Week 3 - 2D meshing for sheet metal

Aim:

The aim of this project is to the mesh the car hood with a good flow of elements and to match the given quality criteria and mesh parameters. To mesh the car hood, mid-surface has to be taken for the given different components in the car hood.

Procedure:

Once the model is imported in Ansa, the model is subjected to various check to clean up the geometry. Under geometry check, 44 errors have been popped up and shown in the figure.

Most of the errors are cleared by specifying the length of perimeter and macros edges to 1. Other errors are fixed by using the auto-fix option. since the geometry topo is cleaned up, the material has to be defined in the material card. Orientation is done to get the same grey colour all over the surface which is shown in the following figures.

As it has been clear from the property card, the car hood has five different properties for five different parts. The mid-surface has to be taken for five different parts to proceed further with the meshing. To take a manual mid-surface, the thickness of each part has been measured at different segments to ensure the part has the same thickness all over the surface. The top and bottom part of the hood has 0.75mm thickness and other parts measured as 1.2mm thickness.

In this project, manual mid-surfacing is adapted to generate the mid-surface for different parts. By using offset and feature option, the surface has been selected to generate the mid-surface and half the thickness value is given to generate the mid-surface. The generated mid-surface has single con on the outside boundaries since it is not stitched to any other surfaces. As soon as mid-surface is generated, by using !not option, the mid-surface was alone taken out from the main part.

By using the Set PID option, the new property PSHELL is set up for the Mid-surface. Likewise, the mid-surface for all other parts is generated and the new property is made. To ensure whether the generated mid-surface is correct, the auto mid-surface is generated by using Mid-surface skin option and the new property is set up for the auto mid-surface. When the topo is made, all the single cons converted to double cons ensures that generated manual mid-surface is in the good form. The model is now ready for meshing.

Before going to the mesh module, the mesh parameters and quality criteria have to be set up as per the given conditions is shown in the figure.

Once the conditions have been set up, the mesh can be generated in the mesh module. Before meshing, the perimeter and macros edges should have the given element of 5. The distance between the nodes has to be checked at critical surfaces to avoid quality failures. If the distance between the nodes falls below the minimum length the defeaturing of the surface should be done. when the mesh is generated and it falls for minimum length by using edge2perimeter option, the new con is generated and joining the cons helps to avoid the quality failure at the critical points like fillets and curvy areas.

The mesh is generated by using Best mesh option. To obtain quality elements and good mesh flow, the different cuts are introduced in the mid-surface. Normal mesh without cuts and with cuts for different parts show in the figure. From the figures, it is clearly understood that parts with manual cut have a good element quality and good mesh flow. After the generation of mesh, reconstructing it helps to get a greater quality element.

The hot points on the features like circle holes have to be set up in even number by using num+/- to avoid the bad patterns and it should be kept parallel to any one of the perimeter edges and it should be marked to avoid the changes in its own position. The opposite trias and trias at the perimeter should be eliminated by using split, reconstruct and smooth option. As soon as the mesh has been generated, shadow mode should be shifted to hidden mode. In hidden mode the falling elements are visible for different qualities and care should be taken to avoid the off elements. All the off elements on the different parts should be cleared and each part should have good mesh flow and patterns.

The mid-surface without any manual cut gives bad mesh quality and bad mesh flow whereas the mid-surface with manual cuts shows good mesh quality and mesh flow and it is shown in the figure.

The top and bottom part of the hood has a symmetrical surface. The Manual cut is generated at the centre of the part. The mesh has been generated at this particular portion and on the other side, the mesh is transformed by using the transform symmetry option. If the nodes are mismatched, the elements should be released and auto paste has to carried out. When it is not applicable the nodes should be pasted manually and the quality of the element should be checked. All parts should be checked for element quality, mesh parameters, opposite trias, trias at the perimeter. From the meshed parts, it is understandable that tria elements are present in the convergence area. 

Learning Outcomes:

I learned how to clean up the geometry in the topo module, how to take the mid-surface for different parts, setting up the mesh and quality parameters, how to deal with the critical surfaces such as defeaturing, and how to achieve good mesh flow by giving manual cuts and I learned how to deal with the generated trias and how to transform the mesh when it has symmetrical faces. More I work I get familiar with the different options in topo and mesh module.

Conclusion:

After completing this specific task, I would like to conclude that I get confidence that I will achieve a good mesh flow and quality on any parts which has uniform thickness all over the body. I conclude that it’s great learning.