- So the final type of element we're going to look at
is the shell element.
And what is a shell element?
Shell elements is a 2D surface element.
There's no representation of space,
this time above and below the surface.
And parametrically, the shell element
only knows about its thickness.
The shell element carries in-plane and bending loads.
The membrane, in-plane stress
is constant through the thickness.
And the bending stress is linear through the thickness.
Applications include thin walled structures,
such as the aircraft fuselage shown here.
Pressure vessels, such as the tanker body shown here.
Casing and cabinets shown here, and any form of bracketry.
All of these represent thin walled type of structures.
The setup for Solidworks, as far as meshing is concerned,
we can use surface-offset type of methods.
This would be picking a geometric surface on the body,
then offsetting that or creating it someway from scratch,
so that we are creating independent surface geometry.
We can also use boss-extrude type techniques,
so here we're using the faces
of thin walled or solid geometry.
Or I can also use sheet-metal bodies,
which actually have automatic mid-surface extraction.
The FEA mesh that's shown in this example here
are triangular elements only, and we want to use
the high-order version of the elements,
those with mid-side nodes.
And we ensure we have that by not selecting
the draft quality mesh.
That's something we want to avoid.
That will have three-noded triangles only.
The orientation of the element is given by
orange is the bottom face.
So we can put this color-coding on,
and paint the bottom face orange to really help understand
the orientation of the shell elements.
Being thin, can sometimes otherwise
be difficult to visualize.
This also controls the stress sense,
in that our top surface stress
and our bottom surface stress are plotted independently.
Or if we have a 3-dimensional representation,
we can plot them together.
But it's important to know which is the top surface
and which is the bottom surface.
Mid-surface offsets.
The shell element offset is virtual.
In this image, we've got a reference surface in blue,
where we're meshing, but we want to offset
the center line of the element to the virtual mesh position,
so that the thickness is properly represented
through the body.
And one of the problems here is that
any in-plane loading applied to this mesh
will create spurious moments.
So I've shown the in-plane loading here
and the resultant moments.
Now if the loading is out of plane,
than the offset shell element doesn't suffer.
If it's predominantly loading in-plane,
then that should really be avoided.
So instead of using offsets,
we'd use a surface representation
where the mesh is already at the center line of the body.
Parent geometry limitation.
If we look at the structure here, for example.
If the bending loads are significant,
then the length to thickness of the shortest member here
should be a ratio of 15 or bigger.
Now there is no warning in solid works,
but that's our usual engineering judgment call on that.
So, for example here, the short vertical upstand,
compared to the thickness of that upstand,
should be about a ratio of about 15:1.