Creo Simulate > Creo Simulate > Modeling Structure and Thermal Problems > Idealizations > Shells > Midsurface Shells > Before You Define a Shell Model > Gaps in Assemblies
  
Gaps in Assemblies
For assemblies, gaps typically occur at locations where two parts meet or overlap. Creo Parametric assembly constraints are such that Creo Simulate treats an assembly as a single body for solid modeling. However, when you treat an assembly as a shell and perform midsurface compression, Creo Simulate sees more than one body, resulting in a gap. For an analysis of a model to be accurate, you must correct these gaps so that the parts of the assembly move together as if they were a single body.
For assemblies, the likelihood of gaps caused by midsurface compression is greater than it is for single parts because an assembly can have gaps between parts as well as within a particular part. For an example of an assembly that has a gap caused by midsurface compression, see Example: Assembly Model with Gap.
To find gaps in your model, be sure to perform a compression test before starting an analysis.
To avoid gaps in assemblies, you must make sure the midsurfaces of the parts connect. You can correct assembly gaps in various ways depending on your model geometry and sensitivities. Here are some methods you can consider:
• In native mode, Creo Simulate creates automatic midsurface connections wherever two components have mated or overlapping surfaces. When you mesh your model, these connections appear as dotted magenta lines along the edges of all connected curves and surfaces. If your meshed model shows automatic midsurface connections in areas where you do not want them, define interfaces for these areas.
In general, you should use automatic midsurface connections only if the size of the connections is small relative to the size of the model.
In a model that uses automatic midsurface connections to connect two overlapping surfaces that are subject to displacement, Creo Simulate can give results that do not reflect the actual displacement. For a comparison of the expected results versus the actual results, see Results When Using Automatic Connections.
• You can use end welds, perimeter welds, spot welds, or Weld feature weld.
• You can use a rigid link.
• You can use a fastener.
• You can change pair placement on the Shell Pair Definition dialog box to define the midsurface placement so the midsurfaces of the parts touch in the merged area. This can result in coincident curves and surfaces, which can cause meshing problems.
When you start your first analysis or design study for an assembly, you should request error checking. Among other things, error checking includes a count of the disjoint bodies in the assembly.
If Creo Simulate finds multiple disjoint bodies during error checking, your assembly may have gaps that you should consider before progressing further. The software does not consider spot welds when determining the number of disjoint bodies.
 
* Do not confuse the term gaps as discussed here with the gap idealizations that you can create in FEM mode.