2D meshing of HOOD of a car
AIM: To create a 2D mesh in the HOOD of a car according to the given quality criteria.
PROCEDURE
The given component is a hood model which is also called bonnet of a car. This part is a sheet metal part and so that thickness is very low. So instead of going for 3D meshing, We can go for 2D meshing. For that we need to extract the mid surface. The hood is having 4 separate parts, which are hinge reinforcement, Latch reinforcement, a inner panel and an outer panel. So we should extract the mid surface of each of these 4 parts before meshing.
WHY we carry out meshing?
The answer is because the original component is having infinite number of atoms in it and when we apply any force the displacement or strain is happened to each and every atom inside it. but infinite number of displacements to find out is impossible. So we need to make the calculation finite. for that we divide the component into a finite number of elements having finite number of nodes. Most useful and popular 2d elements are quad and trias. Trias are being stiffer compared to quads they hesitate to transfer the force applied on it to nearby nodes so they are to be needed to minimize in the meshing process. Each element is having lines at the outer boundary which is based on the specific shape function of the element. We can find out the values of stress and strain at the nodes of the elements. Since we know the shape function of the element we can find out the displacement and stress values at the particular x and y as we need using the shape function.
For QUAD4 element shape function is u = a0 + a1x+a2y+a3xy.
FEA will calculates the values at nodes ,ie = a0,a1,a2,a3 Value of the variable in between nodes can easily be find out using the above shape function. Each element will have several degrees of freedoms. Degree of freedoms is the number of variables with which the position of a node can be represented perfectly. Mostly the total number of degrees of freedom = total number of elements x 6. since the 3 linear motion and 3 rotational motion along x,y,z axis are possible for each and every elements. Then the degree of freedom also depends on the type of Analysis we are to be carried out, Type of element we used in meshing process. Higher number od nodes in the mesh means higher accuracy. But the computational time will be high. Computational time is directly proportional to nth power of the Degree of freedom, and n = 1 to 4 depending upon the solver we used for analysis.
First step in 2D meshing is to open the hyper works software and in the interface it will ask for to select a solver, Here we select Opti struct as solver .Then we need to import the IGES file which is the CAD file of hood.
For that select IMPORT file icon > Then choose the hood .iges from the directory > Then click on import. Then the hood geometry will be appeared in the screen. This file is the Cad model so the file imported may not be proper and some surfaces may get distorted in the process of importing. So we need to do the clean up the geometry. Some free surfaces may be presented in the imported model, some surface duplication may be seen, some glitches in the surface etc… will be presented. For to remove all these issues before the process of meshing we need to do the topology clean up. We can use visualization option to see the free edges and surface duplications. For that uncheck the green lines which is of shared edges in visualization option and then switch the model into the wireframe mode. Then we can see the duplication of surfaces and free edges. If there is free edges in the geometry expect at holes and outer boundary, Then that is the indication of missing surfaces and improper connectivity. And if there is no T connection and still there are yellow lines present ,Then it is the indication that thee are duplication of surfaces. So we need to delete the duplicate surfaces and for removing the free edges which are red lines ,if present we can use EQUIVALANCE option in edge edit ,TOGGLE option in quick edit can also be used, If these two cant solve the problem we can make use of spline and ruled surface options in SURFACE panel. Also, we can use the replace point option in quick edit tab.
After the geometry cleanup we need to take the mid surface of the model. Since this is having Third dimension which is very very less magnitude. We can take the midsurface and mesh it in 2D and then we can add thickness to the mesh which is exactly same as the model. Since this model is having 4 different components, we need to take midsurface of the each components in different collectors.
TO take the midsurface, Go to GEOMETRY tab click on MIDSURFACE, then select the surface for which we need to take the midsurface. Then place the radio button in extraction option and select OFFSEt+SWEPS+PLANES.Then choose auto extraction and click on CREATE. Then we can see the midsurfaces of the component is saved in the current connector. Similarly take the midsurface of each component.
Fig 1 Outer panel midsurface
Fig 2 Latch midsurface
Fig 3 Hinge Midsurface
Fig 4 Inner panel midsurface
Then we need to do the geometry check or clean up .in that process we need to pay attention of our target element size, minimum element size and maximum element size. We need to defeature the fillets which having possibility of elements failing for minimum size. Ask to CAD Design engineers for the importance of those fillets. If those fillets are not that important then it is permitted to defeature those fillets. For defeature go to geometry tab > defeature> surface fillets> select te fillets we need to defeature>remove. If fillets are between 0.1 mm to 5 mm radius, Then we can remove those fillets. And is there is holes present in the surface and the holes id having radius below 3 mm , then it is called a pinhole. We can remove or fill the pinholes for better meshing. To remove PINHOLES Go to geometry tab>DEAFEATURE>PINHOLES>select the pinholes> REMOVE. Then the pinholes will be removed.
Provide WASHERS to the holes above 5 MM radius and if the radius is below 10 mm provide 6 to 8 elements inside the washer .And if the radius is above 10 mm then it should be meshed with 12 or more elements. Washer can be given by Go to QUICK EDIT> provide offset >click on WASHER SPLIT >click on the edge of the hole, then we can see the washer split is appeared over the hole. Since there is possibility of stress concentration near the holes, we need to mesh those areas with only quad elements.That ‘s why we provide washer in those regions. Other critical areas can be find out by run the analysis an observe the results .the red colored areas are of high stress areas and they are critical areas. We should mesh those areas with small element size for proper capturing of surface and with quad only elements.
Cad engineers cannot provide the model in exactly same way as needed in meshing, because the design engineers design the components for purchase engineers and manufacturing engineers. So he kept in mind only the manufacturing perspective while designing. So CAE engineers should remodify the model as per our need.
Then we can start meshing the midsurface. there we can use auto mesh option in 2d panel to mesh the surface.
Go to 2D PANEL> AUTOMESH>Select the surfaces needed to mesh>give the element size AS 5 MM > HIT MESH
Then the whole surfaces got meshed. with the radio button in density we can clean change the nodal densities at various locations so as to get a smooth mesh with lower number of trias. Before accepting the mesh we should change the densities in such a way that the maximum tria removal is possible. Then accept the mesh. Various things to keep in mind in meshing
- If there are any opposite trias ,we should re-mesh these trias and thereby we can remove those trias.
- There should not be any mesh transition at the fillet regions.
- Smoothing command can be used to smoothen the mesh. For that go to TOOL panel >click on SMOOTH> Select the elements needed to smooth>Click on smooth until we get even mesh flow.
- Never place TRIA elements at the edges or inside the washer.
- Adjacent TRIAS must be removed. For that we can use DRAG TRIA option in QUALITY INDEX tab in 2D panel. We can select the tria and drag it to any other position so that we can remove the adjacent trias.
- Rotating Quads are not acceptable in CRASH TEST but acceptable in structural analysis. Same directional following TRIA elements are acceptable in CRASH ANALYSIS but not permissible in structural analysis.
- Always mesh the component keeping in mind the quality requirements.
Using all the steps above ,meshing is done for the 4 components in the hood model. Then we need to check for quality criteria. The quality requirements are given below.
After meshing some of the elements violates the quality criteria . This can be checked by Press F10 which is the check element tab. This can also be entered with Tool>check elements>type the quality in relevant boxes. Then click on the respective options to check whether tab elements fail or not. So we should make them follow the quality criteria .For that the tools used are:
1)replace node option. For that PRESS F3 the replace the nodes with each other .
2)Go to QUALITY INDEX then use ELEMENT OPTIMISE ,this is used to optimize the elements based on the criteria we typed in the criteria file settings in the preferences option in menu bar. Go to PREFERANCES> criteria file setting> edit the Table as the quality criteria> then use element optimize and click on near the element we need to optimize.
Yellow color elements means the element is failed. But the red colour element means the element is worst. Touching near the failed element with element optimize will optimize the element.
3)Also we can use place node option in clean up tools in quality index. We can move the node along the surface /Normal to surface to optimize the element.
4)Also smooth tool in TOOL panel also can be used .
5)in case if element need to split ,Then we can make of split element feauture.For that press F6 place the radio button in split and select the elements we need to split, then click the points through which we need to split the elements. Then click split. Then the elements will get split.
6) PRESS F5 which is the mask option so we can save the elements from check elements tab and then we can retrieve those elements using mask option, And we can work on those failed elements without disturbing the other quality elements.
Since the inner panel, Hinge reinforcement and outer panel is having symmetry in the model, we can make use of reflect feature. For this first mesh the half of the geometry, Then Go to TOOL panel> REFLECT>Select the displayed elements>Duplicate those elements into the current component>Choose the axis over which reflection is needed>choose the base node> Hit reflect.
Fig 5 Using place node to optimize the elements failed
Fig 6 After clearing the failed elements
After the reflecton,we need to EQUIVALENCE the elements at the middle.Since the proper connectivity between the exact middle is not proper after reflection.For this Go to TOOL panel>FIND >choose FIND EDGES>PREVIEW EQUIVALENCE>EQUIVALENCE.This will ensure there are no free edges in between elements.
Fig 7 Checking for free edges
Then assigning the thickness to the mesh.For this we need to create property and apply thickness to it exactly same as the real component thickness .And then assign those properties to the respective components.Here the outer and inner panel have the thickness as 0.75 and the latch and hinge reinforcement have 1.2 mm .So create property with thickness 1.2 and .75 and assign those properties to the respective componenets.
RESULT
Outer panel mesh
Latch reinforcement mesh
Hinge reinforcement mesh
Inner extract Mesh
Complete mesh
After asssigning thickness
Conclusion
By working on this challenge , I learned about the Process of 2D meshing of a sheet metal component.
Link for the files
https://drive.google.com/drive/folders/1i5jrJpi10ShV3UM9x3eexr3-zS6NzS15?usp=sharing