MATERIALS PROCESSING AND DESIGN: Modeling, Simulation and Applications - NUMIFORM 2004 - Proceedings of the 8th International Conference on Numerical Methods in Industrial Forming Processes
Role of Plastic Resistance of Amorphous Covalent Compounds in the Superior Performance of Superhard Nano‐Structured Ceramic Composite Coatings for Cutting Tools712(2004); http://dx.doi.org/10.1063/1.1766493View Description Hide Description
Nano‐structured composite ceramic coatings such as TiN with Si3N4 or TiSi2 prepared by various forms of plasma assisted CVD composed of crystalline components of equiaxed TiN of several nm diameter, surrounded by amorphous Si3N4 intercrystalline layers of roughly 0.2 volume fraction have exhibited hardnesses in the range of 70–100 GPa—quite commensurate with polycrystalline diamond layers—and thermal stability up to 1000C. Best present considerations indicate that such ultra‐hardness is not governed by processes of crystal plasticity in the crystalline component but by the characteristics of flow mechanisms of the often topologically continuous amorphous component exhibiting “liquid‐like” behavior in the constrained spaces between the crystalline components.
712(2004); http://dx.doi.org/10.1063/1.1766494View Description Hide Description
The paper describes some recent developments in finite element methods for analysis of bulk forming and sheet stamping processes. The developments include new stabilized linear tetrahedra for non linear solution of problems in solid and fluid mechanics and an enhanced version of the three node rotation‐free BST shell triangle for analysis of thin shells. Applications of the new elements to casting and sheet metal forming problems are shown.
712(2004); http://dx.doi.org/10.1063/1.1766495View Description Hide Description
Microstructure development of polymer blends in chaotic mixing flows is studied, using a model that idealizes the microstructure as ellipsoidal droplets of the minor phase. The model includes the effects of viscosity ratio and interfacial tension. Calculations are performed for a two‐dimensional, time‐periodic flow between eccentric cylinders, using a protocol that is globally chaotic. A Lagrangian particle method is used to follow the microstructure. The flow strength is described by a global capillary number. Flows with a global capillary number greater than about 30 give exponential stretching of the long axes of the droplets, with a Gaussian global distribution, similar to previous results with zero interfacial tension. Sheet‐like microstructures can be generated by using very large global capillary numbers, but in the absence of breakup these always relax to thread‐like structures at long times.
712(2004); http://dx.doi.org/10.1063/1.1766496View Description Hide Description
Methods for simulating macroscopic and microscopic behaviors in powder forming and sintering processes are presented. In the finite element method, the volume change in deforming material is taken into consideration by obeying the macroscopic constitutive equations allowing for the volume change. The microscopic rotation of powder particles during compaction is dealt with on the basis of the Cosserat continuum theory. In addition, a micro‐macro method for simulating a sintering process of ceramic powder compacts based on the Monte Carlo and finite element methods is presented. The macroscopic non‐uniform shrinkage during the sintering is calculated by the viscoplastic finite element method. In the microscopic approach using the Monte Carlo method, powder particles and pores among the particles are divided into many cells, and the growth of grains in the particles and the disappearance of pores are simulated by means of the Potts model. The microscopic and macroscopic approaches are coupled by exchanging microscopic and macroscopic results in each step.
712(2004); http://dx.doi.org/10.1063/1.1766497View Description Hide Description
The solution of forming processes requires reliable and efficient finite element methods to model the various complex physical phenomena encountered. The objective in this presentation is to focus on the current state of finite element methods with respect to reliability and efficiency in modeling forming processes. The finite element procedures pertain to the simulation of sheet metal forming, bulk forming, extrusion and drawing, rolling, welding, cutting processes, etc. It is emphasized that the appropriate finite element methods for a specific problem should be used, and that indeed procedures are available which are effective in many situations. The presentation briefly considers the state of modeling of solids, shell structures, contact conditions, friction, inelastic material response in large strains, thermo‐mechanical coupling and fluid‐solid interactions, as encountered in forming process simulations. The solutions of the governing finite element models are obtained using sparse direct or iterative solvers. The oral presentation will include the results of various example simulations.
712(2004); http://dx.doi.org/10.1063/1.1766498View Description Hide Description
The development of finite element simulation of material forming processes started about 30 years ago in academic laboratories, while the introduction of the corresponding commercial computer codes in industry is less than twenty years old. The main mechanical integral formulations for solid or viscous liquids are briefly recalled: classical Eulerian, Eulerian with a characteristic function, updated Lagrangian and arbitrary Euler Lagrange, with some comments on the finite element discretization using a mixed formulation and mini tetrahedral elements. The crucial remeshing issues are analyzed for non steady‐state processes with different levels of sophistication: Updated Lagrangian for solids, or Euler and a characteristic function, possibly combined with error estimation and adaptivity. As real problems are usually very complex in industry, we must consider different levels of coupling such as thermal and mechanical coupling with the tools in forging and gas — liquid — solid coupling as in polymer foaming. In an attempt to model microstructure evolution of the work‐piece during the different stages of forming, three approaches are reviewed. In the first example the physical evolution of metal is described by macroscopic parameters and their laws of evolution, while the second one is based on a finite element modeling of the two‐phase material at the microscopic level. Finally the third case is the presentation of a new approach of polymer crystallization during injection molding and its introduction in a computer code.
712(2004); http://dx.doi.org/10.1063/1.1766499View Description Hide Description
It is well known that injection molding is the most effective process for mass‐producing discrete plastic parts of complex shape to the highest precision at the lowest cost. However, due to the complex property of polymeric materials undergoing a transient non‐isothermal process, it is equally well recognized that the quality of final products is often difficult to be assured. This is particularly true when a new mold or material is encountered. As a result, injection molding has often been viewed as an art than a science.
During the past few decades, numerical simulation of injection molding process based on analytic models has become feasible for practical use as computers became faster and cheaper continually. A research effort was initiated at the Cornell Injection Molding Program (CIMP) in 1974 under a grant from the National Science Foundation. Over a quarter of the century, CIMP has established some scientific bases ranging from materials characterization, flow analysis, to prediction of part quality. Use of such CAE tools has become common place today in industry.
Present effort has been primarily aimed at refinements of many aspects of the process. Computational efficiency and user‐interface have been main thrusts by commercial software developers. Extension to 3‐dimensional flow analysis for certain parts has drawn some attention. Research activities are continuing on molding of fiber‐filled materials and reactive polymers. Expanded molding processes such as gas‐assisted, co‐injection, micro‐molding and many others are continually being investigated.
In the future, improvements in simulation accuracy and efficiency will continue. This will include in‐depth studies on materials characterization. Intelligent on‐line process control may draw more attention in order to achieve higher degree of automation. As Internet technology continues to evolve, Web‐based CAE tools for design, production, remote process monitoring and control can come to path. The CAE tools will eventually be integrated into an Enterprise Resources Planning (ERP) system as the trend of enterprise globalization continues.
Implementation of Microstructural Material Phenomena in Macro Scale Simulations of Forming Processes712(2004); http://dx.doi.org/10.1063/1.1766500View Description Hide Description
The paper deals with problems related to full/macro scale simulations of industrial forming processes. Large‐scale numerical simulations and virtual modeling are replacing prototypes in order to reduce costs and time. This requires accurate and reliable predictions. To satisfy these requirements, sophisticated material models including micro‐structural phenomena as phase transitions, aging, and recrystallization have to be considered on macro scale level simulation. Solution strategies are discussed and some examples are given of complex thermo‐ mechanically coupled forming simulations.
712(2004); http://dx.doi.org/10.1063/1.1766501View Description Hide Description
A microstructure‐based finite element formulation for the mechanical response of friction stir welded AL‐6XN stainless steel is presented. The welding process generates regions of substantial variations in material state and properties that contribute to strong heterogeneities in the mechanical behavior of welded components We modeled the system with a multiscale elastoplastic formulation in which polycrystalline behavior is computed as the integrated responses of constituent crystals. Model validation is made through comparisons to post‐test measurements of shape and hardness and to lattice strain measurements from in situ neutron diffraction experiments.
712(2004); http://dx.doi.org/10.1063/1.1766502View Description Hide Description
Internal interfaces in metals and alloys provide a convenient and natural basis for the construction of 3‐D meshes employed in finite element calculations. For moderate anisotropy (orthotropic symmetry) the spatial variation of the intercept density of test lines with internal interfaces can be expressed as an Orientation Distribution Function (ODF) that can be approximated by a polynomial in powers of the components of the unit vector parallel to the direction of the test line. It is suggested that finite element meshes derived from measurements of actual grain boundary networks can be similarly described. A local finite strain can be defined based on the distortion of the representation quadric for the intercept density relative to a hypothetical isotropic distribution having the same average intercept density. This measure can be a useful means of describing the spatial variation of internal total distortion within an inhomogeneously deformed body.
712(2004); http://dx.doi.org/10.1063/1.1766503View Description Hide Description
Many issues in forming are influenced to some degree by the internal structure of the material which is commonly referred to by the materials science community as microstructure. Although the term microstructure is commonly only thought of in the context of grain size, it more properly encompasses all relevant aspects of internal material structure. For the purposes of forming, the most relevant features are the crystallographic orientations of the grains (“texture”) and the locations of the grain boundaries, or, equivalently, the size, topology and shape of the grains. In order to perform realistic simulations one needs to specify the initial state of the material, e.g. on a finite element mesh, with sufficient detail that all these features are reproduced. Measuring microstructure at the scale of individual grains is possible in the synchrotron but scarcely practicable for an analyst. Cross‐sections or surfaces are easily evaluated through automated diffraction in the scanning electron microscope (SEM), however. Therefore this paper describes a set of methods for generating statistically representative 3D microstructures based on microscopy input for both single‐phase and two‐phase materials. Examples are given of application of the technique for generating input structures for recrystallization simulation, dynamic deformation and finite element modeling.
712(2004); http://dx.doi.org/10.1063/1.1766504View Description Hide Description
Crystallographic slip, i.e. movement of dislocations on distinct slip planes, is the main source of plastic deformation of most metals. The Crystal Plasticity FEM combines this basic process with the Finite Element Method by assuming that the plastic velocity gradient is composed out of the shear contributions of all slip systems. To apply the method to forming simulation of “real” parts suffered from the fact, that a huge number of single orientations is needed to approximate the crystallographic texture of such parts. This problem was recently solved by the introduction of the Texture Component Crystal Plasticity FEM (TCCP‐FEM), which uses orientation distributions (texture components) for the texture approximation instead of single orientations. Excellent agreement of experiments and numerical simulations for different forming operations has shown the feasibility of this idea. Most crystal plasticity codes use simple empirical constitutive equations. However, as crystal plasticity is build on dislocation movement it was an obvious idea to introduce a constitutive model based on dislocation densities (internal state variables) instead of strain (external variable) into the crystal plasticity.
The dislocation model used is based on five main ingredients:
1) For every slip system mobile and immobile dislocations are distinguished. 2) A scaling relation between mobile and immobile dislocations is derived. 3) The immobile dislocations are divided into parallel and forest dislocations for every slip system. 4) The Orowan equation is used as kinetic equation. 5) Rate equations for the immobile dislocation densities are formulated based on distinct dislocation processes, e.g. lock formation or annihilation by dislocation climb.
For a wide range of temperature and strain rate the constitutive behavior of single and polycrystals is studied and simulation results are checked by comparison with experiments.
712(2004); http://dx.doi.org/10.1063/1.1766505View Description Hide Description
The main objective of this paper is to survey some recent developments in the constitutive modelling of inelastic polycrystalline solids, which may be used for the description of important problems in modern manufacturing processes. This description is needed for the investigation by using the numerical methods how to avoid unexpected plastic strain localization and fracture phenomena in manufacturing technology. Since modern manufacturing processes lead to very complex state of stress and deformation for a solid body under consideration then in the description we have to take into account the influence of stress triaxiality and plastic spin effects. In this lecture emphasis is laid on experimental and physical foundations as well as on mathematical constitutive modelling for the description of localization of plastic deformation and various modes of fracture phenomena in polycrystalline solids. The description of kinematics of finite deformations and the stress tensors is given. The development of a thermo‐elasto‐viscoplastic model within the thermodynamic framework of the rate type covariance constitutive structure with finite set of the internal state variables is presented. Particular attention is focused on the determination of the evolution laws for the internal state variables. Fracture criterion based on the evolution of microdamage is formulated. By assuming that the mechanical relaxation time is equal to zero the thermo‐elasto‐plastic (rate independent) response of the damaged material can be accomplished. The thermodynamical theory of elasto‐viscoplasticity of polycrystalline solids presented has important features as follows: (i) invariance with respect to diffeomorphism; (ii) finite plastic deformation and plastic spin effects; (iii) plastic non‐normality; (iv) softening effects generated by microdamage mechanism; (v) plastic deformation induced anisotropic effects; (vi) thermomechanical couplings (thermal plastic softening and thermal expansion); (vii) influence of stress triaxiality on the evolution of microdamage; (viii) rate sensitivity; (ix) length scale sensitivity; (x) regularization of the evolution problem; (xi) dissipation and dispersion effects; (xii) synergetic effects generated by cooperative phenomena. All these fundamental features have been carefully discussed. It should be noted that the very important part of the constitutive modelling is the identification procedure for the material functions and constants involved in the constitutive equations proposed.
Development of Crystallographic Homogenization Finite Element Method to Study Crystal Texture Effects on Sheet Formability712(2004); http://dx.doi.org/10.1063/1.1766506View Description Hide Description
A “semi‐implicit” elastic/crystalline viscoplastic finite element analysis (FEA) code based on a “crystallographic homogenization” theory for the multi‐scale analysis are developed, by introducing SEM‐EBSD measured crystal texture. The homogenization algorithm is introduced to calculate macro‐continuum plastic deformations and material properties by considering a micro‐structural in‐homogeneity, which is characterized by the crystal texture and the initial and deformation induced hardening evolutions. Its micro‐structure is defined as the unit cell, which satisfy the periodicity condition, where both compatibility and equilibrium are satisfied. The conventional rate‐dependent type crystal plasticity constitutive equation is employed in a micro‐structure modeling. In the homogenization procedure, the asymptotic series expansion is introduced to define the displacement, which can be decomposed into two parts: the homogenized deformation defined by using the macro‐continuum displacement and the perturbed one, defined only in the micro‐structure. This “two‐scale” analysis code enables to carry out transition from a representative micro‐structure to the macro‐continuum. This consistency model satisfies both the compatibility and the equilibrium in the micro‐structure. This homogenization FE code is applied to study texture evolution induced by the plastic deformation, assess the formability of sheet metal, which has a ferrite phase, and confirm the availability through comparison with EBSD‐SEM observation results.
712(2004); http://dx.doi.org/10.1063/1.1766507View Description Hide Description
Microstructural information is fundamental to determining the critical properties of today’s high performance materials. Hence, there is a need for a representation that can quantify all the microstructural elements through the analysis of digitized images. This paper addresses representation through the creation of a dynamic microstructure library. The paper focuses on the application of machine learning theory for the creation of a library that is trained by experimentally or computationally obtained microstructure snapshots. Support vector machines (SVM) are used to classify microstructure snapshots based on its features into various classes. An incremental‐principal component analysis (PCA) method is employed within image classes to constantly update the microstructure basis and numerically quantify the microstructural features.
712(2004); http://dx.doi.org/10.1063/1.1766508View Description Hide Description
Micro‐forming using meso‐scale press has the potential of bringing up an entire new concept of manufacturing. Compared to macro‐forming process, individual microstructure (i.e., the size, shapes and arrangement of grains) plays an even more critical role in deformation, process variation, interfacial behavior and wear in the micro‐forming process. In this paper, reproducing kernel element method (RKEM) is employed to simulate the micro‐forming process. As a meshfree method, RKEM can handle large deformation of material in the micro‐forming process. The efficiency of RKEM can be improved with using the stabilized conforming nodal integration. Due to the natural conforming of approximation and no requirement of compatible mesh in RKEM, it is easy to remesh the model when moving interfacing (grain boundary) is considered. As the first step of a series investigation, the resulting force of deforming a pin in a micro‐forming process is calculated with RKEM and is compared to the experimental result.
712(2004); http://dx.doi.org/10.1063/1.1766509View Description Hide Description
The unified disturbed state concept (DSC) allows elastic, plastic and creep deformations, and microcracking leading to softening and failure under thermomechanical loading. It is applied for analysis and design of chip substrate systems. Validation of laboratory tests is included as well as a parametric design study in which the influence of the DSC parameters on number of cycles to failure is investigated. Such parametric studies can lead to improved methods for design and reliability.
712(2004); http://dx.doi.org/10.1063/1.1766510View Description Hide Description
Experimental research and molecular dynamic simulation proved that nanofibers can be effectively considered in the framework of continuum mechanics as the homogeneous prolate spheroidal homogeneous inclusions with a large aspect ratio. Nanocomposite is modeled as a linearly thermoelastic composite medium, which consists of a homogeneous matrix containing a statistically homogeneous random field of nanofibers with prescribed random orientation. Estimation of effective thermoelastic moduli of nanocomposites was performed by the version of effective field method (MEF, see for references and details Buryachenko, Applied Mechanics Reviews, 2001, v.54, 1–47) developed in the framework of quasi crystalline approximation when the spatial correlations of inclusion location take particular ellipsoidal forms. These “correlation hole” including the representative fibers are prohibited for the location of centers of surrounding fibers (since they cannot overlap) and compatible with mutual orientations of fibers. The independent justified selection of shapes of inclusions and correlation holes provide the formulae of effective moduli which are completely explicit and easily to use. However, the main advantage of the proposed approach is that it eliminates some of drawbacks of Mori‐Tanaka scheme, which can generate tensors of effective moduli which fails to satisfy a necessary symmetry requirement. The parametric numerical analyses revealed the most sensitive parameters influencing of the effective moduli which are defined by the axial elastic moduli of nanofibers rather then their transversal moduli as well as by the justified chose of correlation holes, concentration and prescribed random orientation of nanofibers.
Role of Forming In Micro‐ And Nano‐Scale Material Removal Mechanisms During Surface Machining of Ductile Materials712(2004); http://dx.doi.org/10.1063/1.1766511View Description Hide Description
The material detachment mechanisms of ductile metal surfaces are studied experimentally during dry grinding operation in a simulated experiment with near single grit contact with the surface. The spectra of the cutting and thrust forces are recorded and analyzed. It is found that the thrust force changes its direction from a compressive to a tensile mode. The ratio between the thrust and cutting force is consistently found to be greater than 1. In the grinding process, the chip is found to be much shorter and thicker than those predicted by traditional continuum cutting theories. From the analysis of chip dimensions and cutting forces, we speculate that the cutting process during a grinding operation comprises of three phases as follows: (i) lifting up of the surface ahead of the abrasive particle, (ii) segmentation through shear instability, and finally (iii) chip tearing from the surface. Accordingly, the heating cycle is much longer with a lower mean temperature, compared to those of macro machining. In addition, the proposed deformation field leads to loss of constraints ahead of the cutting grits, and possibly reducing the thrust to cutting force ratio. This suggests that forming took place prior to material detachment in grinding.
An Analysis of the Effects of Chip‐groove Geometry on Machining Performance Using Finite Element Methods712(2004); http://dx.doi.org/10.1063/1.1766512View Description Hide Description
This paper discusses the influence of major chip‐groove parameters of a cutting tool on the chip formation process in orthogonal machining using finite element (FE) methods. In the FE formulation, a thermal elastic‐viscoplastic material model is used together with a modified Johnson‐Cook material law for the flow stress. The chip back‐flow angle and the chip up‐curl radius are calculated for a range of cutting conditions by varying the chip‐groove parameters. The analysis provides greater understanding of the effectiveness of chip‐groove configurations and points a way to correlate cutting conditions with tool‐wear when machining with a grooved cutting tool.