Spring 2008, MET 415 - FEA Applications I

Prof. Dave Johnson, dhj1@psu.edu
Penn State - Erie, The Behrend College

HW-9A:  3D Modeling with Shells

CONCEPTS:

• 3D Modeling: Shell Elements
• Simple IGES Import of Geometry
• Pressure Loading on Shell Elements (FACE Number)
• Postprocessing of Shells 
  • ANSYS vs. Workbench
  • (Top/Mid/Bott) vs. PowerGraphics
• Mesh Error for non-Planer Surfaces

  A common joist hanger is pictured above.  This 0.050" thick sheetmetal part supports a 2" x 10" x 12' floor joist and the surrounding flooring on a treated lumber deck.

This part was modeled in CADKEY97 as a "mid-surface" and an IGES file of the geometry is available to start your modeling.  You may click here to access the IGES File   BETTER YET: Use a Right-click on this link, then "Save Target As...", then "Save as type: All Files", and use a file name like: joisthanger3d.igs  (You can open this IGES file in Pro/E to examine it for problems also)

In ANSYS Workbench: start an "Empty Project" then simply Link to the geometry file (on the WB Project page), and Open a EITHER New Geometry (if you want to use Design Modeler first) or New Simulation.

The customer desires that the loads and constraints we use represent a "worst case" condition for this part:

• A minimum of three nail holes (worst case scenario) on the flat surface are used to secure the joist hanger to the wall/header - whichever three you feel are best  - but not all three on the same side.
  [Nailed holes may often be treated as "simply supported" - look in Simulation Help to compare simply supported and fixed supports]
• A frictionless surface support should be used on the back surface, against the wall/header.
  [A "compression only" support may be a better choice, but is not available (WB 11.0) for the 3D surface body model]
• Loads: The joist hanger is rated at 60 lbf max load.  The load will be divided between the six cross nail holes and the flat plate on the bottom of the joist hanger.  Approximately 20% of the load will be carried by the cross nails and 80% carried by the flat plate on the bottom of the joist.  See Figure below.
• Make sure that impossible deflection behavior is kept to a minimum. (Specifically, deflections which would fail to represent the presence of the cross nails and the actual joist.)
• Make sure you note any assumptions and why you made them.
• Should you use symmetry ?  Can you use symmetry ?
• After solving and evaluating the mesh error, refine the model carefully to reduce SEPC below 10%

• Plots showing:
  • the element mesh
  • label/describe the mesh refinement plan you used
  • the "environment" [ALL constraints and loads on the model]
  • Fully document your decisions for loads and constraints for a " worst case" analysis.
  • vonMises Stress, by both of these methods^[1]:
    • Powergraphics (WB Geometry Pane): view from both sides (use MAX tag)
    • Full graphics (WB Command Object): use one view, change TOP and BOTT shell element face settings - compare the difference of max. vonMises stress value/location. 
      EXPLAIN why the max value is different depending on the setting TOP/BOTT
  • mesh error evaluations: SEPC, SERR, and based on these: 
    EXPLAIN how reliable you feel the max vonMises stress value is
• Check volume/mass/weight of part by finding a commercial part specification to make sure part size is reasonable.  Compare the volume/mass/weight of the model to the commercial part.

^[1]Shell Element STRESS Results - shell elements lack full geometric representation (FEA programs appear to show the thickness using the real constant data).  When examining Stress results:

• With PowerGraphics, ON (default in WB):
  • We see stress on the side of the part we are looking at (dependent on our view)
  • the plot legend includes max and min stress from BOTH sides
  • the plot legend includes the real maximum stress value
• With PowerGraphics, OFF (default in a WB Command Object):
  • Independent of our view of the model, we see the stress on only one face (either TOP or BOTTOM - listed in the legend
  • used commands: TOP or BOTTOM to request plots of stress on both sides of a part
  • must check both sides to determine the real maximum stress value