2D MESHING FOR SHEET METAL COMPONENTS [CAR HOOD]

2D Meshing for sheet metal components [Car Hood]

AIM-

• For the given Hood-Model taking the mid-surface of the component, and also mesh the middle surface with given element quality criteria

Objective-

• Generating the mid-surface from the parent component.
• Defeaturing the fillets from the extracted surface.
• Performing a meshing operation on a middle surface as per element quality criteria.
• Checking the element quality by comparing with given quality parameters and criteria and improving the quality by various options.
• Assigning the material and proper thickness to it.

ANSA as Preprocessor-

• ANSA is Automatic Net Generation for Structural Analysis software used for preprocessing in the CAE domain.
• It can be used for both FEA and CFD.
• But mostly it is used in CFD meshing.
• It is more user-friendly than other preprocessing software.

Theory:

Why do we do Geometrical cleanup?

While we import any geometry data it generally comes up with little errors, some geometrical issues are there. Surfaces that are not stitched together. ie. (there, is a gap present between the surfaces). Very small surfaces are squeezed between the surfaces. Sometimes duplicate surfaces are present. The junction between the two surfaces also contains gaps, overlaps, and misalignments. In order to remove that issues and errors, we do geometry cleanup.

Mid-Surfacing:

Why did we create the Mid-surface?

While doing 2D meshing we extract the mid-surface from the model. Mid-Surface means the surface between the two surfaces. 2D meshing is carried out on the mid-surface because mathematically element thickness we assigned half in +Z axis (element top) and half in -Z axis (element bottom). Hence for appropriate representation of geometry via 2-d mesh, it is necessary to extract the mid-surface & generate nodes and elements on the mid-surface. We try to create the shell elements (one dimension i.e. thickness is significantly smaller than the other dimensions) which is a 2D representation of 3D elements. Mid-Surface meshing is used when the geometry thickness is very small as compared to its length and width, so we are generally used in most of the industries where the structure is made up of sheet metals and plastic, etc. In that cases, we use 2D elements instead of 3D elements, hence by adopting Mid-Meshing number of nodes decreases, and also computational time decreases.

During meshing, we have to take care about

• The feature should be captured properly
• No back to back trias
• There should be element connectivity between the elements
• No opposite trias
• No rotational quads
• Quality check to ensure that all elements are within good quality

Procedure:

Stage-1

Importing the model:

• Here we will be importing the CAD file into ANSA.
• Here are many types of file formats supported by ANSA for CAD models like IGES, STEP, and sometimes Parasolid also depending upon the requirement of industries.

For importing the model we will go on pull down tab >> File >> open >> select the all format >> select the model from folder

Imported CAD model-

Before Geometry cleanup:

Outer Extract-

Inner extract-

Latch Reinforcement Extract-

Hing Reinforcement Extract-

Stage-2: Geometry check

Visual check for model:

This will check the model visually that there should not be any,

• Damaged Geometry
• Free edges in unnecessary areas.
• Unnecessary points on the lines.
• Unnecessary connectivity and connection in the model.

Stage-3: Geometry cleanup

The above model is with a lot of geometrical errors,

we will check it all in Check >> Geometry >> Execute

• It will show all the geometrical errors over the model then we need to clear it.
• For that, we will select all the errors at one time and right-click and fix it all.
• If any of them is still remains then select on visible on the errors, then for that particular region and fix it manually with Topo, release, project, insert, delete hot points, etc.

Stage-4: Midsurface Extraction

• Before Midsurface extraction, we need to give the perimeter and macros length on the component.
• It will refine the surface and edges so that components get into a proper shape.
• It will ease the mid-surface extraction and give an errorless mid-surface.
• For this Go to Mesh Module >> Perimeter >> Length.

• Perimeter >> select region >> Middle mouse button >> Length as 1 unit
• Macros >> select region >> Middle mouse button >> Length as 1 unit

Components After changing perimeter length are Shown below:

Fig: After perimeter length of 1unit

fig: After perimeter length of 1 unit

Fig: Hing Reinforcement after perimeter length of 1 unit

Fig: Latch Reinforcement after perimeter length of 1 unit

Midsurfacing:

Go to Top Module >> Faces >> Mid.surf. >> select region >> Middle mouse click >> Again middle mouse and select faces >> Offset value >> middle click

Note: While midsurfacing uncheck on delete original surface in Mid. surf. tool

Midsurfaces of every component-

Fig-Outer extract mid surface

Fig- Inner Extract Midsurface

Fig- Latch Reinforcement Extract Midsurface

Fig- Hing Reinforcement Extract Midsurface

Stage-5

Meshing:

After all Midsurface extraction, we will go for meshing of the surface.

Start meshing with outer extract-

• This is the outer extract geometry with a lot of cons which we don't have any requirement for meshing especially.
• So we will toggle these lines using the cons>>cut option in the topo bar.

• Now as we can see it has symmetry along an axis so we can delete the symmetrical part and start working on one part.
• After deleting the symmetrical part we will split this into a number of sections for better mesh flow as shown in the below picture.

• Now we will perform 2D mesh on this surface

Mesh module >> Mesh generation >> Best >> select component and mesh

1. In 2D meshing with ANSA preprocessor easiest way of meshing is by Best mesh option.
2. After giving mesh to the surface usually, we will be using Reconstruction (Recon.) from shell mesh for better mesh flow.
3. Uses of some other tools also like smooth, split, swap, cut, join, project, etc. for better mesh flow.

Given Quality criteria for this model:

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

Outer extract-

• After meshing one side now we will reflect this mesh on another side also.
• So here in ANSA we will transform this mesh using the transform copy method.

Utility >> Transform >> copy >> select Entities >> symmetry >> Apply >> Finish

Inner extract-

We will do this in the same way as we did for the outer extract previously.

Fig- Splited the symmetric Geometry

fig- started meshing for half symmetric geometry

fig- Inner extract complete meshed

Latch Reinforcement extract-

Hing Reinforcement Extract-

Final Hood Meshed Model-

• After creating the mesh, now we have to assign thickness to the component, we have to define the value of thickness on the property of the component, here when we create a component or when we extract a mid-surface, during that ANSA will automatically create a property (default property) for that, so if you want to change it the go-to SET-PID option in Macros panel

Property is applied to the component

• To see whether the property is assigned to the component or not, that go to Presentation parameters in quality criteria and switch on the, draw shell as a solid option to see mesh in 3D

Thickness is applied properly

Result-

• We have achieved zero surface error in the model
• Successfully extracted mid surface for each component.
• Feature lines have connected properly and surfaces have good connectivity.
• A good mesh flow has been achieved with zero elemental failure.
• We have achieved a minimum tria concentration of 1.55%.

Learning Outcomes-

• How to extract the mid surface automatically for the sheet metal components.
• How to clean the geometry.
• How to generate a proper mesh flow with a low tria percentage.
• How to assign thickness to the components.