Week 3 - 2D meshing for sheet metal

AIM :

• To clear geometrical errors for all the components
• To generate a 2D mesh pattern over the mid surface of the given part – HOOD MODEL
• To assign thickness for the generated mid surface

PROCEDURE :

The component that is given here is a hood model called bonnet .A sheet metal part of a car and which has very low thickness . This model hood has 5 separate parts

1.Inner hood panel

2.Outer hood panel.

3.Hinge reinforcement ( Right )

4.Hinge reinforcement ( Left )

5.Latch reinforcement

GEOMETRICAL AND TOPOLOGY CLEANUP :

After importing the cad model to ANSA GUI geometrical clean-up is done by steps shown below.

Go to Check Manager > Geometry > Execute it will show the geometrical errors . this figure shows hood model without topology clean up.

Fig 1 Hood model without topo cleanup

Along with the errors shown we can also isolate the errors and fix them using auto fix command. After topology clean-up is done the model will be show as below .

Fig 2 Hood model after topo cleanup

Topology clean up is done for all the hood parts. As using the same procedure for all the parts.

MID SURFACE EXTRACTION:

Mid surface extraction is carried out after topology clean-up by using automatic and manual methods.

Manual method offset is used for these hood model parts. Offset value for the parts is calculated by the thickness of sheet metal parts.

• Before extracting the mid surface for the components we need to give perimeter and macros length on the components to avoid errors.
• Go to Mesh Module >> Perimeters >> Length.

Before assigning Perimeter and Macros :

Fig 3 Hood model Before assigning Perimeter and Macros

After assigning Perimeter and Macros :

Fig 4 Hood model after assigning Perimeter and Macros

Manual method – Offset :

1. Go to Topo Module >> Faces >> Offset >> Select Face >> Set Offset Value and Direction
2. Here we are extracting midsurface for this inner hood extract.
3. After extracting midsurface we need to again assign perimeter and macros value.
4. After the midsurface extraction of each component, we need to look for sharp edges in the component which will make elements fail for minimum element size.
5. To improve meshing we need to suppress the cons that are forming sharp edges , using the command join
6. Go to Mesh Module >> Macros >> join.

As the below images shows the mid surface that were extracted from the given components and also removed sharp edges.

Fig 5 Mid surface of inner hood panel

Fig 6 Mid surface of outer hood panel

Fig 7 Mid surface of Hinge reinforcement ( Right )

Fig 8 Mid surface of Hinge reinforcement ( Left )

Fig 9 Mid surface of Latch reinforcement

MESHING PROCEDURE :

• Mesh generation of the mid surface from the given component needs to set the quality criteria for a proper mesh to be generated.
• the table Below shows the quality criteria that should be followed during mesh generation.

S. 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

QUALITY CRITERIA AND MESH PARAMETERS :

The below image shows the quality criteria and parameters that were assigned for the entire hood model .As per above mentioned table.

Fig 10 quality parameters

Fig 11 Mesh parameters

Points that I kept while meshing are :

• Any opposite trias should re-mesh and need to remove those trias.
• Any mesh transition at the fillet regions need to eliminated.
• Tria elements at the edges or inside the washer also be eliminated.
• Adjacent Trias must be removed.
• Rotating Quads and Same directional Tria elements create an error in analysis should be eliminated.
• Meshing the components with the quality criteria will help achieve good mesh quality.
• Trias should be eliminated at the edges and perimeters at feature area .
• Smooth command used to smoothen the mesh. When trias fall at different grids.

Fig 12 Meshed mid surface of inner hood panel

Fig 13 Meshed mid surface of outer hood panel

Fig 14 Meshed mid surface of Hinge reinforcement ( Left )

Fig 15 Meshed mid surface of Hinge reinforcement ( Right )

Fig 16 Meshed mid surface of Latch reinforcement

ASSIGNING THICKNESS AND PROPERTY ID:

The thickness assigned to the model can be viewed using utilities -> quality criteria -> presentation parameters -> draw shell as solid. It also viewed from property menu.

Fig 17 property menu bar

Fig 18 Meshed mid surface of Latch reinforcement with thickness

Fig 19 Meshed mid surface of inner hood panel with thickness

Learning Outcomes :

• To use various meshing tools and how to mesh with given quality criteria
• To resolve errors caused by min & max length elements.
• Learnt how to work on hemming regions and capture the features properly
• Learnt basic things like removing Topo errors , generate symmetry , how to mesh , how to create midsurface etc..
• Mid surface extraction methods like offset , middle etc.
• Learnt assigning of thickness on components
• Finally learnt how to work and do mesh on sheet metal on pre-processing the hood model .

Conculsion :

• Proper mid surfacing, good mesh flow, no off elements, thickness assignment are done as per given quality criteria
• Total no. of trias = 1817 as per the percentage of 1.93 % with quality to achieve good mesh.
• Given 2d sheet metal hood model is pre processed as per the requirements.