Thermo-elasto–viscoplastic numerical model for metal casting processes

The finite element method is used to model the thermomechanical behaviour of ductile cast iron using metallic moulds. Heat conduction is assumed for the heat transfer analysis and an elasto-viscoplastic model is employed to predict the development of thermal stresses and strains. Special finite elements with coincident nodes are used to model the heat transfer and the mechanical contact at the metal–mould interface. The local heat transfer coefficient between the casting and the mould may be dependent on the air-gap formation. The latent heat evolution effect is modelled by the use of the enthalpy method. An iterative procedure is required to take into account the material and the contact non-linearity. A real casting has been modelled and numerical results are compared with the experimental measurements.     

Numerical simulation and experimental validation of a ductile damage model for DIN 1623 St14 steel

St14 steel (DIN 1623) is widely used in sheet metal forming industries because of its remarkable formability and also its low price. In this paper, damage behaviour of St14 steel is studied in order to be used in complex forming conditions with the goal of reducing the number of costly trials. Damage parameters of St14 steel have been determined by using standard tensile and Vickers micro-hardness tests. A fully coupled elastic-plastic-damage model has been developed and implemented into an explicit code. With this model, damage propagation and crack initiation, and ductile fracture behaviour of drilled and notched specimens are predicted. The model can quickly predict both deformation and damage behaviour of the part because of using plane stress algorithm, which is valid for thin sheet metals. Experiments are also carried out to validate the results. It is concluded that finite element analysis (FEA) in conjunction with continuum damage mechanics (CDM) can be used as a reliable tool to predict ductile damage and fracture of St14 steel.     

玻璃器皿压制成型的建模与仿真

玻璃器皿的压制成型过程是一个非常复杂的热力耦合弹塑性变形过程,其具有材料非线性和几何非线性的大变形特征。在对玻璃材料进行了传热分析以及受力分析等理论分析的基础上,利用专业有限元CFD软件GAMBIT进行有限元建模;利用POLYFLOW对玻璃器皿压制过程中的温度场、应力场进行有限元数值模拟,得到玻璃熔体和凸凹模的温度分布及应力分布图;最后利用FIELD-VIEWER显示模拟结果,并结合实际对其结果进行分析,以期为优化产品及模具的设计、提高产品的质量提供参考和指导。     

An investigation of descaling spray on microstructural evolution in hot rolling

High pressure water spray is used to remove oxide scales in hot rolling of steel plate. This would reduce the temperature temporarily on the workpiece surface. To investigate the effect of temperature variation due to the descaling spray on the microstructural evolution, a thermal-mechanical coupled FE model has been established. The heat loss due to the spray is experimentally measured and modelled using the established FE model. The heat transfer coefficient is determined using an inverse engineering method. A set of unified viscoplastic constitutive equations is implemented into the commercial FE solver MARC through the user-defined subroutine CRPLAW. The effect of descaling water spray on the surface temperature variation is numerically studied. Furthermore, the effect of surface temperature variation on the evolution of dislocation density, recrystallisation and grain growth is investigated for various hot rolling conditions.     

Investigation of the viscous and thermal effects on ductile fracture in sheet metal blanking process

In this paper, a methodology is proposed to predict the ductile damage in the sheet metal blanking process using a coupled thermomechanical finite-element method. A constitutive material model combined with the ductile fracture criteria was used. The effect of material softening due to the heat generated during plastic work in a specimen was considered in blanking simulations. To verify the validity of the proposed model, several blanking simulations are performed and the results compared with those obtained from an experimental study. The interaction of fracture initiation and temperature distribution in the sheet metal during the process was studied. The effect of velocity and the clearance on the product shape were examined. It was seen that at high punch speeds the viscous and thermal effects have significant effects on product quality.     

On the burst strength and necking behaviour of rotating disks

The behaviour of an elastic-plastic rotating disk is analysed both in the context of three dimensional theory and within the framework of the plane stress approximation. For an axisymmetric disk the possibility of bifurcation into a non-axisymmetric mode is investigated. Computations are also made for the behaviour of a disk with initial imperfections either in the form of a thickness variation or in the form of material inhomogeneities. For a ductile, bored disk of uniform thickness it is found that bursting occurs after the critical bifurcation point, which may occur before or after the point of maximum angular velocity in the axisymmetric solution. Thus, necking has started to develop in the disk, when ductile bursting occurs.     

Development of a combined tension–torsion experiment for calibration of ductile fracture models under conditions of low triaxiality

Developments in computational mechanics have given engineers tools to predict the evolution of damage in complex structures. Damage models have been developed that relate failure strain to stress triaxiality and Lode angle. Calibration of these models has traditionally relied on specimens that exhibit high triaxiality and limited Lode angle. This paper presents a specimen that can be tested in combined tension and torsion to achieve low triaxiality over a range of Lode angle. Numerical analysis of the specimen shows that it exhibits uniformity of stress–strain and stable values of triaxiality and Lode angle as plastic strain develops, both of which are desirable characteristics for calibration of ductile failure models. The design of a new displacement and rotation gage is presented that allows non-contact measurement at the gage section. Experimental results are used to develop the failure surface for 5083 aluminum.     

铝热精轧轧制区温度场三维有限元模拟

根据某铝热连轧厂生产线实际结构参数和工艺参数,应用弹塑性有限元法,考虑轧件金属塑性变形热、摩擦热、界面接触热导等对轧件和轧辊传热的影响,运用大型通用有限元分析软件MSC.Marc建立了铝热连轧精轧机组F2机架的热力耦合三维有限元仿真模型.通过分析轧件温度场的分布规律,为更好地控制轧件的温度分布、提高产品的质量提供依据.     

A micromechanical model for inelastic ductile damage prediction in polycrystalline metals for metal forming

This paper deals with micromechanical modeling of ductile damage and its effects (coupling) on the plastic behavior of FCC polycrystalline metallic materials. The ‘fully coupled’ constitutive equations are written in the framework of rate-dependent polycrystalline plasticity where a ‘ductile’ damage variable has been introduced at a crystallographic slip system (CSS) scale in order to describe the material degradation by initiation, growth and coalescence of microdefects inside the aggregate. Both, theoretical and numerical (FEA) aspects of the proposed micromechanical coupled model are presented. The ability of the obtained model to predict the plastic strain localization, due to the ductile damage effect, in the classical tensile test is carefully analyzed. Application is also made to the fracture prediction in deep drawing of a cylindrical cup using a thin sheet. Finally, some concluding remarks and perspectives are pointed out.     

Numerical simulation of incremental sheet forming with considering yield surface distortion

The incremental sheet forming processes (ISF) are attracting lots of attentions due to their advantages on rapid prototyping, without special dies and short lead time. The numerical simulation can be a valid method to investigate the forming process and predict the defects. In this study, an extended fully coupled ductile damage model with mixed nonlinear hardening was used to simulate the ISF process. At the same time, the yield surface distortion was also considered in this model, which can enhance the capability of modeling metallic material behavior under complex loading paths. Afterwards, some simulations were conducted with the proposed model. Additionally, one tension-shear orthogonal loading test was assigned on the one representative element in order to investigate the loading path effect during ISF process. By comparing the equivalent plastic strain and ductile damage evolution of the blank, the influence of the yield surface distortion on the ISF process was proved.     

损伤力学在塑性加工中的应用

损伤力学是一门新兴的固体力学分支。它目前主要用于机件安全评定与寿命估算。笔者根据损伤力学的概念，原理和方法，在空洞型塑性损伤特征和热力学的基础上导出新的塑性损伤演变方程。理论分析与实验结果表明，损伤变量与相当应变呈线性关系，并且其值受应力三轴变的显著影响。该方程不但能反映应力三轴度为正时的影响，而且能反映应力三轴度为负时的影响，从而使损伤力不学可以与塑性加工学科结合起来，扩大了损伤力学的应用范围。     

Ductile damage micromodeling by particles’ debonding in metal matrix composites

The elastic–plastic behaviour of particle-reinforced metal matrix composites undergoing ductile damage is modelled using a two-level micro-structural approach. The considered heterogeneous material is a polycrystal containing intra-crystalline elastic particles. Ductile damage is initiated by the matrix/particle interface debonding and the subsequent voids growth with plastic straining of the crystalline matrix. Homogenization techniques are used twice: first at mesoscale to derive the equivalent grain behaviour and then to obtain the macroscopic behaviour of the material. Plastic deformation of the crystalline matrix is due to crystallographic gliding on geometrically well-defined slip systems. The associative plastic flow rule and the hardening law are described on the slip system level. The evolution of micro-voids volume fraction is related to the plastic strain. The elastic–plastic stress–strain response of particle composite is investigated. Predictions of the proposed model are compared to experimental data to illustrate the capability of the suggested method to represent material behaviour. Furthermore, specific aspects such as the stress triaxiality and yield surfaces are discussed.     

Moving mesh application for thermal-hydraulic analysis in cable-in-conduit-conductors of KSTAR superconducting magnet

In order to study the thermal-hydraulic behavior of the cable-in-conduit-conductor (CICC), a numerical model has been developed. In the model, the high heat transfer approximation between superconducting strands and supercritical helium is adopted. The strong coupling of heat transfer at the front of normal zone generates a contact discontinuity in temperature and density. In order to obtain the converged numerical solutions, a moving mesh method is used to capture the contact discontinuity in the short front region of the normal zone. The coupled equation is solved using the finite element method with the artificial viscosity term. Details of the numerical implementation are discussed and the validation of the code is performed for comparison of the results with thse of GANDALF and QSAIT.     

基于耦合传热的圆锥液体静压轴承热态性能分析

针对圆锥液体静压轴承的发热问题,利用CFX软件建立了轴承系统耦合传热模型。首先分析了网格数量对计算精度的影响,然后求解获得了轴承的温度场分布和热量传递情况,最后分析了不同参数对轴承热态性能的影响规律。结果表明,采用CFX软件计算圆锥液体静压轴承耦合传热问题是可行的,与绝热假设下的计算方法相比,耦合传热有效地反映了真实的传热情况;圆锥液体静压轴承的端泄散热占总散热量的大部分,轴颈散热能力最弱,在不同的转速和进油压力下,各个方向上的散热比率发生变化。     

Numerical modelling of ductile plastic damage in bulk metal forming

This work addresses the computational aspects of a model for rigid–plastic damage. The model is a modification of a previous established model formulated by Perzyna (Recent Advances in Applied Mechanics, Academic Press: New York, 1966, p. 243–377 (Chapter 9)) which is here extended to include isotropic damage. Such an extension is obtained by incorporating the constitutive equations introduced by Lemaitre (J. Eng. Mater. Technol. 107 (1985) 83; Comput. Meth. Appl. Mech. Eng. 51 (1985) 31; A Course on Damage Mechanics, Springer, Berlin, Heidelberg, New York, 1996) for ductile plastic damage into the original model. In its original version (J. Eng. Mater. Technol. 107 (1985) 83; Comput. Meth. Appl. Mech. Eng. 51 (1985) 31) this model does not distinguish tension and compression in the damage evolution law, so it was necessary to introduce a refinement proposed by Ladevèze (in: J.P. Boehler, (Ed.), Proceedings of CNRS International Colloquium 351 Villars-de-Lans, France (Failure Criteria of Structured Media, 1983, p. 355) and Lemaitre (A Course on Damage Mechanics, Springer, Berlin, Heidelberg, New York, 1996) which takes into account the partial crack closure effect with isotropic damage. The accuracy of the computational model, developed for the analysis of the material degradation in bulk metal forming processes, is shown through the discussion of the results of two examples, allowing to compare the simulation results with experimental and numerical results obtained by other authors.     

Optimisation of springback in bending processes using FEM simulation and response surface method

The aim of this work includes the springback optimisation of bending processes using the concept of experimental design and response surface methodology (RSM). The optimisation method includes two phases. The first involves the objective function prediction using design of experiments and response surface method, while the second is an optimisation process using a FORTRAN gradient algorithm. Springback of sheet parts during bending processes is simulated using finite element model (FEM) including damage evolution effects within the sheet. The numerical simulation of the damage evolution has been modelled by means of continuum damage approach. The Lemaitre damage model, taking into account the influence of triaxiality, has been implemented into ABAQUS/Standard code in order to predict the external fibres rupture evolution during the process and the material characteristics changes after bending. The simulation included die corner radius and punch-die clearance as the main variables.     

Residual stress analysis of the thermal barrier coating system by considering the plasma spraying process

The residual stress is the key factor causing the reliability problem of thermal barrier coating (TBC). The failure of plasma spray coatings due to residual stresses is a serious and recurring problem of TBC. The difference of thermal expansion coefficient between the substrate and each coating combined with temperature evolution and temperature gradients during deposition process determine the residual stress for the whole TBC system. The magnitudes and distributions of the residual stresses are affected by deposition process and deposition characteristics. Most of FEA (finite element analysis) has been performed under the assumption that the multilayer coating system is stacked at once without considering the deposition process during plasma spraying. In this research, FEA for a coupled heat transfer and elastic-plastic thermal stress was performed to obtain the more detailed and reliable result of residual stress of the TBC system using the element activation/deactivation technique. The residual stress variation from the start of plasma spraying to cooling stage with room temperature was obtained systematically considering the deposition process. It can be used as reference data to improve the performance of TBC. In addition, the relationship between residual stress and coating conditions such as cooling rate and time is also examined thoroughly.     

延性材料损伤演化及材料软化的孔洞尺寸影响

应用有限元方法，对延性材料孔洞型细观损伤演化进行探讨。考察具有相同初始孔洞体积分数fo、不同相对尺寸孔洞长大和对材料软化的影响。三种体胞分别为简单立方体胞、体心立方体胞以及非均匀孔洞群状分布体胞。体胞基体为Mises等向强化材料。采用大变形有限元方法，简单加载，应力三维度保持定值。分析结果发现，孔洞群状分布体胞模型所代表小尺寸孔洞的长大速度较慢，且在高应力状态下具有较强的承载力和较低的裂纹敏感性。     

基于数值模拟的线材轧制成形过程研究

通过对线材热连轧过程传热关系进行分析,利用热力耦合弹塑性有限元方法建立了线材与轧辊的三维数值计算模型,结合热轧生产线的实际情况,对高速线材轧制生产线预精轧区六道次轧制过程中的温度场、轧制力与轧制力矩进行了模拟计算。分析了变形区内温度场的分布和变化规律,探讨了轧制过程中所涉及的几何、材料和边界条件等多重强非线性问题,计算结果为生产工艺制度的改进、工艺参数的优化以及微观组织变化规律的研究提供了理论依据,对现场生产也有重要的参考价值。