INTRODUCTION TO FINITE ELEMENT ANALYSIS USING MSC.PATRAN/NASTRAN

Transcription

1 Objective INTRODUCTION TO FINITE ELEMENT ANALYSIS USING MSC.PATRAN/NASTRAN The goal of this exercise is to prepare a finite element model using surface representation of an object from CAD system. Then a linear static analysis of the model is performed. Boundary conditions are applied according to the loading for the real component of the structure. Typical workflow A typical workflow for an analysis using the finite element method and MSC.Patran consists of the following steps: importing the three dimensional model of an object into preprocessing tool (MSC.Patran) - once the 3D model of the part is prepared in CAD system it is imported to FEA preprocessor. It is worth noting that the model is typically imported as a boundary representation of the object (B- REP). surface mesh generation - various meshing tools are used to prepare the surface mesh based on B- REP. The mesh has to be checked for consistency (gaps) and the orientation. volume mesh based on the surface mesh - if a proper surface mesh has been prepared it may be used to create the volume mesh. All three stages must be completed before proceeding to the analysis. The above tasks must be accomplished by using the tools described below. Requirements The following requirements will be checked during the exercise: 1. CAD model import (File - Import) 2. Control of the surface mesh (Mesh seed tool, element size) 3. Surface mesh definition (Elements menu): - closed mesh (no gaps in the model; tools: Nodes equivalence, move, translate etc.) - normal vectors orientation (consistent orientation for the whole surface mesh; tools: Verify element normals, reverse direction) 4. Volume mesh definition (tools: Create Mesh Solid) 5. Boundary conditions (BCs menu) 6. Analysis definition (Analysis menu)

2 Exercise outline This section presents some important points of the modelling process. Generally, the following substages may be distinguished. Model setup Model definition Geometry Mesh Boundary conditions (BCs) Material properties Loadcase Analysis Results evaluation Model setup Warning! Due to specific work organisation of the program it is particularly important to follow exactly the steps outlined below. Otherwise problems with results evaluation may occur! Create new working folder. Create new file, change its name and extension to *.db Double click to open the file in default program - Patran. In Patran choose new file and select previously created file confirm creating new file message

3 Next, choose model preferences Model definition Model geometry Open geometry modeller Import the CAD model file by using File - Import. You may change display mode by using various options from the bar at the top of the screen Lighting can be managed by through Light sources menu Select all light sources and apply

4 Model rotation, panning and zooming may be carried out by selecting the mode (F2-F6 buttons), pressing the middle mouse button and moving the mouse. Mesh definition Once the geometry has been imported, proceed to mesh definition Use the tools described in the tools section of this document to apply mesh seed and create the surface mesh. Then, use the surface mesh to create volume mesh. Boundary conditions Open Loads/BCs menu

5 Define new boundary condition - fix nodes at one side of your model. Specify BC's name, constraints (DOFs), and application region

6 apply changes Fixed nodes should look like in the figure below

7 Prescribe a force at selected node Change BC type to force Specify desired force magnitude and apply

8 Select application region, apply The force boundary condition should look like this

9 Material properties Open material definition menu Specify material's name and its properties Note that you must specify all parameters in consistent unit system. Do you know why?

10 Save changes by clicking apply Open properties menu from the bar at the top of the screen, specify name Select "Input properties", and choose already defined material by clicking material button

11 choose defined material Specify application region select all the elements and apply Loadcase definition Apply specified loads to create a loadcase. Open loadcase menu, select Action: Modify

12 Select default Loadcase check if both previously defined boundary conditions are selected

13 Confirm by clicking apply Analysis proceed to analysis menu switch analysis method to "analysis deck", this will create solver input file, but will not submit it to the solver

14 Check it the solution type is set to static analysis and apply Find the BDF file in your working folder Open file in notepad, try to recognize entries in BFD file. Run Nastran Browse BDF model file

15 Run the analysis After Nastran finishes, check if *.xdb result file has been created Result evaluation In Analysis menu, find "Access Results" option Select results file and apply

16 Go to Results menu Check results you want to display You can change display settings, e.g. turn off undeformed model configuration

17 and geometry in Plot/Erase menu Tools The following tools might be useful when defining a finite element mesh based on the surface representation of an object: Menu: Elements Create - Mesh seed: allows to define a set of points on the surface' edges that will be subsequently used for the mesh definition. The nodes of the final mesh will be created in those points, thus it is possible to control the mesh resolution at the surface outline. This option should be used to adjust the mesh around corners, holes etc. Create - Mesh - Surface (Elem shape: Tria): this option is used for surface mesh definition. Element size may be input here. Surface mesh is defined on a surface, preferably with mesh seed defined in the previous step. Make sure that the triangular element shape is selected. Create - Mesh - Solid: enables the volume mesh definition. Solid mesh requires consistent surface mesh with correctly oriented normal vectors. Make sure that the mesh does not contain any gaps and forms a closed volume. This option uses a set of element as an input. Verify - Element - Boundaries: allows to check whether the surface mesh is correctly defined. If the surface mesh does not contain gaps only the outline and sharp edges should be visible in the preview window. To return to model view use Reset Graphics button. Verify - Element - Normals: the normal direction for surface elements may be checked. The surface mesh is defined correctly if the surface normals are pointing inwards or outwards of the component

18 (direction is not important). The guiding element may be chosen to change the normals according to its normal vector. In order to change the normal vector orientation choose the button in the Test Control box within this tab. Next, choose the guiding element and apply changes. Equivalence - List: this option enables to merge nodes that are close to each other. In this way a gaps in the surface mesh may be closed. If there is a gap between two neighbouring elements (as checked with Verify - Element - Boundaries) the nodes of these elements may be selected and equivalenced. The effect of the equivalencing may be checked by using the Verify - Element - Boundaries tool. Other tools may be also used. Especially those from Transform action of the Elements menu.