Abstract. 1 Introduction

Transcription

1 Linking knowledge based engineering techniques to the finite element analysis of structures Martyn Pinfold, Craig Chapman Advanced Technology Centre, Warwick Manufacturing Group, University of Warwick, Coventry, CV4 7AL ac. uk, c. b. ac. uk Abstract The current design analysis process used by many areas of industry lends itself ideally to the application of a knowledge based engineering (KBE) approach. Currently any geometry provided to an analyst by a computer aided design (CAD) system of a structure usually has to be simplified before the creation of a finite element (FE) mesh and subsequent structural analysis. This simplification and mesh creation process can be very time consuming. It is possible with the use of a KBE system to provide the rules and knowledge such that the geometry produced in the system can first be automatically simplified as required by an FE pre-processor and then meshed. The material properties and boundary conditions can also be applied at this stage if standard, and the elements grouped in such a manner to facilitate ease of manipulation by the analysis system. The work undertaken here shows the feasibility of this process for the design and analysis of a simple gear wheel. However, now that the feasibility of the approach is seen to be valid, it is believed that this process can be applied to much more complex structures such as an automotive body-in-white structure. 1 Introduction The need to consider a number of different alternatives at an early stage in design has been well established. A tool for automatically modelling and analysing the behaviour of a design would aid the conceptual designer by providing the facilities to consider more varied alternatives. Unlike traditional CAD systems Knowledge Based Engineering systems can capture the intent

2 416 Computer Aided Optimum Design of Structures V behind the product design by representing the 'why' and 'how' of a design, in addition to the 'what' of a design'. The geometric description is only one piece of information about the total product model. The focus of KBE is to provide an informationally complete description or representation of a design as well as access to external databases. The knowledge based engineering environment is therefore a framework for capturing and defining the process of design creation. The inputs to the KBE model can contain geometric and nongeometric attributes which can include design specifications, design practices, engineers expertise, material properties and the boundary conditions for mechanical analysis. When given a set of inputs, the KBE model can use the knowledge and rules to create an instance of a design. This can then be modified by changing the input parameters to produce very quickly a new instance of the design which allows for a significant increase in engineering productivity'. Hence reduction in design cycle time through using a KBE system is achieved through the complete automation of tedious, time consuming, error prone and repetitive tasks'. In addition, the model can be made to run external programs, such as finite element analysis, with the results being used to guide design changes'. A KBE system therefore allows for the rapid evaluation of design alternatives and the integration of islands of technology. Development of a KBE application requires a lot of up front investment in time and effort to gather knowledge, rules, dependencies and constraints that are applicable to a family of parts. However, once this information has been acquired and incorporated into the model, it can then be used repeatedly to create new designs'. Due to this initial investment, KBE techniques are not suitable for one-off designs. Previous work^ has indicated the possible gains to be made by linking a KBE system directly to finite element analysis. Desaleux and Fouet* state that the input costs for analysis represent about 80% of the total analysis cost. Whilst what is proposed here is essentially a much simpler problem to be addressed than those discussed in most of the literature it is felt that this area of linking KBE and FE should not be ignored if only for the potential time savings achievable for the FE mesh generation. 2 Knowledge Based Engineering Linked To Structural Analysis The goal of structural analysis is to understand the physical response of structural systems when subjected to loads\ The finite element mesh geometry is, in general, an approximation of the model geometry\ The simplification of the CAD geometry to make it suitable for analysis and the generation of the FE mesh is often the most time consuming part of undertaking an analysis. Whilst many FE pre-processors will automatically create an FE mesh, such

3 Computer Aided Optimum Design of Structures V 417 automatic creation still requires data such as mesh density and the position and type of boundary conditions to be applied. However, if the same generic problem is to be analysed over and over again time can be saved in the creation of the FE mesh if the rules regarding the mesh creation are created once and then referred to. The simplification of the CAD geometry to facilitate mesh generation can be regarded in a similar manner. The task of modelling and interpretation for FE analysis therefore meets all the criteria for the application of knowledge based engineering technology^. Many papers are available in the literature which discuss the linking of knowledge based engineering systems to structural analysis. The systems described in the literature range from the use of artificial intelligence techniques to build a prototype training system for structural analysis that provides the capability of adaptively learning from training experience when performing that structural analysis, to a system to convert aeroplane designs to FE models and solve the models produced*. Many of the systems described as either rule based expert systems or knowledge based systems are described as a tool to be used by the structural analyst in creating the finite element model^"^. Given the structural geometry, material properties, loading, boundary conditions, (and possibly pre-specified maximum allowable error and/or number of degrees of freedom,) the systems aid in the creation of the FE model by advising on suitable element types and mesh density^\ Other systems'* develop a structural analysis programme to create an intelligent FE model such that the elements know what they are connected to, how the loads are applied, and which joints they are attached to etc. Gregory and Shephard in two papers*'^ describe ideas behind a system designed to bridge the gap between the geometric modellers and the analysis programs. The system which encompasses a geometric model translator expert system, FE generation software, FE translator will they claim automate the creation of finite element models and hence reduce the time taken. The geometry is automatically translated from the form supplied by the geometric modeller into the form expected by the analysis software. Automatic mesh generators are then used to mesh subparts of the geometry and the material properties are automatically processed. In many cases the output from the KBE system to the FE program consists of a set of rules, or attributes, for automatic generation of the FE model, boundary conditions mapping and automatic generation of the input decks. The automatic generation of the boundary condition data and the mapping of this data to the FE model is handled by external software that is integrated with the KBE model. Once the design parameters are specified the complete process of generating the analysis models starting from creation of model geometry, through automatic mesh generation, boundary conditions generation and mapping, to the generation of the analysis decks, is totally automated and can be made transparent from the designer.

4 418 Computer Aided Optimum Design of Structures V From the available literature it can be seen that whilst there are many examples of linking KBE or expert systems to structural analysis, these are done in two different systems. The KBE system will output a set of geometry maybe with attached parameters which will guide the creation of an FE mesh in another software package. In addition, it is clear from the literature that the creation of the FE mesh is not seen as a major part of work. More time and effort would appear to be spent upon optimising the FE mesh or upon optimising the design. What is described in this paper is an example where the geometry creation and FE mesh generation are both done in the same system, which then outputs the data in the format suitable for the analysis package. In addition, the work proposed here concentrates initially much more on the conversion of the CAD model into an FE mesh. For the types of analysis considered for future work this is a very time consuming process that can take many man weeks of effort whilst the geometry, FE mesh and boundary conditions for the analysis are generically essentially the same each time. 3 Example As an example to consider the feasibility of integrating the geometry creation and FE mesh creation in a single system some simple geometry was used to create the rules for model simplification and mesh generation. The example shown here is of a standard gear wheel. This component was previously generated as a KBE model and the rule based defined as a part of a previous project^ and will thus not be discussed here. The work presented here takes the previous work a stage further with a rule base concerning the creation of first a simplified model for mesh generation, and then the generation of the FE mesh itself. The KBE system used is AML^ which has a module for FE mesh generation. Whilst the mesh generated can be seen to consist of triangular or tetrahedral elements, work is undergoing to generate a quadrilateral mesh in the case of a surface model, or a hexahedral mesh in the case of a solid model as these element shapes may be felt to be more acceptable to industry. The CAD design and typical KBE interface can be seen in Figure 1. A simplified model for analysis where the teeth are assumed trapezoidal to reduce complexity, and the fillet radii deleted, can be seen in Figure 2 whilst an example of an FE mesh produced automatically within the KBE system can be seen in Figure 3.

5 Computer Aided Optimum Design of Structures V 419 Figure 1: CAD design and KBE interface for the gear wheel. Figure 2 : Simplified model for analysis.

6 420 Computer Aided Optimum Design of Structures V Figure 3 : Example of an FE mesh produced by the KBE system. Alternatives to the design of the component can be seen in Figure 4 which shows design variants based on using different rule sets^ which can be produced in seconds when different selections, maybe regarding material, are made from the interface. As the design simplification and FE meshing rules are also captured in the KBE rule base than the FE mesh can be created automatically in seconds from the rules previously created - see Figure 5. In addition, once the rules have been created many different instances of the gear wheel can be created with for example different numbers of gear teeth or gear teeth pitch without having to create the FE mesh again. The mesh only has to be re-generated which takes seconds to create a new instance of it. Figure 4 : Alternative designs of component.

7 Computer Aided Optimum Design of Structures V 421 Figure 5 : Alternative design of component. This initial work has assessed the capabilities of the KBE system to produce an output in the required format and also the ability to group selected features in the FE mesh for ease of manipulation in the FE system. 4 Discussion The example of a gear wheel is shown with different designs depending upon the options chosen by the user from the menu buttons for the creation of the mesh. The options could represent different standard types of analysis, different standard materials, different standard loading conditions etc. In this case they represent different designs corresponding to different material requirements. The rules for model simplification and mesh generation apply to all of the instances of this component such that if the gear wheel is required to have 30 teeth instead of 20 then the creation of a new FE model is achieved automatically in seconds. Whilst this is a simplistic example and the time saving achieved by generating the FE mesh in this manner may not be significant when compared to the time taken to generate the mesh in a standard FE pre-processor, there is still a time saving when different instances of the model are created and meshed as these would have to be meshed separately in

8 422 Computer Aided Optimum Design of Structures V the pre-processor. This example has been used to demonstrate the possibilities of the integration of KBE and FE systems. If this technique can be applied to a more complex and realistic problem then the potential time savings in mesh generation could be significant. The time taken to gather and create a rule base can be justified only if the same generic problem is faced many times. This is the case, for example, in the analysis of an automotive body-in-white structure where essentially the same generic components are analysed for the same types of boundary conditions. All that may differ between analyses are the effects of the particular vehicle styling upon the body-in-white package and the values of the applied loads. The number of elements for such a structure is typically around 80,000, and the mesh generation can entail up to fifteen man weeks of effort upon receipt of the CAD model. Currently, due to the time and cost required to generate the analysis models, they are often used in a post design phase to evaluate a final design that will only be modified if the results are unacceptable. If the geometry simplification and mesh generation is done in a KBE system then the FE mesh can potentially be achieved in seconds. This would reduce enormously the overall time taken for the analysis of the structure which means that the analysis department can respond much more quickly to potential design changes and can evaluate more design options in the same time period. If the work envisaged shows that the KBE system cannot produce an acceptable FE mesh then the system should at least be able to produce a 'cleaned up' set of geometry for meshing elsewhere, hence also saving time. Thus the possibilities of quickly creating a useable FE mesh directly from the KBE system for more complex and realistic components will be investigated. From the work undertaken so far it would appear that it is easier to create the required FE models in parallel with the geometric models, and thus the rules for creation of the FE models are required at the start of the KBE rule definition process. 5 Conclusions This work has demonstrated the feasibility of producing acceptable finite element meshes within the KBE system using rules to control the mesh generation. It is now the intention to apply this methodology to a more complex structure for which substantial savings in the time to create the finite element mesh can be made. The next logical step would be to automate the post-processing phase, giving the designer recommendations for design modifications and pointing out possible errors in the analysis to facilitate re-analysis^.

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