Meshing of plastic component of side door of a car
OBJECTIVE
To create a 2D mesh in the side door plastic component according to the quality criteria.And apply thickness to the mesh.
PROCEDURE
The given component is a SIDE DOOR model which is a plastic component. And these type of plastic components having varying thickness . And they are having large number of fillets and ribs inside the model.Since the third dimension which is thickness is very low , instead of going for 3D meshing, We can go for 2D meshing. For that we need to extract the mid surface. Since the component is having the thickness which is varying ,we cannot go for auto midsurfacing Since that will give only distorted Midsurface.So we should go for manual midsurfacing.
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 Side door.
For that select IMPORT file icon > Then choose the Side door file from the directory > Then click on import. Then the Side door model will be appeared in the screen.
Fig 1 Impoted CAD model of side door
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.
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.
To take midsurface of the fillets ,first measure the thickness of the fillet.This can be done by using DISTANCE option by pressing F4 .Then place the radio button in two points.Then choose the two points at the extremum on opposite sides to find the thickness.Then create a new collector named MIDSSURFACE and make it as current component.Then measure the side door body thickness by pressing F4.
Then Go to GEOMETRY>SURFACE EDIT>OFFSET(or use the shortcut shift+F9)>select the outer surfaces off the body>choose duplicate to current component>click on vector normal and see whether the offsetting in right direction,if not then click on reverse normal>Then type in the offset distance as half of the thickness>Press offset.
Similarly each and every fillets and ribs also midsurfaced to the midsurface collector.
And there will not be proper connectivity between the ribs and fillets with lower body part.So we can use the Drag the line option to drag the surface through the lower body part of side door.Fot that go to GEOMETRY>SURFACES>DRAG LINE>SELECT THE LINE OF FILLET WE NEED TO DRAG THROUGH THE SIDE DOOR>TYPE in the distance to drag as some value so that the surface made by the line cuts the surface of side door. Then we need to remove the additional potruding surface of fillet midsurface from the side door .we can use TRIM WITH SURFACES/PLANES option for that .Go to GEOMETRY panEL>SURFACES EDIT>place the radio button in trim with sufaces /planes>then select the surfaces we need to trim(Here it is the protruding surface of the fillet and the side door surface)>Then hit TRIM.Then the protruding surface can be deleted by using delete option by pressing F2.This process should be done for each and every fillets and ribs so that the meshing will be done with proper connectvity with the side door and fillets and ribs.
Fig 2 midsurface of side door (front view)
Fig 3 Midsurface in another orientation
IF T connection cannot be made using tHE TRIM with surfaces option.we can simply mesh the fillet midsurface and the side door seperately.And then PROJECT the nodes of the fillets to the side door surface.This can be done using the project option .For this Go to TOOL panel>select PROJECT>Place the radio button in to surface >Select the lowe nodes of the fillets>Turn on the midsurface of side door>select the surface to which we need to project the nodes>Hit ptoject.
Thiis will project the nodes of fillets to the surface but then also the connectivity between the mesh in the sillets and the side door are not proper.There we can solve this issue by using REPLACE NODE option.HIT F3 replace nodes of fillets with that of side door will ensure proper connectivity of the meshes.Then we use SMOOTH option to smooth the mesh.For that press SHIFT +F12>Select the elements we need to smooth>Hit smooth until require smoothing is obtained.
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 4 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.
Then we should check the connectivity of the elements in the msesh .For that we should check whether there is any free edges present in between elements.For that GoGo to TOOL panel>FIND >choose FIND EDGES . this will show the free edges in between the elements.To equivalence those free edges,Go to TOOL panel>FIND >choose FIND EDGES>select the elements in between which the free edges are present > PREVIEW EQUIVALENCE>EQUIVALENCE.This will ensure there are no free edges in between elements.
Fig 4 2d mesh after equivalencing the free edges
Fig 5 2d mesh after equivalencing the free edges(in other orientation)
Using all the steps above ,meshing is done for all the components in the model.TRIAS are reduced as possible and at the end out of 38336 elements 425 are Trias and 37911 are quads elements.
Then we need to check for quality criteria. The quality requirements are given below.And the target element size is 4 mm.
Fig 6 quality criteria
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 I 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.
7)TRANSLATE option can be used to solve the element failing issue.FOR this first switch to element quality mode and find out the failed nodes .And press SHIFT+F4, so that TRANSLATE OPTION appears.Then Choose the node And select the nodes in giving direction of movement of the node .This option if used in logical way can solve many element failing situations.
Fig 7 using translate option to optimise the failed elements
Then next is assign the thickness for mesh just like the thickness of actual model.For thet create properties with thicknesses as 2,2.8,3,3.5,4,4.5,4.8,5,5.7,6.6.8 mm .These are the exact thickness of the various parts in the plastic component side door.Then Create 10 seperate collector in name specifying the vrious thicknesses to identify the thickness of elements in that collector without cross checking it.
Then Go to TOOL>ORGANISE>select the elements of the surface having same thicknesses>Choose destination component as collector of appropriate thickness >Hit MOVE.Similarly every newly created collector must be occupied with the elements which requred the appropriate thickness as stated in its collector name.
Then right click on the collector >ASSiGN>choose property>choose property having thickness same as indicated in collector name>Then apply>Ok.Similarly every collector must be applied with its appropriate property of appropriate thickness.
Go to 2D detailed representation will show as the mesh having thickness exactly same as that of the actual component.
Fig8 collector and properties created
RESULT
After meshing
After applying thickness and organising the respective elements into the newly created collectors depending upon the thickness
CONCLUSION
Process of meshing the complex plastic components is learned with this project.In some plastic components, there may be waste parts present in model,CAE engineers should find out the exact geometry in the plastic components by properly removing those waste parts .And then carry out the meshing step by step and at last we should apply the respective thicknesses to the respective parts of the plastic component ,
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