Plastic flow localization in mechanism-based strain gradient plasticity

The theory of mechanism-based strain gradient (MSG) plasticity is used to study plastic flow localization in ductile materials. Unlike classical plasticity, the thickness of the shear band in MSG plasticity can be determined analytically from a bifurcation analysis, and the shear band thickness is directly proportional to the intrinsic material length, (μ/σ_Y)²b associated with strain gradients, where μ is the shear modulus, σ_Y is the yield stress, and b is the Burgers vector. The shear band thickness also depends on the softening behavior of the material. The analytical solution of the shear strain rate yields that the maximum shear strain rate inside the shear band is two orders of magnitude higher than that outside, which is a clear indication of plastic flow localization. The limitation of the present model is also discussed.     

Nonlinear behavior of capacitive micro-beams based on strain gradient theory

This paper studies the size dependent behavior of materials in MEMS structures. This behavior becomes noticeable for a structure when the characteristic size such as thickness or diameter is close to its internal length-scale parameter and is insignificant for the high ratio of the characteristic size to the length-scale parameter, which is the case of the silicon base micro-beams. However, in some types of micro-beams like gold or nickel bases, the size dependent effect cannot be overlooked. In such cases, ignoring this behavior in modeling will lead to incorrect results. Some previous researchers have applied classic beam theory on their models and imposed a considerable hypothetical value of residual stress to match their theoretical results with the experimental ones. The equilibrium positions or fixed points of the gold and nickel micro-beams are obtained and shown that for a given DC voltage, there is a considerable difference between the obtained fixed points using classic beam theory, modified couple stress theory, and modified strain gradient theory. In addition, it is shown that the calculated static and dynamic pull-in voltages using higher order theories are much closer to the experimental results and are higher several times than those obtained by classic beam theory.     

Finite element analysis of the influence of tool edge radius on size effect in orthogonal micro-cutting process

The size effect in metal cutting is evident in the nonlinear scaling phenomenon observed in the specific cutting energy with decrease in uncut chip thickness. It has been argued by many researchers that this scaling phenomenon is caused mainly by the cutting tool edge radius, which purportedly affects the micro-cutting process by altering the effective rake angle, enhancing the plowing effect or introducing an indenting force component. However, the phenomenological reasons why the tool edge radius causes size effect and the relationship between the tool edge radius and the characteristic length scale associated with the size effect in micro-cutting has not been sufficiently clarified. In this paper, a strain gradient plasticity-based finite element model of orthogonal micro-cutting of Al5083-H116 alloy developed recently is used to examine fundamentally the influence of tool edge radius on size effect. The applicability of two length scales—tool edge radius and the material length scale l in strain gradient plasticity—are also examined via analysis of data available in the literature.     

Strain gradient plasticity based finite element analysis of ultra-fine wire drawing process

Steady-state rigid-plastic finite element analysis coupled with strain gradient plasticity theory has been performed to examine the size effect of material on its plastic deformation behavior and find an optimal semi-cone angle of die which minimizes the drawing energy in the ultra-fine wire drawing process. A stream-line tracing method was adopted to calculate strain component at each element and a strain surface function was introduced to compute the equivalent strain gradient of each element. Introduction of this function enables us to use an established FE code without renewal of its main structure. Hence, the constitutive equation in FE formulation is changed to couple the strain gradient plasticity. A series of FE simulation reveals that significant differences in drawing stress are observed when material size approaches its intrinsic material length. When the strain gradient plasticity theory is reflected on the steady-state FE analysis, the optimal semi-cone angle of the die is reduced by 30%. The variation of optimal semi-cone angle is attributable to considerable increment of homogeneous deformation when the material size reaches its intrinsic material length.     

Study of flow instability in a centrifugal fan based on energy gradient theory

Flow instability in a centrifugal fan was studied using energy gradient theory. Numerical simulation was performed for the threedimensional turbulent flow field in a centrifugal fan. The flow is governed by the three-dimensional incompressible Navier-Stokes equations coupled with the RNG k-ε turbulent model. The finite volume method was used to discretize the governing equations and the Semi-implicit method for pressure linked equation (SIMPLE) algorithm is employed to iterate the system of the equations. The interior flow field in the centrifugal fan and the distribution of the energy gradient function K are obtained at different flow rates. According to the energy gradient method, the area with larger value of K is the place where the flow loses stability easier. The results show that instability is easier to generate in the regions of impeller outlet and volute tongue. The air flow near the hub is more stable than that near the shroud. That is due to the influences of variations of the velocity and the inlet angle along the axial direction. With the decrease of the flow rate, instability zone in a blade channel moves to the impeller inlet from the outlet and the unstable regions in different channels develop in opposite direction to the rotation of impeller.     

Deformation analysis of micro-sized material using strain gradient plasticity

To reflect the size effect of material (1–15 μm) during plastic deformation of polycrystalline copper, a constitutive equation which includes the strain gradient plasticity theory and intrinsic material length model is coupled with the finite element analysis and applied to plane strain deformation problem. The method of least square has been used to calculate the strain gradient at each element during deformation and the effect of distributed force on the strain gradient is investigated as well. It shows when material size is less than the intrinsic material length (1.54 μm), its deformation behavior is quite different compared with that computed from the conventional plasticity. The generation of strain gradient is greatly suppressed, but it appears again as the material size increases. Results also reveal that the strain gradient leads to deformation hardening. The distributed force plays a role to amplify the strain gradient distribution.     

Length-scale-dependent strengthening of particle-reinforced metal matrix composites with strain gradient plasticity

The strength of particle-reinforced composites is size-dependent such that the composite strength increases as the particle size decreases. This length scale is attributed to the geometrically necessary dislocations punched around a particle during cooling after consolidation due to CTE mismatch between the particle and the matrix. Additional geometrically necessary dislocations due to the elastic-plastic mismatch between the particle and the matrix during extensive deformation also contribute to the particle-size-dependent length scale. In this study, the length-scale-dependent strengthening of particle-reinforced metal matrix composites are examined using two-step, strain gradient plasticity based finite-element computation for considering the effects of these two types of dislocations. The effective plastic strain gradient for the calculation of enhanced strengthening is evaluated with the low order elements using an isoparametric interpolation of the averaged-at-nodal plastic strain. The results from the proposed method along with the interface decohesion for SiC_p/A356-T6 composites are in good agreement with the published experimental data. Additional predictions for different particle sizes, shapes and arrangements show qualitatively that the length scale according to the particle size obviously affects strengthening of the particulate-reinforced metal matrix composites.     

利用摩擦学系统理论对磨粒流加工过程的分析

简介当前磨粒流加工技术的现状,得到了磨粒流和工件表面的摩擦以及导致的材料去除是该加工过程的核心问题.利用摩擦学系统理论,构建了针对磨粒流-工件表面的摩擦学系统框架,得到了系统的主要结构元素、元素的特性和相互之间的关系.归纳了磨粒流加工过程中材料去除的摩擦学机理,并讨论摩擦学机理与该摩擦学系统的关系,对从摩擦学角度认识磨...     

微切削加工中切削力的理论与实验

微切削过程中的切削力严重影响刀具寿命及零件的加工精度,因此,深入研究微切削过程中的切削力变化规律及影响因素是确定合理的加工参数、加工工艺及提高加工系统性能的基础.本文在考虑刀具钝圆半径存在的条件下,采用轴对称原理建立了微切削力理论公式及微切削模型,实验研究了切削用量、刀具材料及工件材料对切削力的影响,验证了理论分析的正确性.研究结果表明:在切深ap为0.002～0.032 mm,进给量f为0.01～0.20 mm/r,切削速度v为20～120 m/min情况下,切削力Fz的变化范围为100～1030 N,Fy的变化范围为40～700 N;减小刀具钝圆半径会减小刀具后刀面与工件的接触长度,并且会减小切削刃以下部分金属的变形,有利于获得高质量的加工表面;控制切削速度对切削力的影响可以通过控制切削层厚度与刀具钝圆半径的比值来实现,控制切削力比值Fz/Fy则可以通过控制走刀量、切深与刀具钝圆半径的比值来实现.     

Modeling surface roughness for polishing process based on abrasive cutting and probability theory

The surface roughness is a variable used to describe the quality of polished surface. This article presents a surface roughness model based on abrasive cutting and probability theory, which considers the effects of abrasive grain shape, grit and distribution feature, pressure on surface roughness. The abrasive grain protrusion heights are thought to close to Gaussian distribution, and then the relationship between the indentation depth and the pressure based on Hertz contact theory is obtained. Surface roughness prediction model is established by calculating indentation depth of the abrasive grains on workpiece surface. The maximum surface profile height (R_y) is approximately equal to the maximum indentation depth of the abrasive grain. The arithmetic average surface roughness (R_a) is equal to the average indentation depth of the abrasive grain. The effects of process parameters such as pressure and grit on R_y and R_a were simulated and analyzed in detail.     

基于过程代数的可变业务流程建模方法

为实现灵活的业务流程重组,提出了一种面向流程变化调整的业务流程模型。该模型从业务逻辑中抽取出流程相关性语义,用连接器的模式来实现一种可配置的流程属性。业务人员可以通过调整模型中的连接器对流程进行重新配置,从而实现业务流程重组的软件调整。给出了模型的过程代数描述,并介绍了建模步骤和流程调整方法。最后,以机场塔台调度流程的建模和调整过程为例,对相关问题进行了讨论。     

基于正交实验的工业纯钛研磨工艺研究

为了提高工业纯钛的表面加工质量,需要优化研磨工艺.采用单因素实验法探索磨粒形状对工业纯钛研磨效果的影响规律;利用正交实验法定量地对工业纯钛的研磨加工工艺参数进行优化.结果 表明,采用不规则形状磨粒进行研磨的材料去除速率可达60.45 nm/min,高于球形磨粒的23.70 nm/min,而两者的工件表面粗糙度基本相同;研磨压力对材料去除速率的影响最为显著;采用最优的工艺参数可得到的材料去除速率和表面粗糙度分别为81.90 nm/min和Ra 0.099 μm.研究结果可为工业纯钛研磨时磨粒形状的选择以及工艺参数的制定提供依据.     

基于正交实验的工业纯钛研磨工艺研究

为了提高工业纯钛的表面加工质量,需要优化研磨工艺.采用单因素实验法探索磨粒形状对工业纯钛研磨效果的影响规律;利用正交实验法定量地对工业纯钛的研磨加工工艺参数进行优化.结果 表明,采用不规则形状磨粒进行研磨的材料去除速率可达60.45 nm/min,高于球形磨粒的23.70 nm/min,而两者的工件表面粗糙度基本相同;研磨压力对材料去除速率的影响最为显著;采用最优的工艺参数可得到的材料去除速率和表面粗糙度分别为81.90 nm/min和Ra 0.099 μm.研究结果可为工业纯钛研磨时磨粒形状的选择以及工艺参数的制定提供依据.     

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.     

槽道非定常流场脉动特性的POD分析

针对槽道内非定常流场的脉动程度是影响传质、传热的效率问题,通过构建2DPIV测量非定常流场瞬态速度矢量场的实验系统,分析了2DPIV测量的各个时刻矢量场和涡量分布的形态,获取了非定常流场的循环周期和频率;利用本征正交分解(POD)技术分解了2DPIV的测量非定常流场的瞬态速度矢量场,根据各个模态的能量比,选取前16阶模态作为非定常流场的主导结构模态,并对POD分解的前16阶模态系数进行功率谱分析;结果表明POD分解的低阶模态分别表征了非定常流场的基频和相应的倍频,阐明了非定常流场的脉动机理;非定常流场的脉动是由于一些涡列和余弦形式的波状主流形成的,并将这些涡列以及伴随的波状主流的脉动频率映射到POD分解的低阶主导模态系数上。     

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。     

基于分时段参数修正的发酵模型的建立

发酵过程的自身复杂性以及菌体生长的不可逆性导致在单批发酵过程中模型的参数变化较大，本文在丝状菌体生长的产物结构模型基础上，利用检测到的数据对发酵过程模型中的参数分时段进行修正。通过灵敏度分析得到对状态变量影响较大的参数，采用经繁殖操作改进的粒子群算法对参数进行寻优修正。实验证明，基于分时段参数修正的发酵模型能适应单批发酵过程中的参数变化，具有精度高、修正速度快等优点。     

基于粗糙集理论的系统建模与故障诊断

应用粗糙集理论,可以从原始的数据中提取有用的知识或规则.依据这一思想,本文建立了一个实际非线性系统的粗糙集模型.在建模过程中,首先用系统输入输出的采样数据构成原始信息表,然后离散化,再利用粗糙集算法得到系统粗糙集模型的不完备规则集,通过实验和线性插补法实现规则集完备化,最后完成模型的设计和校验.校验结果表明所建的粗糙集模型是有效的,并利用该模型实现了系统的一种故障诊断.     

A piecewise linear theory of plasticity for an initially isotropic material in plane stress

A 20-faced polyhedron is chosen as a reasonable piecewise linear approximation to either the Mises or Tresca yield criterion. Strain hardening and motion of the faces during hardening are assumed to be linear functions. Subject to the above assumptions, to initial isotropy and to some reasonable symmetry requirements it is shown that the most general possible theory contains 11 material constants. Some simple experiments with a thin-walled tube are suggested for determining these constants.     

基于热力耦合模型的金属切削过程有限元分析

基于有限元理论和热力耦合模型的研究,通过讨论切削过程中的关键技术,主要包括切削加工有限元方程的建立：构件材料的Johnson-Cook本构模型;切屑分离准则;材料断裂准则;接触摩擦模型;切削热的产生和分布;残余应力的分析和切削力的比较分析等,建立了二位金属切削过程模型,通过采用粘结．滑移摩擦模型,有效地模拟了航空钛合金的切削加工过程,对此类材料加工的切削力、切屑温度以及应力场和应变的分布进行了分析。