Hood Meshing Using ANSA Pre-processor

Aim: A sheet metal component is to be meshed for structural analysis by using ANSA(a Structural analysis tool).

Procedure: The components is a car hood which has very less thickness and comes in sheet metal components category.

The mesh on such components can be efficiently carried on by taking its midsurface, cleaning the geometry, creating a mesh on this surface and then assigning its thickness to it.

The step wise procedure for this type of model is explained in detail below.

1.Topology cleanup: Firstly, the model imported in the ANSA file should be checked for geometric errors. This is done in the checks manger and selecting the geometry option. The window will throw errors if present. These errors can be eliminated by auto fix option or fixing it manually.

2. Taking Midsurface: The midsurface of a part can be extracted by using either Midsurface command or offset command. Here the midsurface is taken by using offset command. A surface is to be selected first and distance for it should be half the components's thickness. There are three parts in hood with there specific thickness.

1. Outer extract : Thickness=0.75

2. Inner extract : Thickness=0.75

3. Latch and joint attachments : Thickness=1.2

The offset distance should be half of these thicknesses for the specific part. Both, inner and outer extracts are symmetric and it will save time if we create half of its midsurface and after meshing we can mirror the mesh.

Midsurface is also generated using the midsurface command. In this, software itself detects one upper surface one lower surface and the distanvce between them as thickness. Software assigns three colors as green, red and blue for upper, lower and thickness side respectively. Now this automatic midsurface generating command can be tricky and not suitable for intricate parts. Also, in this model, we can save time by midsurfacing half part of the symmentric components and mirroring it. Hence, offset face command proves to be more reliable in this case.

After generating the midsurface, it should be checked for geometric errors. It is convenent to make the necessary changes in the geometry so that the mesh flow is maintained.

Hence, a topology cleanup is necessary after generating the midsurface.

3. Setting PID for each midsurface: Here are three midsurfaces which need to have their own identity so as to make the process easier and accesible. While setting PID, the name assigned and thickness of the component is the most important aspect.

4. Meshing: The first thing to do during meshing is to set the quality of the mesh. In simple terms, the quality parameters or quality criteria is a set of instruction that determine the tolerable element size and shape and allowable percentage of tri elements in the mesh.

Sr.no Quality Criteria Value

1	Target/Average length	5
2	Minimum Length	2
3	Maximum Length	7
4	Aspect	3
5	Warpage	15
6	Skewness	45
7	Jacobian	0.7
8	Minimum Quad Angle	45
9	Maximum Quad Angle	135
10	Minimum Tria Angle	30
11	Maximum Tria Angle	120
10	Tria Percentage	15

Above are the criterias where the components should not fail in its quality.

In mesh module there are different mesh types that can be generated on any entity. The Best option creates a mesh that suits best to the quality criteria set. After the generated mesh it will show the elements in the respective color of the criteria that is not met. For improving this mesh, reconstruct mesh and smooth options are best to improve it. Whenever there are tria elements in the mesh they should be removed as much as possible. They can be eliminated by bringing together two trias, spliting them to edges etc. The concentration of tri-elements should not be high at any place. This can be done by swaping them away from each other. There are a set of rules to follow when handling these tri-elements. Tria elements should never be opposite to each other. They should not be connected back to back. No tria elements should be at the edge of the geometry, at any hole, on any feature, and at the fillets.

The above images show same area before and after improving the mesh. After inproving mesh, the concentration is less, number of trias is also less and opposite trias are also removed.

5. Assigning the thickness to the midsurface: The tickness can be assigned to the part from the property list, selecting the midsurface, putting the thickness value to it. For example, in this component, the inner and outer extract are given the thickness of 0.75 and the latch and joints midsurface is given 1.2 thickness.

The thickness is visible when the 'Draw Shell as Solid' option is checked in the quality criteria.

Problems encountered: The common peoblems that one may face during entire process is listed below.

1. Geometric errors: Though geometric errors can be fixed in the auto fix mode, one has to understand the error and should get the idea behind the fixture made by the software. This will ensure that further there will not be any errors that the operator can not fix.

2. Hot points that are extra on any edge: The image attached below shows this type of error that cause the mesh elements to fail in minimum length.

Removing the hot points which are near each other and fail in minimum length in between should be deleted. For avoiding this type of errors the geometry of the parts should be cleaned properly by taking into consideration the minimum length of the mesh.

3. Topo errors: The red line in the image shown tells that it is single con. Though, looking at it one can tell whether it should be a single or double con. This red line should be converted to yellow, i.e., it should be a double con(meaning the con is sharing two surfaces).

This is done by releasing the con and performing a topo between the surfaces the con shares.

Learning outcomes:

While processing this component one can learn many new things. Some are listed below.

The fixing of the geometric errors manually.

Creating midsurface with the use of offset command.

Time saving techniques by understanding the symmetry of the model.

The shape of the inner extract was intricate and thus mesh flow is to be monitered thoroughly. Thus, maintaining mesh flow can be mastered in such parts.

Conclusion:

The hood model is checked for all geometrical errors and those have been removed. In this model, the offset command has proven to be more efficient than the midsurfacing command for extracting the midsurface. Meshing is done precisely by taking into consideration all the quality criterias and the tria element handling rules.

Below is the link to download the meshed model of the hood.

https://drive.google.com/file/d/1cSsThcQ7EWKh_xw1m2GgxV3A82j462Q-/view?usp=sharing