Comparative study between crack closure model and Willenborg model for fatigue prediction under overload effects

A comparative study is performed between a crack closure model and the Willenborg model, which can calculate the fatigue crack growth rate under the overload effects. The modified virtual crack annealing (VCA) model is briefly reviewed, which is based on the equivalent plastic zone concept. In this method, the retardation phenomenon is explained by the crack closure level variation, which is derived from the interactions between forward and reverse plastic zones ahead of the crack tip. As a comparison, the Forman equation in conjunction with the Willenborg model is also reviewed. The retardation phenomenon is described by directly modifying the stress intensity factor. It is known that the large plastic zone created by the overload can decelerate the fatigue crack growth rate until the crack grows beyond this region. A relationship between the plastic zone and the modified stress intensity factor is developed, which is a mathematical fitting equation instead of physical-based formulation. The experimental data in aluminum alloys are used to val-idate these two models. Overall, good agreement is observed between the model predictions and the testing data. It is noted that the approach based on modified VCA model can give more accurate prediction curves than the Willenborg model.     

我国国债宏观经济效应的实证研究

采用多元Granger因果测试模型和1980～2003年的相关数据实证检验我国国债的宏观经济效应。实证结果表明，我国国债融资实际上并未对宏观经济产生显著影响。这主要是因为我国利用国债融资的时间较短，国债的宏观经济效应尚未显现；利用国债融资的多为基础设施项目，周期较长，真正发挥效应尚需一段时间；尤其是国债资金管理不到位，使得国债融资未对实体经济作出积极贡献。因此，政府当务之急是完善国债投资管理和审批体制，增加国债资金运用的透明度，加大对国债资金管理和使用的监督和检查力度，提高国债项目的投资效益。     

奥运会对北京经济的挤出效应

举办2008年奥运会,是发展北京经济强有力的助推器,对北京经济具有巨大的提升作用.然而,奥运对北京经济同时也具有不利的影响--挤出效应.文章具体分析了奥运对北京经济的挤出效应,以便更好的利用这次机遇促进北京经济的进一步发展.     

压力探针临壁效应的试验研究

在窄通道中测量时（如全台压气机试验），临壁效应会使压力探针的测量结果发生误差。对几种常用型式的压力探针的试验研究表明，由于探针多属非流线形物体，所以其临壁效应的机理与流线形的机翼不同，不应简单套用机翼临壁效应的结论。临壁效应所造成的测量误差与探针的形状有关。     

论疲劳短裂纹

 <正> 1.裂纹扩展阶段划分及影响因素 总结已有的实验分析表明,影响短裂纹扩展的可能因素有:(1)微观结构因素和微观力学因素;(2)力学因素和环境因素。微观因素是影响晶粒量级裂纹扩展的主要因素;远大于晶粒量级的裂纹则主要受宏观因素的影响。因此将短裂纹按其控制因素的性质分为微观结构短裂纹(MSC)和物理短裂纹(PSC)较为科学(图1)。 通过对许多实验结果的分析表明:MSC的上界视局部应力,晶界和夹杂的性质等的不同为1～3M(M为微观结构特征尺寸)。     

银膜的原子氧剥蚀效应及其防护的试验研究

 空间太阳电池板作为空间飞行器电源系统的重要组成部分,它的正常工作对飞行器的运行起着非常重要的作用,而作为互连片的银材料,可能会与空间环境中的原子氧发生反应,生成不导电的氧化物,从而影响空间太阳电池板的有效寿命。本文在原子氧效应地面模拟试验设备中,对银以及镀有不同防护层的银膜进行了原子氧效应地面模拟试验研究,对试验前后试样的外观、质量进行了比较,获得了银在设备中的反应特点,同时对不同防护镀层的有效性进行了验证,为银互连片在空间太阳电池板上的应用及其防护提供了设计依据。     

稀薄流热化学非平衡效应的DSMC研究

以钝体绕流驻点线流场为例，应用直接仿真蒙特卡罗法，模拟了五组元混合气体（Ｎ２，Ｏ２，Ｎ，Ｏ，ＮＯ）中的碰撞能松弛，离散反应，交换反应及复合反应，给出了碰撞分子各自由度间的能量交换模式及各类化学反应的选择模式。ＤＳＭＣ仿真结果与宏观粘性激波层方程计算结果，在飞行高度较低时基本相符，表明采用的热化学非平衡非模式是令人满意的，随着飞行高度增高稀薄气体效应增大，二者在激波附近有显著差异，表明高空飞行必须考     

微重力燃烧研究进展

微重力燃烧是一门非常新的学科,它研究在微重力环境下的基本燃烧过程。本文概括最近几年刚发展起来的以下几方面的进展:微重力燃烧研究所用的设备;固体材料的着火性;预混气燃烧;液滴燃烧;颗粒群燃烧;空间飞行器着火安全性。最后对我国开展微重力燃烧研究提出建议。     

碳纤维/环氧复合材料的原子氧剥蚀效应试验研究

碳纤维/环氧树脂用在空间飞行器上时,面临着空间环境中各种因素特别是原子氧效应的影响,因而在地面模拟设备中对这种材料进行了原子氧剥蚀效应试验研究,并且考察了温度变化的影响.通过比较试验前后试样的质量及表面形貌,得出了材料在设备中的反应特点以及温度变化对材料原子氧效应的影响规律.同时,还对设备中存在的离子氧在材料的质量损失中所起到的作用进行了定性的研究.     

Different space weather effects in anomalies of the high and low orbital satellites

Preliminary results of the EU INTAS Project 00810, which aims to improve the methods of safeguarding satellites in the Earth’s magnetosphere from the negative effects of the space environment, are presented. Anomaly data from the “Kosmos” series satellites in the period 1971–1999 are combined in one database, together with similar information on other spacecraft. This database contains, beyond the anomaly information, various characteristics of the space weather: geomagnetic activity indices (Ap, AE and Dst), fluxes and fluences of electrons and protons at different energies, high energy cosmic ray variations and other solar, interplanetary and solar wind data. A comparative analysis of the distribution of each of these parameters relative to satellite anomalies was carried out for the total number of anomalies (about 6000 events), and separately for high (5000 events) and low (about 800 events) altitude orbit satellites. No relation was found between low and high altitude satellite anomalies. Daily numbers of satellite anomalies, averaged by a superposed epoch method around sudden storm commencements and proton event onsets for high (>1500 km) and low (<1500 km) altitude orbits revealed a big difference in a behavior. Satellites were divided on several groups according to the orbital characteristics (altitude and inclination). The relation of satellite anomalies to the environmental parameters was found to be different for various orbits that should be taken into account under developing of the anomaly frequency models.