基于Cowper-Symonds本构模型铁路车轮扁疤激发的轮轨冲击仿真分析

轮轨关系是高速铁路工程领域最重要的研究课题之一,车轮扁疤作为一种主要的轮轨失效形式,严重影响了高速列车运行的平稳性和安全性。利用Hypermesh软件建立了带有扁疤的三维轮轨滚动接触模型,并基于轮、轨钢Cowper-Symonds本构模型,采用LS-DYNA 3D显式算法进行了相应的有限元仿真分析,重点研究了车速、扁疤长度和轴重对轮轨冲击响应的影响。仿真结果表明：车轮扁疤缺陷引起的最大轮轨垂向冲击力显著大于相应的准静态垂向载荷,最大von Mises等效应力和最大等效塑性应变通常发生在轮轨接触表面;轮、轨钢的应变率效应对轮轨垂向冲击力没有影响,但由于应变率的强化效应,与不计应变率效应的结果相比,基于Cowper-Symonds模型得到的最大von Mises等效应力明显增大,而最大等效塑性应变略有降低;车速、扁疤长度和轴重对轮轨冲击响应均有显著的影响。研究结果可为轮轨系统的安全性设计与评估提供技术支持。

     

Backscattering Mössbauer spectroscopy of Martian dust

We report on the determination of the mineralogy of the atmospherically suspended Martian dust particles using backscattering ⁵⁷Fe Mössbauer spectroscopy on dust accumulated onto the magnets onboard the Mars Exploration Rovers. The spectra can be interpreted in terms of minerals of igneous origin, and shows only limited, if any, amounts of secondary minerals that may have formed in the presence of liquid water. These findings suggest that the dust has formed in a dry environment over long time in the history of the planet.     

永磁流变抛光纳米精度非球面技术研究

利用永磁流变抛光技术制造高精度光学元件是一项极具前景的超精密制造技术。对一台五轴联动磁流变数控抛光系统的结构特点、功能特色及关键部件的设计进行了阐述。在此基础上,结合装置开展基础试验,对磁流变抛光过程中的主要控制参量如抛光轮下压量、抛光轮速度等对材料去除特性的影响进行了研究。开展了磁流变抛光对提高工件(K9玻璃)表面粗糙度效果的抛光试验,结果证明该套系统具有良好的磁流变抛光特性,抛光23min后工件表面粗糙度降低到0 6739nm。     

Relative position determination of a lunar rover using high-accuracy multi-frequency same-beam VLBI

Multi-frequency same-beam VLBI means that two explorers with a small separation angle are simultaneously observed with the main beam of receiving antennas. In the same-beam VLBI, the differential phase delay between two explorers and two receiving telescopes can be obtained with a small error of several picoseconds. The differential phase delay, as the observable of the same-beam VLBI, gives the separation angular information of the two explorers in the celestial sphere. The two-dimensional relative position on the plane-of-sky can thus be precisely determined with an error of less than 1 m for a distance of 3.8×105 km far away from the earth, by using the differential phase delay obtained with the four Chinese VLBI stations. The relative position of a lunar rover on the lunar surface can be determined with an error of 10 m by using the differential phase delay data and the range data for the lander when the lunar topography near the rover and the lander can be determined with an error of 10 m.     

Evaluating shielding effectiveness for reducing space radiation cancer risks

We discuss calculations of probability distribution functions (PDF) representing uncertainties in projecting fatal cancer risk from galactic cosmic rays (GCR) and solar particle events (SPE). The PDFs are used in significance tests for evaluating the effectiveness of potential radiation shielding approaches. Uncertainties in risk coefficients determined from epidemiology data, dose and dose–rate reduction factors, quality factors, and physics models of radiation environments are considered in models of cancer risk PDFs. Competing mortality risks and functional correlations in radiation quality factor uncertainties are included in the calculations. We show that the cancer risk uncertainty, defined as the ratio of the upper value of 95% confidence interval (CI) to the point estimate is about 4-fold for lunar and Mars mission risk projections. For short-stay lunar missions (), SPEs present the most significant risk, however one that is mitigated effectively by shielding, especially for carbon composites structures with high hydrogen content. In contrast, for long duration lunar () or Mars missions, GCR risks may exceed radiation risk limits that are based on acceptable levels of risk. For example, the upper 95% CI exceeding 10% fatal risk for males and females on a Mars mission. For reducing GCR cancer risks, shielding materials are marginally effective because of the penetrating nature of GCR and secondary radiation produced in tissue by relativistic particles. At the present time, polyethylene or carbon composite shielding cannot be shown to significantly reduce risk compared to aluminum shielding based on a significance test that accounts for radiobiology uncertainties in GCR risk projection.