外文翻译--有限元分析软件的发展

中文3240字Steps in Finite Element AnalysisIntroductionRecently there is a trend towards using it in the early stages of design. A designer may use FEA just to validate the structural integrity of a design or she may use it for structural optimization along with the parametrized design techniques.This paper examines the requirements of a structural analysis agent and proposes an architecture to facilitate FEA in a concurrent design environment. The next section briefly describes how FEA is used in a typical industrial set up.Section 3 presents a survey of existing FE tools. Section 4 discusses some issues related to the development of an FEA agent. Section 5 proposes an architecture for the FEA agent that addresses the issues described in Section 4 and finally Section 6 presents the concluding remarks.Steps in Finite Element AnalysisThe process of FEA starts with identification of the region of interest and the formulation of the physical problem。

[1]. The region of interest might be an assembly, a component or a portion of a component (or an assembly). The interaction of the rest of the assembly and the environmental conditions with the region of interest is captured in two ways. One way to represent this interaction is to idealize them as loads and displacement constraints on the region of interest. For example a spot weld fixing a component to a bigger structure will result in a constraining all the degrees of freedom at that point. The other commonly used method is to use spring and/or gap elements.Analysts often draw a Free Body Diagram of the region of interest to clarify its interaction with the rest of the assembly and to gain more insight into its structural behavior.Required components and assemblies are then retrieved from the Solid Modeling system into the finite element package.In recent years a number of commercial systems have started offering both: FEA and Solid Modeling capabilities.In this case, the data exchange may occur between two modules of the same package.The original design geometry is sometimes too complicated for the purpose of analysis. The analyst may choose to simplify it so that it is easier to mesh and incurs less computational cost.This task of simplifying the design geometry is referred to as Global Idealization.Global Idealization may involve deletion/modification of some of the geometric features. The analyst may choose to take advantage of the symmetry and analyze only a portion of the model. If the problem is axi-symmetric, she may choose to reduce a 3D problem to 2D by analyzing the radial cross-section.If the analyst intends to make significant modifications in the geometry, she may choose to import the geometry in a drafting package first and then read the modified geometry in the analysis package.Global Idealization is often followed by Element Idealization. Element Idealization consists of characterizing the finite element dimensionality of the globally idealized object. The original 3D geometry may be transformed into a collection of 1D, 2D and 3D entities depending on the characterization of various geometric parts as beams, plates/shells, and solid elements respectively. Element Idealization decisions are based on two factors: shape of the object and the boundary conditions.The next step in the modeling process is selection of type of elements and their material properties. Based on this decision, the user discretizes the idealized geometry into finite elements.This step is commonly referred to as Mesh Generation.Traditionally the loads and boundary conditions are applied to the nodes and the element boundaries. In the proposed system they are applied to the geometry. Finally, he user has to select the type of analysis (static, modal,etc.) and the solution method and the finite element model is ready for analysis.The raw answers computed by the finite element solver have to be processed further. This includes calculation of derived quantities (such as stress and strain values), computing error estimates, creating创建graphical displays showing deformed shapes , stress contour plots, etc. All these tasks are collectively referred to as post-processing. Based on the post-processing results the user may modify the model at any stage of idealization (including the original design itself)and start the loop once again.An overview of the analysis process is shown in Figure 1.Figure 1 Steps in Finite Element AnalysisDevelopment of Finite Element ToolsDue to the obvious pay-offs associated with speeding up of the analysis process, there is almost an explosion of both research and commercial systems supporting FEA. The development of FEA tools has followed a path very similar to the development of Design Automation tools. The early software supporting FEA was primarily meant to automate tasks in the detailed analysis stage, namely Mesh Generation and Post-Processing. A survey of earlier work in automatic mesh generation methods can be found in references [2] and [3]. Earlier mesh generator would simply discretize the analysis geometry into a bunch of elements with almost no regard for the solution accuracy implied by the mesh.Adaptive meshing methods improved the reliability of mesh generation process.These methods use one of the several error estimation techniques [4,5] to estimate the discretization error for a trial mesh and improve the mesh quality either by refining the mesh in certain areas (called h-refinement methods) [6], or increasing the order of element interpolation (called p-enrichment methods) [7] or a combination of both (called h-p methods) [7].The raw FE data is usually too difficult to interpret due to its large volume.Post-Processing tools aid visualization and facilitate easier interpretation of the data. Post-processing features provided by today’s commercial packages include display of deformed shapes; calculation of useful engineering quantities such as V on Mises Stress, principal stresses, etc.; contour and shaded plotsshowing distributionf numerical parameters over the analysis domain.The emergence of Expert Systems technology saw the development of a new generation of FEA tools. The researchers became interested in applying Expert Systems techniques to automate early stages of the finite element modeling process. These systems try to capture the experiential and often subjective knowledge used by expert analysts and act as “modeling assistant” to a novice user. Fenves [8] suggested a framework for developing a knowledgebased system to assist FE analysis. Bennett et al. [9] developed a rule based system called SACON to suggest an analysis strategy to a novice user of MARC (a commercial FE code).Today’s commercial systems have incorporated most of the research in Mesh Generation and Post Processing. Also,the trend is towards developing integrated Computer Aided Engineering (CAE) packages which offer a range of facilities (solid modeling, drafting, analysis, etc.). This has greatly helped to ease the transition from a Solid Model of a design to its finite element model.Issues in Developing a Finite Element Analysis AgentAn FEA agent must surely support all the FE activities described in Section 2. But we prefer to use a commercial package for Mesh Generation and Post-Processing because today’s FE packages are fairly sophisticated in these areas and it seems pointless to duplicate this work. On the other hand, commercial codes are not suitable for合the Model Preparation tasks in any specific domain and the proposed FEA agent is intended to fill this gap. As a result, the following discussion primarily focuses on the model preparation tasks in FEA.1Exchanging Finite Element Modeling InformationThe FEA agent should facilitate exchange of an FE modeling problem at different levels of descriptions. A very high level description would consist of the sketch of the part to be analyzed with a verbal description of the operating conditions and the analysis requirements. Another form of description may consist of a B-Rep of the part with a descriptionof the boundary conditions with reference to the B-rep entities of the part. An ontology for describing assemblies, components, boundary conditions, etc. will have to developed to provide a formal language for the data exchange.The agent should have the capability to read and write IGES and STEP files. Both the standards have the capability of handling Constructive Solid Geometry (CSG) and Boundary Representation (B-Rep) formats. STEP is claimed to have the capability of exchanging FE entitiesas well.2Representation of a Finite Element ModelA representation of an FE modeling problem would have to include the following information:1. Geometry2. Boundary Conditions3. Material Properties4. Geometric Properties5. Type of Analysis6. Accuracy DesiredThe geometry should be maintained at different levels of FE idealizations. This would require the use of a non-manifold geometric modeler since FE models are often composed of elements of different dimensionality (e.g. a model may consist of plates and beams). The geometric description also needs to be maintained in the b-rep form since all of the mesh generators require it in this form.Traditionally, the boundary conditions are applied to model after it has been meshed even though the analyst knows what they are in the beginning and uses this knowledge in creating an appropriate FE mesh for the object. The Design Representation System (DRS) that we have developed allows us to prescribe boundary conditions along with the geometry and attachthem to the b-rep of the FE model.In practice, the FE problem seldom involves a single mechanical component, therefore it is desirable to maintain a symbolic representation of the assemblies and connections. The representation of connections can also be used to automatically derive the boundary conditions due to the interaction of mating components.3Geometry EditingAn analyst often wants to delete/modify certain geometric features of the model to simplify analysis procedure. This has been referred to as Global Idealization.Therefore the FE agent should provide feature-editing facility and quickly compute the b-rep of the resulting object. Certain higher level commands that convert one FE model into another should be provided (E.g. building a 2-D model by extracting the radial cross section of an axi-symmetric model).Geometric properties such as feature volumes, centroids, etc. should be automatically calculated. Direct addition/ deletion of FE entities such as beams, plates, etc. should also be possible for a quick what-if analysis in structural design.4 Interface with Commercial Finite Element PackagesThe proposed agent would use a commercial code for Mesh Generation, Analysis and Post-Processing.Therefore, the interface with this package may not be limited to just IGES or STEP files. The FEA agent must incorporate the knowledge needed for the effective use of the chosen FE package. This knowledge can then be used to write “program files” that w ill direct the abovementionedactivities in the FE code.5Knowledge-based AssistanceThe FE modeling decisions are primarily based on the two factors: shape of the components being analyzed and the boundary conditions. Since the geometry is maintained in terms of it’s boundary representation, the b-rep informationcan be used to infer about the shape attributes of the component. DRS also allows the user to attach boundary conditions to the b-rep entities (vertex, edge, face) in the geometric model, therefore the system has an integrated representation of the geometry and the boundary conditions. At the very least this information can be used to intelligently limit the options given to the user. For example, if the geometry is not axi-symmetric, the option for axi-symmetric 2- D elements may not be shown to the user. Going a step further a knowledge-base can be developed which makes use of the geometric and boundary conditions representation and the available domain specific information. This knowledge-base can be used to provide expert advice to a novice user.Architecture of the Proposed FEA AgentA schematic of the proposed architecture is shown in Figure 2. The Central Representation Module will be implemented in CLIPS. It will maintain all the aspects of a FE model listed in Section 4.2. The Geometry Kernel will be provided by the non-manifold geometric modeler called NOODLES. The graphics display programs will be written using TK/TCL interface builder. I-DEAS will be used for Mesh Generation, Analysis and Post-Processing. The Knowledge-based Module will be written using CLIPS rules and facts.The Interface Module will be able to read and write IGES and STEP files, write I-DEAS program files and use EIT’s software for agent communication.Figure 2 Schematic Diagram of the Proposed FEA Agent (Arrows indicate information flow.)Concluding RemarksThe implementation of the proposed agent will be CLIPS/C/TK/TCL based.Some of the features described in the earlier section have been previously implemented in Lisp.These are as follows:• Data structures for attaching boundary conditions to b-rep entities• Simple shape recognition and b-rep updating• Calculation of inertial properties such as volumes, centroids, moments of inertia, etc. from the solid b-reps• Creation of I-DEAS program files for modal analysis of beam models有限元分析软件的发展介绍最近有一种将有限元分析用在设计早期的趋势。