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 Process for an Automotive Injection Molded Plastic
Midsurface Creation for Beginners
The video below shows fundamental concepts that can get you started in your midsurface extraction, creation, and meshing process.
Many automotive interior trim components such as headliners, door trim, and consoles are characterized as thin structures that can be difficult to model. The process of performing a structural analysis on such trim components via Finite Element Analysis often requires the creation of midsurface geometry and meshes for very complex, curved, variable-thickness sections. Unfortunately with existing finite element pre/post processors, creating the midsurface geometry and mesh can require anywhere from a few hours to days, depending on the complexity of the model, and the limitations of the software being used.
This presentation covers the newest methods and CAE technologies available to expedite the midsurface geometry and meshing process up to ten times faster (10x). An actual midsurface and meshing demonstration will be performed on an automotive headliner and best practices will be discussed to expedite the process. The use of MSC Apex will be highlighted throughout this presentation.
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.
This presentation discusses best practices for expediting the midsurface geometry process. Specifically, a new semi-automatic or incremental method for creating midsurface geometry will be presented. The concepts mentioned in the presentation will be demonstrated on an injection molded plastic component. The same component required 10 hours to complete with existing pre/post processors, but once the incremental midsurface method is adopted, the process required 1 hour. The MSC Apex incremental midsurface method will be used throughout the live demonstration.
Constructing midsurface geometry and meshes for Finite Element Analysis (FEA) is a process often requiring hours to days to complete. Existing midsurface extraction methods in pre/post processors, while automated, often produce very incomplete midsurface geometry. As a consequence, significantly more time is required before midsurface geometry is completed and meshed.