运行模态分析方法综述

运行模态分析(OMA)是结构动态设计和机械故障诊断的一种重要方法,是系统辨别方法在工程振动领域中的应用.为了在结构动态设计和机械故障诊断中更好地运用该方法,从频域方法、时域方法以及时频分析方法三个方面,综述了运行模态分析国内外研究现状,指出了其关键技术、存在问题以及研究发展方向.     

微量Y(0.2 wt.%)对铸造Mg-Sn合金热变形行为影响

采用MTS-CMT5105电子万能试验机,在温度为250~400℃、初始应变速率为10_-4~10_-1s_-1,最大变形量为40%的条件下,对铸态Mg-2.5Sn(wt.%)和Mg-2.5Sn-0.2Y(wt.%)合金进行压缩变形。结合真应力-应变曲线、峰值应力、显微组织、本构方程、DMM加工图,研究了微量Y(0.2 wt.%)对铸造Mg-Sn合金热变形行为的影响。结果表明：微量Y的添加,在较低应变速率时,会使Mg-Sn合金热压缩峰值应力增加30%以上;但在较高应变速率时,却对其影响不大。会改变合金塑性变形机制的控制方式,即由扩散控制变为位错的滑移和攀移。会抑制动态再结晶。会降低功率耗散系数,扩大加工失稳区。     

镁-稀土耐热镁合金活塞的开发研究

采用Mg-11Y-5Gd-2Zn-0.5Zr镁合金重力铸造发动机活塞,确定了浇注温度、模具温度、热处理工艺等参数。对镁合金活塞本体的组织研究表明,原始铸态合金组织中有大量的第二相分布于晶界处,经过固溶和时效热处理,在晶界处析出大量的层片状析出相。在时效态,活塞裙部的硬度(HV)达到131.4;活塞顶部的高温抗拉强度大,在350℃时的抗拉强度为161.9MPa;在250℃、80MPa条件下经过100h镁合金活塞材料的蠕变变形量只有0.428%;Mg-11Y-5Gd-2Zn-0.5Zr镁合金活塞的密度为1.995g.cm-3,100℃时比热容为850J.kg-1.K-1,室温热导率为23W.m-1.K-1,20～100℃的线膨胀系数为25.6×10-6K-1。     

Stress intensity factors for semi-elliptical surface cracks in plates and cylindrical pressure vessels using the hybrid boundary element method

At first, a hybrid boundary element method used for three-dimensional linear elastic fracture analysis is established on the basis of the first and the second kind of boundary integral equations. Then the concerned basic theories and numerical approaches including the discretization of boundary integral equations, the divisions of different boundary elements, and the procedures for the calculations of singular and hypersingular integrals are presented in detail. Finally, the stress intensity factors of surface cracks in finite thickness plates and cylindrical pressure vessels are computed by the proposed method. The numerical results show that the hybrid boundary element method has very high accuracy for the analysis of surface crack.     

热壁加氢反应器运行过程中的材质劣化问题

堆焊层的表面开裂、氢致剥离，器壁母材的回火脆化、氢脆和氢致裂纹扩展等材质劣化问题是影响热壁加氢反应器使用安全的主要问题。对这些问题产生的原因及其影响因素作了概要的分析和讨论。并在理论分析和实验研究的基础上，探讨了热壁加氢反应器安全运行的保障技术。     

Stress intensity factors for internal semi-elliptical surface cracks in pressurized thick-walled cylinders using the hybrid boundary element method

The purpose of this paper is to present a comprehensive range of results of mode I SIFs of three-dimensional surface cracks in internally pressurized thick-walled cylinders. The hybrid boundary element method is summarily reviewed and used to calculate the SIFs of surface cracks in pressurized thick-walled cylinders. The analyzed ratio of crack depth to wall thickness ranges from 0.2 to 0.8; the ratio of crack depth to crack length ranges from 0.25 to 1.0; and the ratio of wall thickness to cylinder radius is 0.5, 1.0 and 2.0. The present normalized SIFs are also compared with other solutions from the literature. The recent results of the body force method and the finite element method agree well (ca 3%), and the early ones of the boundary integral equation and the finite element method agree fairly well (ca 10%) with the present results.     

The Ductility Variation of High-Pressure Die-Cast AE44 Alloy: The Role of Inhomogeneous Microstructure

In the study, the through-thickness microstructure and its effects on the ductility and strain heterogeneity in high-pressure die-cast AE44 alloy were investigated. The results show that the studied alloy had a gradient microstructure, where two fine-grained skins sandwiched a core with coarse externally solidified crystals (ESCs) embedded in fine grains. In the core, where porosity concentrated, the ultra-coarse ESCs with sizes up to 600 μm were observed. A great amount of Al₁₁RE₃ phase, as the predominant intermetallic phase, was distributed in homogeneously through the thickness. High-resolution digital image correlation (DIC) measurement coupled with electron backscatter diffraction (EBSD) was employed to reveal the deformation inhomogeneity and its root cause. It was found that considerable strain localization mainly appeared in the ultra-coarse ESCs with soft orientation for basal slip and the regions where porosity appeared. Unlike the yield strengths and ultimate tensile strengths, the elongations showed a significant variation. Not only defects but also the ultra-coarse ESCs were the primary factors responsible for the variation in ductility.

     

Extra Strain Hardening in High Pressure Die Casting Mg-Al-RE Alloy

Metallurgical and Materials Transactions A - High pressure die casting Mg-Al-RE alloy has an intrinsic and macroscopic gradient structure regarding the grain size and the area fraction of second...     

基于云服务实现智能家居应用控制的方案

智能家居设备如何接入网络,智能控制终端如何控制智能家居设备,智能家居设备及应用管理平台如何对用户和设备进行管理,这些问题的解决方案伴随着智能家居应用的快速发展不断变化。首先讨论了智能家居业务的应用控制方式,然后引出应用控制的关键逻辑分层结构,在关键逻辑分层结构的基础上,讨论基于云服务的应用控制方案,最后给出总结。     

亚麻织物的无氯漂白工艺研究

以乙酸、双氧水为原料制备了过氧乙酸,通过单因素分析和正交实验确定较佳合成条件：n（C2H5COOH）：n（H2O2）为1．2：1,反应时间为26h,硫酸的用量为4％,并研究了过氧乙酸的用量及漂白液pH对亚麻织物漂白效果的影响。