Finite Element Analysis (FEA) is an excellent method for evaluating the strength and performance of reinforced and unreinforced thin wall structures such as plastics. FEA simulation models often take the form of shell meshes that are based on the extracted midsurfaces of original solid geometry. Unfortunately the process of constructing such midsurface geometry often requires hours to complete. In extreme cases, the process requires days to complete.
We invite you to attend our webinar as we explore advanced midsurface methods that will enable analysts and designers to construct midsurface geometry and meshes up to 10x faster for Finite Element Analysis (FEA).
Join this webinar to learn about:
Semi-automatic midsurface extraction methods to produce continuous midsurfaces
Midsurface extraction methods for thin wall sections with draft angles or tapering
Direct Modeling and Meshing technology to rapidly move vertices and close free edges
Exporting geometry from MSC Apex to other CAE applications such as FEA pre/post processors
During this webinar:
A live demonstration will be performed to reinforce the concepts discussed in the presentation.
MSC Apex will be used throughout the presentation to perform the midsurface extraction and meshing.
Commentary will also be given regarding the use of midsurface geometry in injection molding simulation workflows.
Merging nodes is a step common in Finite Element Analysis and is traditionally done when adding a mesh transition or connecting meshes together. Unfortunately the process of merging nodes or equivalencing nodes is a time consuming process.
In this post, an alternative to merging nodes is mentioned that is easier and can get you faster to your goal of calculating stresses, deformations, and loads.
To start off, why are merging nodes necessary.
Refer to the animation below. At first, the FE mesh representing the structure appears as one continuous structure, but in fact, has a piece of mesh that is disconnected from the rest of the structure. The interfaces of this separate piece contain points or nodes of its own that overlap with the nodes of the other meshe. A Finite Element Analysis (FEA) software application will be unable to perform a strength analysis on such a mesh configuration. The mesh has to be intact or one continuous piece before an FEA application can evaluate the model. For the FE mesh to be one continuous structure, the nodes at these interfaces have to be merged or equivalenced.
A mesh such as the one shown below is not suitable for most FEA software packages. Reason being, many of the nodes are unaligned and duplicate nodes exist. For clarification, such unaligned nodes and duplicate nodes have been marked in red.
In order to evaluate this structure, the mesh must be such that all the nodes align and that nodes are not overlapping. The image below is an example of a mesh that has aligned nodes with no duplicate nodes. As mentioned at the beginning of the post, this process is quite time consuming and brings us to our alternate method of merging nodes.
For the remainder of this article, we will be focusing on this example. A plate with fillets is fixed on one end and a total load of 300 lbf is applied on the opposite end. The goal is to determine the maximum stress, which occurs at the fillets. Material properties: E = 30e6 psi; v = .3.
Here is the example using shell elements. Since we know the maximum stress is at the fillets, the region around fillets should have a fine mesh. The rest of the plate can have a coarse mesh. Earlier I mentioned meshes should either be intact or continuous in order for an FEA application to successfully analyze.
Since this mesh is not continuous, it should be intact in order to be solved by an FE application. This is where we use Glue.
When I use Glue in this post, I am not referring to an actual adhesive used to connect physical components. Glue in MSC Apex is a virtual method of attaching FE meshes. Generally, when Glue is applied, it enforces a requirement that the interfaces of the meshes act in unison when displacing. This achieves a comparable result compared to continuous meshes that were equivalenced.
Below is a depiction of the Glue region for the bottom fillet.
Now, how does the maximum stress of the mesh with Glue compare to a normal continuous mesh? As can be seen below, the max von Mises stress values of 2060 psi and 2070 psi are aligned with each other. This is good. It means:
The stress result of the Glue mesh can be used for writing margins of safety.
The process of creating a mesh with Glue is significantly faster than creating a continuous mesh that would require merging nodes or equivalencing meshes.
You get to go home early since you saved yourself all the time of merging nodes.
How applicable is Glue? Below are variants of the same example using a variety of different shell and solid elements. Note that the maximum stress in each example are aligned with each other. Glue can be used in many different meshing scenarios, which means you can apply Glue to many different structural models, including yours. Instead of merging nodes and equivalencing meshes, perhaps take some time and explore the use of Glue next time you have a chance. Feel free to click on the image to get a closer look.
Edit: After I finished this post, I forgot to include one meshing scenario that is definitely worth mentioning. As the image below shows, meshes of different topologies can be glued together and can yield useful answers. In the image below, shell and solid elements are used. Solid elements are used in the region where accurate stress answers are necessary. Note the maximum von Mises stresses of 2080 and 2060 psi are in agreement with the results from the previous mesh examples.
Watch a video demonstration on YouTube
Another tip, for more complex geometry, feel free to TET mesh portions of the geometry that are otherwise difficult to HEX mesh. Then use Glue to connect the meshes. This can help you move through your FEA workflow faster and help you write your stress margin reports sooner.
The finite element method (FEM) is increasingly used for structural calculations. For the most part, FEM is used to calculate structural deformation, stress, fatigue life, vibration, or temperature. The manufacturing of a product may be simulated, and one can calculate the acoustics, crash behaviour, and many other disciplines. The FEM model answers questions like “will the product satisfy the design requirements?”,”will the component fail?” or later “why did it fail?”.
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.
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.