Week 6

AIM: 

1. To establish connections on various parts of the rear door FE model using the NASTRAN deck module.

PROCEDURE:

1. The given file contains a extension .nas which means it is a nastran file. To open such files go to the FILE menu in the ANSA software and click on the INPUT option in the drop down menu. A detailed list of options appear.

Since our file extension has .nas, we will select the nastran deck option and select the downloaded file. Now our rear view FE model will look like this.

2. Since the component is already meshed we have to only establish the connections.

3. There are various types of connections in the model such as rbe2(RIGID BODY), spot weld connection, bush connection,bolt connection and the concentrated mass connection. Let us look at where these connections need to be established.

PROCEDURE(1): DEPLOYING BOLT CONNECTIONS

1. Bolts are used in assemblies to connect two or more parts. We can represent the bolt in an assembly as a connection. First identify the area where the bolts have to be mounted in the model and then use the NASTRAN deck to deploy a connection wherever necessary. Now let us look at an area where there should be a bolt connection.

2. You can see that this is the hinge area of the door model which is used as a lock mechanism. This needs to be bolted to the main frame of the door. So a bolt connection is necessary. 

3. To create a bolt connection we must first create a 3D point. To create a 3D point click ON COG option from the POINTS menu. It is available in the TOPO module. Select the perimeter where the bolt connection needs to be deployed and click on the middle mouse button. Now we have to convert the 3D point into a bolt point. 

4. To convert a point cilck on CONVERT option in the assembly menu which is available at the top. Select 3D points option and click on the created point. It will now be converted into a bolt connection. A screenshot is displayed below for reference.

You can see that a pink coloured point is created exactly in the middle of the hole where the bolt connection needs to be deployed.

5. Now click on CONNECTIONS manager. A dialog box appears. We have to select the surfaces in which there will be a bolt connection in between. After selecting the surfaces, click on the BOLT option in the FE representation type menu. After selection click on apply and realise the connection. The screenshots below will give a better understanding on the bolt connection.

A blue coloured line is seen along with a yellow line which represents the bolt head and the stem head in the FE model. This method has to be followed throughout for areas wherever the bolt connection is necessary.

PROCEDURE(2): CREATING A RBE2 ELEMENT

1. RBE2 is nothing but a rigid body element connection. This connection is deployed mostly at holes as they will be used for fastening and assembling purposes. So those areas must be hard and rigid. An example is given below.

2. To create a RBE2 connection go to the NASTRAN deck. Under elements there will an option called RBE2. There are many sub-options such as many nodes, two nodes etc. If the hole contains more than 2 nodes use the many nodes option. Select the perimeter and click on OK. A spider like connection is created. The meeting point of the nodes is called the master node while the nodes covering the perimeter of the holes are called slave nodes.

3. In case of a two noded RBE2 connection, a yellow line is connected. This is required whenever we need to represent a bolt stem connection. The hinge areas connected with the door main frame is an example of a two noded rbe2 connection. 

4. There are also cluster rbe2 connections ie, bolt connections without the stem representations. A picture of those connections are attached below.

These are a detailed overview of rbe2 connections and its types.

PROCEDURE(3): REALIZING A WELD CONNECTION

1. Weld connections are of two types ie, spot weld and seam weld. A spot weld is a point weld and the seam weld is a continuous weld. In this model we will be realizing only spot weld connections. The areas where spot weld connections are necessary are shown below.

2. To realize a spot weld connection, we must first create a spot weld point. Create a 3D point where the weld needs to be realized and convert the 3D point into a spot weld point. This can be done using the CONVERT option in the assembly menu.

3. Then click on the CONNECTIONS manager and select the spot weld point. A dialog box appears. Specify the base and the attachment part of the weld (in this case the cylinder becomes the base and the feature above it becomes the attachment).

4. To create a spot weld connection click on the RBE3-HEXA-RBE3 option in the FE representation type. Enter a search distance value and also create a seperate property ID for the weld. After realizing the connection the weld will be represented as a shell element. A picture is attached below.

5. Follow the same technique for all other areas that require spot weld connections. Store them in seperate PID's.

PROCEDURE(4): CREATING A CON-MASS ELEMENT

1. A concentrated mass element connection depends on the center of gravity. The best example of a con-mass element connection would be the door handle area. 

This is the area where the door handle needs to be represented. If we know the mass of the door handle we can create the connection.

2. The first step is to create the rbe2 connection for both the holes of the door handle as seen in the picture above. Use the many node rbe2 connection so that the connection gets captured like this.

3. A master node will be seen in the middle. Now this master node needs to be converted into a concentrated mass element. To convert it, click on CONM2 option under ELEMENTS. Select the master node and click OK. A property card will be displayed asking to specify the mass. Since we know the mass of the door handle to be 100gms convert it into tons and enter the value. It will be 0.0001tons.

4. Now a COG point is created and the entire mass will act on that point. This is the procedure of a CON-MASS connection.

PROCEDURE(5): CREATING A BUSH CONNECTION

1. An example of a bush connection will be the door glass part. The door glass needs to be connected to the door frame. This can be achieved only with the CBUSH connection.

2. The glass frame needs to be captured in the form of a closed curve. To do that click on the FEAT2CURVE option from the PERIMETERS menu. It is available under MESH module. Click on the edge perimeter of the door glass and click on the middle mouse button. A closed curve is created. To view it switch off the FE model.

3. Now this curve needs to converted as a SPOT LINE. Go to the CONVERT option under ASSEMBLY menu and click on this curve. A spot line is created. 

4. Now go to the CONNECTIONS manager and select the curve. Specify the surfaces to create a connection in between them. Now click on RBE3-CBUSH-RBE3 option under the FE representation type. Specify the search distance and also store the connection under a seperate PID. Now realize the connection and it will look like this.

5. A spring like connection is realized between the door glass and the frame. This is how you will deploy the cbush connection.

These are the various connections deployed in the rear door FE model. 

LEARNING OUTCOMES AND CONCLUSION:

1. We learnt on how to create connections for an entire system.

2. We got to know about the different connection methods and the type of connection that needs to be deployed in different parts of a system.