Week 3 - 2D meshing for sheet metal

Pre- Processing of Car Hood Assembly Using Ansa

Objective –

• Geometry clean-up with no topological error
• Mid Surface Generation
• Meshing the geometry with the target/average length of 5mm

The quality criteria to be followed –

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
12	Tria Percentage	15

Abstract – A car hood is the part that covers the engine of the automobile. Its function is to protect and access the main engine, battery and other parts for repair and maintenance. It is important to optimize the design and stiffness of the hood to minimize air drag. For this challenge, we are performing 1^st step in CAE i.e Pre-processing using ANSA.

Methodology –

• Import Geometry
• Topological clean-up
• Mid-surface extraction
• Quality criteria setup
• Meshing and Mesh improvement

Introduction to car hood and geometry parts –

The car hood car hood consists of two major parts i.e an inner part and outer part. The inner part is for providing the structural strength, while the outer part is for providing design, maintain aerodynamic of the vehicle. While designing a car hood certain safety aspect should be taken into consideration such as pedestrian safety, crash worthiness, etc.

Fig – Car hood panel

The imported geometry consists of 5 parts –

1. Outer panel
2. Inner panel
3. Latch reinforcement extract
4. Right hinge
5. Left hinge

Procedure –

Geometry clean-up -

After importing geometry into ANSA we perform geometry check on the model, below mentioned some topological error was captured.

As shown in the figure the features that was check are single cons, cracks, overlap faces, needle faces, triple cons, collapsed cons, unchecked faces. There are two options for fixing the errors, manual fixing these errors one by one or using auto fix by selecting all the errors at once and selecting the command fix and ANSA fixes the errors. Also helps in reducing the time consumption by using auto fix option. As shown in the figure below for the reference.

After performing the auto fix or performing the manual fix, check again for the topological error as shown in the figure below that was no error found after executing the command.

Once the errors are fixed store the lock view for all the parts for easy excess and working on the model. I) Outer panel, ii) Inner extract, iii) Latch and hinges (since both the latch and hinge are of same thickness.

Further after seperating the parts in the store lock, measure the thickness throughout the part and assure uniform thickness across the parts, if there is change in thickness of parts, smilar parts needs to be kept together.

Mesurement of individual parts are shown in the figure attached below for ready reference.

Fig – Outer panel (thickness – 2.69 mm aprox)

Fig – Inner extract ( thcikness 0.75mm aprox)

Fig – Latch and Hinge (thickness 1.2 mm aprox)

Orientation –

Must be checked thoroughly by the user every time working on a model. All models should have the correct orientation because the orientation issue will cause the final time issue when you merge the mode with the final product. It is always preferred to use the grey orientation (entity model) on the top side as grey is for positive direction as per ANSA environment.

Mid Surface Generation –

There are two ways to extract mid surface-

• The manual mid surface method -This can be done by manually selecting the surface and giving an offset option in the direction opposite to the grey surface. The offset is -0.375, since the grey area is positive. Similarly, it can be done for other subparts there for inner extract the thickness measured approximately 0.75mm and for the hinges and latch thickness is 1.2mm.

• Auto melt surface generation - If the geometry is closed then we can go for auto mid surface generation option, but it will delete the parent geometry. Press the mid surface tab under face. Select the full geometry and confirm the mid Location

Once the mid surface is generated from either of the methods now, we have to set the New property ID different for all 3 subparts with a respective thickness (as shown in the figure below).

Quality Criterion Setup -

We can say that result quality is directly proportional to the element quality. The parameters for quality criteria are Target/average length, minimum length, maximum length, aspect, skewness, included angle, Jacobian, tria percentage, etc.

So, now we can start our meshing process on ANSA

As we can see the inner extract is the symmetrical part so we can compare them with the help of Rmdbl, later after the completion of the meshing process it can transform and mirror copy about the centre line the rest of the symmetrical part.

similarly, can be done for the Outer panel.

Now we can finally start meshing the component and can get the better mesh flow by keeping in mind the best practices followed in meshing.

We have multiple options to mesh (shell mesh), Best mesh, Spot mesh, all of them works on the different algorithms but here in this model we will use the best mesh algorithm Also after creating the best mesh we can reconstruct and smooth the mesh to get better mesh flow.

Please find below the meshed parts result with quality criteria.

Outer Panel -

Inner Extract -

Latch and Hinges -

Mesh Results
Description	Outer Panel	Element %	Inner Extract	Element %	Latches	Element %
Quads	49013	99.09%	37487	96.23%	5194	97.19%
Trias	451	0.91%	1470	3.77%	150	2.81%
Off	0	0	13	0.03%	0	0

Mid-Surface to Solid conversion –

The model is not ready for FEA analysis since the meshing is done only for the surface and not on solid geometry. These elements needs to converted into solids by assigning the thickness of the generated mid surface,

Learning outcome -

• Able to fix topological error
• Able to generate mid surface for any complex geometry
• Aware of the 2D elements and shelling meshing(Quads and Tria importance) 4 Aware of the quality parameters and there meaning during mesh generation

Conclusion -

From the challenge, it can be concluded that methodology for any geometry is moreover very similar in case of shell meshing, one has to follow the sequence stated above to get a better geometry clean-up followed by mid surface and finally meshing keeping in mind the good practices that needs to be followed in order to get better/smooth mash flow. Few areas we need to compromise due to design constraints by the help of defeaturing and construction line adjustments, but it should be only done until and unless it does not damages your real geometry.