Critical wall thickness in electrical discharge machining

Electrical discharge machining (EDM) is capable of fabricating thin wall structures due to the absence of physical contact between the tool and workpiece. However, the presence of impulsive force does exist during the sparking process, which could give rise to limitations in fabricating small features. The L9 Taguchi method together with the analysis of variance was used to determine the significance of various machining parameters on the specific impulsive force. Through the statistical analysis, it was observed that discharge energy was the most significant factor that influenced the specific impulsive force. Hence, it is important to understand the effects of specific impulsive force in EDM and to investigate its influence that limits the fabrication of thin wall structures. The limit in which the machined part is able to maintain its geometrical integrity is described as the critical wall thickness. The effects of specific impulsive force on thin wall structure were studied through determining the critical wall thickness for various discharge energies. The experimental work showed that both electrothermal erosion and brink erosion, which is attributed by the specific impulsive force was responsible for the loss of geometrical integrity of the wall structure. As the wall thickness falls below a critical value in a given discharge energy setting, the brink erosion was observed. The limit that established the critical wall thickness was elucidated by conflating the thermal erosion and brink erosion.     

Collective buckling of an elastic beam array on an elastic substrate for applications in soft lithography

We analyze the collective buckling of an array of vertical elastic beams with their lower ends built into an elastic substrate. The beams interact between themselves through the deformation of the elastic substrate. The present analysis is more sophisticated than previous ones on rigid beams on an elastic substrate in that the beams are regarded as elastic rather than rigid. From the linear theory for elastic beam buckling and the linear theory of elasticity, an eigenvalue problem is formulated and solved. Calculations show that the deformability of the beams lowers the critical height of the beams, but it does not affect the buckling pattern much. Our work also suggests that the collective buckling is dominated by the interaction of neighboring beams through the deformation of the substrate rather than whether the beams are rigid or elastic. The results are useful for the better understanding, design and application of the nanostructures produced by soft lithography.     

议总监如何有效提高项目队伍的稳定性

从工程监理企业在公司统一薪酬制度、统一企业文化的背景下,不同项目的运作管理存在员工队伍的稳定性较大差异这一现象入手,通过深入分析,论证了项目总监的管理水平、意识和能力是造成项目员工稳定性差异的内在根本原因。进而提出了如何通过提高总监协调、沟通和管理能力,从而提高员工队伍稳定性的可能措施和途径。     

Owl Shaped You

The Shang Dynasty(16th Century B.C.-11th Century B.C.)Height:20.5 cmBronzePreserved in Shanghai MuseumYou is a vessel to contain wine.This You is in the zoomorphic shape of two owls standing back to back.The cover is shaped as     

Experimental methods designed for measuring corrosion in highly resistive and inhomogeneous media

Corrosion measurement in highly resistive and inhomogeneous media can be very challenging due to practical difficulties in setting up and maintaining testing cells and sensors, problems associated with IR potential drops and nonuniform polarisation current distribution, and limitations associated with localised corrosion monitoring and detection. Many technological innovations have been made over the past decade to address these issues with reports scattered throughout the literature. This paper provides an overview of innovative experimental methods designed for corrosion testing and monitoring in highly-resistive and inhomogeneous media, with particular focus on localised corrosion measurement using electrode array and electrochemical noise techniques.     

Understanding the effects of electrode inhomogeneity and electrochemical heterogeneity on pitting corrosion initiation on bare electrode surfaces

The effects of electrode inhomogeneity (EI) and electrochemical heterogeneity (EH) on pitting corrosion initiation have been analysed by revisiting research findings reported in the literature and experimental evidences obtained in our laboratories using the wire beam electrode (WBE) method. Two mechanisms of pitting corrosion initiation have been identified on bare metal surfaces exposed directly to electrolytes. For WBE surface under free corrosion or low anodic polarisation conditions the initiation of pitting corrosion was found to be due to the disappearance of minor anodes, leading to accelerated dissolution of a few remaining major anodes. The nucleation stage of pitting corrosion appeared to be controlled by EI, while the propagation stage appeared to be determined by EH. For WBE surface under large anodic polarisation the initiation of pitting corrosion was found to be due to the formation of active new anodic sites, which is in agreement with the conventional mechanism of pitting nucleation.     

基于多孔介质热弹性理论的井壁诱导缝成因

常规纯弹性理论通常把钻井过程中直井井壁诱导缝的形成归结为钻井液密度过高或者水平主应力差值过大,然而该理论却并不能完全合理地解释深井井壁诱导缝的成因。为揭示井壁诱导拉伸缝形成机理,考虑钻井液循环期间低温钻井液与高温井壁围岩的热交换,基于多孔介质热弹性力学理论,建立了适于深部地层的井壁稳定分析模型,研究了流固热耦合作用下的井周周向有效应力、孔隙压力与温度分布计算模型。计算结果表明:(1)在最大水平地应力方位,井眼钻开初期多孔弹性作用居于主导,孔隙压力在近井壁地带出现谷值,对诱导缝的形成起到了抑制作用;(2)然而随着钻井液与地层热交换的进行,钻井液的冷却作用愈加重要,井周周向有效应力由挤压状态逐渐转变为拉伸状态,从而导致近井壁地带诱导缝的形成。结论认为,为避免诱导缝的形成,在工程上一方面需加强钻井液的封堵性,控制液柱压力向井周的扩散;另一方面也可以通过调整优化钻井液密度与流变性能,控制井底钻井液当量密度(ECD)。     

Deflection behaviour of plate girders loaded in shear at elevated temperatures

This paper presents a theoretical investigation about out-of-plane and in-plane deflection behaviour of plate girders loaded in shear at elevated temperatures at the pre- and post-buckling stages. Theoretical elastic analysis of plate out-of-plane deflection employs an approximate approach using a combination of classical plate theory and Galerkin’s method. Effects from initial out-of-plane imperfections have been addressed, with the considerations of various varying degrees of thermal restraint. These elastic analytical solutions are compared with experimental results and numerical predictions. In addition to an out-of-plane deflection investigation, a combination of shear strength theory for plate girders and energy method is adopted to describe the in-plane vertical deflection of plate girders. The advantages and implementations of these methods are also discussed.     

我国城市空间可持续利用问题探析

该文从城市空间基本经济属性入手,在我国城市空间急剧膨胀的前提下,分析我国实现城市空间可持续利用的诸多动因,指出城市空间可持续利用是我国城市化可持续进行的重要内容。