FE modelling of floor panel using HyperMesh

AIM : To mesh the given floor CAD model as per required mesh element size.

OBJECTIVES :

1. To extract the midsurfaces of the given part.
2. To perform geometry clean-up on the midsurface.
3. To deploy 2D mesh on it as per the required element size.
4. To improve the mesh according to the required quality criteria and minimise tria element concentration.
   
   

PROCEDURE :

• The CAD model is imported in Hypermesh.
  
  
  
  
• First of all, all the solids present are deleted. This is done because we are going to perform 2D mesh over the model by extracting the midsurface.
• The thickness of each of the parts is measured at different positions by using Geom - Distance or Geom - Dimensioning command.
  This is to check if the model is of uniform thickness (sheet-metal) or not (plastic).
  Also, this thickness will be added while defining the property for the midsurfaces later on.
• Here, the component is found to be of unifrom thickness. (0.75mm)
  
  

(1) Extracting the midsurface -

• Before extracting the midsurface, the geometry of the parent CAD is checked for any surface irregularities, as these might cause problems in front of the Hypermesh automid algorithm.
  
  
• Any unnecessary surfaces are deleted.
  Similarly, if any adjacent surfaces are not connected properly, they are stitched together by using the toggle option.
  
  
  
  
• Also, there are very small small impressions on the CAD surface. These are removed off and a plane surface is created at such places.
  If these are not removed, mesh elements will fail for quality at these places.
  
  
  
  
• Now, since this is a uniformly thick sheet metal component, Hypermesh should be able to perform auto midsurfacing operation.
  Accordingly, Geom - midsurface command is used to extract the midsurface.
  Here, offset+planes option is chosen in the extraction options panel.
  
  
  
  
• After generating the midsurfaces, the first thing done is assigning property to the midsurface..
  This is done so that the midsurface, despite being a 2D geometry, will represent its respective 3D parent geometry.
  
  
  
  
• As can be seen in the above picture, there are some geometrical issues in the extracted midsurface.
  For this, a proper geometry clean up has to be performed on it.
  
  

(2) Geometry clean-up -

• Now, the midsurface geometry needs to be cleaned up before meshing.
• For this, Hypermesh provides an automatic tool. However, here a manual geometry cleanup is performed.
• Accordingly, the surfaces are checked for any inconsistencies or irregularities by changing into different visibilty modes.
• Toggle command is used where adjacent faces were not stitched properly.
• A proper geometry would result in the follwing visibilty in Topo mode -
  Red edge - where only one face is attached (outer periphery or holes).
  Green edge - where 2 surfaces are connected.
  Yellow edge - where more than 2 surfaces are connected.
  
  
• After the cleanup, defeaturing is done. This is essentially the removal of unnecessary cons/lines. This is done with the help of quick edit - toggle command.
  The purpose of doing this is, this reduces the complexity of the mesh afterwards.
  
  
  
  
• Unnecessary points are removed using the quick edit - add/remove point command.
  If not removed, these will act as nodes while the mesh is deployed and will eventually lead to a quality failure of lots of elements.
• Also, unnecessary close cons are suppressed by using the toggle command.
  While doing this, the lines capturing the features such as curvatures or fillets should not be removed.
• Any wrongly created surface is deleted and extracted manually by using the surface edit - offset command.
  
  
  
  
• Finally, a fine midsurface is available.
  
  
  
  

(3) Meshing -

• Now, the midsurfaces are ready to be meshed.
• Before this the required quality criteria is provided in the preferences - criteria file settings.
  
  
  
  
• Here, the mesh is deployed with the help of 2D - automesh command. The element types selected are mixed.
  
  
• Washer cuts are established around holes by using the quick edit - washer split command.
  This reduces the possiblity of formation fo rotating quads around the hole region.
  
  
• To ensure better mesh flow, the nodes around the hole are so positioned, that the edge in between remains parallel to the nearby edge.
  For this, the Tool - rotate command is used.
  
  
• Also, cuts are performed at some places to divide the faces into rectangular sections with the help of surface split command. This results in better mesh flow.
  
  
  
  However, these cuts should be suppressed afterwards. The surface edit - untrim command can be used for this purpose.
  If done directly, the mesh will get reconstrcuted as per the pre-fed algorithm in Hypermesh. To avoid this, the topology revision option in the 'options' tab should be changed to keep mesh.
  This will retain the mesh even after the cut is removed.
  
  
• After deploying the mesh, improving the mesh flow is very important.
  A better mesh flow will result in higher accuracy of the analysis.
• Removing opposite tria elements is the first step in mesh flow improvement.
  These opposite trias can be very easily removed by remeshing the region by considering the elements in a rectangular shape as shown below.
• Similarly, rotating quads are also removed by remeshing upto the nearby free edge.
• 2 adjacent trias can be combined into a single quad element using the element edit - combine option.
  Similarly, single trias can be dragged out of the periphery using the drag tria elements inside the qualityindex tab.
  
  
  
  
• Also, the overall tria percentage is kept as low as possible.
  
  
  
  
• While remeshing, sometimes, the nodes tend to change their position from their corresponding edges.
  To deal with this, Tool - project command is used. This results in proper geometry capturing.
  Also, node edit - align command is used to align the nodes in porper line.
  
  
• Now, the failed elements as per the provided quality criteria are to be dealt with.
• For this, the cleanup tools in the qualityindex tab is used.
• The elements which are failing can be dealt with by specifically using the check elems command in Tool tab.
• The element optimize command can be used at the failed element region to auto-optimize it.
  
  
• Now, if some of the elements do not have any connectivity in between them, such elements are connected properly by using the Tool - edges - equivalence command.
  
  
  
  The find edges and delete edges commands are used to deal with the unnecessary edges at those places.
  
  
  
  
• Thus, a desired mesh with proper flow is achieved.
• Before finalising, the elements normals are checked using the Tool - normals command to see if all the element normals are in the same direction or not.
  If any element is having the opposite normal direction, it should be reversed.
  
  
• Finally, the model is checked for any duplicate elements using the Shrink Element option.
  If found any, such elements are deleted.
  
  
• This completes the entire meshing procedure of the side door component.
  
  
  
  
• The 2D detailed element representation option can now be enabled to visualize the mesh in 3D representation.
  
  

CONCLUSION :

We have succesfully done FE modelling of the given 3D component by extracting the 2D midsurface of it. Also, learnt quite a lot of ineteresting things like

1. How to perform manual misurfacing and what role does a midsurface play in FE modelling.
2. How to deploy 2D mesh and improve the flow. Also, how to deal with the off/failed elements.
3. What are different types of quality failures in case of 2D meshing.
   
   

(The model file is attached with the report for reference.)