Analysis of crack tip plasticity for microstructurally small cracks using crystal plasticity theory

Crack tip plastic zone sizes and crack tip opening displacements (CTOD) for stationary microstructurally small cracks are calculated using the finite element method. To simulate the plastic deformation occurring at the crack tip, a two-dimensional small strain constitutive relationship from single crystal plasticity theory is implemented in the finite element code ANSYS as a user-defined plasticity subroutine. Small cracks are modeled in both single grains and multiple grains, and different crystallographic conditions are considered. The computed plastic zone sizes and CTOD are compared with those found using conventional isotropic plasticity theory, and significant differences are observed.     

Fabrication of sharp silicon tips employing anisotropic wet etching and reactive ion etching

We developed a process to obtain sharper silicon tips by employing anisotropic etching in a KOH solution followed by SF₆ plasma etch. The tips were further sharpened using the established thermal oxidation technique to decrease the cone angle and, therefore, obtain smaller curvature radii. We have analyzed the impact of such changes in geometry on a figure of merit associated with the field emission characteristics. An increase in the figure of merit by a factor of three was found in relation to the tips before sharpening.     

Oscillatory crack tip triangular elements for finite element analysis of interface cracks

In this paper a special crack tip element has been developed in which displacements and stresses have the same behaviour as those of bi‐material interface cracks with open tips. The element degenerates into a traditional triangular quarter point element in cases of homogeneous cracks. An isoparametric co‐ordinate system (ρ, t) is defined in this study, and numerical techniques using these co‐ordinates to evaluate Jacobian matrices, shape function derivatives, and element stiffness matrices are developed. Also, equations calculating the complex stress intensity factor using displacements are obtained in this study. Numerical results are in good agreement with known analytical solutions in two examples. Copyright © 2003 John Wiley & Sons, Ltd.     

Experimental and analytical study on the spray characteristics of dimethyl ether (DME) and diesel fuels within a common-rail injection system in a diesel engine

This paper investigates the effect of injection parameters on the characteristics of dimethyl ether (DME) as an alternative fuel in a diesel engine with experimental and analytical models based on empirical equations. In order to study macroscopic and microscopic characteristics of DME fuel, this work focuses on the atomization characteristics of DME and compares experimental and predicted results for spray development obtained by empirical models for diesel and DME fuel. Detailed comparisons of spray tip penetration from three different empirical correlations and from visualization experiments of diesel and DME fuels were conducted under various fuel injection conditions. In comparison with the results of different empirical equations for measured spray tip penetration, the experimental results of this study provide good agreement with the calculation results based on empirical equations, except during the earliest stage of the injected spray sequence. The results of atomization characteristics indicate that DME showed better spray characteristics than conventional diesel fuel. Also, the fuel injection delay and maximum injection rate of DME fuel are shorter and lower than those of diesel fuel at the same injection conditions, respectively.     

扫描探针制备技术的研究现状及发展

探讨了近几年来探针制备技术的发展和趋势,分别就探针检测性能的改进、探针结构、探针检测功能拓展及多通道信息采集组合探针等几方面,对相关制备技术的方法、特点以及相应检测条件、适用环境展开讨论,这些方面反映探针制备技术正朝着尖锐化、多样化、功能化和组合方向发展。     

RETARDATION AND REPAIR OF FATIGUE CRACKS BY ADHESIVE INFILTRATION

Abstract— This paper presents results which demonstrate that polymeric filler materials, such as low-viscosity epoxies, can be vacuum-infiltrated into fatigue cracks in 7050 aluminium alloy to produce significant levels of fatigue crack retardation. It was found that the main test variable affecting the degree of retardation was the stress level at which the adhesive was introduced and cured. Two infiltrated adhesives were tested.
Infiltration at 0% (of the original) peak fatigue stress level produced negligible retardation, while infiltration at the 80% stress level produced about 300% increase in fatigue life for one adhesive and 3000% for the other adhesive. For the highest infiltration stress level both crack-face wedging and adhesion contributed initially to the retardation, but the adhesive component ceased after a crack grew through the adhesive to the original crack tip position. The results are discussed in terms of the applicability of the technique to highly-stressed aircraft components.     

抗弹陶瓷的犁削机理和自锐作用研究

研制了铬刚玉抗弹陶瓷，其抗弹能力比标准均质钢有很大的提高，通过分析讨论，其主要抗弹机制为犁削机理和自锐作用。     

Fe calculations of stress fields from cracks located at the fusion line of weldments

This paper investigates the stress fields for a crack located at the fusion line of a weldment. The strength mis-matching and the size of the HAZ were varied, and the corresponding distribution of the maximum principal stress was examined. The weld metal strength was globally overmatched with respect to the base material, but locally over- and undermatched with respect to the heat affected zone. Three cases of mis-match were compared, and it was found that reducing the strength of the HAZ lowered the maximum principal stresses.     

IN-SITU OBSERVATION OF CRACK TIP DEFORMATION IN A DUPLEX STAINLESS STEEL

The development of dislocation structures in the plastic zone ahead of a crack tip has been in-vestigated in a duplex stainless steel during in-situ deformation experiments in a scanningtransmission electron microscope.It was found that the dislocation distribution wassignificantly different in the ferrite and in the austenite.In the ferrite grains,the dislocationsemitted by the crack tip may cross-slip out of the original slip planes and form a broad plasticzone.However,in the austenite,the dislocation free zone is small and the dislocations emittedby the crack pile up in its slip plane.The selection of slip systems at the crack tip depends onthe crack tip Schmid factors in both phases.But after large deformation,the selection of thesecond slip systems at the craek tip in austenite does not depend on the Schmid factors.