超声衍射时差法探伤和定量技术--焊缝超声检测最新欧洲标准介绍

超声衍射时差(TOFD)法是焊缝超声检测和缺陷定量很有发展前景的新技术。它根据入射纵波在缺陷端部产生的衍射波传播时差进行测深定高,是一种可靠性甚高的方法,有A,D,B三种显示方式,探伤结果记录较直观和客观。根据最新欧洲标准(草案)ENV583 6:2000的有关规定,介绍超声TOFD技术应用要领,企望该技术在国内无损检测(NDT)领域迅速推广应用。     

Buckling of cylindrical open-topped steel tanks under wind load

Vertical cylindrical welded steel tanks are typical thin-walled structures which are very susceptible to buckling under wind load. This paper investigates the buckling behavior of open-topped steel tanks under wind load by finite element simulation. The analyses cover six common practical tanks with volumes of 2×10³ m³ to 100×10³ m³ and height-to-diameter ratios H/D<1. The linear elastic bifurcation analyses are first carried out to examine the general buckling behavior of tanks under wind load, together with comparison to that of tanks under uniform pressure and windward positive pressure (only loaded by positive wind pressure in the windward region). The results show that for larger tanks in practical engineering, the stability carrying capacity of wind load is relatively lower. It is also indicated that the buckling behavior of tanks under wind load is governed by the windward positive pressure while wind pressure in other region of tank essentially has no influence on the buckling performance. The geometrically nonlinear analyses are then conducted to investigate the more realistic buckling behavior of tanks under wind load. It is found that the buckling behaviors of perfect tanks and imperfect tanks are much different. The weld induced imperfection only has little influence on the wind buckling behavior while the classical buckling mode imperfection has significant influence, leading to a considerable reduction of wind buckling resistance. The influences of thickness reduction of cylindrical wall, liquid stored in the tank and wind girder on the buckling behavior are also examined. It shows that the thickness reduction of cylindrical wall considerably reduces the wind buckling resistance while sufficient liquid stored in the tank and wind girder significantly increase the wind buckling resistance.     

Nonlinear mechanical,thermal and thermo-mechanical postbuckling of imperfect eccentrically stiffened thin FGM cylindrical panels on elastic foundations

This paper presents an analytical approach to investigate the nonlinear stability analysis of eccentrically stiffened thin FGM cylindrical panels on elastic foundations subjected to mechanical loads, thermal loads and the combination of these loads. The material properties are assumed to be temperature-dependent and graded in the thickness direction according to a simple power law distribution. Governing equations are derived basing on the classical shell theory incorporating von Karman–Donnell type nonlinearity, initial geometrical imperfection, the Lekhnitsky smeared stiffeners technique and Pasternak type elastic foundations. Explicit relations of load–deflection curves for FGM cylindrical panels are determined by applying stress function and Galerkin method. The effects of material and geometrical properties, imperfection, elastic foundations and stiffeners on the buckling and postbuckling of the FGM panels are discussed in detail. The obtained results are validated by comparing with those in the literature..     

带有初缺陷的弹性圆柱壳冲击扭转屈曲

本文以B-R运动准则为基础给出带有初缺陷的弹性圆柱壳冲击扭转屈曲研究。研究表明,随着初缺陷的增加以及矩形脉冲扭矩作用时间的减少,B-R运动准则意义上的冲击屈曲不再有意义,另外随着矩形脉冲矩作用时间的减少,周向屈曲波纹数增加,但初缺陷的大小对周向波纹数没有影响。     

预制-再制

文章集中探讨了二战以后也就是所谓"战后"时期的建筑设计与建造。涉及的不仅是实践层面的问题,同时也有伦理层面的思考。因为正是这段时期,由于大量房屋毁于战争,要给成千上万的居民提供住房。同一时期,建造材料稀少而紧缺,于是一种新的现实主义应运而生,并且在施工体系上也有所创建:不再是此前乌托邦时期现代建筑的"封闭"体系,而是一种被认为是"开放的"施工方式。这一时期的建筑师们也认同建筑设计应该自我调节以适应有限的资源与手段,当然,他们也使用了一定的技术措施尤其是那些新的技术,但并非不分场合的滥用。项目的急迫性往往要求对工业化制品或者构件进行变更,使得要对"预制"的东西进行"再制"。这一时期,人们允许更大的容差,接受偶发因素,以积极的眼光来看待"不完美";建筑的物质性层面变得非常重要,而再现性则必须后退;使用的材料都是那些唾手可得的,有时是工业化材料(钢),有时是传统材料(砖);另一改变是将那些仅起服务作用的东西暴露在外而不再遮蔽。以"粗野"一词,来表达这种直接的、裸露的、混合的、讲求实际的建造方式。文章研究了埃利森和彼得·史密森、柯布西耶、詹姆士·斯特林、亚历杭德罗·德-拉·索塔、希哥德·勒沃伦兹等建筑师,最后还研究了弗兰克·盖里——虽然他的作品显然不属于这一时期,但可以这一背景为基础来加以解读,就像今天的诸多先锋建筑一样。     

Sensitivity analysis of steel plane frames with initial imperfections

The article presents the sensitivity and statistical analyses of the load-carrying capacity of a steel portal frame. It elaborates a typical stability problem of a system comprising two single-storey columns loaded in compression. The elements of this system mutually influence each other, and this fact, in conjunction with the random imperfections, influences the load-carrying capacity variance. This mutual interaction is analysed using the Sobol’ sensitivity analysis. The Sobol’ sensitivity analysis is applied to identify the dominant input random imperfections and their higher order interaction effects on the load-carrying capacity. Majority of imperfections were considered according to the results of experimental research. Realizations of initial imperfections were simulated applying the Latin Hypercube Sampling method. The geometrical nonlinear solution providing numerical result per run was employed. The frame was meshed using beam elements. The columns of the plane frame are considered with two variants of boundary conditions. The dependence between mean and design load-carrying capacities and column non-dimensional slenderness is analysed.     

Elastic local post-buckling of elliptical tubes

The elastic local post-buckling behaviour of elliptical tubes under compression is analysed in this paper. A brief outline of the local, distortional and global buckling behaviour of EHS tubes is firstly provided, where it is shown that local buckling modes govern the stability of short to intermediate length tubes while distortional modes control the stability of intermediate length to moderately long tubes and global buckling dominates the behaviour of longer tubes. Following this, an in-depth numerical study employing shell finite element modelling, of the elastic local post-buckling behaviour of compressed elliptical hollow section (EHS) tubes is presented. It is concluded that EHS tubes with a low to moderate aspect ratio can support loads up to their limit loads but are imperfection sensitive (shell-type behaviour), while EHS tubes with a moderate to high aspect ratio can carry loads higher than their limit loads (plate-type behaviour) and are imperfection insensitive. The slope of the ascending post-buckling path increases with the EHS aspect ratio and can reach values up to 40% of the slope of the linear primary path. The bound imperfection amplitude concept, separating the imperfection amplitude ranges where the EHS tube is sensitive and insensitive, is proposed. It is also found that, for increasing EHS aspect ratio, the compressive stresses grow and accumulate near the zones of minimum radius of curvature while the zones of maximum radius of curvature possess an approximately uniform and relatively low compressive stress level. Therefore, it is expected that an approach based on the effective width concept widely used for the evaluation of the strength of flat plates may be adapted to the design of EHS tubes with moderate to high aspect ratios.     

Behavior of CFST short column and beam with initial concrete imperfection: Experiments

Gap between steel tube and concrete core could be recognized as a type of initial concrete imperfection in concrete-filled steel tubular (CFST) structure. This paper studies the influence of the gap on the compressive and flexural behavior of CFST members. A total of 21 specimens, including 14 short columns subjected to axial compressive loading and 7 beams subjected to bending, were tested. The main parameters of these tests were the gap type (circumferential and spherical-cap) and gap ratio. The influence of gap on the failure mode, ultimate strength and flexural stiffness of the CFST specimens were investigated.     

Structural sensitivity of circular sewer liners to geometrical imperfections

The impact of geometrical defects of liners on their structural performance has been examined in this article. The major common imperfections, such as gap, ovality and wavy imperfections, have been taken into consideration. In the first step, the influence of separate imperfections is presented. Then, the combination of coinciding defects is examined. The formula for weighted influence of individual defects has been found, based on a wide range of FEM calculations and statistical interpretation. All the examinations performed led to improving the general formula for critical groundwater pressure.