Midsurfacing: Using Shell Elements

      

      

If you work with thin-walled solids, using a midsurface shell model can reduce the degrees of freedom in your model by factors of ten and save hours of time in your analysis and postprocessing.

But you’re probably thinking:

“Isn’t creating a midsurface model just going to add more time to my workflow?”

Well yes, it will probably take you a little longer to create a midsurface model. However, it is rare that your geometry will not need any modification, so you can just include it in your geometry cleanup process.

Also, the truth is, it doesn’t have to take that long.

Today we’re going to show you how to quickly create a midsurface, discuss common problems you might encounter, and demonstrate tools you can use to alleviate those issues.

Why use shell models?

Shell models are useful when working with a solid that is thin in relation to the overall size; such as when working with sheet metal parts or plastic injection modeling parts.

      

Even in a small part like this, a shell model can reduce the model by 40,000 nodes!

Creating Midsurfaces

There are a number of tools available to help you create midsurfaces available in Femap, but we’re going to focus on the three commands that are used most often when creating midsurfaces: Automatic, Single in Solid, and Offset Tangent Surfaces.

Automatic Midsurface Creation

The Automatic Midsurfacing function attemps to use face pairining technology in the Parasolid modeling engine to automatically create a midsurface.

You can get a midsurface simply by selecting the surfaces and specifying a target thickness (midsurface tolerance).

If you do not know the target thickness, there is a Distance function that will determine the distance between two nodes or curves.

Automatic midsurfacing works well for simple geometry that has constant thickness.

However, if you have a more complex model, you might want to use one of the manual options like Single in Solid or Offset Tangent Surfaces.

Single in Solid

Single in solid creates a single midsurface between two surfaces of a solid. The surface is trimmed by the solid so that it is completely contained within the solid.

The dialog will ask you to select the two surfaces that it will create the midsurface between.

Offset Tangent Surfaces

Offset Tangent Surfaces should only be used on solids with constant thickness.

The dialog will prompt you to select a “seed surface” and “tangency tolerance”. All of the surfaces tangent to the seed surface within the tangency tolderance will be chosen and highlighted.

You then need to set an offset value. This value is the distance used to offset the selected surfaces towards the middle of the solid part.

The offset surfaces will be automatically stitched together and you will be asked if you want to delete the original solid.

Finishing Touches

After you create your midsurface, you may find that some edges do not connect, or there might be coincident edges at an intersection.

To remedy this, we can use the Extend Surface, Extend Merge Mesh, and Non-Manifold Add tools. If you find that two edges are not meeting up, use the Extend Merge Mesh or Extend Surfaces tools.

Extend Surface

Extend Surface - Merge Mesh

Stitch together disconnected surfaces using Non-Manifold Add

If you have midsurfaces that you want to stitch together, use the non-manifold add tool.

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