Wednesday, 10 December 2014

Using a Shell Mesh with Thin Components

I’ve always been clumsy when decorating my Christmas tree, consistently working my way through my decorations at a rate of two to three baubles per year.  This inconvenience of having to replace broken baubles has taken its toll and I’ve decided to design the world first structurally sound bauble. 
How can SOLIDWORKS Simulation help? Well stage one will be to determine how much force one of these baubles will withstand before breaking, that way we will have a basis for comparison.  In order to do this we’ll set up a simple stress test to begin with, with a fixture to simulate my hand holding the top of the bauble and a force to represent the moment when I idly bump it into something and turn another shiny ornament to a glittery mess on the floor.  This is where I face problem one:

In order to achieve accurate results, it is recommended to have at least two high quality mesh elements across the thickness of your part, with our wall thickness of 1mm we therefore require an element size of 0.5mm.  This would result in an incredibly dense mesh across the surfaces of the model, which in turn would take an unreasonable amount of time to both mesh and solve for a part of this simplicity. 
We can therefore use an alternate mesh type for thinned walled parts (such as our bauble) known as a shell mesh.  By utilizing a shell mesh, it is possible to drastically reduce the mesh complexity resulting in significantly faster solve times in models which feature a thin cross section.
To define your thin walled solid bodies as shell’s follow this process:
Step 1: Right click your chosen body in the Simulation tree and select ‘Define Shell by Selected Faces’

Step 2: Select the faces you would like to define as a shell.  Also input the thickness of your material as well as the material offset to determine where the top and bottom faces lie.

Step 3: Re-mesh your model to create your shell mesh.  Ensure you remove any small element size you may have added trying to achieve two solid elements across the material thickness as these are not necessary for a shell mesh.  Once meshed you will notice both the ‘inside’ faces and ‘outside’ faces are highlighted in different colours, ensure the ‘outside’ and ‘inside’ colour is consistent and doesn’t swap between faces. 

If you have a mismatch of shell colour select the face, right click the mesh in the simulation tree and choose to ‘Flip Shell Elements’

Step 4: Run your simulation and get your results in a fraction of the time it would have taken using Solid Elements.

Whilst I continue my design quest of the unbreakable bauble (You hear it here first…) why not give shell elements a try for yourself and see how this technique could save you analysis time.

Merry Christmas from everyone at TMS CADCentre!

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