Geometry cleanup and meshing of IP substrate models in ANSA for structural analysis.

Objective:

      The goal is to clean the surface geometry to avoid errors and extract the FE structure of the mid surface of the plastic trim that accurately captures the input domain geometry(An IP substrate) 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 the 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.

Instrument panel(IP):

A dashboard (also called dash, instrument panel (IP), or fascia) is a control panel usually located directly ahead of a vehicle's driver, displaying instrumentation and controls for the vehicle's operation.

The dashboard of a car has three layers. the bottom layer which is a glass-fiber reinforced thermoplastic. On top of that is a layer of polyurethane foam. The top layer on old cars is, of course, PVC.

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 or structural analysis.

        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 modeling software is imported into ANSA. The Procedure is File>Open and select the required solid part. The substrate is divided into several parts and each part is meshed separately.

 

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. 

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. Most of the cad models had clean geometry. For geometries that have errors are cleaned up. After performing the clean up the geometry is again checked for error using geometry checks.

 

 

 

Steps involved in extracting FE structure of mid surface in ANSA:

Extracting FE structure of mid surface in Ansa can be done in various steps:

• Extracting mid surface manually and extracting the FE from the middle surface
• Using ANSA's automatic middle surface FE generator option.
• Performing mesh quality checks to meet the quality criteria

1. Extracting mid surface manually and extracting the FE from the iddle surface 

• In order to extract mid surface from the model, the Middle/offset option is used. Before performing the said operation the thickness of the plastic trim is measured at every possible location.
• In order to measure the thickness at different locations, Weldspots are created at random locations and it is projected on the opposite surface so the thickness can be measured as shown below.

• The mid surface of the IPl is extracted using the offset/ middle option under the faces tab. 
• Incase if the offset option is used, The offset window appears and the offset distance is specified. Since the thickness of the IP substrate is measured to be as 3mm the offset distance is half of the thickness of inner panel. Next, the offset direction is specified.
• the mid surface direction has to be in the opposite direction so '-' sign is specified in the offset distance tab.
• If middle option is used, The faces are selected and the ANSA displays the middle surface preview shown below.

• If the preview is correct the surface is accepted.
• the extracted mid surface is checked for geometry errors and it is manually meshed according to the quality criteria.
• Then, The release option under the elements tab of the mesh deck is used to extract the FE structure.
• The released FE is assigned the thickness based on the parent face.

2. Using ANSA's automatic middle surface FE generator option.

Ansa provides an automatic mid surface generator option, This option saves a lot of time by performing multiple operations namely

a. Extracting the middle surface

b. Checking the middle surface for deviation

c. Calculating and auto assigning the thickness of the middle surface.

A. Extracting the middle surface.

1. PLastic trims are usually molded to form the desired shape. Molding process doesn't provide a geometry of uniform thickness, So ANSA provides aan option named casting under the middle surface option for molding and casting components as shown below

 

 

2. The above shown window appears after selecting the CAD model and the inputs are provided. The minimum thickness is the least thickness of the substrate and the mesh element type and the target element size is assigned. Also the preferred T-joint treatment is selected.

B. Checking the middle surface for deviation.

1. Ansa also provides the option for checking the extracted middle surface for deviations from the parent CAD.

2. The deviations are detected by ANSA and it can also be fixed automatically using fix option as shown below.

3. The generated mid surface is compared with the parent cad model. Transparency can be switched on for the geometry under the properties tab as shown below.

4. The middle surface extracted by ANSA may not be 100% correct, The User has to intervene and check for errors. Some of the manually found deviations are shown below.

5. The above-seen deviations are corrected using various mesh correction options provided by ANSA.

6. The manually corrected mid surface is shown below.

 

C. Calculating and auto assigning the thickness of the middle surface.

1. As already said the IP doesn't have a uniform thickness, Here the substrate thickness is found to be 3mm and the rib thickness to be 1.5 mm.

2. The extracted middle surface has to be assigned the same thickness to that of the parent CAD.

3. Ansa also provides an automatic thickness calculation option as shown below

4. The maximum known thickness of the CAD model is provided and the increment value of the thickness for the varying thickness is assigned. Also the material from the parent CAD is automatically assigned by ANSA.

5. As seen in the below image ANSA has automatically assigned different thicknesses for the substrate and the ribs. Also, they are assigned to the respective PID's automatically.

6. The FE model can be cross-checked with parent CAD for thickness by using the display shells as solids option as shown below.

7. Ansa's thickness assignment may not be correct at the areas of fillets and chamfers, So the user has to intervene and do some manual thickness assignment. Some of the ANSA's deviation in thickness assignment is shown below.

8. As already said it is seen in the above image that ANSA has assigned different thickness for a rib.

9. The thickness assignment is corrected by the user by assigning the elements to the respective PID's manually as shown below

3. Performing mesh quality checks to meet the quality criteria:

a. The mesh parameters and the quality criteria are provided and fed into ANSA as shown below.

b. After the quality criteria are provided the mesh is checked for quality as shown below

c. 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.

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

e. 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.

f. 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 using casting option for the plastic trim component.

2. Mid surfaces are cleaned and the FE structure is released. The quality criteria are satisfied and the model is ready for structural analysis.

3. Performed the above-said operations both manually and automatically.