A Review of Dynamic Fracture Studies in Functionally Graded Materials

Abstract:  This article presents a review of dynamic fracture studies on functionally graded materials. A brief literature review on the fracture mechanics of graded materials is presented first. This is followed by a discussion on the higher-order asymptotic analysis of the transient elastic field surrounding the tip of a dynamically growing crack in a functionally graded material. A comprehensive experimental study of dynamic crack growth in model functionally graded material using the optical method of reflection photoelasticity and high-speed photography is then presented. The results are analysed to establish a generalised relationship between the crack velocity and the dynamic mode-I stress intensity factor (SIF). This relationship is found to be unique and is distinctly different from that previously established for the matrix material (polyester). Finally, an innovative experimental procedure is used to demonstrate the necessity of employing a fully transient stress-field representation in the analysis of optical data for an accurate prediction of the dynamic SIF history.     

Study on the anodic film formation process of AZ91D magnesium alloy

The formation of anodic film of AZ91D magnesium alloy has been investigated by means of electrochemical impedance spectroscopy (EIS), cyclic voltammetry, anodic polarization curve, current-time transients and SEM technique. The results show that, under our experimental conditions, the formation of AZ91D anodic film follows the mechanism of 3D nucleation with diffusion controlled growth. With the increase of applied anodizing potential, the nucleation type of anodic film changes from progressive to instantaneous. The results also show that the initially formed anodized film is threadlike and porous, and high potential is essential for the formation of good anodic film with excellent properties.     

A kinetic model for gas adsorption at solid surfaces by surface transient formation

In this paper is presented a model for the kinetics of gas adsorption at solid surfaces, in which transient formation is involved. In the case of ideal adsorption where the rate constants are independent of the surface coverages, the kinetics can be solved exactly in the framework of the first order kinetic law. The expression of the sticking coefficient for the final chemisorbed state is obtained in terms of the rate constants introduced in the model and compared to the expression commonly adopted in the literature. By means of a fitting procedure the kinetics were used to describe experimental data on the dioxygen adsorption at the Zn(0001) surface. An appropriate analysis of the fitting parameters, derived from adsorption data collected at different temperature of the substrate, allows for the activation energy of the processsurface transient final adsorbed species to be estimated.     

Evidence for transient composition variations at GaAs/Ga1−xAlxAs heterostructure interfaces prepared by metal-organic chemical vapour deposition

The composition profiles of GaAs/Ga_1?xAl_xAs heterostructures prepared in two different Metal-Organic Chemical Vapour Deposition (MOCVD) reactors have been studied. Transmission electron microscopy (TEM). and Auger and secondary ion mass spectrometry (SIMS) sputter profiling results are in good agreement and interface widths below 20 å have been achieved. Significant new results on transient phenomena have been obtained. showing that large and sometimes very sharp excursions in composition can be associated with valve switching actions. Direct evidence is presented that gas pressure or flow transients can occur during valve operations. and it is suggested that such instabilities are the cause of the effects observed.     

三电平逆变器供电永磁同步电机直接转矩控制研究

针对临近空间飞行器电驱动系统的高效控制问题,构建一种三电平逆变器供电永磁同步电机直接转矩控制系统,以减少输出电流的谐波含量,降低转矩脉动,提升驱动系统整体效率。针对三电平逆变器空间电压矢量调制(SVPWM)实现复杂的问题,通过对大、小电压矢量选择的归纳研究,利用矢量的对称性和归一化处理方法,提出一种易于通过可编程器件实现的简洁计算方法,实现不同扇区的统一计算;构建参考矢量的复数表达形式,利用极坐标相角信息简化扇角区间判别。采用Matlab/Simulink分别构建传统两电平逆变器和三电平逆变器直接转矩控制系统进行对比分析,仿真结果验证了该算法在减小电流谐波和削弱转矩脉动方面的有效性。     

Numerical Explanation of Slow Transients in an AlGaN/GaN HEMT

观察了AlGaN/GaN HEMT器件在短期应力后不同栅偏置下的一组漏极电流瞬态,发现瞬态的时间常数随栅偏压变化很小,据此判断这组瞬态由电子陷阱的释放引起.为了验证这个判断,采用数值仿真手段计算了上述瞬态.分别考虑了在器件中不同空间位置的电子陷阱,分析了应力和瞬态中相应的陷阱行为,对比和解释了仿真曲线与测量结果的异同.基于上述讨论,提出测量的瞬态可能是表面深陷阱和GaN层体陷阱的综合作用的结果.     

A unified formulation for continuum mechanics applied to fluid–structure interaction in flexible tubes

This paper outlines the development of a new procedure for analysing continuum mechanics problems with a particular focus on fluid–structure interaction in flexible tubes. A review of current methods of fluid–structure coupling highlights common limitations of high computational cost and solution instability. It is proposed that these limitations can be overcome by an alternative approach in which both fluid and solid components are solved within a single discretized continuum domain. A single system of momentum and continuity equations is therefore derived that governs both fluids and solids and which are solved with a single mesh using finite volume discretization schemes. The method is validated first by simulating dynamic oscillation of a clamped elastic beam. It is then applied to study the case of interest—wave propagation in highly flexible tubes—in which a predicted wave speed of 8.58 m/s falls within 2% of an approximate analytical solution. The method shows further good agreement with analytical solutions for tubes of increasing rigidity, covering a range of wave speeds from those found in arteries to that in the undisturbed fluid. Copyright © 2005 John Wiley & Sons, Ltd.     

泵管渠池复杂抽水系统的过渡过程计算

探讨了复杂抽水系统中有压流与无压流数值模拟的求解方法,将其统一于一种特征线求解格式,解决了在渠管衔接处的明满流问题.用该模型对大型泵站中泵管渠池组成的复杂抽水系统的水力过渡过程进行了计算.     

带调压井水电站水力过渡过程试验研究

从工程实际出发,经模型试验和数值仿真计算对带调压井水电站水力过渡过程进行了深入的研究和分析,提出了带调压井水电站水力过渡过程模型试验的研究方法.研究表明,在一定的条件下,利用球阀进行水电站水力过渡过程试验,完全能满足水电站实际工程的需要.