On Nodal Displacement Boundary

[Lonelytown]

Hello, guys

This problem has been giving me a had time.  I want to put displacements on several nodes of my model.  Acturally, I can do this in "Static stress with nonlinear material models".  Well, here comes the problem.  In "Static stress with linear material models", there is an option of "Node displacement boundary E..." when I right click an node and select add.  In the dialog of the "Node displacement boundary E..." , besides the displacement type and magnitude, I am asked to input a "stiffness", which makes no sense to me.  How comes the stiffness is needed when we just want to put a displacement on nodes?  What the exact meaning of a stiffness for displacement?

Anyway, I input an arbitrary number, say 10000, in that, and tried to run the analysis.  In the results, that node stays at its original location rather than moves to the location expected.

So what is wrong with this Nodal Displacement Boundary?

I will really appreciate if you have the same experience and would like to share your thoughts?

Thank you.

[Petr.Krysl]

The node displacement is enforced through a spring linked to another (auxiliary) node, you can think of it as ground, which moves by the prescribed amount.  The greater the stiffness that you need to input in the dialogue, the closer the displacement of the node to which you applied this device is to the motion of the ground.  For zero stiffness the displacement would not be enforced at all.  For infinite stiffness it would be enforced "exactly".

[Lonelytown]

Thank you for response.  I can understand the philosophy of this stiffness with your explaination.  But how come I still cannot get the displacement enven after assigned a rather large number in it?

[Lonelytown]

100000000 lb/ft is the number I input.

[algor_neil]

Try something much bigger like 1e12, needs to be several orders of magnitude larger than the material stiffness.

[Lonelytown]

Thanks. But not working.

[John_Holtz]

Just about all loads in linear stress can be adjusted from one load case to another, and prescribe displacements (displaced boundary elements prior to version 2012) is one of those. My guess is that you did not do the following (from the documentation)

• For prescribed displacements to be used in the analysis, you must specify a Multiplier in the Displacement column of the Multipliers tab in the Analysis Parameters dialog box. The value entered is multiplied by the value in the Magnitude field. The product is the displacement applied to the new node of the displacement boundary elements.

P.S. One of the nice things about the wiki documentation is that anyone can add their ideas and explanations. I can see a few improvements myself, so I am going to draft them right now.  🙂

[Lonelytown]

Thank you.  This is turely the solution.  I put the multiplier paramter of the boundary to 1, and the node displaces to the location that I expected.