Extracted mid surface & Performed shell mesh manually for a tailgate of a passenger vehicle using HYPERMESH

AIM:

To mesh the given Tail gate of a passenger vehicle meeting the given quality criteria along with good mesh flow.

Figure 1. Tail gate

GIVEN DATA:

The given data are the inputs to work. Here the given data are,

• CAD model.
• Quality criteria.

• CAD MODEL: Here the given cad model is sheet metal - Back door/ Tailgate assembly (BIW components).

Figure 2. Given data (CAD model)

• QUALITY CRITERIA: Every product which is been produced will go through the set of quality check. If the quality check fails other steps like boundry condition & apllying loads will not be feasible, So for this reason the element quality check is performed. To identify the quality failure the quality criteria will be entered as an input data. The given quality criteria are shown in the below table. The elements should be maintained within the below given value. If the given quality value is not maintained there will be a failing in qulaity.

Figure 3. Quality Criteria (Given data)

METHODOLOGY:

1. Import CAD geometry.
2. Geometry cleanup.
3. Mid surface generation.
4. Shell or solid meshing.
5. Element quality check.
6. Mesh improvement.
7. Apply load & Boundry condition.
8. Export input file.

Figure 4. Tail gate - METHODOLOGY

1. Import CAD geometry:

Input the given cad model. Here the given cad model is sheet metal - Back door/ Tailgate assembly (BIW components).

Figure 5. Cad model

2. Geometry cleanup:

• To generate FEM we clean up the geometry.
• Surface patch up, creating new surfaces, eleminating of triple cons in unwanted area, converting single cons to double cons in necessary area are comes under geometry cleanup phase. HYPERMESH has some option to rebuilt the surfaces for some extent using auto clean-up option.

3. Mid surface generation:

• This will be done once after the geometry cleanup & extraction of mid surface is an important step in CAE which has to be followed.
• Mid is the neutral pahse which is been generated from the CAD which is imported or given to us.
• The conversion of 3D cad to 2D cad is happening in Skin / mid surface generation.
• The mid surface is generated to reduce the complexity as by simplifying the cad model we get more accurate simulation results in shorter time.
• Once mid surface is extracted create property card & feed in the properties of materials & assign thickness for particular model.

Figure 6. Mid surface

4. Shell or solid meshing:

• Shell meshing is achieved after MID extraction but we do not mid surface all components based on requirements we should also be doing volumetric mesh.
• For casting & molding products we generally go with solid meshing.
• In this project, I have used a shell mesh for meshing the given model since it is a sheet metal component.

Figure 7. Shell mesh

5. Element quality check:

• Every product which is been produced will go through the set of quality check. If quality check fails other steps like B/C & applying loads will not be feasible, So element quality check will be performed.
• The given quality criteria colours are customizable and can be seen while meshing if the elements are beign failed. These failed elements should be corrected and the elements should matches the quality criteria.

6. Mesh Improvement:

• In this phase, the quality of FEM will be improved.
• After quality check any improvements that is been addressed that will be done in mesh improvement stage.

7. Apply load & BC: Loads & boundry conditions will be applied on this phase. Will perform these phases in upcoming challenges/projects.

8. Export input file:

• Have to choose which software will read the file. As different solvers follow seperate algorithms & process.
• Decks: NASTRAN - NVH, PAM CRASH - Safety, LS DYNA - safety, ABAQUS - durability.

THEORY:

TOPOLOGY Vs GEOMETRY:

Topology: Describes how elements are bounded & connected.

Geometry: Describes the shape of each element.

MESHING :

• Meshing - Automated net generated.
• A mesh is a network that is formed of cells & points. It can have almost any shape in any size & is used to solve partial differential equations. Each cell of the mesh represents an individual solution of the equation which, when combined for the whole network, result in a solution for entire mesh.
• FEM allowed for mixed types of mesh cells & unstructured meshes became feasible.
  • 2D - Structural mesh.
  • 3D - Unstructured as well as structured mesh.

Process of Meshing:

• Definite element size.
• Choose types of element.
• Shell mesh (2D - tria & quad)
  • Various methods.
  • Surface less meshing.
• Solid mesh (3D - tetrahedron, hexahedron, pyramid, wedge)
  • Tetra mesh ( Unstructured mesh).
  • Hexa mesh ( Structured mesh).
• Quality cruteria check.
• Remesh - Quality improvement.

PROCEDURE:

• Import the cad geometry - Input the given cad model. Here the given cad model is sheet metal - Backdoor panel (BIW components).

Figure 8. CAD MODEL

• Geometry clean up -
  • To generate FEM we clean up the geometry. Surface patch up, creating new surfaces, eleminating of triple cons in unwanted area, converting single cons to double cons in necessary area are comes under geometry cleanup phase.
  • HYPERMESH has some option to rebuilt the surfaces for some extent using auto clean-up option.
  • For this project I have performed geometry cleanup manually.
  • Go to --> Geometry ---> autocleanup.
• Mid surface generation -
  • Since its a sheet metal component which has uniform thickness here the mid surface is extracted by auto mid option.
  • Go to ---> midsurface --> automid --> Extract option (Skin offset) ---> select surface (By displayed) --> Extract.

Figure 9. Mid surface

• Create property & Material card - Assign propetry, Material & thickness.

Figure 10. Property card

• Quality Criteria -
  • Every product which is been produced will go through the set of quality check. If quality check fails other steps like B/C & applying loads will not be feasible, So element quality check will be performed.
  • The given quality criteria colours are customizable and can be seen while meshing if the elements are beign failed. These failed elements should be corrected and the elements should matches the quality criteria.
  • Go to Preferences ---> Enter the given quality criteria values.
• Defeaturing -
  • In CAD setup defeaturing plays a vital role. Defeaturing is nothing but to remove the unnecessary surface radius, holes, edges. Often, your CAD model does containmany detailed features because of manufacturing constraints. Very small entities can be a problem. If you have very small faces with sharp angles the surface meshing might fails for qulaity. Therefore, check your CAD model for very small entities and remove them before trying to create a mesh for it.
  • Go to Geometry ---> Defeature
  • Also use toggle edges to remove construction lines. Go to --> Geometry --> quick edit --> Toggle.
  • In this project, The fillet region in outer panel has been failed for quality. So the fillet region has been defeatured for meeting the qulaity criteria values.
• Meshing -
  • Automated net generated.
  • Go to 2D --> automesh --> enter target element length ---> Select the surface to mesh --> Mesh.
  • To reconstruct the mesh use rebuild mesh. Go to 2D --> Rebuild mesh.

Figure 11. Shell Mesh

• Quality check -
  • To get visibility of quality failure. Change the element colour mode to by element quality.
  • If any element fails make use of element cleanup tools like place node, swap, split trias, combine trias, drag trias, element optimize,etc.
  • To remove element failure GO to --> 2D --> Quality Index --> Cleanup tools.

Figure 12. Quality check

The above shown model has zero / no failed elements.

• Elimination of trias - Mostly the trias should be eliminated whereever possible.
  • Two opposite trias should be eliminated.
  • Tria element should not lies on perimeter line.
  • Nearer trias must carried out to some extent.
  • To achieve above three points in step 4. Split, swap, combie, replace, drag tria options can be used. To find these options GO to --> 2D --> Quality Index --> Cleanup tools.

MESH FLOW

Figure 12. Mesh Flow

• 2D DETAILED ELEMENT REPRESENTATION -
  • Change 2D TRADITIONAL ELEMENT REPRESENTATION to 2D DETAILED ELEMENT REPRESENTATION
  • 2D DETAILED ELEMENT REPRESENTATION should be activated after finishing challenge. Since it is a shell elements, After activating 2D DETAILED ELEMENT REPRESENTATION the meshed shell elements will convert to solid, by considering the thickness assigned on PROPERTY CARD. This is a final stage in this week-7 challenge which has to done before submiting.

Figure 13. 2D detailed element representation

CHECK LIST:

After completing the meshing the below mentioned checks have to be performed.

• Check for free edges
  • Go to ---> Tool ---> Free edges.

Figure 14. Check for free edges - Tail gate assembly

• Check for comparison
  • Go to ---> mesh ---> Elements ---> Comparison ---> (Source : elements, Target : surface)

Figure 15. Check for comparison - Tailgate assembly

• Check for penetration & Intersection -
  • This check will be done for whole assemblies.
  • Go to ---> Tool ---> Penetration.

Figure 16. Check for Penetration & Intersection - Tailgate assembly

• Check for Normals -
  • Go to ---> Tool ---> Penetration

Figure 17. Check for Normals - Tailgate assembly

• Check for duplicates.

• Check for dependency.

CONCLUSION :

Hence, the geometrical errors and mesh, with the given element Quality criteria has been performed successfuly. Also after meshing the component the thickness has to be assigned for the given component. As per methodology, From import cad, geometry clean up to mesh improvement phases have been performed successfuly. The Applying load & Boundry condition phases will be performed on upcoming challenges.

Figure 18. SHELL MESH

LEARNING OUTCOMES :

At the end of this challenge, I can able to

• Perform geometry cleanup for any complex sheet metal & BIW components.
• Generate midsurface for any complex sheet metal & BIW components.
• Generate shell mesh meeting the given quality criteria for any complex sheet metal & BIW components.
• Identify the quality failure and resolve the failure & improve the mesh quality.
• Work with HYPERMESH with a flow.