Modelling of inelastic bending of a metal sheet with thermal coupling

The modelling and numerical analysis of inelastic bending of a metal sheet are presented. The distribution strains and stresses are found for slow and fast bending while taking into account the geometrical and material non-linearities. The constitutive relation of Klepaczko, (Int. J. Plast. 17 (2001) 87, Comput. Methods Appl. Mech. Eng. 175 (1999) 19) was used which includes strain hardening, strain rate sensitivity and temperature effect in material behaviour. The large strains are assumed under plane strain conditions. The results with complete thermal coupling are compared with solutions obtained in the isothermal conditions of bending.     

Influence of processing parameters on wear resistance of nanostructured OFHC copper manufactured by large strain extrusion machining

The tribological behaviour of three ultra-fine grained oxygen-free high conductivity (OFHC) copper materials manufactured by large strain extrusion machining (LSEM), under variable shear strain, is studied under ball-on-flat reciprocating configuration and compared with that of conventional microstructured copper. The results are discussed as a function of microstructure and sliding direction. The lowest wear volume is obtained when the sliding takes place in the perpendicular direction to that of grain orientation. The highest wear resistance is observed for nanostructured copper material with an elongated grain structure in the extrusion direction. The wear resistance of this anisotropic material depends on the sliding direction. Wear mechanisms are discussed from scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) observations.     

GENERATION AND TEXTURE OF SURFACES IN ULTRAPRECISION CUTTING OF COPPER

Abstract

The texture of diamond-machined optical-quality surfaces on polycrystalline metals is strongly influenced by the anisotropic elastic and plastic properties of the work material. Plunge-cut experiments were carried out on coarse-grained OFHC copper specimens to examine the correlations between surface topography, process forces, and the orientation of individual grains. Three distinct surface microtopographies were found on individual grains regardless of cutting speed and tool radius. These microtopographies are closely linked with different cutting force levels, which also appear to influence burr formation in the cutting process. The crystallographic orientations of individual grains, which reveal different microtopographies after machining, were determined by X-ray diffraction to correlate the generated surface texture with the material properties.     

The influence of atmospheric humidity on the abrasive wear of metals

A pin-on-disc apparatus has been used to obtain continuous simultaneous measurements of the wear and friction (sliding force) behaviour of metals on bonded silicon carbide abrasive paper under conditions of controlled humidity. Iron, mild steel and copper exhibit qualitatively similar wear behaviour: the wear rate decreases progressively with the number of passes over the same track. In contrast, the wear rate of titanium remains constant. Variation in atmospheric humidity has little effect on the wear rates of copper or titanium, although a slight effect was found in mild steel and iron. A stronger dependence on humidity was found in the friction behaviour of all four metals, as well as a corresponding relationship between humidity and the specific energy required for metal removal by abrasion. Preliminary results from single-particle scratch tests reveal changes in the contact between a single silicon carbide particle and a polished iron surface at different humidity levels. Although only tentative explanations can be made at this stage for these effects, it is evident that any proposed mechanism must account for the behaviour of both the metal and the abrasive together, rather than of one component of the system alone.     

GENERATION AND TEXTURE OF SURFACES IN ULTRAPRECISION CUTTING OF COPPER

The texture of diamond-machined optical-quality surfaces on polycrystalline metals is strongly influenced by the anisotropic elastic and plastic properties of the work material. Plunge-cut experiments were carried out on coarse-grained OFHC copper specimens to examine the correlations between surface topography, process forces, and the orientation of individual grains. Three distinct surface microtopographies were found on individual grains regardless of cutting speed and tool radius. These microtopographies are closely linked with different cutting force levels, which also appear to influence burr formation in the cutting process. The crystallographic orientations of individual grains, which reveal different microtopographies after machining, were determined by X-ray diffraction to correlate the generated surface texture with the material properties.     

Plastic bulging of sheet metals at high strain rates

The biaxial bulge test is a material test for sheet metals to evaluate formability and determine the flow stress diagram. Due to the biaxial state of stress induced in this test, the maximum achievable strain before fracture is much larger than in the uniaxial tensile test. A new dynamic bulge testing technique is simulated and analyzed in this study which can be performed on a conventional split Hopkinson pressure bar (SHPB) system to evaluate the strain-rate dependent strength of material at high impact velocities. Polyurethane rubber as pressure carrying medium is used to bulge the OFHC copper sheet. The use of hyperelastic rubber instead of fluid as a pressure medium makes the bulge test simple and easy to perform. The input bar of SHPB is used to apply and measure the bulging pressure. The finite element simulation using ABAQUS/explicit and analytical analysis are compared and show good correlation with each other. The results clearly show that as the strain-rate increases, the strength of the OFHC copper increases. From the study, a robust method to determine the material behavior under dynamically biaxial deformation conditions has been developed.     

一种用于热端部件高温蠕变分析的统一型粘塑性本构模型──模型分析之一

为了描述韧性材料的率相关力学行为，提出了一个具有“过应力”特征的统一型粘塑性本构模型。通过模型分析的方法，将该模型与典型的Ｂｏｄｎｅｒ－ｐａｒｔｏｍ统一型粘塑性本构模型进行了对比分析。重点考察了该模型对工程材料三种基本变形过程的描述能力：（１）瞬时塑性。（２）循环塑性。（３）蠕变变形（包括应力松驰、蠕变恢复）。     

Representation of uniaxial creep curves using continuum damage mechanics

A methodology has been developed for the numerical determination of the material constants in the uniaxial creep constitutive equations based on continuum damage mechanics. The method developed overcomes former problems with convergence, and is based on a normalization technique. Temperature variation is included in the model.An algorithm for the digitization of continuous creep curves has been developed that enables a true representation of uniaxial creep behaviour using only 20 data points. Consequently large amounts of data covering many tests may therefore be stored in data bases, and, the method leads to short CPU times for fitting the material model.The algorithm for digitization and the constant determination methods have been applied to data for alloy 800H at 850°C; and, to at 550°C. For both materials good comparisons have been obtained between experimental and predicted uniaxial creep behaviour. The material model has been shown to be suitable for large strain behaviour.Uniaxial creep tests have been carried out on grade 1 cast copper nominal composition 99.99% Cu, 0.005% O₂, B.S. 1035–1037 at 150, 250 and 500°C, and an anisothermal continuum damage mechanics creep model developed for the temperature range 150–500°C. Model predictions are in good agreement with test results.     

Optimization of brazing conditions for OFHC Cu and ASTM A501 low carbon steel by taguchi method

This paper concerns the optimization of brazing conditions for joining the two dissimilar materials Oxygen-free high conductivity (OFHC) copper and ASTM A501 low carbon steel usually used for an Oil-Separator of an air-conditioner using Ag-based (BAg) alloy as a brazing filler metal (BFM). A mixture of 70% N₂ and 30% H₂ gases was used to prevent oxidation of the joints during furnace brazing process. Brazing joint clearance, length, temperature, and time were selected as design parameters that have significant effect on the bonding strength of a brazed joint. Taguchi method was used as a statistical technique for design of experiment (DOE) in an attempt to optimize brazing conditions in terms of shear strength. L₉ (3⁴) orthogonal array was designed for conducting the experiments. The relative influence of design parameters and their interaction on the response were also discussed. The experimental results verified that the brazing conditions predicted in this study by Taguchi method could produce the brazed joint between OFHC copper and ASTM A501 low carbon steel with maximum shear strength.     

An incremental formulation of hereditary constitutive laws applied to uniaxial problems

The paper describes an incremental formulation of hereditary constitutive laws for uniaxial stress states which admits convenient ‘exact’ treatment of this type of constitutive relation for linear and nonlinear spring/dashpot models. The formulation and solution technique introduced here are applied to the detailed analysis of imperfect viscoelastic columns made of ice or concrete.The difference between such an ‘exact’ treatment and solutions based on approximate constitutive laws, in which hereditary effects are neglected, is analyzed. It is shown that the apparent increased material rigidity associated with such a constitutive approximation, referred to as Shanley's hypothesis, can significantly affect the behaviour of columns made of this type of material and may even lead to totally erroneous conclusions.     

DEVELOPMENT OF AN X-RAY FLUORESCENCE SPECTROMETER FOR ENVIRONMENTAL SCIENCE APPLICATIONS

The performance of a new X-ray spectrometer is presented. The device was designed originally to be employed in environmental sciences, and allows the use of different types of primary radiation sources, such as a radioactive source (²⁴¹Am) or tubes with anodes of different metals (Rh or W). Among the advantages of this spectrometer are the possibility of exchanging detectors to improve efficiency at different X-ray energy ranges [such as Si (Li), Si-PIN, hyperpure Ge, or CdTe detectors], to attach it to a wavelength dispersive system, and its use in vacuum or atmospheric pressure. Furthermore, it is feasible to modify the geometry so a secondary target may be installed for using polarized X-rays as exciting radiation.     

切削深度对超精密切削过程影响的有限元分析

基于大变形有限元理论和更新的拉格朗日方程式 ,建立了热耦合的平面应变正交切削有限元仿真模型。采用通用的商业非线性有限元软件 ,对无氧铜超精密切削加工过程进行了仿真 ,研究超精密切削过程中切削深度对切削力、残余应力、等效应力、等效应变和切削温度的影响。通过对超精密切削有限元仿真结果的分析 ,可以优选出合理的切削参数。     

HIGH STRAIN RATE SHEAR EVALUATION AND CHARACTERIZATION OF AISI D2 TOOL STEEL IN ITS HARDENED STATE

High strain rate mechanical testing on fully hardened AISI D2 tool steel (at 62 HRc) was performed utilizing the Compressive Split Hopkinson Bar technique (CSHB) incorporating a punching shear strain state. The high strain rate conditions were comparable to those encountered in machining processes, with shear strain rates on the order of 5 × 10⁴ s^-1 and shear strains in excess of unity (100% mm/mm). The tests were performed at various initial temperatures ranging from 296-873 K to investigate the flow stress behavior of the hardened tool steel as a function of temperature. The high strain rate experimental shear stress-strain data was used to fit the flow stress by; i) an empirically based constitutive law in the form proposed by Johnson and Cook; as well as, (ii) a physically based constitutive law proposed by Zerilli and Armstrong which accounts for strain, strain rate, and temperature dependence of flow stress. The data incorporated the adiabatic temperature rise in the shear zone and was used in the constitutive law modeling. The deformed microstructure was investigated using optical and scanning electron microscopy to determine the extent of the shear localization zone and the final fracture mode.     

Influence of electronic structure on the friction in vacuum of 3d transition metals in contact with copper

Fundamental mechanisms of the friction in vacuum of 3d transition metals in contact with copper were investigated by Auger emission spectroscopy. The electronic structure of the metals was shown to be a better indication of frictional behaviour than compatibility ratings previously obtained     

Use of crystal plasticity in metal forming simulations

Some recent work on the implementation of crystal plasticity constitutive relations in the simulation of metal deformation processes is discussed. The rate-dependent crystal plasticity theory employed is first briefly presented. We then consider some special cases of fcc metals with certain “ideal textures”, for which closed-form analytical solutions are given for rolling and biaxial sheet stretching modes. Finally, the incorporation of the crystal plasticity constitutive relations into the finite element analysis of large-strain boundary-value problems is illustrated. The example treated is the behaviour of axially constrained and freely elongating solid circular bars subject to finite strain torsion.