2D meshing of car hood (Sheet metal) in ANSA pre-processor.

Aim:

To Mesh the given Hood Model.

Objective:

Take the mid surface for all the components after checking the geometrical errors.

Mesh the mid surface with the given element Quality criteria. (Refer the table Below for the Quality criteria).

The features in the model have to be captured properly with good mesh flow.

Assign the thickness to the model surface.

Sl. No.	Quality Criteria	Value
1	Target/Average length	5
2	Minimum Length	2
3	Maximum Length	7
4	Aspect	3
5	Warpage	15
6	Skewness	45
7	Jacobian	0.7
8	Minimum Quad Angle	45
9	Maximum Quad Angle	135
10	Minimum Tria Angle	30
11	Maximum Tria Angle	120
10	Tria Percentage	15

Introduction:

Meshing (nodes and elements) is nothing but the discretization of a continuous system (a model or a geometrical shape) with infinite degrees of freedom to a finite degree of freedom.

A mesh is made up of nodes and elements. A Nodes are limited number of points where calculations are made. The entity joining nodes and forming a specific shape such as quadrilateral or triangular is known as an Element.

Basic Terminology:

In the TOPO menu the following entities appear:

The Surface is the underlying mathematical CAD description of the Face where the actual mesh takes place. The CONS (Curves ON Surfaces) are the boundaries of the Faces. According to their connectivity, they are coloured in Red (single edge), Yellow (double connectivity) or Cyan (triple -or more- connectivity). The Hot Points are the end points of CONS. 3D Curves are curves in space that are used for CAD constructions, but they are not connected to the Faces. 3D Points are also used for geometrical constructions.

Correspondingly, in the MESH and DECK menus:

A Macro Area may consist of one or more joined Faces. The Perimeters are the boundaries of the Macro Areas. The Hot Points are the end points of the Perimeters. FE-model mesh is a mesh that is not associated to geometry (dead-mesh). Finally, a yellow Connecting Spot indicates connectivity of a Macro area with FE-model mesh.

Procedure:

1) Importing the CAD model.

2) Geometric Clean Up of the CAD Model.

2.1) CONES in the geometry

From figure 5, it can be observed that there are, Double cones (Yellow Lines) i.e., edges shared with only single surface and Single cones (Red Lines) i.e., edges shared with two surfaces.

To make things clearer, we can activate and deactivate the visibility of each of the three types of CONS separately using the “Single”, “Double” and “Triple” flag buttons located in the FOCUS toolbar.

2.2) Orientation of the Geometry.

On activating ‘shadow’ view mode flag in the ‘Drawing styles toolbar’, we can the Faces in grey and yellow (as you are always in ENT view mode). Gray colour represents the positive side of the Faces while Yellow the negative one.

For this model the orientation of all the visible Faces is uniform.

To toggle the orientation, go to, Faces>Orient> [Visible] function.

For this model ‘Positive or Grey’ orientation is kept.

2.3) Checking the Geometry.

To identify and isolate on the screen the problematic areas that require the user’s attention, checks manager is used.

In the toolbar on the top of the screen, go to, Checks -> Geometry.

This will open ‘Checks Manager’ window.

Here we can select the type of problems that may exist in the geometry of the model to be investigated by activating the corresponding flag in the lower part of the window. Keep the default options and click Execute.

ANSA identifies a total of 397 errors in the geometry of the model.

To auto fix the errors select all the errors and click on fix. This will resolve some of the errors.

Even after auto Fix, we can see 125 remaining errors that ANSA could not resolve. These errors need to be resolved manually.

Some of the geometrical errors along with manual fixes are as shown below.

Note in case that there are Faces that fail the Shadow operation, the so-called Unchecked Faces, then these Faces are reported on the Screen.

Such Faces usually are thin needle shaped Faces or Faces that have bad Surface description.

To resolve Unchecked faces, delete this face by selecting the double CONES bounding the unchecked face. Then create a new face by going to, Faces -> New -> COONS.

Cyan CONS show connectivity of more than two Faces and are usually present at T-junctions. If there are no T-junctions present, then usually cyan CONS imply the existence of multiple defined Faces. To resolve the Triple CONES error, we can delete the unwanted face.

Single CONES error arises when there is a gap or dis-connectivity between the surfaces.

The gap here is larger than the tolerances (the CONS matching distance is set to 0.2 when the “middle” tolerance mode was selected). As a result, ANSA did not connect the Faces at this location. We will have to connect (paste) these CONS manually.

Activate the CONS -> Paste function. Then paste both the CONES together.

Single CONES error can also be resolved by using TOPO operation. Activate the Faces>TOPO function, select with a box selection the area shown on the left and confirm with the middle mouse button.

Crack between CONES Errors can be resolved by either creating a new face or pasting the CONES.

Sometimes, while pasting the first CONS on the second one. OR creating a surface while not selecting all the boundary CONES. A white dot appears. This is a topological problem, as a white dot in the TOPO menu represents a collapsed CONS, that is a CONS that has its start and end points coincident. To fix such a problem we must first release all topological conditions at that location. Activate the Hot Points -> Release function and make a box selection of the area. Then recreate the surface.

After, clearing all the geometrical errors we can see in the ‘Checks Manager’ (Figure:14) that geometrical errors are resolved.

Now, to identify any intersecting geometry that needs to be corrected, go to, Checks -> Penetration -> Intersections.

Upon running the Checks, some intersecting surfaces are identified.

To resolve the intersecting geometry we can,

Activate the Auxiliaries -> Fine function in TOPO Menu and left click on the crosshatch of the big Face twice.

Each time you press the left mouse button the Resolution length of the selected Face is halved. This makes the Face appear with more detail. The curvature of the Face is now clearly visible.

Note that if the resolution length is dropped too much, you may lose the shadow of the Face. To fix this, you can use the right mouse button to do the opposite, i.e., increase the Resolution length and make entities appear coarser. Now the geometry appears much clearer.

3) Extracting Mid Surface.

To Extract the mid-surface of the Model, we would use ‘Offset’ command.

But prior to that the thickness of the Model should be determined. By using ‘Measure’ function in the utilities Menu.

Since, the thickness is 0.75, the Offset distance of the surface should be 0.375.

Activate the Faces -> Offset function in TOPO Menu.

Now, in the feature section, select the ‘feature area’.

Select the face, enter the offset value as 0.375.

In the options list, under the Property dropdown select “Ask always”.

Then create a new property, and rename it as ‘Midsurf’ and enter thickness value as 0.75.

Now, a mid-surface with thickness 0.75 has been created as shown below.

4) Geometric Clean Up of the Mid-Surface.

As show in Figure 25, majority of the mid surface is symmetrical, except the key hole region. Hence, for Geometric clean up and meshing as well, only one half of the model including the key hole area will be considered.

Now, delete one half of the symmetric region so that one half and key hole remain.

Now, again checking the problematic areas in the geometry using Checks manager. As shown in Figure: 28, there is no geometrical issues.

5) Assigning mesh element length.

In the Mesh menu, go to, Perimeters -> Length -> Box select the model -> set Perimeter length as 5.

Inspecting the model area by area, to check if the perimeter length is matching our mesh length requirement.

We can see from the above figure that the perimeter length is deviating from our ideal element length. To fix this we need to first release the Hot points. Activate (in the Mesh menu), Hot Points -> Insert -> Right click the Hot point. Now, again assign the perimeter length.

To stop the nodes from sliding during meshing, we can mark the nodes. To mark the nodes, activate, (in mesh menu) -> Mark/un -> make a box selection -> click middle mouse button.

6) Supressing Feature Lines.

There are regions where the two feature lines are too close to each other, this will result in element failure after meshing. In order to avoid this some feature lines, need to be supressed. To avoid excessive geometric modification and to make sure there is geometric adherence, we can always capture the important regions of the feature such as peak of the fillet.

To suppress the feature lines, Activate, Mesh -> Macros -> cut -> Right click aa feature line to supress.

7) Setting up Quality Criterion.

In order to avoid deviation from the quality after meshing, we need set up quality ranges. In Utilities tab, select ‘Quality criterion’.

Setting up mesh parameters. In Utilities tab, select ‘Mesh parameters.

8) Meshing.

To mesh the Model area by area, switch to the mesh module, then go to, Mesh Generation -> Best. In the ‘Feature section, select “Macro Area”. Now one by one click on the areas to mesh.

Following rules for should be kept in mind to obtain a good mesh:

• Features should be captured properly.
• Proper Tria Management, i.e., Tria should no touch each other, There should be no opposite trias, Trias near edges and free edges should be avoided.
• Proper Mesh Flow.
• Check if there is any dysconnectivity in the mesh.
• Quality Criteria should be satisfied.

9) Reflecting the Mesh.

Now we need to reflect the model along with the mesh on the other side to get the entire mesh. In the ‘utilities’ menu, go to, transform -> Copy. -> Symmetry.

The reflected mesh is as shown below.

10) Assigning thickness.

To assign thickness, in the property card, put T = 0.75.

Now, to view the thickness, go to, -> Quality criteria -> Presentation parameters -> check ‘Draw shell as solid’.