Week 4 - 2D Meshing for Sheet Metal in ANSA.

Aim:

To Mesh the given CCB CAD Model.

Objective:

Check for the Geometry clean-up (if required).

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	3
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:

• Importing the CAD model.

The model comprises 34 individual components.

• Geometric Clean Up of the CAD Model.

2.1) CONES in the geometry

From figure 3, it can be observed that there are Single cones (Red Lines) i.e., edges shared with only single surface.

To establish connectivity between the surfaces, go to, Faces -> Topo, in theTopo module. Select all the surfaces.

Now, from figure 4, we can see Double cones (Yellow Lines) i.e., edges shared with two surfaces.

2.2) 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 6 errors in the geometry of the model.

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

Even after auto Fix, we can see 1 remaining error that ANSA could not resolve. This error needs to be resolved manually.

The geometrical error along with manual fix are as shown below.

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.

The Triple CONES error in Fig 9 has occurred while performing the TOPO action initially. Since the faces are close enough the cones got joined while performing TOPO action.

Figure 10: Resolving Triple Cones Error.

As seen in Figure 10, Triple cone error can be resolved by first, releasing the cones (b), then individually performing TOPO action the individual component of the model (c). Figure 10 (d) shows the Triple cones error is resolved.

Now, again running the geometric check we can see that the errors are resolved.

• Extracting Mid Surface.

To Extract the mid-surface of the Model, we would use ‘Skin mid-surface’ command. Go to, Faces -> Mid Surface -> Skin.

In the “option list” at the bottom, change the “Skin Property” to “Ask always”. This will help in creating a new property card of the model component one by one. And we can simultaneously put the thickness of the model in the new property card.

Select the model and press enter.

It will prompt us to identify the red, blue and green surfaces. Select outer face as green, inner face as red and face in between as blue.

After mid-surfacing one sub component, ANSA will prompt to create the mid-surface of the next component as shown in the Figure 15.

Now after completing the mid-surface of all the sub-components, we have to individually check the mid-surface of all the sub components. We can see in Figure 16; the mid surface is incomplete in one of the sub-component.

To create the mid surface this time we will use the “offset” command using appropriate distance i.e., half of the sub-component thickness.

Finally, we can see the mid-surface of the entire component in Figure 18.

• Geometric Clean Up of the Mid-Surface.

Now, again checking the problematic areas in the geometry using Checks manager. As shown in Figure: 19, there is one geometrical issues (Needle face).

Needle face (Figure 19) can be resolved by deleting the needle face.

Now, again running the geometric check we can see that the errors are resolved.

• 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.

• 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’.

Figure 24: Quality Criterion menu.

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

• 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 a feature line to supress.

• Meshing.

Since the Model comprises multiple sub components, we need to individually mesh the sub-components one at a time. To do this use “Or” command from the “Focus” Commands section in the “Visibility Status buttons”.

Switch to the mesh module, then go to, Mesh Generation -> Best. In the “Feature section / Selection assist”, select “Macro Area”. Now one by one click on the areas to mesh.

Out of 34 sub components some of the sub-components with mesh are as shown below.

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.

• Ensuring Quality criteria and Mesh flow.

We can use Cut lines using “Zone cut” command to ensure proper mesh flow.

Symmetrical surfaces can be divided into one half to save meshing time and then the mesh can be reflected to get a symmetrical mesh on the both sides.

Elements failing around the holes can also be resolved by moving the “Hot Points” around the hole perimeter by a small margin. To do this, go to, Grids -> Move -> Free, in the Mesh menu.

Failed elements can also be resolved by reconstructing the mesh. To do this, go to, Shell Mesh -> Reconstruct, in the Mesh menu.

• Assigning thickness.

• To assign thickness, in the property card, put T = 1.6.

  

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

  

  

  After ensuring no elemental failure as per our quality criteria, the final mesh is as shown below.