Is it possible to change connector behavior? 3

[an318is]

Hi,

I have been working on the same problem for quite some time. I have attempted a variety of ways to solve the problem but have not had much luck.

The current method I am hoping is possible requires the modification of connector behavior definition. Basically, I have a simulation set up with a very weak connectivity between the nodes, I would like to make the components very stiff or possibly set the behavior as rigid at a later point in the simulation.

The aim is to simulate a simplified spot weld, turning it on after weld tips come into position. As stated in some other posts I am trying to do this in Abaqus/EXPLICIT so cant use *MODEL CHANGE to turn on/off the connectors.

I am open to any suggestions to achieve this, whether they be during a step, at the start of a restart analysis or modifying the odb with python.

Any help would be extremely useful.

Thanks

 

[akabarten]

Hi,

To change connector behavior you can use field variables.
Simple example how to define it and use:

**
** model data
**
...
** connector definition
*ELEMENT,TYPE = conn3d2, ELSET = connector
10, 1, 2
*CONNECTOR SECTION, ELSET=connector, BEHAVIOR=beh
axial,
** connector behavior definition, in example elastic behavior
*CONNECTOR BEHAVIOR, NAME=beh
** we need to add number of dependencies
*CONNECTOR ELASTICITY, COMPONENT=1, DEPENDENCIES=1
** if field variable is 0.0 connector use this def
1.0e+06, , , 0.0
** if field variable is 1.0 connector use this def
1.0e+01, , , 1.0
** definition node set, it will be use with *FIELD and *INITIAL CONDITIONS keyword
*NSET, NSET=conn_nodes_field
1, 2
** define initial condition for field variables
*INITIAL CONDITIONS, TYPE=FIELD
conn_nodes_field, 0.0
** definition of amplitude to control field variable with time
*AMPLITUDE, NAME = field_amp
0.0, 0.0, 24.0e-03, 0.0, 25.0e-03, 1.0, 100.0e-03, 1.0
...
**
** history data
**
...
*FIELD, VARIABLE=1, AMPLITUDE=field_amp
conn_nodes_field, 1.0
...

As you can see value of field variable is control by amplitude so connector will change behavior from stiffness to weak at time 25 ms.
Field variables are really powerful tool but unfortunately in my opinion in abaqus documentation is not enough information about it. Please take a look here: 28.6.1 Predefined fields.

Regards,
akaBarten

 

[an318is]

Thanks for your help akaBarten.

The code did exactly what I wanted. It was very useful to learn about field variables too. However, there was one side effect that I didnt think about. Once the connector is stiffened up, the nodes are pushed back to their original positions. They dont maintain their relative positions in the new clamped (from clamps and weld tips) state.

Any suggestions on how to get around this?

Thanks Again

Adam

 

[an318is]

I made some attempts to get around the observed behavior. Basically, as I understand it, the force calculation connecting the two nodes is performed based on the relative distance between them. Since their original position was about 10mm apart, when I changed the connector stiffness component the same force calculation pushed the nodes back outwards.

I attempted to try using the *CONNECTOR CONSTITUTIVE REFERENCE command with 0,0,0 for the three displacement components. However, if I do this, It will pull the two nodes together. This will infact be hiding the non-linearities I am searching for. I would rather have the connector displacement characteristic reset to 0 at the same time the stiffness is increased. This would mean the distance between the nodes would stay the same when the stiffness is increased.

Alternativly is there any way I can change the weak spring to a BEAM element?

Thanks again

Adam

 

[akabarten]

Hi Adam,

You are right, due to new high stiffness new high force is generate in the connector. The idea with *CONNECTOR CONSTITUTIVE REFERENCE will work only when the reference length is set to connector length at time when you switch characteristic.

Maybe you can try do it in different way and use *CONNECTOR LOCK keyword.

...
*ELEMENT,TYPE = conn3d2, ELSET = connector
10, 1, 2
*CONNECTOR SECTION, ELSET=connector, BEHAVIOR=beh
axial,
*CONNECTOR BEHAVIOR, NAME=beh
** use only weak characteristic
*CONNECTOR ELASTICITY, COMPONENT = 1
1.0e+01,
*CONNECTOR LOCK, LOCK=1, DEPENDENCIES=1, COMPONENT = 1
**if field is 0.0 do not lock connector
, , , , , , ,0.0
**if field is 1.0 lock connector when velocity is higher then 0.0
** since connector with low stiffness has movement, at time when field is switch the condition will be true
, , , , , 0.0, ,1.0

Axial connector with lock should has similar behavior to *MPC type PIN, you can change connector to beam after that it should be similar to *MPC, BEAM type.

Regards
akaBarten

 

[an318is]

Good idea akaBarten. I have implemented it now and it is working perfectly. I changed the LOCK to ALL and implemented rotational degrees of freedom. Couldnt ask for a better implementation.

Thank you very much.

Regards
Adam