In Finite Element Analysis, structures can be represented with a variety of elements including 0D, 1D, 2D and 3D elements. Thin structures in particular are commonly represented with 2D or surface elements.
Different dimension elements can be used represent the same geometry. An I-Beam (W Shape) can be idealized as 1D, 2D, or 3D elements.
Surface elements, also called two-dimensional (2D) elements, are used to represent a structure whose thickness is small compared to its other dimensions. Surface elements can model plates, which are flat, or shells, which have single curvature (e.g., cylinder) or double curvature (e.g., sphere). Surface elements can also be used to model sections that are uniform or non-uniform in thickness.
Below are various examples of thin structures and their respective midsurface representations. Click on the thumbnails for a closer look.
Built up connection of an aerospace application
Injection molded plastics used in an automotive headliner
Machined rib used in aerospace applications
Injection molded plastic used in an electronic box
The midsurface is displayed within the original solid geometry
Wing tie used in aerospace applications
Midsurface Creation Process
A common way to produce a surface element mesh is to first create midsurface geometry that represent the midplanes of the thin walls and then mesh the midsurface geometry. For more details on the process, refer to 3 Steps to Create Midsurface Geometry.
Midsurface Creation Process
Midsurface Creation for Beginners
The video below shows fundamental concepts that can get you started in your midsurface extraction, creation, and meshing process.
The goal of this post is to bring you up to speed on how to create midsurface geometry for finite element analysis. While the process requires numerous actions, each action can be generalized into one of 3 categories. Video explanations complement each section.
1) Extract Midsurfaces
Thin structures will be composed of numerous thin walls. Individual midsurfaces may be extracted manually, but automatic and semi-automatic methods can extract midsurfaces for numerous thin sections. The advantages of each method are given below.
Automatic Midsurface Extraction
This method traditionally works well for designs that have uniform thicknesses or have been through a stamping manufacturing process.
Semi-automatic Midsurface Extraction
This method works well for designs that have multiple thickness changes, non-uniform thicknesses and the walls have a large variation of position to one another.
Individual Midsurface Extraction
There may be the situation when certain midsurfaces were excluded from the automatic or semi-automatic and must be created manually. In other cases, when the extracted midsurfaces are poor and severely distorted, it is best to delete the midsurfaces and recreate it individually.
2) Free Edges
A free edge is an edge that is not connected to another edge or face. If a continuous mesh is intended at an edge and it happens to be a free edge, it can produce a discontinuous mesh. The goal is to resolve the necessary free edges.
Individually Resolving Free Edges
If it is one edge, closing a gap is as simple as taking a free edge and dragging it to a nearby edge or face. The benefit of this dynamic behavior is that you do not have to created and delete construction geometry each time an edge operation is performed.
Suppose the scenario when an edge is already resting on an edge or face. A stitching function can be used to connect the free edge to the edge or face.
Extending Free Edges
After you have extracted numerous midsurfaces, there will be numerous free edges to resolve. Individually moving and closing each free edge is too time consuming. An automatic method can be leveraged to extend the free edges up to nearby edges or faces, and in addition, a simultaneous option to stitch the edges can be used.
3) Final Clean Up with Mesh Quality
To confirm the midsurface geometry is suitable, the resulting mesh must have elements that are within satisfactory levels of quality. In other words, nothing in the midsurface geometry should cause poorly distorted elements. With the mesh superimposed on the midsurface geometry, the mesh quality may be viewed and edits can be continuously made without having to delete and recreate the mesh.
Meshing is long and hard. You probably spend hours using a pre/post processor to fix and mesh geometry. Save your self the time and move through your meshing process faster with MSC Apex.
To illustrate an example, Figure 1 shows a bracket where the CAD to mesh process involved extracting midsurfaces, connecting the midsurfaces, meshing, and attributing it with thickness and offset properties. With a traditional Pre/Post processor, the CAD to Mesh process of this bracket required 5.5 hours. With MSC Apex, the CAD to mesh process required only 24 minutes.
How long is the trial?
The free trial is good for 30 days. The trial period begins once you receive a trial license from MSC Software.
The process of constructing finite element meshes can often require hours or days to complete. Part of the challenge is that original geometry is in a state not suitable for meshing. The original geometry must be significantly edited before a mesh can be created. On average, this CAD to FE mesh process can constitute over 60% of the entire FEA workflow.
Existing pre/post processors contain sophisticated meshing functionality, but lack modern geometry editing tools necessary to expedite the meshing process.
MSC Apex is the first in the CAE platform in the industry to feature both direct modeling and meshing technology that has been demonstrated to show performance gains of up to 50x (Click here for benchmark). Below are a collection of presentations for your viewing that cover MSC Apex functionality.
Everyone likes a challenge! At MSC we believe that challenges bring out your best. Your best people – Your best process – Your best tools. MSC Apex Modeler is a CAE-specific direct modeling & meshing solution that streamlines CAD clean-up, simplification & meshing workflow. We are so confident in its ability to accelerate your CAD to Mesh (C2M) performance were willing to put our money where our mouth is.
MSC is offering to conduct a C2M challenge on your parts at 50% off. That’s right – we’ll take your CAD geometry and deliver back to you mid-surfaced meshed parts of comparable quality at a cost savings of 50% of your current expense. This offer is only available in Canada and the United States.
Simple – give us your dirtiest, ugliest looking CAD parts or your most difficult geometry and challenge us to outperform your current process. We will deliver a mid-surfaced meshed part in a fraction of the time and cost. You have nothing to lose – except 50% of what you’re spending today!
As another awarding winning product from MSC, Apex is the world’s first computational parts based CAE system. It is transforming the way engineers perform simulation by reducing critical CAE modeling and process time from days to hours.
The table below showcases the actual productivity gains achieved from CAD to Mesh for an aviation bulkhead. The steps performed included CAD import, geometry clean up, extraction of mid-surfaces, connection of separate surfaces, meshing, and assignment of thicknesses and offsets.
The process of creating dozens of mid-surfaces from solid geometry, connecting the surfaces, meshing, and attributing thickness and offset properties is a very time consuming process. This process is also very common when taking advantage of Finite Element Analysis for structural analysis. We have developed new technology to expedite this workflow. Below is a quick demonstration. For more information, please visit www.mscapex.com.
Geometry and CAD can be very complex, and sometimes the complexity of the model can hinder meshing for FEA. I present three common obstacles and solutions to overcome them.
1) Features and Details
Often features and details can be disregarded in a subsequent structural analysis with FEA.
Threads such as these is an example of how detailed a model can be, but can be disregarded in FEA. In MSC Apex Modeler, we can use the Direct Modeling technology to remove the threads.
Dips or extrusions like these can also be ignored in FEA. MSC Apex Modeler can be used to raise the surface so it is flush with the rest of the surface.
Fillets are not always necessary, and can be easily removed with defeaturing tools such as the one built into MSC Apex Modeler.
2) Extra Edges on solids
Meshers will naturally follow the edges of a solid. If a model has numerous edges, meshers will force elements to follow the edges leading to severely distorted elements. MSC Apex Modeler has a tool that allows you to suppress edges such that they do not conflict with the mesher. This results in meshes that have fewer distorted elements.
In this example, you see the elements are severely distorted, but supress operation hides the edges from the mesher.
3) Spikes through the model
In some scenarios, you may find that a sharp spike is piercing the solid geometry. These spikes are usually unintended, but may be remedied in MSC Apex Modeler as the demo below shows.
The content in this post was orignally inspired by a webinar titled Easy Editing and Repairing Solid Geometry for FEA Meshing by Mica Parks.