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.     

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.     

基于ALE方法的AZ31B镁合金微铣削有限元仿真

基于任意拉格朗日—欧拉(ALE)法,以AZ31B镁合金为研究对象,采用考虑应变梯度理论修正的Johnson-Cook本构模型作为描述力学性能的材料模型,使用ALE自适应网格技术有效克服仿真过程中网格畸变过大的问题,运用ABAQUS/Explicit对微铣削加工过程进行非线性弹塑性有限元模拟,分析切削过程中的应力应变以及温度分布,探究微细加工过程中的尺度效应和最小切削厚度,为实际加工参数的选择提供参考。     

Analytical prediction of cutting forces in orthogonal cutting using unequal division shear-zone model

This paper presents an analytical method based on the unequal division shear-zone model to study the machining predictive theory. The proposed model only requires workpiece material properties and cutting conditions to predict the cutting forces during the orthogonal cutting process. In the shear zone, the material constitutive relationship is described by Johnson?CCook model, and the material characteristics such as strain rate sensitivity, strain hardening, and thermal softening are considered. The chip formation is supposed to occur mainly by shearing within the primary shear zone. The governing equations of chip flow through the primary shear zone are established by introducing a piecewise power law distribution assumption of the shear strain rate. The cutting forces are calculated for different machining conditions and flow stress data. Prediction results were compared with the orthogonal cutting test data from the available literature and found in reasonable agreement. In addition, an analysis of the deviation from experimental data for the proposed model is performed, the effects of cutting parameters and tool geometry were investigated.     

On the inadequacy of the single-shear plane model of chip formation

This paper argues that the single-shear plane model is inadequate to the real cutting process. The model has been developed in the late 19th century on the basis of simple observations of the cutting process. Although a number of other models are known to the specialists in this field, the single-shear plane model survived all of them and, moreover, is still the first choice for studies on metal cutting, computer simulations programs and students’ textbooks. Although it is usually mentioned that the model represents an idealized cutting process, no information about how far this idealization deviates from reality is provided. This paper lists and discusses the following principal drawbacks of the single-shear plane model: infinite strain rate; unrealistically high shear strain; unrealistic behavior of the work material; improper accounting for the resistance of the work material to cut; unrealistic representation of the tool-workpiece contact; inapplicability for cutting brittle work materials; incorrect velocity diagram; incorrect force diagram; inability to explain chip curling. The paper concludes that any progress in the prediction ability of the metal cutting theory cannot be achieved if the single-shear plane model is still in the very core of this theory.     

Analytical modeling of grinding wheel loading phenomena

Wheel surface condition plays an important role in the grinding operation. Grinding wheel loading, meaning chip accumulation in the space between grains, leads to deteriorating wheel cutting ability and causes excessive force and temperature. This paper presents an analytical model of wheel loading phenomena as a function of cutting parameters, wheel structure, and material properties. The model is based on the adhesion of workpiece material to abrasive grain surface. It is validated by experimental results from grinding nickel-based superalloy with cubic boron nitride vitrified wheel. This model considers wheel specifications including abrasive grains size and the number of cutting edges. Cutting parameters and process temperature are the other determinant factors. On the basis of this model and empirical results, the effects of the various process parameters are presented.     

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.     

An Experimental Analysis and Optimisation of the CO2 Laser Cutting Process for Metallic Coated Sheet Steels

An experimental investigation is presented which analyses the CO2 laser cutting process for difficult-to-cut metallic coated sheet steels, which are called GALVABOND. It shows that by proper control of the cutting parameters, good quality cuts are possible at high cutting rates. Plausible trends of the energy efficiency (percentage of energy used in cutting) with respect to the various process parameters are analysed. Visual examination indicates that when increasing the cutting rate to up to 5000 mm min⁻¹ , kerfs of better quality than those produced using the parameters suggested in an early study can be achieved. Some kerf characteristics such as the width, heat affected zone and dross, in terms of the process parameters are also discussed. A statistical analysis has arrived at the relationships between the cutting speed, laser power and workpiece thickness, from which a recommendation is made for the selection of optimum cutting parameters for processing GALVABOND material.     

Monitoring and processing signal applied in machining processes – A review

In machining processes several phenomena occur during material cutting. These phenomena can affect the production through the reduction of quality or accuracy, or by increasing costs (tools, materials, time). Thus, an understanding of machining phenomena is needed not only to define the cutting parameters for maximizing production, but also to ensure worker safety. An easy way to identify these phenomena is by monitoring machining processes, such as the measurement of cutting force, temperature and vibration. The acquired signal can have information about tool life, quality of cutting and defects in the workpiece. This review paper discusses the first steps involved in choosing and defining various techniques that may be used to monitor machining processes. Furthermore, this paper also outlines the techniques to acquire and process the signals of the monitoring processes. Hence, the objective of this paper is to help the reader understand the procedures for monitoring machining processes, and define methods, parameters, targets and other factors involved in doing so.     

Critical cutting thickness in ultra-precision machining of single crystal silicon

This paper makes use of a strain gradient theory to obtain excellent consistency between observed experimental phenomena and theoretical calculations in exploring the brittle–ductile transition mechanism of single crystal silicon (SCS). The critical cutting thickness in the ultra-precision machining of SCS is then derived by means of theoretical calculations. SCS was first subjected to nanoindentation, and it was observed that under a particular scale of deformation, the silicon not only underwent plastic deformation, but more importantly also experienced strain gradient effects. This can be attributed to different types of dislocation motion present in the crystal, suggesting that the plastic deformation of SCS is caused by geometrically necessary dislocations, and that a size effect fulfills the necessary conditions for plastic region machining of SCS. Subsequently, the ability of scale gradient theories to link together microscopic mechanisms with observable mechanical properties was utilized to calculate the critical cutting thickness in the ultra-precision machining of SCS as approximately between 110 and 220 nm, a result which was then verified by experimental means.     

不同金属材料在磨料水射流加工时的可加工性的试验研究

通过对磨料水射流切割金属材料过程的能量转移过程的分析,并且将粉碎原理中的比表面能的理论引入切割深度的分析。得出了不同金属材料被切割时由于比表面能的不同,在相同的切割参数的情况下达到相同切割深度时候的切割速度不同。并且通过试验加以验证,同时得出316不锈钢和6061铝合金板材切割时的可加工性特征系数。该系数可以很好的用于磨料水射流切割时候的参数设定。     

陶瓷刀具高速干式切削等温淬火球铁的表面粗糙度研究

结合现有等温淬火球墨铸铁(ADI)材料的生产情况,制备三组试样并测定力学性能;采用CC650陶瓷刀具实施高速切削试验,探讨ADI高速切削时加工材料-刀具材料-切削用量-表面粗糙度之间的关系,基于微粒群算法建立了ADI高速切削过程中工件表面糙度与切削参数之间的理论模型,为高速切削加工ADI的最佳生产工艺提供理论指导。     

基于ABAQUS的二维直角切削加工有限元分析

运用ABAQUS有限元分析软件对二维切削加工过程中非线性、弹塑性进行了有限元分析,结合ABAQUS中材料失效准则、剪切失效、单元删除等实现了切削过程中切屑的分离,同时得出了切削过程中应力、应变、切削力的变化趋势,符合一般切削理论。     

The mechanism of ductile chip formation in cutting of brittle materials

A theoretical analysis for the mechanism of ductile chip formation in the cutting of brittle materials is presented in this paper. The coexisting crack propagation and dislocation in the chip formation zone in the cutting of ductile materials are examined based on an analysis of the geometry and forces in the cutting region, both on Taylor’s dislocation hardening theory and the strain gradient plasticity theory. It was found that the ductile chip formation was a result of large compressive stress and shear stress in the chip formation zone, which shields the growth of pre-existing flaws by suppressing the stress intensity factor K _I . Additionally, ductile chip formation in the cutting of brittle materials can result from the enhancement of material yield strength in the chip formation zone. The large compressive stress can be generated in the chip formation zone with two conditions. The first condition is associated with a small, undeformed chip thickness, while the second is related to the undeformed chip thickness being smaller than the radius of the tool cutting edge. The analysis also shows that the thrust force F _t is much larger than the cutting force F _c . This indicates that large compressive stress is generated in the chip formation zone. This also confirms that the ductile chip formation is a result of large compressive stress in the chip formation zone, which shields the growth of pre-existing flaws in the material by suppressing the stress intensity factor K _I . The enhancement of material yield strength can be provided by dislocation hardening and strain gradient at the mesoscale, such that the workpiece material can undertake the large cutting stresses in the chip formation zone without fracture. Experiments for ductile cutting of tungsten carbide are conducted. The results show that ductile chip formation can be achieved as the undeformed chip thickness is small enough, as well as the undeformed chip thickness is smaller than the tool cutting edge radius.     

基于滑—停—滑机理的锯齿形切屑高速成形分析

高速车削时被切削材料以极高的应变速率产生连续的塑性变形,产生大量的切削热,在出现集中剪切滑移的情况下,产生了连续型带状锯齿形切屑。根据高速外圆车削中碳淬硬钢切屑试样的SEM照片和金相显微组织照片分析了锯齿形切屑周期性形成的变形机理。被切削材料的变形过程由普通的剪切滑移变形和集中剪切滑移变形组成,切屑沿前刀面的流出可细分为“滑—停—(再)滑”三个阶段。切削速度和材料硬度是决定切屑变形的两个主要影响因素。只要能够使材料应变率增加、致使切削温度升高的因素改变达到某种临界状态都能促成锯齿形切屑的形成,锯齿形切屑的形态随着切削用量的改变而变化。     

基于DEFORM的钻削力预报研究

应用Deform软件的单元去除技术和网格自适应重划技术,考虑工件材料的机械物理性能随温度的变化和流动应力受应变、应变速率和温度影响的特性来模拟材料的非线性,建立了适于钻削过程的三维有限元模型,分析了钻削加工中切削用量对切削力的影响规律;并采用YD-21/4动态电阻应变仪进行了钻削实验,验证了仿真结果的正确性和可靠性。     

Prediction model and simulation of cutting force in turning hard-brittle materials

The removal mechanism of hard-brittle material was studied in this paper. The shear strain and specific shear work of brittle material cutting were analyzed. The cutting force model of hard-brittle material was developed based on the fracture mechanics. Johnson-Cook model was modified and applied to finite element simulation of hard-brittle material cutting. The cutting force of machinable ceramics was predicted by BP neural network. The turning experiments of machinable ceramics were carried out. The influence of processing parameters on cutting force was investigated. The results show that the modified constitutive model well reflects the fracture removal process of brittle material. The simulation results are in well agreement with experimental data and theoretical data. The effects of cutting depth and feed speed on cutting force are larger than those of cutting speed and tool cutting edge angle.     

基于改进温度模型的300M钢本构方程参数识别

为了提高通过切削实验获取材料本构方程参数的精度,提出了将基于移动热源理论的温度分布模型沿剪切面积分计算剪切区平均温度的方法,结合不等距剪切区模型求得等效应变和应变率,建立了材料Johnson-Cook(J-C)本构方程参数的求解模型。根据切削实验获取的切削力和切屑厚度数据并采用遗传算法求得了300M钢J-C本构方程参数。与AdvantEdge FEM软件自带的300M钢本构模型相比,用所求模型参数仿真得到的主切削力、进给力和切屑厚度的精度有显著提高,验证了所建本构方程参数求解模型的有效性。     

基于ABAQUS的金属切削过程模拟

基于ABAQUS系统强大的大变形分析功能,对A6061铝合金材料的正交切削过程进行了有限元模拟分析.讨论了切削过程中切削层内部应变场和工件中残余应力的分布,分析了不同参数对切削力、残余应力的影响.模拟结果与切削试验数据相互吻合.     

Optimization of cutting parameters for energy saving

Correct selection of cutting parameters is one of effective approaches to achieve optimum machining process, including reducing energy consumption. For the close relationship between cutting parameters and energy consumption in machining process, energy consumed is modeled and to be reduced based on analyzing the energy consumption in this paper. According to the different requirements in roughing process and finishing process, corresponding multi-objective optimization functions are formulated considering energy consumption. Taking the optimization of milling operations on aluminum alloy as an example, experiments are carried out to analyze the energy consumption and production rate with sets of optimized/un-optimized cutting parameters for different objectives. The experimental results show that the objectives of low consumed energy and high production rate can be simultaneously achieved by optimization of cutting parameters.