Finite element modeling of erosive wear

Material damage caused by the attack of particles entrained in a fluid system impacting a surface at high speed is called ‘Erosion’. Erosion is a phenomenon that takes place in several engineering applications. It also can be used in several manufacturing process such as abrasive waterjet machining. Erosion is a complex process dependent on particle speed, size, angle of attack as well as the behavior of the eroded material. Extensive experimental results have been reported in the literature on the erosion of different materials. Simulating the erosion process through finite element enables the prediction of erosion behavior of materials under different conditions, which will substitute the need of experimentation, and will enable the identification of constants required for existing analytical models.In this paper, an elasto-plastic finite element (FE) model is presented to simulate the erosion process in 3D configuration. The FE model takes into account numerical and material damping, thermal elastic–plastic material behavior and the effect of multiple particle impacts as well as material removal. The workpiece material modeled was Ti–6Al–4V. The effects of strain hardening, strain rate and temperature were considered in the non-linear material model. Comparison against results reported in literature and erosion models by Finnie, Bitter and Hashish are made. It is shown that the predicted results are in agreement with published results obtained experimentally and from analytical erosion models.     

The influence of friction models on finite element simulations of machining

In the analysis of orthogonal cutting process using finite element (FE) simulations, predictions are greatly influenced by two major factors; a) flow stress characteristics of work material at cutting regimes and b) friction characteristics mainly at the tool-chip interface. The uncertainty of work material flow stress upon FE simulations may be low when there is a constitutive model for work material that is obtained empirically from high-strain rate and temperature deformation tests. However, the difficulty arises when one needs to implement accurate friction models for cutting simulations using a particular FE formulation. In this study, an updated Lagrangian finite element formulation is used to simulate continuous chip formation process in orthogonal cutting of low carbon free-cutting steel. Experimentally measured stress distributions on the tool rake face are utilized in developing several different friction models. The effects of tool-chip interfacial friction models on the FE simulations are investigated. The comparison results depict that the friction modeling at the tool-chip interface has significant influence on the FE simulations of machining. Specifically, variable friction models that are developed from the experimentally measured normal and frictional stresses at the tool rake face resulted in most favorable predictions. Predictions presented in this work also justify that the FE simulation technique used for orthogonal cutting process can be an accurate and viable analysis as long as flow stress behavior of the work material is valid at the machining regimes and the friction characteristics at the tool-chip interface is modeled properly.     

材料模型对成形过程一步法模拟结果的影响

采用大变形塑性理论和理想变形假设,给出了用于复杂形状拉深件成形过程分析的所谓“一步分析法”的有关公式和有限元表达。在此基础上,分别对各向同性刚塑性材料、厚向异性刚塑性材料的复杂形状拉深件成形过程材料应力、应变进行了分析研究,对拉深件毛坯初始形状进行了优化,讨论了材料模型对分析结果的影响。     

Modeling linear guide systems with CoFEM: experimental validation

In this work a new efficient finite element (FE) model for rolling contact [RiBEM (rigid ball with equivalent material)] is shortly presented. The RiBEM model is used to efficiently compute the eigenmodes of a linear guide system. Experimental modal analysis of the real linear guide shows with 5?% error a very good correlation with the numerical results. Although the new contact model reduces computational time by more than ten times compared with the standard FE model, it is still too expensive for use within whole machine tool models. For this reason an equivalent, mesh independent and geometrically scalable model (rolling contact spring) based on the Hertz theory is introduced and also validated with the help of modal measurements. As a justification for the presented work, results with stiffness data from the manufacturer are also presented and less correlation with the experiment is found in this case.     

The finite element simulation of high-temperature magnesium AZ31 sheet forming

Finite element (FE) simulations will be vitally important to advancing magnesium alloy sheet forming technologies for vehicle component manufacturing. Although magnesium alloy sheet has been successfully formed into complex components at high temperatures, material constitutive model development for FE simulations has not kept pace with the needs of forming process design. This article describes the application of a new material constitutive model in FE simulations for hot forming of magnesium AZ31 alloy sheet. Simulations of forming both simple geometries from laboratory studies and complex parts from production trials are presented and compared with experimental results.     

Modeling linear guide systems with CoFEM: equivalent models for rolling contact

Today’s machine tools are highly complex mechatronic systems capable to exert large translational and rotatory movements. In most cases rolling bearings are used to connect the moving parts to each other. As full FE models of rolling bearings can consume a large amount of degrees of freedom (DOF) efficient methods for reducing the DOF consuming rolling elements to more simple equivalent models are needed. As an example a linear guide system is used. A special feature of the considered linear guide is that the runner block consists of three separate parts, which are hold together only by pretension and friction. FE simulations of such linear guide system were not reported before in the literature. Beside the full FE model three equivalent contact models are presented. The first two equivalent contact models feature novel characteristics. Advantages and disadvantages of the equivalent models are discussed using as reference a slice of the full model and simulation results of static stiffness. The validation of the numerical models is also done using the general analytical solution of Hertz.     

An integrated process for modelling of precipitation hardening and springback in creep age-forming

Creep age-forming (CAF) process has been developed and used to manufacture complex-shaped panel components in aerospace applications. CAF is based on the complex combination of stress relaxation, creep and age hardening. The aim of this paper is to introduce an integrated technique to model stress–relaxation, creep deformation, precipitate hardening and springback in a CAF process. Firstly, a new set of physically-based, unified creep-ageing constitutive equations is presented, which is based on the high temperature creep and ageing kinetics, and, is determined for a solution-treated and quenched AA7010. This new material model is then implemented in the commercial FE solver ABAQUS through a user defined subroutine. An integrated FE simulation process is introduced for the simulation of CAF and springback. In addition to the stress relaxation, creep-age precipitate growth and yield stress evolution during CAF are predicted.     

Modeling the effects of microstructure in metal cutting

Continuous chips from experimental orthogonal cutting of materials with a heterogeneous microstructure such as 1045 steel are better represented by finite element (FE) models that incorporate material microstructure into the model. A macroscale FE model that incorporated the material microstructure into the model was developed. This approach was found to be more accurate in reflecting the chip formation process than conventional homogeneous models. The heterogenous model showed a rippled chip free surface and defects on the machined surface. The plastic strain was much larger from the heterogeneous FE model versus the homogeneous model due to strain localization during chip formation.     

Failure of high strength steel sheets: Experiments and modelling

Failure in sheet metal structures of ductile material is usually caused by one of, or a combination of, ductile fracture, shear fracture or localised instability. In this paper the failure of the high strength steel Docol 600DP and the ultra high strength steel Docol 1200M is explored. The constitutive model used in this study includes plastic anisotropy and mixed isotropic-kinematic hardening. For modelling of the ductile and shear fracture the models presented by Cockroft–Latham and Bressan–Williams have been used. The instability phenomenon is described by the constitutive law and the finite element (FE) models. For calibration of the failure models and validation of the results, an extensive experimental series has been conducted including shear tests, plane strain tests and Nakajima tests. The geometries of the Nakajima tests have been chosen so that the first quadrant of the forming limit diagram (FLD) were covered. The results are presented both in an FLD and using prediction of force–displacement response of the Nakajima test employing element erosion during the FE simulations. The classical approach for failure prediction is to compare the principal plastic strains obtained from FE simulations with experimental determined forming limit curves (FLCs). It is well known that the experimental FLC requires proportional strains to be useful. In this work failure criteria, both of the instability and fracture, are proposed which can be used also for non-proportional strain paths.     

Extraction of flow properties of single-crystal silicon carbide by nanoindentation and finite-element simulation

A method is presented for estimating the plastic flow behavior of single-crystal silicon carbide by nanoindentation experiments using a series of triangular pyramidal indenters with five different centerline-to-face angles in combination with two-dimensional axisymmetric finite-element (FE) simulations. The method is based on Tabor’s concepts of characteristic strain and constraint factor, which allow indentation hardness values obtained with indenters of different angles to be related to the flow properties of the indented material. The procedure utilizes FE simulations applied in an iterative manner in order to establish the yield strength and work-hardening exponent from the experimentally measured dependence of the hardness on indenter angle. The methodology is applied to a hard, brittle ceramic material, 6H–SiC, whose flow behavior cannot be determined by conventional tension or compression testing. It is shown that the friction between the indenter and the material plays a significant role, especially for very sharp indenters.     

考虑材料韧性损伤的中厚板半冲孔成形过程分析

为研究半冲孔成形过程中韧性损伤的演化以及部分工艺参数对成形质量的影响规律,本文在ABAQUS有限元模拟软件中建立了半冲孔轴对称有限元模型,并通过VUMAT用户子程序引入GTN(Gurson-Tvergaard-Needle-man)损伤模型,结合同时考虑空穴形状与体积变化影响的韧性断裂准则,进行弹塑性大变形有限元分析.基于该有限元模犁,预测了半冲孔工艺中静水压力、等效应力、等效应变、应力三轴度以及损伤断裂的产生和发展趋势,分析了反顶力、压边力和冲裁间隙对零件的影响规律,并与实验结果进行对比分析,验证了数值模拟的准确性.     

A new Thermo-viscoplastic Material Model for Finite-Element-Analysis of the Chip Formation Process

A new material model for describing the thermo-viscoplastic flow behavior of workpiece material in metal cutting is presented. In order to express the complex flow behavior which depends on the local strain, strain rate and temperature, a new methodology for sequential formulation is proposed. The material parameters which are achieved by using the flow stress data available at low strain rates are enhanced by matching the results of the experimental investigations and finite element simulations of the orthogonal cutting process. As a result, a material model which has a wide validity range of strain, strain rate and temperature is established.     

Prediction of damage in small curvature bending processes of high strength steels using continuum damage mechanics model in 3D simulation

Sheet metal bending of modern lightweight materials like high-strength low-alloyed steels (HSLA) is one major challenge in metal forming, because conventional methods of predicting failure in numerical simulation, like the forming limit diagram (FLD), can generally not be applied to bending processes. Furthermore, the damage and failure behaviour of HSLA steels are changing as the fracture mechanisms are mainly depending on the microstructure, which is very fine-grained in HSLA steels composed with different alloying elements compared to established mild steels. Especially for high gradients of strain and stress over the sheet thickness, as they occur in small curvature bending processes, other damage models than the FLD have to be utilised. Within this paper a finite element (FE) 3D model of small curvature bending processes is created. The model includes continuum damage mechanics model in order to predict and study occurring failure by means of ductile coherence loss of the material and crack formation with respect to influencing process parameters. Damage parameters are determined by inverse numerical identification method. The FE-model is strain based validated considering the deformation field at the outer bending edge of the specimen by using an optical strain measurement system. The Lemaitre based damage model is calibrated against the experimental results within metallographic analysis adapting the identified damage parameters to the bending process und thus adjusting the crack occurrence in experiment and simulation. Using this model the bendability of common HSLA steel, used for structural components, is evaluated with respect to occurring damage and failure by numerical analysis.     

Finite element modeling of microstructural changes in dry and cryogenic cutting of Ti6Al4V alloy

This paper presents a customized FE model for describing the microstructural changes during dry and cryogenic cutting of Ti6Al4V. It addresses the importance to modify the material behavior taking into account the microstructural changes and the cooling/lubrication effects during the cutting process. With this aim, a user subroutine is implemented in the FE code to describe the surface and subsurface modifications taking place during the cutting process and to implement them in order to properly modify the material flow stress. Thus, the material flow stress is continuously updated during the simulation according to the new microstructure characteristics. The proposed FE model is calibrated and validated by comparison with experimental results.     

微裂纹群损伤的超声非线性评价数值仿真

材料早期力学性能退化总是伴随着某种形式的材料非线性力学行为,从而导致超声非线性特征,即高频谐波的产生。建立长度小于半波长的随机分布微裂纹群模型以描述材料早期损伤状态,计算了微裂纹群超声非线性特征,分析了衍射效应对超声非线性特征的影响。计算结果表明,在早期损伤几乎未对材料性能造成影响的情况下,检测信号已经表现出明显非线性特征;随着裂纹数量的增加,二次声源叠加产生更为复杂的频率成分,显著影响超声非线性特征。研究工作可为材料早期损伤评价提供技术支撑。     

Thermal and mechanical modeling analysis of laser-assisted micro-milling of difficult-to-machine alloys

This study is focused on numerical modeling analysis of laser-assisted micro-milling (LAMM) of difficult-to-machine alloys, such as Ti6Al4V, Inconel 718, and stainless steel AISI 422. Multiple LAMM tests are performed on these materials in side cutting of bulk and fin workpiece configurations with 100-300 μm diameter micro endmills. A 3D transient finite volume prismatic thermal model is used to quantitatively analyse the material temperature increase in the machined chamfer due to laser-assist during the LAMM process. Novel 2D finite element (FE) models are developed in ABAQUS to simulate the continuous chip formation with varying chip thickness with the strain gradient constitutive material models developed for the size effect in micro-milling. The steady-state workpiece and tool cutting temperatures after multiple milling cycles are analysed with a heat transfer model based on the chip formation analysis and the prismatic thermal model predictions. An empirical tool wear model is implemented in the finite element analysis to predict tool wear in the LAMM side cutting process. The FE model results are discussed in chip formation, flow stresses, temperatures and velocity fields to great details, which relate to the surface integrity analysis and built-up edge (BUE) formation in micro-milling.     

Modeling the properties of closed-cell cellular materials from tomography images using finite shell elements

Closed-cell cellular materials exhibit several interesting properties. These properties are, however, very difficult to simulate and understand from the knowledge of the cellular microstructure. This problem is mostly due to the highly complex organization of the cells and to their very fine walls. X-ray tomography can produce three-dimensional (3-D) images of the structure, enabling one to visualize locally the damage of the cell walls that would result in the structure collapsing. These data could be used for meshing with continuum elements of the structure for finite element (FE) calculations. But when the density is very low, the walls are fine and the meshes based on continuum elements are not suitable to represent accurately the structure while preserving the representativeness of the model in terms of cell size. This paper presents a shell FE model obtained from tomographic 3-D images that allows bigger volumes of low-density closed-cell cellular materials to be calculated. The model is enriched by direct thickness measurement on the tomographic images. The values measured are ascribed to the shell elements. To validate and use the model, a structure composed of stainless steel hollow spheres is firstly compressed and scanned to observe local deformations. The tomographic data are also meshed with shells for a FE calculation. The convergence of the model is checked and its performance is compared with a continuum model. The global behavior is compared with the measures of the compression test. At the local scale, the model allows the local stress and strain field to be calculated. The calculated deformed shape is compared with the deformed tomographic images.     

Finite Element Prediction of Creep-Plastic Ratchetting and Low Cycle Creep-Fatigue for a Large SPF Tool

Industrial experience shows that large superplastic forming (SPF) tools suffer from distortion due to thermal cycling, which apparently causes high temperature creep and plasticity. In addition to distortion, thermomechanical fatigue and fatigue-creep interaction can lead to cracking. The aim of this study is to predict the life-limiting thermomechanical behavior of a large SPF tool under realistic forming conditions using elastic-plastic-creep FE analyses. Nonlinear time-dependent, sequentially coupled FE analyses are performed using temperature-dependent monotonic and cyclic material data for a high-nickel, high-chromium tool material, XN40F (40% Ni and 20% Cr). The effect of monotonic and cyclic material data is compared vis-à-vis the anisothermal, elastic-plastic-stress response of the SPF tool. An uncoupled cyclic plasticity-creep material model is employed. Progressive deformation (ratchetting) is predicted locally, transverse to the predominant direction of the creep-fatigue cycling, but at the same spatial location, due to creep and cyclic plasticity, during the so-called minor cycles, which correspond to comparatively small-amplitude temperature changes associated with opening of the press doors during part loading and unloading operations.