ME 505 FINITE ELEMENTSIN MECHANICAL ENGINEERING AND APPLIED MECHANICS

Fall 1999

Creating Hypermesh Models From Scratch

In this tutorial, you will build from scratch a simple heat conduction model usingHypermesh and ABAQUS. This tutorial will introduce you to the geometry creationand mesh generation features of Hypermesh.

1. It is always a good idea to create a new directory when you are starting a new problem,so that you minimize confusion as to what files are where and also to minimize thechance that you will overwrite an important file. The UNIX command to changedirectories is cd dir where dir is the name of the directory you want to move to.UNIX is case-sensitive! Change to your Private directory (cd Private). The UNIXcommand to create a new directory is mkdir dir where dir is the name of the directoryto be created. Create a new directory titled ME505F99 (mkdir ME505F99). Changeto this new directory (cd ME505F99). Then create a new directory titled Tut1 andchange to this directory.

2. Start up Hypermesh by typing hm; be certain that you have done the steps discussed inthe Getting Started with Hypermesh, NASTRAN and ABAQUS at CAEN

Web page.

3. Recall from class that a finite element model consists of nodes, elements, material prop-erties, geometry properties, loads, and boundary conditions (constraints). Hypermeshorganizes this information in what it denotes as collectors. The best way to think ofa collector is as a label for identifying different parts of a model. Since we are workingwith ABAQUS, you need to load a template file that will enable Hypermesh to translateits Hypermesh data into an ABAQUS-compatible input file. In the lower right cornerof the Hypermesh window, the blue buttons are known as the permanent menu block.Click on the global button Double-click, using your left mouse button, thetemplate

= button and in the subsequent menu, first click on the abaqus/ button and then clickthe standard.2d button that appears. Click the return button to leave the globalmenu.

4. Let’s start to create the necessary collectors. Click on the collectors menu, which isthe 2nd menu item in the 1st column. You will see upward pointing arrows, these are

referred to as switches. Click on the toggle next to the comps button. You will seesix choices for collector types: comps, props, mats, loadcols, systcols, vectcols.Click on mats. The structure of Hypermesh is such that it prefers starting with thespecification of material properties.

5. If not already active, click on the circle next to the create choice at the left of thepanel so that the circle is filled; these fill in the circles type of buttons are referred toas toggles. Click on the name = button and enter the name steel. Then, click on thecard image = field and choose ABAQUS MATERIAL in the pop-up. Then, clickthe green create/edit button. In the subsequent menu, since this is a heat conductionproblem, toggle on the check box next to Conductivity. Just above this, you willsee a line labeled *CONDUCTIVITY, TYPE=ISO; below this is a label K, whichrepresents the conductivity coefficient. Enter a value of 50.0, by clicking on the 0.0 fieldbelow the K and typing in the new value. Toggle on the SpecificHeat button; youwill need to scroll down the top window pane to see its field; leave these fields at theirdefault zero values. Finally, click on the Density toggle and set the DENSITY fieldto 1.0 (again you will need to scroll down the top window panel to see the DENSITYfield). Click the red return button.

6. Now that you have a material collector specified, you can create a component (comp-

s) collector. A component collector actually is used to collect Hypermesh geometry

data and elements. We need to distinguish between geometry data and geometry

properties. I will use the term geometry properties as I have used it in class todenote cross-sectional information such as thickness, area, etc. Geometry data con-sists of the lines and surfaces you generate in Hypermesh to represent the geometricattributes of the part you are modeling. The best way to think about geometry data isthe information you would generate from a CAD model of a component. Geometry da-ta does not include finite element nodes and elements. However, when creating finiteelements in Hypermesh, they are created from the geometry data, so the componentcollector is used to simultaneously store both geometry data and finite element nodesand elements.

While you are still in the collectors menu, click on the switch next to mats (rememberswitches are the arrows), and choose the comps item. The create toggle on the leftshould be active. Click on the material = button and in the subsequent menu panel,click on the steel choice you created in the above steps. Click on the name = buttonand type in plate. Click on the color button. You will get a pop-up menu with abunch of color choices. You may choose any of the colors shown. This color choice

will be used to display any geometry and element entities that are created as platecomponents. You may choose any color you wish. However, bad choices are the brightwhite and the bright red, since these colors are typically used by Hypermesh to depictother types of operations. Interestingly, maize and blue are two of the choices; note theabsence of the shade of green of that school somewhere west of UM! Click on the card

image = button and in the pop-up, choose SOLIDSECTION. Finally click on thecreate button; you should see the message The collector was created.

7. To specify the loading on the model, we need to create a loadscols collector. Clickthe switch and choose the loadscols item. Click on the name = button and type instep1. Click on the card image = button and in the pop-up, choose the ABAQUS

STEP option. Then click the create/edit button. Click the HeatTransfer toggleand set the HTStep and HTPeriod fields to 1.0. Scroll down the menu of togglesuntil you reach NodeResults. Click this toggle and click the NT toggle for nodaltemperatures. Scroll down and click on the FileFormat toggle and choose the ASCII

switch if not already set. Click the return button to leave the loadcols card imagemenu page and click the return button again to leave the collectors menu.

8. At this point, it is a good idea to save your work in terms of a a Hypermesh databasefile. You can call your Hypermesh database files anything you want; for my sanity, Ialways use the file extension .hm to denote files that are saved and retrieved using thefiles menu. Click on the files button. With the hm file toggle active, click once onthe file = button; in the box to the right of this button, type tut1.hm. Click on thesave button; finally, click on the return button at the bottom right of the menu panel.

9. You are now ready to create the geometry data for the plate. This will be done bycreating the 4 corner nodes of the plate. Use the F8 function key to quickly get to thecreate nodes menu.

10. If not already set, click on the toggle next to type in. Click on the field next to x =

and type in 0.0; click on the field next to y = and enter 0.0 (Actually these valuesare the default, so you don’t really need to enter anything). Then click the create

button. You should see a yellow circle appear in the graphics window. If you don’tsee the circle, do not continue to hit the create button. First, click on the fit to screenbutton (f) found in the permanent menu panel to be certain that the point can beviewed. Every click on the create button creates a new node, and you can run intoproblems if you have more than one node at a point.

11. Following the same sequence as in step 10, create a node with (x,y) coordinate of (150.0,0.0). In this case, you will almost certainly need to click on the fit to screen button to

see this node. Create two more nodes with coordinates (150.0, 50.0) and (0.0, 50.0).Again, if you do not see a new node after hitting the create button, click the f buttonto change your graphics window. At this point you should have 4 nodes viewed onscreen, representing the corners of the plate. Click on the return button.

12. You are now ready to create a finite element mesh to fill in the space between thesenodes. Go to the 2D menu page, which contains the 2D element generation capabilitiesof Hypermesh.

13. Click on the spline menu. If not already set, click on the leftmost switch and choosethe nodes option. What you will be doing is creating a geometry surface and a mesh

of finite elements between the 4 nodes.

14. The switch in the center of the menu panel should be set to mesh, keep surf. If not,use the switch to choose this option.

15. The word nodes should have an aqua border around it, indicating that Hypermesh isready for you to choose a set of nodes. Click on each of the 4 nodes in the graphicswindow. You will know you have selected them when their color changes to white,indicating they have been selected (this is why choosing white for your collector coloris not a great idea). Now click on the create button. After a short interval, you shouldbe taken to a rather busy menu panel titled Element Densities. There are a lot ofoptions for creating a mesh of finite elements. For now, click on the element size =

button and enter the value 10.0. Then click on the recalc all button. The numberson the edges connecting the 4 vertex nodes are the number of elements that will becreated along each side. Finally, click on the mesh button.

16. Click on the return button; the green lines should change to whatever color you selectedas your component color. You have just created a finite element mesh. Finally, clickon the return button to get back to the menu pages.

17. Recall in class the discussion about the ordering of the node numbers and how mostfinite element codes have a preferred numbering direction. If the numbering is incorrect,the elements may be perceived as having a negative area. The mesh generation used inHypermesh is optimized for shell elements. The orientation of shell elements is definedby its normal to the surface. For the heat conduction problem we are building, thisnormal must point in the positive-z direction for the element node numbering to becorrect. To check this, go to the normals menu on the Tools page. Make sure thetop switch is set to elems; click on this collector and in the pop-up, choose all. Clickon the view button in the permanent menu block and choose the iso1 viewing option.Then click the display normals. If any of the normals, represented by arrows, point

in the negative-z direction , they will need to be changed. If all the arrows point in thenegative-z direction, simply click the reverse normals button.

18. Now comes a very important step that can cause you great confusion and trouble ifyou don’t do this. When you created nodes using the create nodes menu, whatyou were actually doing is creating what Hypermesh calls temporary nodes. Whenyou generated the mesh in the last few steps, Hypermesh created new nodes at the4 plate vertices. Thus, at each vertex, there are actually 2 nodes; one node attachedto an element and one “floating” node. The trouble arises if you do not clear outthese floating, temp nodes. When you are applying boundary conditions and loads tonodes, it is very easy to select these temp nodes rather than the “real” nodes that areconnected to the elements. As a result, at best, the finite element analysis (NASTRANor ABAQUS) will not run, giving you some type of error message. What is worse is thatthe analysis will actually run. You will then have run a problem with the wrong loadingand constraint conditions. This often produces bizarre looking plots; more insidious isthe case where you believe these wrong results and end up with a completely incorrectanalysis.

19. To see that there are double nodes, go to the Tools menu page, and choose the num-

bers menu. Set the switch to nodes; click on the nodes button and in the pop-upmenu, choose the all button. Then click the on button and you will see displayed allnode numbers. Note that at the 4 corners it is hard to read what the node numbersare since they are printed on top of each other. Click the all off button to erase thenode numbers and then click the screen refresh button p in the rightside permanentmenu block.

20. To clear these temp nodes, use the temp nodes menu panel, which can be found onthe Tools menu page. Click on the clear all button to remove those nodes that are notattached to any elements. You can see that the temp nodes are removed by repeatingstep 18 above.

21. To complete the model, we need to and boundary conditions, including prescribedtemperature, prescribed heat flux, and convective heat flux. Hypermesh organizesthese entries on the BCs menu page. To have the Hypermesh buttons produce properentries in the ABAQUS input file,go to the load types menu on the BCs page. Clickon the temperature = button and choose the BOUNDARY selection. This willenable g-type temperature boundary conditions to be specified at nodes. Click on theflux = button and choose the CFLUX option. This will enable (concentrated) heatfluxes to be specified at nodes. Click on the pressure = button and choose the FILM

selection. This will enable convective flux boundary conditions to be specified on edgesof elements (although we won’t use this for the tutorial but you need this step for thehomework). Then click the update button.

22. While we are updating quantities, we will need to change the element types createdin the steps above. For heat conduction analyses, the element types are DC2D4 inABAQUS lingo. Choose the elem types menu found on the 2D page. Click on thequad4 = button and choose the DC2D4 option from the pop-up. Click on theelems collector (collectors are green buttons surrounded by aqua borders) and choosethe all option in the pop-up. Finally click the update button. All of the 4-nodedquadrilaterals are now of type DC2D4.

23. Return to the BCs page and choose the temperatures menu. With the nodes collectorhighlighted, click on the nodes along the left edge of the mesh. They will highlight towhite circles. An easier way to get all these nodes is to click on the nodes collectorand in the pop-up, choose by window. Move your cross-hair cursor into the displaywindow and click the mouse at a point to the left and below the left corner of the mesh.Then move the mouse up to the upper left corner of the mesh, and with the cursor justoutside the mesh, click again. You will see a white “rubberband” drawn between yourtwo points. Continue to move and click until you have surrounded the nodes alongthe left edge. Then click the select entities button. The nodes inside the lasso willhighlight. Click the return button.

There are two entry fields on the temperatures menu. The top field controls the sizethat the temperature boundary condition is displayed on the screen. This is for visualpurposes only. The actual temperature value is prescribed in the value = field. Set thisfield to 10.0 and click the create button. You will see labels added to the highlightednodes indicating that these nodes have prescribed values. Click the return button.

24. Now choose the flux menu on the BCs page. In the value = field, enter the value 1.0.Click on the upper right node of the mesh and then click the create button. Changethe value field to 2.0; click the node below the upper right and click the create button.Similarly for each of nodes along the right edge, specify an increasing value of flux inunit values from top to bottom.

25. You can now export the Hypermesh database to an ABAQUS input file. You will wantto save your current Hypermesh database using the files menu. When asked if youwant to overwrite, choose yes. While in the files menu, toggle to the export button.Click on the file = button and in the field, enter tut1.inp. ABAQUS requires itsinput files to have the file extension .inp.

26. Back in the UNIX window, type abaqus. At the prompt Identifier : enter tut1; donot add the .inp extension. At the Input file (w/o .inp) :, type tut1.

27. If all went well, you should see a tut1.fil file after a short time. Check the tut1.log filefor any possible error summaries. The tut1.dat file contains the output from ABAQUSin print form, including more detailed error messages.

28. To post-process results for display in Hypermesh type the following:

/usr/caen/hyperworks-3.1/altair/translators/3.1/bin/HPUX/hmabaqus tut1.fil

tut1.res

where all of the above is typed on 1 line.

The file tut1.res is the file the translator program hmabaqus produces that can beread by Hypermesh.

29. Return to the Hypermesh window. In the global menu, double click on the results =

button and in the list that follows, choose the tut1.res option. Then click the return

button.

30. Go to page 5 of menus and choose the contour menu. Click on the data type =

button and choose the Nodal Temperatures option. Then click the contour buttonto see the contour plot of temperature distribution.

31. You have now completed a simple yet complete finite element heat conduction analysisusing Hypermesh and ABAQUS. It is worthwhile to look at the tut1.inp file to studythe format of the ABAQUS input file.