CLEANUP THE GEOMETRY AND MESHING THE FRONT HOOD FOR STRUCTURAL ANALYSIS

OBJECTIVE

Clean up the Hood geometry and then mesh the geometry with the element size of 5 mm for structural analysis using ANSA

ABSTRACT

In this project we need to clean up the geometry of the inner panel and outer panel surface of the hood and then extract the mid-surface and perform hemming process on the mid surface of the front hood and perform meshing for structural analysis of the front hood using ANSA.

What is hemming process?

Hemming is a technology used in the automotive industry to join inner and outer closure panels together (hoods, doors, tailgates, etc.). It is the process of folding the flange of the outer panel over the inner one. The accuracy of the operation significantly affects the appearance of the car\'s outer surfaces and is therefore a critical factor in the final quality of a finished vehicle.

PROCEDURE

1. STEPS TO OPEN A FILE IN ANSA

• Click on File
• Select Open
• Select File (FORMAT .stp file.)
• Click Open
• A translator window appears CLICK OK

As the hood geometry is opened in ANSA, initially we will goto the right side deck menu and change it to NASTRAN and then under quality criteria menu we will give the values for the quality criteria to be maintained.

INNER PANNEL OF THE FRONT HOOD

GEOMETRY CLEANUP

Initially, we will start with inner panel of the hood and go for the Geometry check for checking the errors present in the model and rectify it for extracting the mid surface. As we will fixed the errors at one side of the inner panel of hood and perform symmetry for reflecting the geometry.

JUNK GEOMETRY

Junk geometry popped up due to translating of file in ANSA, we need rectift it by deleting these junks using universal delete. As these should come along with the parts of the geometry but due to translation issues they might have moved to some other location or coordinate.

SURFACE ERROR

After geometry checks the another error showed up was the absence of surface, holes and improper faces as these damaged surfaces are needed to be fixed by deleting it using universal delete , use Topo under Faces and recreating it using New under Faces. The clean surface will be attained.

HOTSPOT

Hotspot is a white spot shown at the geometry when there are number of hot points are present at a point and ANSA is not able to combine it by itself. It is an issue and needed to be fixed by releasing the cons and checking it carefully, also release the hot points and deleting irrelevant hot points will rectify the errors of hotspot.

FIG. GEOMETRY CHECKS OF MID SURFACE INNER HOOD

As all visible damages are cleaned up, now go to the Geometry manager and select Geometry Checks. Use Auto fix to fix some issues as ANSA fixes these issues by itself. But if the issues are not solved by ANSA using Auto fix, then we have to fix those errors manually to clean the geometry. After cleaning the geometry completely again run the checks as it should show no errors in the results.

CREATING MID-SURFACE OF INNER PANEL OF THE HOOD

As before creating mid surface we need to check the thickness of the geometry at any random points as we know it is a hood made up of sheet metal so the thickness will be uniform.

STEPS

• In Topo mode
• Under Faces
• Select offset and choose faces under this option
• Select the geometry using feature selection
• Set the offset value
• Click ok

Then the mid surface of the inner panel of the front hood (child surface) is extracted, now ANSA will ask to delete the parent surface, as we require it so we won’t be deleting the parent surface (original cad model)

Lock the view of mid surface of the Hood

• Go to Geometry checks
• After execution it will show some errors
• Initially auto fix all the errors if it persists then manually fix those errors

SYMMETRY

As we have attained a clean geometry of the mid surface of inner panel, now we will perform symmetry using Transform option but before performing symmetry we will check the symmetry of the geometry using RmDbl and then we segregated the unsymmetrical parts and symmetrical parts of the geometry as in this we need to retain the unsymmetrical part so we made it invisible and performed the symmetry and again made all those unsymmetrical part visible using All view option with the symmetrical parts to attained a clean geometry of mid surface.

ASSIGNING PID FOR INNER PANEL

STEPS

• Goto Property option
• Select New
• Select PSHELL
• Set the name as hood inner panel
• Assign the thickness as 0.75mm
• Now creating new material by putting values of young’s modulus, poisons ratio and density of the material a new MID card is generated under the Material dialog box
• Assign the MID of the new material
• Click ok
• Goto the PID created and right click on it and select apply
• Select the model using left drag and middle click to apply the new property with new material is assigned to the mid surface

NOTE

For NASTRAN all MID cards should be same i.e. If MID1=100 then MID2 and MID3 = 100.

We can also change the material by changing the young’s modulus, poisons ratio and density of the material.

FIG. MESHED MID SURFACE OF INNER HOOD

OUTER PANEL FRONT HOOD

Clean the outer surface geometry of the hood and execute geometry check as no error is detected. So we can extract the mid surface of the outer panel of the front hood using Mid-surface under Faces option and then use Convert tool to break the link between the parent and the child surface. Now we can assign the thickness and material by creating new PID using Property option under PSHELL.

ASSIGNING PID FOR OUTER PANEL

STEPS

• Goto Property option
• Select New
• Select PSHELL
• Set the name as the hood outer panel
• Assign the thickness as 0.75mm
• Now creating a new material by putting values of young’s modulus, poisons ratio and density of the material a new MID card is generated under the Material dialog box
• Assign the MID of the new material
• Click ok
• Goto the PID created and right-click on it and select apply
• Select the model using left drag and middle click to apply the new property with new material is assigned to the mid surface

FIG. GEOMETRY CHECKS OF MID SURFACE INNER HOOD

HEMMING PROCESS OF THE HOOD

Now, we are ready to perform the hemming process on the front hood. We will be pasting the mid surface of the inner panel and outer panel of the hood using Paste under Cons, as a result, a triple con will be formed around the hood and then delete the irrelevant part from the hood geometry to attain clean hemming of the front hood.

FIG. MID SURFACE OF THE HOOD

We need to calculate hemming thickness using the below formulae,

HEMMING THICKNESS =2*thickness of outer panel + thickness of the inner panel

Hemming thickness of hood = 2*0.75+0.75 = 2.25

Now, we have to assign thickness to the hemming region by using show only option and assign the properties to it

ASSIGNING PID PROPERTY TO THE HEMMING REGION

STEPS

• Goto Property option
• Select New
• Select PSHELL
• Set the name as hood_hemming
• Assign the thickness as 2.25mm
• Putting the same material as of the inner and outer panel of the hood, for changing the material we can put values of young’s modulus, poisons ratio and density of the material a new MID card will be generated under the Material dialog box
• Assign the MID of the new material
• Click ok
• Goto the PID created and right-click on it and select apply
• Select the model using left drag and middle click to apply the new property with new material is assigned to the hemming region.

FIG. ASSIGNING PID TO HEMMING REGION

NOTE

To check the thickness of Hemming region open the Property option and check the PID created for Hemming region it will show the assigned thickness.

MESH QUALITY

We need to maintain the mesh quality given below as the target element size for meshing is 5mm with mixed element type. As we will now go to mesh parameters and set values under the general mesh and then select quality criteria to set the given criteria and maintain the mesh quality. Now, we need to clean up the mesh errors of the hemmed hood under the Hidden mode. As the errors in the hidden mode is shown by the offs, here our objective is to completely remove this off from the geometry to attain a clean mesh of the hood also the mesh quality will be enhanced and will provide us with an accurate result of the structural analysis.

FIG. MESHED MID SURFACE OF THE HOOD

STEPS TO SET THE MESH PARAMETER AND QUALITY CRITERIA

MESH PARAMETER

• Select the tool Mesh parameter
• Under Basic submenu, select General and select the Element type as mixed
• Set the Target length as 5mm
• Click ok

FIG. MESH PARAMETERS

QUALITY CRITERIA

• Select the tool Quality criteria
• Under Shells set all the given values of the criteria to maintain the quality of the mesh.
• Click ok

FIG. QUALITY CRITERIA

SKEWNESS

Skewness is defined as the ratio of the difference between optimal cell size and cell size to the optimal cell size. It is only applied to triangles and tetrahedral.

SKEWNESS = (Optimal cell size – Cell size) / Optimal cell size

FIG. SKEWNESS OF ELEMENTS

WARPING

Warping of the quadrilateral element is defined as the deviation of the quad element from its best-fit plane. If the node of the quadrilateral element is formed outside of the plane, then warping of the element takes place. the warpage is the measurement of how much the node has deviated from the best-fit plane and as it gets closer to zero, the element becomes more planar.

FIG. WARPING OF ELEMENTS

ASPECT RATIO

Aspect ratio is defined as the ratio of longest edge length to shortest edge length. If the aspect ratio is equal to 1 then it is ideal for an equilateral triangle or a square.

FIG. ASPECT RATIO

These quality errors were rectified during the mesh clean up, as these affect the quality of mesh which needed to be fixed for better and accurate results of the analysis.

FIG. SURFACE MESH CHECKS OF HEMMED HOOD

CONCLUSION

The geometry clean-up and the meshing of the hood model with 5mm element size and mixed element type for structural analysis is successfully attained using ANSA.

FIG. MESHING OF FRONT HOOD FOR STRUCTURAL ANALYSIS