Assignment 1-2D Meshing Challenge

OBJECTIVE

To mesh the given components as per the given quality criteria after geometry cleanup and midsurfacing. The three components are:

• Practice Component
• Clip Repair
• Mounting Bracket

The 2D mesh is to have a target length of 5 mm with a minimum of 3.5 mm and a maximum of 7 mm. The thickness will also need to be assigned after meshing.

IMAGES OF MODELS

PRACTICE MODEL


CLIP REPAIR MODEL


MOUNTING BRACKET MODEL

PROCEDURE

1. The models are imported to Hypermesh with the RADIOSS solver active. We switch to topo mode to check to see if there are solids - they are to be deleted if any exist. These models did not have solids.

• Geometry Cleanup

But there definitely is some geometry cleanup required, considering the number of free edges (red lines).

2. Geometry cleanup is carried out first. To ensure the models are closed solids (for using the auto-midsurface tool later), we need to make sure there are no imperfections.

For example, in the below case, we can see multiple free edges (red lines). This is because there is no connectivity between this surface and the adjacent one.

To fix this, we simply need to toggle the line. The toggle tool can be accessed from the quick edit section in the geometry panel. The tolerance should be adjusted accordingly. The higher the tolerance, the higher the range of adjacent edge detection for the algorithm.

Another case of geometry cleanup that was encountered in this assignment was fixing missing surfaces. This is fixed by using the filler surface tool from the same quick edit section. We simply need to select one of the free edges for the algorithm to detect and create a surface using the free edges available.

Another thing to take care of was excess surfaces, such as this one here. One of the surfaces was not filleted along with the others and it resulted in this protrusion. Since there is no actual connection between the two surfaces, it simply cannot be deleted. I used the extend tool (geometry > surface edit > extend) to create the connection and the delete tool from the quick edit section to get rid of the vestigial region. After that, it was just a matter of toggling the resulting free edges using the toggle tool from the quick edit panel.

Finally, there were duplicate surfaces as well. These simply need to be deleted (using the delete tool).

• Extraction of Mid-surface

3. After the models were removed of imperfections, we can go ahead with midsurfacing. This can be either done using the offset tool (Geometry > Surface Edit > Offset) or through the auto-midsurface tool (Geometry > Midsurface). I went for the auto-midsurface tool for a couple of them. The algorithm detects the thickness and creates a midsurface (if it is a closed solid).

For the mounting bracket, I used the manual offset method. First, we need to create a new component in the component tree (right-click > create > component) and let's name it 'Middle Surface'.

The offset value is half the thickness of the bracket. We can select any one face from any one of the faces. And then, right-click surfaces and select 'by face'. When this is done, the entire face is selected. Not the entire component.

Before carrying out this step, we need to ensure that the newly created 'Middle Surface' component is our current component. After confirming, we can select 'duplicate' by selecting surfaces again. And now, we can select 'current component'. The offset of the component is now transferred to our newly created component. But we are yet to offset the face.

Before selecting the offset button, we can verify the upcoming offset's normal, which could be in the opposite direction. If so, we can use the 'reverse normal' option to reorient it in the opposite direction. And then we can click offset. We now have our midsurface generated onto the newly created component.

4. After midsurfaces are generated for all the components, all that's left before meshing is to prepare the geometry for the mesh, we need to prevent all possible element failures, primarily due to minimum length restrictions.

We can toggle (quick edit > toggle) feature lines that may result in minimum length failures. That being said, it must be noted that we must avoid toggling lines at the cost of feature capturing as much as possible. But in cases as follows, they have to be toggled to ensure the mesh doesn't fail.

In other cases, we can do the opposite and make cuts ourselves to ensure proper mesh flow and feature capturing. In this case, instead of two feature lines, we can use a midline to represent it. The mesh will still capture the feature pretty well. We can make use of the 'add point on line' tool to first create the points and then the split-surface node, split-surface line tools to make the cuts (all present in the quick edit section)

After the midline is created, we can simply toggle the parent feature lines.

Then, in this case, where we have nodes to each other, we can simply use the replace point tool (from the quick edit panel) to combine the two points into one.

Also, as per the requirement, we can ignore holes with diameters less than 5 mm. Meaning, we can fill these holes and toggle the boundaries.

• Quality criteria

5. After preparing the surfaces for meshing, we can assign the quality criteria for the meshing. This is done via Preferences > Criteria File Settings. The values can then be entered as per our requirements. Our focuses are the target, minimum and maximum lengths.

• Meshing

6. Now we can start meshing each of the components. This is done using the automesh tool from the 2D panel. The surface to be mesh is selected, element size is specified (5 mm) and we can ensure mesh flow align and size options are checked. Mesh type will be mixed.



We must start meshing from a certain region and work our way from that region (preferably one with more free edges). In more complicated models, we can make use of cut lines (quick edit > split surface node) to split the surface into smaller surfaces for better mesh control. Both mesh generation direction depictions and surface splitting have been depicted in the following image:

After the mesh is created, we can clean the mesh using various tools. Primarily, we will be using the element optimize tool, the drag tria tool and to an extent, the swap edge tool. These are accessed from the quality index section in the 2D menu. We can also make use of the 'smooth' tool, which can be accessed from the 2D menu directly.

After we take care of each surface (meshing and cleaning up), we can move onto the next. The process is repeated until we mesh the entire component.

7. In the case of the mounting bracket component, due to its symmetry in 4 directions, we can mesh one instance properly and simply rotate around a central reference point adding 3 more duplicate instances. This is done using the 'rotate' tool from the 'tool' section.

The first step is to select, duplicate and rotate the selected elements in a 90-degree angle. N1, N2 & N3 are nodes selected to define the plane of rotation. The basepoint (B) can be created using Geometry > Nodes > Arc centre. Then the central hole can be selected to generate a node in the centre, which will be our base point.

Clicking either 'rotate+' or 'rotate-' creates an adjacent instance of the same set of meshed elements. This process is repeated again (with elements from both instances selected) but the angle this time being 180 degrees. This creates elements on the opposite side and completes the whole set.

After clearing element quality criteria

part 1

Part 2

Part 3

Check for free edges

8. We will now check for equivalency by going to the edges section in the tools menu and previewing equivalence for varying tolerances (ranging between 0.2 mm to 2 mm). This is to ensure there are no misaligned elements or free edges, which we can also check using a different option in the same section. The preview equivalence option shows us the nodes that require equivalencing. They are rectified by clicking the 'equivalence' option in the menu. If some nodes haven't been tended to, it may require manual fixing, using the replace node option (F3).

Check for duplicate elements

9. After equivalencing, we can check for duplicates through the check elements tool in the same menu. If duplicates exist, we can click 'save failed', move on to the tool menu, select the 'delete' tool, select elements, right-click the element and select retrieve and then click the delete button. This should handle the duplicates.

Check for Normals

The next step is to check for "Normals". The normals option is available in the "Tool" panel. After entering into the normal option, all elements are selected. In order to view the normals for every element.

In the below image we can see that one side face elements have different normals(in blue colour), in order to change the normals of that elements, select the elements that needed to be changed and click reverse.

After correcting normals

similarly checked normals of other parts

Assigning Thickness

10. Finally, we can assign a thickness for each of our midsurfaces. We can do this by right-clicking and creating a component in the component tree. After a property is created, we can go to its section in the bottom left and scroll down to assign the aforementioned thickness. Then we can assign the thickness by selecting the midsurface, going to its properties box on the bottom left and assigning a prop_id, by selecting the thickness property. Similarly, we can assign the material as well through the same process.

After assigning thickness

Part 1

Part 2

Part 3

Conclusion:

• Performed the geometry clean-up on the given components and Extracted the mid surface.
• Performed de-featuring and cleanup on the mid-surface with respect to the element quality criteia.
• Generated a 2D mesh on the given components by following the element quality criteria.
• Finally, assign the thickness for the given components.

Learning outcomes:

By performing this task, the following aspects can be known:

• Performing geometry clean up
• Extracting the mid-surfaces
• 2D meshing
• Maintaining element quality criteria
• Assigning thickness