static unloading

[huans37]

Hi all,

I have a beam modeled, and I want to load it using displacement control, and then unload it in another load step.
So far I used two methods and there are some problems:
In the first try, the first load step is like: d,all,uy,-0.05; and in the second, the previous displacement is deleted and a new displacement is added: d,all,uy,0; The issue with this is during the unloading, typical response like reaction force shows that the reaction is increasing rather than decreasing. It seems that the beam is still under loading rather than unloading.

In the second try, the first load step is still: d,all,uy,-0.05; while the second is ddele,all,uy. But this results in a convergence issue. Since no result file can be obtained, I am not sure if it is right.


Any idea what the problem is? Your help is highly appreciated.

 

[flash3780]

Your first method should work fine... I wonder if you have changed the nodes which are selected when you call your second "d" command. Since it's a beam, it may be a better idea to call the node number explicitly:

d,my_node_number,uy,-.05
solve
d,my_node_number,uy,0

Deleting a constraint as you do in your second method will make it difficult for your model to converge... step changes like that are generally difficult for the solver. Alternatively, you could switch to a load control? If you apply a force in step 1 and then set the force to zero in step 2, the load will be ramped, and you'll have an easier time getting a converged solution. This would be the best method if you have plastic deformation that you're trying to capture.