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Once
you have identified that mixed meshing is the way to go for
analysis of a model, perhaps the most important step of the
analysis process is to prepare the model for meshing. Solid
Tetrahedral elements have 3 degrees of freedom (3 translational)
while shell elements have 6 degrees of freedom (3 translational
and 3 rotational). Because nodes of solid elements do not
have rotational degrees of freedom as compared to nodes of
shell elements, an attempt to connect shell and solid elements
results in unintentional hinge along the common edge.
With
the hinge joint present, we have discontinuous displacement
field and rigid body modes in the model. This incompatibility
between solid and shell elements not only creates hinge, but
solid and shell portions of the mesh remain completely detached.
Geometry preparation
Portions intended for shell meshing must be modeled as surfaces.
Or if modeled as a solid body use midsurface and surface offset
features to create the midsurface which will represent the
surface geometry. Finalize the surface or mixed geometry using
extend, trim, etc. and suppress or delete the solid body.
The example below shows one of the methods to create a surface
model from an existing solid model.
Mesh
considerations
It is a good practice to have split lines defined at the interface
of solids and shells. This forces nodes to be generated at
the incompatible interface. Using split lines created on the
face of the solid, bonded contact conditions should be specified
between solid and shell bodies. This forces the solid and
shell elements to move as one entity. In the global contact
conditions select incompatible mesh option.
Solved
example
Thermal analysis was conducted on a three plate assembly.
Each plate has thickness
5 mm, 10 mm and 15 mm respectively. A heat power input is
applied at the center, and convection condition on the bottom
of 15 mm plate. First (left) model was constructed using solid
elements only. The second (right) study was modeled using
shell mesh for 5 mm plate and solid elements for 10 and 15
mm plate.
Both studies use direct sparse solver, standard mesher, with
default element size. All parameters discussed above were
used to get this solution. The temperature distribution plot
shows same pattern for both studies. The 8 % difference in
values is due to different modeling criteria used for two
solutions.
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