2.8 THE COSMOS CONVERTER Z88G

Sometimes 3D CAD programs include so-called automeshers which devide a CAD model into finite elements. This generated mesh can be stored in some output format to fit the needs of the various FEA programs.

Typical output formats are the COSMOS and the NASTRAN format for the COSMOS or the NASTRAN FEA program.

Z88G is developed and tested for Pro/ENGINEER by Parametric Technology. USA. Pro/ENGINEER must include the option Pro/MECHANICA. Be sure to define the material data (e.g. for steel, only Young's Modulus and Poisson's Ratio is really needed) in Pro/ENGINEER.

Then you may activate FEM in the Pro/ENGINEER program after designing your 3D model , define a coordinate system (which must be in harmony with Z88 !) and add forces and boundary conditions to single points. Create these single points with Feature > Datum > Point.

Modify the mesh control values, if necessary. Create the mesh with Make Model and choose the element type e.g. Tet Mesh or Shell Mesh. Store the mesh with Output Model, choose NASTRAN or COSMOS/M and linear or parabolic. Enter z88g.nas for NASTRAN files or z88g.cos for COSMOS files for the output file name.

Then lauch the converter Z88G. The converter produces the Z88 input files Z88I1.TXT, Z88I2.TXT and Z88I3.TXT automatically. You may then enter the Z88 input files and edit values e.g. material data and integration orders, if necessary.

Test the Z88 input files generated by Z88G with the filechecker Z88V. Plot Z88I1.TXT with the plot program Z88O or Z88P. If you find a 3D model totally flat: You've defined a coordinate system CS0 in Pro/ENGINEER which does not fit Z88's needs. Simply define a new correct coordinate system in Pro/ENGINEER and define it as datum when outputting the model.

You may create the following Z88 element types with Z88G:

Tetrahedron No.16 (Tetrahedron, parabolic in Pro/ENGINEER)

Tetrahedron No.17 (Tetrahedron, linear in Pro/ENGINEER)

Plane stress No.14 (Shell, triangle, parabolic in Pro/ENGINEER)

Plane stress No.7 (Shell,quadrangle, parabolic in Pro/ENGINEER)

Plate No.18 (Shell, triangle, parabolic in Pro/ENGINEER)

Plate No.20 (Shell,quadrangle, parabolic in Pro/ENGINEER)

Torus No.15 (Shell, triangle, parabolic in Pro/ENGINEER)

Torus No.8 (Shell,quadrangle, parabolic in Pro/ENGINEER)

How to proceed?

First step: Choose NASTRAN or COSMOS file format: If you choose NASTRAN the file Z88G.NAS is loaded, in case of COSMOS the file Z88G.COS is loaded. You must know which file type did you file in your former Pro/E session.

(Choose file type before start. The UNIX version of Z88G operates in console mode)

Next step: Pro/ENGINEER makes no distinction between plane stress elements, torus elements and plate elements, so, it's up to you to feed Z88G with the right information; choose the proper element type (the type you prepared in your former Pro/E session) in Z88G before starting the conversation run:

(Before running the conversation choose the right type of elements; works similar for the UNIX version of Z88G in console mode)

The generation of volumes is easy but the generation of plane stress elements, plates and torus elements is tricky: Firstly, build a volume with (small) thickness in Pro/E. Set reference points, especially for axisymmetric elements. Launch Pro/MECHANICA and idealize the volume into shells: Model > Idealizations > Shells > Midsurfaces. This eliminates the depth. When working with axisymmetric elements keep in mind that you are working in cylinder coordinates: Your coordinate system coincidates with the axis of rotation and the "volume” lies on the corresponding radiuses:

(Here you see the generation of torus elements in Pro/ENGINEER (Wildfire). Proceed similar for plane stress and plate elements)

Please keep in mind: These FEA output data formats, especially the NASTRAN format, are really monthly modified. However, why should they kept for a while in the same state? Would be too easy. The COSMOS format is more stable but is missing in Pro/ENGINEER Wildfire. Thus, if you’re using Pro/E up to version 2001 you should store COSMOS files and starting from Wildfire store NASTRAN files. If you’ll store NASTRAN files for Pro/E versions up to 2001 you must check and modify the NASTRAN files: especially material properties are printed as wrong floating point values e.g. 2.06+5. Change such stuff to e.g. 2.06E+5 or 206000.

Anyway: Z88G looks quite harmless, but proper operated Z88G is a mighty tool which allows you to file very large FEA structures to Z88.