FE modelling of front car-hood using Hypermesh

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

OBJECTIVES :

1. To extract the midsurfaces of all the individual parts.
2. To perform geometry clean-up on the midsurfaces.
3. To deploy 2D mesh on each midsurface 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.
  Since, it is an STP file, the file type is changed to STEP while importing the geometry.
  
  
  
• First of all, all the solids are deleted. This is done because we are going to perform 2D mesh over the model by extracting the midsurface.
• The complete model consists of 5 parts -
  Outer extract,
  Inner extract,
  Latch reinforcement extract, and
  2 hinge reinforcement extracts.
• 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.
  
  
  
  
• Now, midsurfaces of each of the individual parts are extracted. This is done by using Geom - midsurface command.
  
  
  
• After generating the midsurfaces, the first thing done is assigning properties to each individual parts. Proper thicknesses are assigned to each part in their property tabs.
  This is done so that the midsurface, despite being a 2D geometry, will represent its respective 3D parent geometry.
  
  
  
• 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.
  Unnecessary edges are tracked and deleted using the Tool - edges command.
• Toggle edges 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.
  
  
  
  
  
• While doing this, the lines capturing the features such as curvatures or fillets should not be removed.
  
  
  
• Also, the unnecessary points are removed by using the Geom - point edit - suppress command. If these are not removed, these will act as nodes when the mesh is deployed resulting in quality failure of elements.
  
  
• 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.
  
  
• 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.
  
  
  
• 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.
  
  
• 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 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.
  
  
• The outer and the inner extracts have symmetrical geometries. So, these two parts are not modelled completely.
• Either of the halves is selected and the meshing is done over it.
  After performing the quality cleanup, the mesh elements are first duplicated and then are mirror-ed with respect to the central plane.
  For this, the Tool - reflect command is used.
  
  Such modelling of a symetrical part reduces the overall time upto a considerable extent.
  
  
• However, the junction elements do not have any connectivity in between them. For this, the Tool - edges - equivalence command is used.
  
  The find edges and delete edges commands are used to deal with the unnecessary edges.
  
  
• Thus, a desirable 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.
  
  Checking the normal direction is quite important as it affects the simulation results.
  
  
• Finally, the FE can be visualized with corresponding thickness using the 2D Detailed Element Representation option.
  
• Thus, the midsurface extraction allows 2D modelling for a 3D parts, in case of thin sheet-metal or plastic components.

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 wuality failures in case of 2D meshing.
4. How to effectively and quickly model symmetrical parts.
   
   

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