微细特征模压成形实验研究与数值模拟

采用微孔模具,改变润滑条件和微孔尺寸,通过万能试验机对不同厚度的纯铜试件进行了微细特征模压成形的实验研究,同时对成形过程进行了数值模拟,分析并给出了微细特征模压成形时的主要变形区域以及成形力的变化特点.实验和模拟结果表明:微细特征尺寸越大,润滑状态越好,成形越容易;适当的坯料厚度可以降低成形的难度;微细特征成形在坯料较...     

A stress distribution remodelling technique

The stress distribution ahead of a notch is of great practical interest when undertaking fatigue and fracture analyses. In particular it is generally the first principal stress close to the notch which is desired. For a sharp notch this can be characterized by the stress field parameter K ^N which is referred to as the notch stress intensity factor (or N-SIF). The finite element method is a very powerful tool which is commonly used to determine K ^N . However, unless specialized methods are used the finite element mesh must be extremely refined in the region of the notch in order to calculate an accurate value. In practical situations, the degree of mesh refinement necessary is often not possible, due to either time or computer limitations. The following describes a simple technique which can be used to accurately determine the stress distribution close to a sharp notch, by remodelling or reshaping a stress distribution that has been obtained from a finite element analysis using a coarse or inadequate mesh. A theoretical equation for defining the principal stress distribution ahead of a sharp notch, which has been developed by Atzori et al. (2005) is used to do this. It is shown that the theoretical distribution can be explicitly determined from the finite element distribution by using global equilibrium conditions. It is shown that this technique is independent of the finite element mesh size. The method is used to calculate K ^N for seven different combinations of geometry and loading condition, using various FE mesh refinement. It is shown that the results are accurate to within 15%.     

Strain behaviour of aluminum 99.5 in micro-forming conditions of the coining process

The coining process can be identified as a shallow die forging. By using this process, it is possible to produce very fine surface geometries. This leads to dimension scaling into micro dimensions and causes the specific material behavior. In the micro area, continuum laws are not strictly followed anymore. The leading role in determining the deformation and strain level is taken by the fine surface geometry dimension and the size of the crystalline grain of the workpiece material. This is known as the size effect and defines all micro-forming processes. In order to provide monitoring of the filling of the smallest die dimensions, additional supporting procedures should be used. In this case, radiography testing procedures for observing the specific points of the workpiece geometry are applied, characterized with very small dimensions (less than 0.5 mm). Testing aluminum samples are formed using two techniques – open and closed die coining, with identical surface geometry. Their crystalline structures are in two-grain sizes of 34 μm and 80 μm. Scanning results show different material behaviour and different surface deformation for the same level of forming force in all four testing cases.     

A superconvergent stress recovery technique for accurate boundary stress extraction

An element‐base superconvergent stress recovery technique is developed for accurate boundary stress extraction. In the present method, higher‐order stress fields are assumed for all stress components and higher order elements are used for the construction of necessary matrices. Unknown coefficients for the assumed stress field are obtained by minimizing the sum of the stress errors and two equilibrium residuals in the least squares sense. The two residuals are derived based on the principle of virtual work. Numerical examples including a three‐dimensional problem have demonstrated that the present method can stably extract very accurate boundary stresses even for coarse meshes. Copyright © 1999 John Wiley & Sons, Ltd.     

An experimental-numerical hybrid technique for two-dimensional stress analysis

A numerical procedure for obtaining the stress components in the interior of a two-dimensional body from the values of stresses measured along the boundary, is presented. In this method the two-dimensional elasticity equations are re-written in a convenient form and solved using the finite difference technique along with the boundary stress values. The application of the method is illustrated with examples.     

Oscillating-cup technique for yield stress and density measurement

We discuss a possibility for simultaneous measurement of different viscometric properties by the oscillating-cup technique. The emphasis is on the following problems related to liquid metal viscometry: identification of non-Newtonian fluid flow and combined viscosity/density measurement. The cases of Bingham and Newtonian fluids are analysed in detail. The basic features of nonlinear fluid flow, viscometer oscillations, modelling of experiment and data processing are systematized and interpreted in terms of linear viscous fluids.     

A controlled-unloading technique for determination of the long-range internal stress

A new method has been presented for measuring the internal stress of a metal for uniaxial deformation. The technique has the benefits that long times for measurement are not necessary nor are instantaneous load decrements. One drawback is that special electronic equipment is necessary to facilitate the experiments.     

The stress analysis of Si MEMS devices by micro-Raman technique

In this paper, the micro-Raman technique was used to measure the residual stress and its variation in Si MEMS devices and carried out on two platforms. Firstly, it was observed that there is a good correlation between the wafer's centre deflection and Raman shift. Secondly, using diaphragms fabricated from a SOI wafer, it was observed that a thinner diaphragm results in more Raman shift and vice versa. After a thin metal film layer (Cr/Au) is sputtered onto the diaphragm, it was observed that the Raman shift is more significant as the metal layer represents an amplification effect. These results show that the micro-Raman technique is a powerful method to investigate the residual stress of Si MEMS devices, especially how the stress changes during the fabrication process. All these could help select and optimize the fabrication parameters during the device manufacturing process.     

TERSA—A new technique for assessing residual stress

This paper outlines a completely new technique which could provide the basis of an instrument for the non-contacting and non-destructive determination of static and residual stresses in engineering components, assemblies and structures. The method has been given the acronym of TERSA, standing for Thermal Evaluation for Residual Stress Analysis and would be applicable to a range of metals, both ferrous and non-ferrous and also to plastics. Some experimental work is described and results are presented which confirm the principle of the proposal.     

Theory of the double-edge technique for Doppler lidar wind measurement

The theory of the double-edge technique is described by a generalized formulation that substantially extends the capabilities of the edge technique. It uses two edges with opposite slopes located about the laser frequency. This doubles the signal change for a given Doppler shift and yields a factor of 1.6 improvement in the measurement accuracy compared with the single-edge technique. Use of two high-resolution edge filters reduces the effects of Rayleigh scattering on the measurement by as much as an order of magnitude and allows the signal-to-noise ratio to be substantially improved in areas of low aerosol backscatter. We describe a method that allows the Rayleigh and aerosol components of the signal to be independently determined. The effects of Rayleigh scattering are then subtracted from the measurement, and we show that the correction process does not significantly increase the measurement noise for Rayleigh-to-aerosol ratios as high as 10. We show that for small Doppler shifts a measurement accuracy of 0.4 m/s can be obtained for 5000 detected photons, 1.2 m/s for 1000 detected photons, and 3.7 m/s for 50 detected photons for a Rayleigh-to-aerosol ratio of 5. Methods for increasing the dynamic range to more than +/-100 m/s are given.     

A modified mapping-collocation technique for accurate calculation of stress intensity factors

A technique combining the advantages of conformal mapping and boundary collocation arguments for calculating stress intensity factors for cracks in plane problems is described. The difficulty of finding the mapping function on a rigidly prescribed parameter region is avoided at the expense of using boundary collocation methods on part of the boundary. Conventional collocation arguments are modified by prescribing stress, force, and moment conditions in a least-square collocation sense. These pseudo-redundant conditions provide a reasonable basis for estimation of the effects of inaccuracy of the boundary conditions. The technique is applied to the problem of a circular disk with an internal crack under a loading of external hydrostatic tension.

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Zusammenfassung Es wird ein Verfahren zur Berechnung von Spannungsintensitä tsfaktoren für Riße in einem ebenen Zustand beschrieben, welches sowohl die Vorteile der Methode der konformen Darstellung als auch die der Verfahren zur Bestimmung der Grenzbedingungen miteinander verbindet.Die Anwendung der Verfahren der Festlegung von Grenzbedingungen für einen Teil der Außenlinie ermöglicht es die Schwierigkeiten zu umgehen, welche sich dann ergeben wenn man versucht die darstellende Funktion für einen Bereich mit streng auferlegten Parameter zu bestimmen. Die konventionelle Behandlung dieser Verfahren wird dadurch abgeändert, daß die Bedingungen für Spannungen, Kräfte and Momente im Singe der kleinsten Quadratzahlen auferlegt werden.Diese pseudo-überflüssige Bedingungen ergeben eine Basis zur Beurteilung der Auswirkung einer Ungenauigkeit in der Definition des Umrisses.Diese Methode wird auf das Problem einer runden Scheibe mit inneren Rissen, welche der Wirkung von äußeren Spannungen unterworfen ist, angewendet.

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Résumé On décrit une technique de calcul des facteurs d'intensité de contraintes pour des fissures en état plan, qui combine les avantages de la méthode de la representation conforme et des méthodes de fixation des conditions aux limites.La difficulté que l'on rencontre à trouver la fonction de représentation qui correspond à une région à paramètres imposés est levée par l'emploi des méthodes de fixation des conditions aux limites sur une partie d'un contour. Le traitement conventionnel de ces méthodes est modifié en imposant les conditions de contraintes, de forces et de moments en un ajustement par moindres carrés.Les conditions pseudo-redondantes ainsi réunies procurent une base d'appréciation des effets d'une inexactitude dans la définition du contour.La technique est appliquée au problème du disque circulaire comportant une fissure interne et soumis a faction d'une tension extérieure uniforme.

     

A technique for experimental evaluation of mixed mode stress intensity factors

High sensitivity moiré interferometry was used to obtain whole-field orthogonal components of displacement in the vicinity of cracks in centre-cracked plates of aluminium 6082. The plates had been prepared in such a way as to enable the application of load at angles of 75° and 60° to the planes of the fatigue pre-cracks. The form of the moiré fringe data enables convenient separation of the components of stress intensity factor K _I and K _II. The results are compared with analytical predictions. The method is, in principle, suitable for use with specimens of arbitrary geometry and loading.     

Theory of the double-edge molecular technique for Doppler lidar wind measurement

The theory of the double-edge lidar technique for measuring the wind with molecular backscatter is described. Two high-spectral-resolution edge filters are located in the wings of the Rayleigh-Brillouin profile. This doubles the signal change per unit Doppler shift, the sensitivity, and improves measurement accuracy relative to the single-edge technique by nearly a factor of 2. The use of a crossover region where the sensitivity of a molecular- and an aerosol-based measurement is equal is described. Use of this region desensitizes the molecular measurement to the effects of aerosol scattering over a velocity range of +/-100 m/s. We give methods for correcting short-term, shot-to-shot, frequency jitter and drift with a laser reference frequency measurement and methods for long-term frequency correction with a servo control system. The effects of Rayleigh-Brillouin scattering on the measurement are shown to be significant and are included in the analysis. Simulations for a conical scanning satellite-based lidar at 355 nm show an accuracy of 2-3 m/s for altitudes of 2-15 km for a 1-km vertical resolution, a satellite altitude of 400 km, and a 200 km x 200 km spatial resolution.     

Application of moiré technique in the stress analysis of cutting tools

Knowledge of the stress field in cutting tools is necessary for their strength evaluation. However, the experimental analysis of stresses in cutting tools during machining poses a number of problems in view of the steep stress gradient present. Photo-elasticity and the 'built-up tool' methods are the common techniques used for this purpose. In the present paper results on the stresses in cutting tools obtained during actual machining through the successful application of the transmitted moiré technique, are presented. In these experiments transparent epoxy tools carrying moiré gratings were used to machine commercial lead alloy at low speeds. The tool rake angle was varied. The effectiveness of moiré method, basically a displacement analysis technique, in providing useful data on tool stresses comparable with the other direct stress analysis techniques such as photo-elasticity is also discussed in the paper.