密码装备和系统的建设概况,试探了其发展趁势。     

煤焦油市场现状及后期趋势

一、煤焦油市场现状 原油期货价格于7月4日创历史新高，盘中达146．69美元／桶，加之全国煤炭市场价格暴涨300元／吨，以及绿色奥运的来临，在多重内外因素的叠加效应下，煤焦油市场价格出现了一波大幅上涨行情，同时也创下煤焦油价格的历史新高。全国煤焦油市场状况、地域性特征十分明显，按地理划分为四类市场。     

两种塑料纸芯管结构与性能比较分析

测试与分析了两种塑料管材的拉伸与弯曲性能、热变形温度、结晶性和化学组成,分析结果表明:管材B较管材A具有更高模量与热变形温度,但管材B较高的分子量与较小的分子量分布抑制管材产品内在的均匀性,且管材B中的α晶在高温下向β晶的转变可能会影响管材长期使用性能.     

Synthesis of TiO<Subscript>2</Subscript> Hybrid Molecular Imprinted Polymer for Ethofumesate Linked by Silane Coupling Agent

TiO₂ hybrid molecular imprinted polymer (MIP) for ethofumesate using methacrylic acid (MAA) as the functional monomer and silane coupling agent 3-(trimethoxysilyl) propylmethacrylate (KH570) as organic–inorganic connective bridge was synthesized via photo-excitation method. Hydrogen bond was proved to act between MAA and ethofumesate for pre- and post-polymerization binding properties as testified by UV spectrometric method. KH570 modified TiO₂ nanoparticles were prepared via sonochemical reaction, which can accelerate hydrolysis, increase collision chance for the reactive system and improve the dispersion of the nanoparticles. Scanning electron microscope (SEM), transmission electron microscope (TEM), binding and the adsorption kinetics experiments as well as thermogravimetric analysis (TGA) were employed for characterization. The results indicated that the hybrid MIP revealed a larger surface area and more ordered imprinting cavities with improved thermal stability compared to organic-only MIP. Furthermore, faster adsorption kinetics and enhancive adsorption capacity were achieved, which made it promising in chemical sensor applications.     

Formation mechanism of 21R AlN-polytypoids in aluminothermic reduction and nitridation process

Synthesis of 21R AlN polytypoids was investigated using Al and ultrafine SiO₂ powder in a flowing nitrogen atmosphere by means of thermal gravity (TG), differential scanning calorimeter (DSC), X-ray diffractometer (XRD) and scanning electronic microscope (SEM) linked with energy dispersive spectrometer (EDS). The results showed that the formation mechanism of AlN polytypoids was different from that in reaction sintering process using Si₃N₄, AlN, Al₂O₃ and other sintering additive as raw materials. It was suggested that firstly Al reduce SiO₂ into Si and is also nitrided into AlN, then AlN, Al₂O₃ and SiO₂ dissolve into silicon liquid until the AlN polytypoids precipitate in saturated liquid in a flowing nitrogen atmosphere at lower than 1700 °C.     

蓝牙技术及应用简析

蓝牙（Bluetooth）技术是一种短距离无线电技术,利用“蓝牙”技术,能够有效使数据传输变得更加迅速高效。本文主要介绍蓝牙的关键技术及其未来的发展趋势。     

新型刮刀钻头的设计及现场试验研究

在理论分析的基础上，综合PDC钻头和刮刀钻头的优点，设计了一种新型的刮刀钻头，并进行了现场试验。现场试验结果表明，该钻头运转平稳，采用低钻压、高转速，钻进速度快，井眼规则通畅，有较明显的防斜作用，该种钻头的推广应用对提高钻井工程的整体效益具有十分重要的意义。