Hemming and surface meshing of frontal hood of the car using ANSA

Objective:

The goal is to clean the surface geometry to avoid errors and carry on the hemming process and create a mesh that accurately captures the input domain geometry(A hood) with high-quality cells so as to make subsequent calculations intractable.

The above-said goal is achieved by using the most widely used Pre - processing software named ANSA.

What is ANSA?

ANSA is an advanced multidisciplinary CAE pre-processing tool that provides all the necessary functionality for full-model build up, from CAD data to ready-to-run solver input file, in a single integrated environment.

ANSA is the user\'s preference due to its wide range of features and tools that meet their needs. The list of productive and versatile features is long and the alternative tasks and processes to be completed using them are countless.

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 bending/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.

Types of hemming process:

1. Press Hemming.
2. Table Top hemming
3. Robot(roller) hemming.

Mesh generation:

Mesh generation is the practice of creating a mesh, a subdivision of a continuous geometric space into discrete geometric and topological cells. Often these cells form a simplicial complex. Usually the cells partition the geometric input domain. Mesh cells are used as discrete local approximations of the larger domain.

Purpose of meshing:

Meshes are used for rendering to a computer screen and for physical simulation such as finite element analysis or Computational fluid dynamics.

Finite element calculations cannot be done directly on solid parts. We know to perform those calculations only on points created by the mesh generation process. Thus generating mesh is a must and most important step for any CFD or FEA calculations to be performed.

Steps involved in geometry clean-up by ANSA:

1.The solid part made using any cad modelling software is imported into ANSA. The Procedure is File>Open and select the required solid part.

2.The solid part is initially imported as a wireframe model in ANSA and it is changed into a solid model by clicking on surface icon as shown below.

3.The geometry is checked using the checks button and selecting the geometry checks option as shown

4. The geometry check dialog box displays various errors in the geometry.

5.You can highlight any of the problematic areas by left-clicking on it, or isolate it on the screen by right-clicking on it and selecting “Show only”. Depending on the type of problem ANSA also offers the option to automatically fix it.

6.Close the Checks Manager window. Click “All” from the focus toolbar to make all the faces visible again.

7.De-activate “Shadow” mode and the visibility flag button for “Double” CONS. You will now isolate a region of your part to work on. Activate the “Or” function located in the Focus Toolbar. Make a box selection of the area shown on the left using the left mouse button. The “Or” function leaves visible only the selected Faces. (If you accidentally select wrong Faces, press the All function of the same Group to bring all the entities back to visible).

8. Activate the “Lock” flag button in the Focus Toolbar to lock the currently visible entities. Now when you press the “All” button, only the selected and locked entities will become visible.

9. To view the complete Faces that you have left visible, activate again the “Double” CONS visibility flag. Activate also the “Cross Hatches” visibility flag. This allows the visualization of two green dashed lines along the center isoparametric of the Faces. Apart from visualization purposes, the crosshatches are also used for selecting Faces.

10. After visualizing the surface to be cleaned using various topo corection tools provided in topo mode. Some of the visualized errors are shown below

11. The topo mode provides following menus to perform geometry cleanup

TOPO menu

In this menu, the user can create, modify and clean up the geometry. The menu is divided in groups of functions according to the entities that they refer to.

Hot Points

These functions are used to create or delete Hot Points or Weld Spots.

CONS

Functions that are applied to CONS (Curves ON Surfaces).

Faces

Functions to create or modify Faces.

Surfaces

Functions that create Faces and modify Surfaces.

Curves

Functions to create 3D Curves.

Points

Functions to create 3D Points.

Auxiliaries

Functions to create Working Planes in order to draw in 2D mode.

12. After performing clean up the geometry is again checked for error using geometry checks.

Steps involved in performing hemming process in ANSA:

Hemming process in Ansa consists of floowong steps:

• Extracting mid surface from inner panel
• Extracting mid surface from Hood
• Projecting cons and pasting them

1. Extracting mid surface from inner panel

• In order to extract mid surface from the inner panel, The inner panel has to be isolated. To isolate the inner panel i-not function is used.
• Once the inner panel is isolated the view is locked and stored using store locks option under locks button.
• The mid surface of the inner panel is extracted using the offset option under faces tab.
• The offset window appears and the offset distance is specified. Since the thickness of the inner panel is measured to be as 0.75mm the offset distance is half of the thickness of inner panel. Next, the offset direction is specified as shown below

• the mid surface direction has to be in the opposite direction so \'-\' sign is specified in the offset distance tab.
• the extracted mid surface is checked for geometry errors and the procedure followed in cleaning the mid surface is to clean the half of the geometry and perform symmetry operation
• the geometry is cleaned and checked for errors as shown below

• The errors are cleaned and the symmetry check is performed by clicking on RMDBL option under faces tab in the topo menu.

• After performing the symmetry check the Inner panel is assigned to a separate PID and the thickness of 0.75 mm is assigned.
• The default material properties for mild steel is assigned for the inner panel.

2. Extracting mid surface from Hood

• Similar to the inner pane mid surface of the hood is extracted, But a different approach is used here.
• First, the hood is separated from other entities using the i-not button.
• then using the mid surf option from the faces tab of topo menu, The mid surface of the hood is automatically extracted as shown below.

• the extracted mid surface is isolated and checked for geometry errors as shown below.

• In this process, ANSA assigns a default property id for the skin automatically as shown below.

• The default material properties for mild steel is assigned for the hood.

3. Projecting cons and pasting them

• The extracted mid surfaces of the hood and the inner panels are stored as a separate locked view.
• The hood\'s bend is removed by deleting the entities.
• The outermost cons of the hood are projected on to the inner panel as shown below.

• now the outer layer of the hood and the projected cons are pasted using paste option under cons tab in topo menu and a separate hem region is created and geometry check is done as shown below.

• Now the hem region is separated and assigned to a separate PID with thickness of 2.25 and the mild steel materialis assigned as shown below

• The above said thickness is calculated by the formula 2 * thickness of the outer layers + thickness of the middle layer of the parent object.

Steps involved in mesh generation by ANSA:

1.Now, to generate the mesh, change from TOPO mode to MESH mode. In Mesh Generation select best and define the element type as mixed.

2. After this in Mesh module, under Perimeter, in length option, the size of the mesh is given after selecting the edges. here different mesh sizes are generated for different PID\'s.

3. Now, Individual PID\'s are selected and assigned the mesh length as shown above.

4. After meshing the surface mesh checks is performed and the errors are as shown below

5. The above-seen errors are rectified by performing surface checks where we rectify the errors for the model by cut option (in the case for the rims), delete the surfaces in case of sharp edges and change parameters for intersection pid.

6. After that we define mesh cleaning parameters i.e.skewness,warping,min & max length, min & max ang.

7.In order to clean the geometry, we use the reconstruction feature, for some elements we MV free or paste, we rectify some elements by using the cut option.

8. Thus by this, all the errors for mesh parameters are cleaned and the final mesh quality check is performed as shown below

Conclusions:

1. Thus the geometry of the parent object is cleaned and the mid surface is extracted.

2. Mid surfaces are cleaned and the hemming process is done.

3. Hemmed front closure of the car is surface meshed and the quality criteria are maintained.