3D Meshing using Tetra and Hexa Mesh

While saving this model, the maximum number of nodes was exceeding the 100000 limit and therefore could not be saved. Hence the ‘hub’, and ‘knuckle arm’ components from the reflected side had to be deleted.

Objective:  

To create tetra mesh (3D meshing) for the given models and check for the criteria given below.

• Minimum element length: 2mm
• Target element length: 5mm-Housing
                                    6mm-Crank Housing, Axle assembly         
• Maximum Element length: 7mm
• Tet collapse: 0.15

Introduction:

• In the 3D meshing, ‘tetra’ (short for tetrahedral) elements, or ‘hexa’ (short for hexagonal) elements are created in the model.
• In this type of meshing, the elements are created inside the component too, unlike in the 2D meshing, where the elements are created only on the surface.
• There are three methods to create ‘tetra’ meshing:
  1. 2D to 3D tetramesh: In this method, first 2D mesh is created on the surface of the component and later it is converted to 3D meshing.
  2. Volumetric Tetras: In this method, the tetra meshing is created directly by selecting the solids or the surfaces of the component.
  3. Quick Tetras: This method is similar to the one above and is done using ‘utility’ tab.
• The tetra meshing done using the last two methods takes less time, but the quality of the mesh is not proper, and requires remeshing or tweaking the elements. The first method ensures a good quality mesh as the 2D mesh done before can be checked for the quality criteria, and other such factors and can be optimized if needed.
• Quality criteria can be entered in the ‘Tetramesh parameters’ to ensure good quality elements pertaining to the limits.
• There are 3 models in the challenged which were to be tetrameshed.
  1. Housing
  2. Crank Housing
  3. Axle Assembly

Procedure:

1. Housing:

• The ‘housing’ model was provided in the ‘STEP’ format and was imported by using ‘import geometry’. It consisted of 2 components as shown in the image below which had to meshed.
  
  
• Hiding the housing component (yellow), the hub (blue) had to be meshed.
• The hub was meshed for shell trias by ‘automesh’ keeping the element size as 5mm and the element type as ‘trias’ as the component had to be ‘tetra’meshed.
• The number of nodes was tweaked around the component and the mesh was checked for any failing elements using ‘check elems’ option in the ‘tools’ tab, by entering the minimum and maximum element length.
• There were few elements failing which were again remeshed and optimized in the ‘quality index’ and ‘elem cleanup’ options. Upon ensuring there were no failing elements for min. and max. length, the mesh was finalized as seen in the image.
  
  
• These shell trias had to be converted to 3D elements. For this, ‘tetra mesh’ from the ‘3D’ tab was used. The ‘comp’ option was changed to ‘elems’ and all the elements on the component were selected and meshed.
• These 3D elements were checked for ‘tet collapse’ of 0.15 in the ‘check elem’>’3D’ tab and ensuring it to be 0, the mesh was finalzed.
  
  
  
  
• Later, the housing had to be meshed in the similar manner. First, the shell trias were created of 5mm length and later they were converted to 3D elements.
• Again like the hub, the housing was checked for tet collapse, and the mesh was finalized.
  
  
  
  

2. Crank Housing:

• The ‘crank housing’ was as seen in the image below
  
  
• For this ‘crank housing’ component, the method of ‘quick tetras’ was used. To do this, the ‘utility’ tab was switched on to show from the ‘view’ > ’browsers’ > ’hypermesh’ > ’utility’. In this, ‘geom/mesh’ option was selected and under the mesh tools bar, ‘quick tetramesh’ was selected.
• The component to be tetra meshed was selected, the mesh size was set as 6mm, minimum edge size as 2mm, mesh type was changed to ‘trias only’ and the component was meshed as shown in the image below.
  
  
• This created a separate collector ‘Quick Tets’ in which the elements were created, represented by red color.
• These elements were checked for ‘tet collapse’ from the ‘check elem’ tab (F10) and few elements were found to be failing for tet collapse of 0.15
• To resolve this, the component was remeshed by using ‘tetra remesh’ option where both the 3D and 2D elements were selected, the ‘tetramesh parameters’ were specified and the component was remeshed.
• This time there were 0 elements failing for tet collapse. Thus the mesh was then finalized.
  
  
  
  

3. Axle Assembly:

• The following image shows the axle assembly model provided. As seen, the model has symmetry all over. Therefore it was advisable to mesh one half of the model and then reflect the elements to the other half.
  
  
• First the hub from the left side was meshed. The geometry of the hub was observed for any edge fillets below 2mm and those less than that were removed, and free edges were toggled. To remove the edge fillets, ‘defeature’ option was used. Those fillets which were not removed by defeature, were removed manually.
  
  
• Similar to the ‘housing’ model, the hub was 2D meshing using the ‘automesh’ feature and the nodes were tweaked accordingly. The elements were then checked for failure of minimum and maximum length, were remeshed and optimized.
• This mesh was then converted to 3D tetra mesh using ‘tetra mesh’ in the ‘3D’ tab by entering given ‘tetramesh parameters’.
• Later, the elements were checked for ‘tet collapse’ and upon ensuring it as 0, mesh was finalized.
  
  
• After the hub, the ‘knuckle arm’ was meshed. For meshing the knuckle arm, ‘Volume tetramesh’ was used.
• The tetramesh parameters were entered according to the given values, the ‘solids’ in the model were selected and it was meshed.
• When checked for tet collapse, there were some tet collapses found and were cleared.
  
  
• Later, the ‘lower arm’ was meshed using ‘quick tetra’ method. The model was selected in the ‘components’, the mesh size was set as 6mm, the minimum edge size was kept as 2mm, and the mesh type was selected as ‘trias only’ and the model was meshed.
  
  
• The model was then checked for failure of the elements. There were around 300 tet collapses found in the model (tet collapse is the height of the tetra elements)
• To clear the tet collapses, the model had to be remeshed. This was done by ‘tetra remesh’ option by selecting all the 3D and 2D elements.
• The model was thus cleared for tet collapse and the mesh was finalized.
  
  
• Later the remaining part of the model was meshed similarly as above.
• The mesh of this side was then reflected to the other side using the ‘reflect’ option. A midpoint was created between the two sides from the ‘distance’ option by selecting two similar points, one from each side and ‘points between’ was used. By selecting the y-axis, the mesh was thus reflected.
  
  
• The mesh of the lower arm component could not be reflected as its 3D elements were stored in separate collector due to the ‘quick tetra’ method. Therefore, the lower arm of the other side was meshed separately. The image below shows the final meshing of the ‘axle assembly’.
  
  
• ERROR: while saving this model, the maximum number of nodes was exceeding the 100000 limit and therefore could not be saved. Hence the ‘hub’, and ‘knuckle arm’ components from the reflected side had to be deleted.

Result and Conclusion:

• In this challenge, the 3D meshing types and its methods were learnt, along with the operation of correcting the failure of the elements like tet collapse was understood.
• Tet Collapse is the height of the tetra elements which can be corrected by remeshing the component or manually using the ‘distance’ option, by changing the distance between two nodes of the element.
• Apart from this, hexa meshing was also practiced for some components.

Gdrive link for file: https://drive.google.com/open?id=1PGF5jn0nWP52SVwo9v4PLk5NkV2SkXdq