Analysis of strain localization for ductile materials with effect of void growth

In the framework of thermodynamics and damage mechanics, an elastoplastic constitutive relation of ductile materials is developed. Based on this constitutive relation, the properties of strain localization for ductile materials are investigated with the effect of void growth included. The influences of void growth on strain localization are analyzed based on the unified strength criterion. Finally, the effects of yield criteria on the properties of strain localization are discussed.     

Modeling of flow stress in orthogonal micro-cutting process based on strain gradient plasticity theory

The rapidly increasing demand for miniature components machining processes has drawn more attention to micro-machining research. Flow stress has always been a significant base for analyzing plastic deformation in machining processes. However, few studies have been conducted to predict accurately the material flow stress in the micro-cutting processes. In order to describe size effect in micro-cutting, this paper discusses the development of a circular primary deformation zone model, calculates the strain gradient in the primary zone, and presents a new flow stress model based on the theory of strain gradient plasticity. First, a series of orthogonal cutting experiments are performed and flow stress is calculated from the experiment data. Results from the proposed model have been successfully validated with experimentally determined results. It shows that the flow stress in micro-cutting is influenced greatly by the feed rate and the cutting edge radius.     

考虑应变梯度效应的低碳钢试样单向拉伸尺寸效应理论研究

采用二阶应变梯度塑性理论，得出伸长率、峰值应力后工程应力应变曲线和真应力应变曲线的解析解，结果表明三者均具有尺寸效应。将低碳钢试样的单轴拉伸简化为轴向的一维问题。采用拉伸过程中体积不变假设及平衡条件，将应变梯度引入屈服函数，得到伸长率、平均应变及真应变的解析式。理论结果完全反映伸长率与标定长度呈反比的客观事实。工程应力应变曲线尺寸效应可以在数值结果中找到佐证。拉应变局部化是三者尺寸效应的本质原因。     

基于递推梯度控制的智能二级张力控制系统

对慢走丝电火花线切割机床的放丝轴电机的仿人智能控制系统给出了一种递推梯度形式的控制规律。分析了其基本性质,以此为基础,给出了一种能保持张力恒定的反扭矩放丝轴电机控制系统的控制规律和算法,并进行了仿真研究。     

Hall effect on unsteady couette flow with heat transfer under exponential decaying pressure gradient

The unsteady Couette flow of an electrically conducting, viscous, incompressible fluid bounded by two parallel non-conducting porous plates is studied with heat transfer taking the Hall effect into consideration. An external uniform magnetic field and a uniform suction and injection are applied perpendicular to the plates while the fluid motion is subjected to an exponential decaying pressure gradient. The two plates are kept at different but constant temperatures while the Joule and viscous dissipations are included in the energy equation. The effect of the ion slip and the uniform suction and injection on both the velocity and temperature distributions is examined.     

A modified micromechanical approach to determine flow stress of work materials experiencing complex deformation histories in manufacturing processes

Work materials experience a broad range of strains, strain rates, and temperatures in many manufacturing processes such as machining, forming, etc. Strain rate has an important effect on the yield and flow stress of work materials, especially metals, since at higher strain rates there is less time for thermally activated events; consequently, it is equivalent to a lowering of the temperature of the materials. On the other hand, it is also true that, for high strain rate deformations such as metal cutting, adiabatic plastic flow may produce significant temperature changes in the materials. Flow stress is significantly affected by the strain rate history; hence, mechanical behavior may not be fully described in terms of a mechanical equation of state relating the instantaneous stress, strain, strain rate, and temperature.Based on the concept of dislocation mechanics, a micromechanical approach with the new concept of temperature coefficient has been explored to overcome the model issues such as negative or constant flow stress above the critical temperatures. The flow stresses of aluminum 6061-T6 and titanium Ti-6Al-4V have been predicted, for the first time, using the modified micromechanical model based on the available baseline high strain rates test data. The constitutive model was further modified and extended to predict flow stress below as well as above the critical temperature. The corresponding model predictions were compared with the experimental data, attaining good agreement.     

能坡法流量计算在水文在线测流系统的应用研究

通过石头水文站使用能坡法流量计算在水文在线测流系统的应用研究,总结、寻找出一种实时性强、测验精度又高的在线测流方法,从而为受人类活动影响较大的测站提供一种高效、快速的水文测验新方法。     

Microstructure evolution and strain localization in Cu and Cu-8Al single crystals subjected to channel-die compression

Single crystals of pure Cu and Cu‐8%Al with two initial orientations, {112}〈111〉 and {112}〈110〉, were subjected to monotonic compression in channel‐die at room temperature (293 K). The dislocation microstructure and local crystallography were investigated by transmission electron microscopy after different amounts of deformation. Various factors, such as initial single crystal orientation, chemical composition and amount of plastic deformation, were analysed in order to determine their influence on the microstructure evolution, local orientation variations and strain localization phenomena.     

Effects of texture gradients and strain paths on localization phenomena in polycrystals

Applications of crystal plasticity theory to the numerical modelling of large strain plasticity phenomena are considered. In particular, instabilities and localized deformation phenomena for FCC polycrystals subjected to various deformation modes are investigated. In-house finite element analyses based on a rate-dependent crystal plasticity model have been developed to simulate the large strain behaviour for sheet specimens subjected to plane strain, plane stress, and simple shear deformation modes. In the formulation, the plastic deformation of an individual crystal is assumed to be due to crystallographic slip. In the simulations, polycrystalline aggregates are modelled at various scales. This formulation accounts for initial textures, as well as texture evolution during large plastic deformations. The numerical analyses incorporate parallel computing features. The results of simulations for the above-mentioned deformation modes are discussed, and the formation of localized deformation in the form of shear bands is investigated.     

Effects of axial temperature gradient on momentum and heat transfer with oscillating pressure and flow

Effects of axial temperature gradient on heat transfer, momentum transfer and energy conversion mechanisms within a closed cylinder-piston apparatus are analyzed. Assuming that the gas density change is small, the first-order and steady second-order solutions of continuity, momentum and energy equations are obtained. The solutions show that there exists a steady circulating flow and the magnitude of the steady axial velocity increases as the axial temperature gradient increases. There exists not only an oscillating component of heat flux between the gas and the wall, but also a steady component whose direction depends on axial temperature gradient. It is shown that heat is pumped from the wall near the piston to the wall near closed-end for negative axial temperature gradient. Heat transfer relation for both oscillating pressure and oscillating flow conditions is proposed.     

Vibration analysis of nano-structure multilayered graphene sheets using modified strain gradient theory

In this paper, for the first time, the modified strain gradient theory is used as a new size-dependent Kirchhoff micro-plate model to study the effect of interlayer van der Waals (vdW) force for the vibration analysis of multilayered graphene sheets (MLGSs). The model contains three material length scale parameters, which may effectively capture the size effect. The model can also degenerate into the modified couple stress plate model or the classical plate model, if two or all of the material length scale parameters are taken to be zero. After obtaining the governing equations based on modified strain gradient theory via principle of minimum potential energy, as only infinitesimal vibration is considered, the net pressure due to the vdW interaction is assumed to be linearly proportional to the deflection between two layers. To solve the governing equation subjected to the boundary conditions, the Fourier series is assumed for w=w(x, y). To show the accuracy of the formulations, present results in specific cases are compared with available results in literature and a good agreement can be seen. The results indicate that the present model can predict prominent natural frequency with the reduction of structural size, especially when the plate thickness is on the same order of the material length scale parameter.     

Numerical simulation of fully developed flow and heat transfer characteristics in a curved tube with pulsating pressure gradient

Characteristics of fluid flow and convective heat transfer of a pulsating flow in a curved tube have been investigated numerically. The tube wall is assumed to be maintained at a uniform temperature peripherally in a fully developed pulsating flow region. The temperature and flow distributions over a cross-section of a curved tube with the associated velocity field need to be studied in detail. This problem is of particular interest in the design of Stirling engine heat exchangers and in understanding the blood flow in the aorta. The time-dependent, elliptic governing equations are solved, employing finite volume technique. The periodic steady state results are obtained for various governing dimensionless parameters, such as Womersley number, pulsation amplitude ration, curvature ratio and Reynolds number. The numerical results indicate that the phase difference between the pressure gradient and averaged axial velocity increases gradually up to π/2 as Womersley number increases. However, this phase difference is almost independent of the amplitude ratio of pulsation. It is also found that the secondary flow patterns are strongly affected by the curvature ratio and Reynolds number. These, in turn, give a strong influence on the convective heat transfer from the pipe wall to the pulsating flow. The results obtained lead to a better understanding of the underlying physical process and also provide input that may be used to design the relevant system. The numerical approach is discussed in detail, and the aspects that must be included for an accurate simulation are discussed.     

Effect of strain on atomic-scale friction in layered MoS2

The atomic-scale friction in MoS₂ is investigated employing the density functional theory calculation including the dispersion correction (DFT-D). Energy corrugations and lateral frictional forces of the lamellar MoS₂ are derived, suggesting that the in-plane compressive MoS₂ exhibits lower friction than the tensile system. The reduced friction is attributed to a stronger coulombic repulsive interaction enabled by the transferred charge to the sliding interface. In-depth understanding of the relationship between friction and interfacial interaction shows that friction can be tuned in layered MoS₂ by applying an in-plane strain to the sliding interface.     

Gas flow visualization by means of sheared beam interferometry: sensitivity, maximum measurable gradient of the density fluctuation, and integrated density estimation

A technique for visualizing the density distribution of a gas flow using sheared beam interferometry was analyzed. The interference pattern which arises in the shearing interferometer encodes information about the gradient of the line integral of the density fluctuation along the propagation path of the light. While this method of flow visualization has been previously used, the sensitivity and dynamic range of this technique have not been analyzed. It is also shown that it is possible to obtain quantitative information about the integral of the density fluctuations along the path of light propagation using shearing interferometry. This paper presents the theoretical aspects of this technique, and provides an analysis of the sensitivity and maximum measurable gradient of the line integral of the density fluctuation. Representative results from a laboratory experiment conducted on a shear-layer gas flow are shown.     

Discussion of cyclic plasticity and viscoplasticity of single crystal nickel-based superalloy in large strain analysis: comparison of anisotropic macroscopic model and crystallographic model

The inelastic behavior of nickel-based superalloy is investigated in detail by application of a macroscopic anisotropic plasticity model developed here, and the results are compared to predictions based on crystal plasticity, which incorporates the kinematic hardening. Uniaxial deformation processes and simple shear deformations at large strain are considered. The plastic spin concept coupled with an anisotropic Chaboche model is provided in the framework of macroscopic viscoplasticity. The plastic spin formulation used here is based on the concept of the noncoaxiality between the stress and plastic rate of deformation. The present model succeeds in reproducing the inelastic behavior during large deformation. It is shown that the plastic spin associated with the anisotropic flow rule plays a key role in the macroscopic model. Simulation results find these two different scale models provide similar predictions under uniaxial deformation for [0 0 1] and [1 1 1] orientation, while their predictions for simple shear deformation at large strain exhibit quantitative difference, but their trends are the same. The interpretations for simulation results are pursued in detail.     

Software objects in distributed flow measurements

Increasing complexity of the contemporary industrial measurement systems is a common characteristic. Partitioning of the measurement process into software components that can be realized as separate objects facilitates the development of the measurement system. Using a client/server approach, the measurement system components become shareable and accessible across the network. The components can be deployed anywhere on the network and shared by a considerable number of applications. The paper describes a realization of the flow measurement system with distributed software components, i.e., measurement procedures, which include the concurrent calculation of the measurement results and the corresponding uncertainties.     

Multiple necking in superplastic Zn-22Al under uniaxial tension

The phenomenon of multiple necking in superplastic Zn-22Al was studied experimentally and analytically. Experiments revealed a proportional relation between the wavelength of multiple necking and the square root of the ratio between the strain rate and strain, which is explained on the basis of diffusion of defects under inhomogeneous strain distributions. The order of magnitude of the diffusion coefficient and the activation energy suggest that strain diffusion can result from the flow of atomic vacancies accompanied by grain boundary sliding in the superplastic state. Moreover, the analytic derivation shows the relation between the present model and second-order strain gradient theory.     

Supersonic flow of hydrogen and air in a duct with variable area

A chemically nonequilibrium supersonic flow of hydrogen and air has been investigated in a duct with conically divergent or convergent walls. Elementary reaction schemes of radicals involved in reaction of hydrogen-air have been considered and solved through the CHEMKIN code. The aim was to promote an understanding of characteristics of chemically nonequilibrium supersonic flow by introducing a simple mathematical formulation. The temperature, pressure, and density all were found to decrease for divergent ducts as the flow was accelerated, whereas they increased for a slightly convergent duct or a constant cross-sectional area duct. For the divergent nozzle with a greater degree the flow became chemically frozen. But it was quite necessary to take account of the effect of chemical nonequilibrium in a moderately expanded or all convergent conical ducts. As was expected, it was found that the temperature, pressure and Mach number were reduced for a fuel-lean mixture.     

Optimal localization of complex surfaces in CAD-based inspection

Complex surface inspection requires the optimal localization of the measured surface related to the design surface so that the two surfaces can be compared in a common coordinate frame. This paper presents a new technique for solving the localization problem. The basic approach consists of two steps: 1) rough localization of the measured points to the design surface based on curvature features, which can produce a good initial estimate for the optimal localization; 2) fine localization based on the least-square principle so that the deviation between the measured surface and the design surface is minimized. To efficiently compute the closest points on the design surface of the measured points, a novel method is proposed. Since this approach does not involve an iterative process of solving non-linear equations for the closest points, it is more convenient and robust. The typical complex surface is used to test the developed algorithm. Analysis and comparison of experimental results demonstrate the validity and applicability of the algorithm.