Visualization of solution results (post-processing)

Transcription

1 What Is Needed d To Run A Finiteit Element Analysis? This chapter includes cudesmaterial a from the ebook Practical ca Finite Element e Analysis. s It also asohas been reviewed e ed and has additional material included by Matthias Goelke and Jan Gras smannsdorf. Basic Information Needed To Run A Finite n a high level summary, the working steps involved in a finite element analysis may be categorized as: Modeling (pre-processing) processing) Solution Visualization of solution results (post-processing) Element Analysis This image depicts the three elementary working steps involved in a FEM analysis. Some details about the individual steps are summarized below. 2. Modeling / Pre-ProcessingP CAD Data Most commonly, an FEM simulation process starts with the impor rt of the component s (or part s) CAD geometry (e.g. CATIA, STEP, UG, IGES, solidthinking, etc.) into the pre-processor processor i.e. HyperMes sh In many cases, the imported geometry is not ready for meshing. Qu uite often the geometry needs a cleanup first due to broken surfaces surfaces which are not stitched together redundant (multiple) surfaces surfaces which are too small to be meshed in a reas sonable way later on 1

2 Another issue related to geometry is depicted in the following image: In the image on the left, the imported geometry is shown. Note the lateral offset of the green edges. Here, the surface edges (in green) do not meet at a single point i.e. there is a very small lat teral offset of the surface edges. As meshing is carried out with respect to the surfaces, this small offset will be automatically take en into account during meshing, which, unfortunately will result in very poor quality elements. The image in the middle depicts the meshed hd initial iti geometry. Note how the mesh is locallyll distorted. t d The updated (cleaned) and meshed geometry is shown on the righ ht. Here, the surface edges (in green) do not meet in a single point, i..e. there is a very small lateral l offset of the surface edges. As meshing is carried out with respect to the surfaces, this small offset will be automatically taken into account during meshing, which, unfortunately will result in very poor quality elements. Once these hurdles are mastered, one needs to ask whether all l the CAD information i is really needed. d What about little l fillets and rounds, tiny holes oesor even e company logos which can often be found in CAD data? Do they really contribute to the eoverall ea performance of the component? Meshing Once the geometry is in an appropriate state, a mesh is create ed to approximate the geometry. Either a beam mesh (1D), shell mesh (2D) or a solid mesh (3D) will be created. This meshing step is crucial to the finiteit element analysis as the quality of the mesh directly reflects on the quality of the results generated. At the same time, the number of elements (number of nodes) affects the computation time. That is the reason why in certain cases a 2D and 1D mesh is preferred over 3D mesh. For example, in sheet metals a 2D approximation of the structure re uses mchless much elem ments and thus reduces the CPU time (whichisis the time while you are waiting for your results). See the picture above for structures that are typically y meshed with 1D, 2D and 3D elements. Which element type would you choose for which part? 2

3 Despite the fact that meshing is (at least optionally) a highly automated process, mesh quality, its connectivity (i.e. compatibility), and element normals needs to be checked. If necessary, these element issues may need to be improved by updating (altering) the underlying geometry or by editing single elements. Material And Property Information After meshing is completed, material (e.g. Young s Modulus) and property information (e.g. thickness values) are assigned to the elements. Loads, Constraints And Solver Information Various loads and constraints are added to the model to repr resent the loading conditions that the part(s) () are subjected to. Different load cases can be defined to represent different loading conditions on the same model. Solver information is also added to tell the solver what kind of analysis is being run, which results to export, etc. To determine your relevant loads, your engineering skills are nee eded. Think of all kinds of load situations that can occur on your structure and decide whether you want to use them in your sim mulation or not. To determine the load from a static or dynamic event, a Multibody Simulation (MBD) might be helpful. l The FEM model (consisting of nodes, elements, material propert ties, loads and constraints) is then exported from within the pre- processor HyperMesh. The exported FEM model, typically called solver input deck, is an ASCII file based on the specific syntax of the FEM solver chosen for the analysis (e.g. RADIOSS or OptiStruct). A section out of an OptiStruct solver deck is depicted in the figure below. As you will see, the bulk of information stored in the analysis file is related to the definition of nodes (or grids). Each single node is defined by its nodal number (ID) and its x-, y- and z coordinates. Each element is then defined by its element number (ID) and its nodes (IDs are referenced). This completes the pre-processingprocessing ph hase. 3

4 3. Solution During the solution phase of a simple linear static analysis or an eigenfrequency study, there is not much for you to do. The default settings of the Finite Element program do handle these classes of problems pretty well. Practice will show you that if the solution process is aborted by an error, it is due to mistakes you have ma ade during the model building phase. Just to mention a few typical errors: Element quality ( 2.wistia.com/medias/rm mretoumym) Invalid material properties Material property not assigned to the elements Insufficiently i constrained model dl (the model dl shows a rigid iid body bd motion due to external loads) Some of these model issues are discussed in a free video series s available on the Academic Training Center ( altairuniversity.com/e-learning/by-altair-_2/hypermesh-related/free-hyperworks-starter-kit-video-series/) ki id i 4.44 Visualization / Post-Processing P Once the solution has ended successfully, post-processing (in HyperView for contour plots and HyperGraph for 2D/3D plots) of the simulation results is done next. Stresses, strains, and deform mations are plotted and examined to see how the part responded d to the various loading conditions. Based on the results, modifications may be made to the part and a new analysis may be run to examine how the modifications affected the part. This eventually completes the FEM process. Remarks Practice will show, that in many projects, the above depicted d process must be re-entered entered again, because simulation results indicate that the part is not performing as requested. It is quite obvious that t going back to CAD (to apply changes) and working through h the entire FEM process becomes tedious. A very efficient (and exciting) technology to speed up this process is called Morphing. Employing morphing allows the CAE engineer to modify the geometry of the FEM model, e.g. change radii, thickn ness of ribs, shape of hard corners, etc. Quite often the morphed 4

5 FEM model can be exported instantaneously (without any remeshing) allowing the CAE engineer to re-run the analysis of the modified part on the fly. An example of morphing a given finite element model is depicted below ( A nice introduction in morphing is given in the video below ( Note: The individual id working steps of the FEM process are not only sub bjected to many user errors e.g. typo while dfii defining material or loads. A lot of attention must also be paid to the chosen modeling assumptions (for instance, simplification of geometry, chosen element type and size, etc.). Even though the FEM solver may detect some of the most striking errors, the likelihood that your results have bypassed errors is high. The following chapters aim at creating awareness about FEM cha llenges and pitfalls. 5