Inventor Nastran – Shell element creation

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When working with Inventor Nastran, you need to analyse the model and decide whether you are going to do the analysis to a solid, surface or line element.  Depending on the shape, choosing between these 3 elements can result in varying results and how long  it will take to calculate the results.

So how do we choose these different elements?   The general rule that I use is if it is a long element with the same profile running the length then we can use a line element.  For a complex part with no uniformity we can use a solid element and for anything that has a uniform or constant thickness then we can use shell elements.

There are 3 methods to create shell elements.

  • Offset surfaces
  • Find thin bodies
  • Midsurfaces

If you have numerous thin looking bodies you can try the Find Thin Bodies function.  This will find all the thin bodies it thinks are correct. Inventor Nastran automatically calculates thin bodies with the following equation.

L/D ratio is greater than 20.  L is length and D is the uniform thickness throughout.   The automatic selection does not mean that those are the only thin bodies to be created.  If you have other thin bodies which did not match up with the ratio that Inventor Nastran used to automatically create the surface then you can select and even deselect bodies you feel are not going to be used as thin bodies.

The offset surfaces function allows you to select the face you want to create a surface element from and then specify the thickness as well as how far to offset.

The Midsurfaces function will allow you to pick a component which has a uniform thickness and then create a midplane throughout that body of your profile.