By Hawk Ridge Systems Engineering Team
One of our favorite features in SolidWorks Simulation was introduced in SolidWorks 2013, and it's the ability to Render Shell Thickness in 3D. This capability makes defining shells more user friendly by allowing us to visually verify and ensure the defined shell mesh correctly represents the solid CAD geometry. In addition, we can review both the top and bottom shell results with rendered thickness plots simultaneously, making post-processing less tedious and more intuitive. Before we get into that; a quick introduction for readers that are new to shells.
Shells are a type of triangular mesh element that are ideal for analyzing parts that are considered “thin” or have a High Aspect Ratio, where one dimension (thickness) is much smaller than the others. Normally, a reliable mesh using solid elements requires at least two layers of elements across the thickness of the part. Notice below as the part becomes thinner, smaller elements are needed to fit two elements across the thickness of the part. The same elements are also used to span the length and width of the part. These excessive elements can negatively affect the time it takes our analysis to mesh and solve. So instead, we can use surfaces to idealize the thin solid geometry and apply a shell mesh which accounts for the part thickness in the formulation of the shell.
When dealing with High Aspect Ratio geometry there are generally two ways to define shells:
- Create a mid-plane surface body that represents the solid, exclude the original solid body from analysis, and mesh the surface body using shell elements. The “Mid Surface” feature in SolidWorks is ideal for creating a surface at the mid-plane of a solid body.
- Use the “Define Shell By Selected Faces” command to use existing model faces to define the shell mesh. Ideal for complex geometry where modeling mid-plane surface is not practical.
Ideally, we want the shell mesh on the mid-plane surface, but creating mid-plane surface bodies can be very time consuming and increasingly challenging as the geometry becomes more complex. In these situations “Define Shell By Selected Faces” can be used to select either the top or bottom model faces to define the shell mesh.
When using “Define Shell By Selected Faces” the Shell Offset option can be used to account for the fact that the selected face is half the thickness above or below the mid-plane surface. This is important since shells created using the top or bottom faces may not precisely represent the solid geometry, but the difference is less significant as the aspect ratio of the part increases. The subsequent difference in results should only be a few percent. If the difference is substantial it means the part is too thick for the shell idealization to be valid and applying a solid tetrahedral mesh is advised.
“Render Shell Thickness in 3D” shows how the orientation of the top (typically grey) and bottom (typically orange) shell faces influence the direction of the Shell Offset. In other words, top and bottom surface directions are based off the shell alignment. The command “Flip Shell Elements” can change the direction of the thickness the shell represents and is often a necessary step when defining shells using existing model faces.
Using a 5mm thick plate, where the Top Plane passes through the mid-plane of the plate, we select the top face of the plate and use “Define Shell By Selected Faces” to define a 5mm thick shell mesh. The logical assumption is that since the top face of the plate is selected, we can select the Top surface Offset option.
However, the resulting mesh does not quite represent the plate. In the figure below, the rendered shell thickness shows our 5mm shell representation of the plate is sitting above the Top Plane, which was supposed to be passing through the mid-plane of the plate. The bottom shell face, shown in the default color orange, is sitting above the face we selected when defining the shell. It appears our virtual thickness is being extruded in the wrong direction.
The face selected when the shell was defined does in fact become the top face of the shell mesh, but since the top and bottom surface directions are based off shell alignment the thickness is being added in the wrong direction. The Top surface Offset option adds the virtual thickness in the direction of top shell face to bottom shell face, or in the direction normal to the bottom shell face. Since the bottom shell face lies above the top shell face in this mesh, the virtual thickness extrudes away from the Top Plane (+Y direction). In order to resolve this issue we edit the definition of the shell and select the Bottom surface Offset option instead. Alternatively, use the command “Flip Shell Elements” to position the bottom shell face below the top shell face making the virtual thickness extrude in the correct direction.
To “Flip Shell Elements,” we select the shell face and Right-Click the Mesh folder from the Simulation Tree. The selected face remains the top shell face but the bottom shell face (orange) now lies correctly underneath the top shell face. The thickness direction will also follow from top shell face to bottom shell face. The rendered virtual thickness no longer sits above the Top Plane and our shell mesh correctly represents our original 5mm plate.
Once we are satisfied with the shell mesh, we can run the study and use “Render Shell Thickness in 3D” to post-process both the top and bottom shell results at the same time. This will save us from having to endure the repetitive task of creating and switching between top and bottom result plots. In addition, probing rendered plots will report both the top and bottom values simultaneously allowing for easier comparison. The results in the thickness direction are linearly interpolated to provide the rendered thickness result plots.
The example above shows the value in the “Render Shell Thickness in 3D” feature when setting up a shell mesh and post-processing shell results. Rendering shell thicknesses helps remove uncertainties when defining shells, allowing us to mesh with confidence. We encourage everyone to try out this new feature and compare results from shell meshes created from inner, outer faces, and mid-surface bodies to gain more confidence in your results. For more information, or if you are new to using shell elements in Simulation, we recommend reviewing our Simulation Training course offerings.
Summary of Offsets
Shell Offset controls where the face defined as a shell is located:
On the mid-plane of the actual geometry
Ideal for mid-surface bodies; virtual thickness is extruded in both directions
Exactly on the top face of the actual geometry
Virtual thickness is extruded down toward the bottom shell face from the top surface of the created shell
Exactly on the bottom face of the actual geometry
Virtual thickness is extruded up toward the top shell face from the bottom surface of the created shell
At a user-specified distance above or below the mid-plane of the actual geometry
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