Shell Pairs in mechanica 1

littlebum2002 · Jan 4, 2008

I am having a minor issue with Mechanica, and I am hoping someone who has had the same issue can share their experience with me.

I have a simple assembly of a solid object sitting on a sheetmetal object. I simplify the sheetmetal part down to a shell pair (not doing so is not an option, the part is too big and too thin), and run the model. Success.

However, the problem is, due to the shell pair my result shows a 1/2 thickness "gap" between the top of the sheetmetal part and the bottom of the solid, since Mechanica shrunk the sheetmetal part down 1/2 thickness on each side of the midsurface.

So, to solve the problem, I went into my assembly, and moved the solid part "into" the sheetmetal part 1/2 thickness, and run the model again, with vastly different results.

Now, my only problem is, I have no idea which one of these represents the true results. Does anyone have some insight into this, as to which of these is actually correct, and why? Any help would be greatly appreciated. Thanks!

seymours2571 · Jan 8, 2008

When mixing solid and shell elements make sure that in your assembly the face of the solid part should be coincidently constrained to the face of the sheetmetal part. I belive mechanica recommends the use of the mate constraint (not sure why though).

During the meshing process Mechanica will compress the sheetmetal part as you have seen into a midsurface shell model. The edges of the solid part that were originally touching the sheetmetal part will be connected to the shell elements with link elements, which are shown in pink if you view the mesh. An excessive number of link elements is not recommended by PTC, given that link elements are computationally intensive. They start out with an element order of 9 (the maximum equation order possible in Mechanica).

Moving the solid part into the sheetmetal part will not bond the two surfaces together when using mechanica to perform mid surface compression. Thus, the explination on why your results differed.

Good Luck,

Steve

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Shell Pairs in mechanica 1

littlebum2002

Mechanical

seymours2571

Mechanical

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