Hi all,
I am facing some difficulties in applying the displacement boundary condition on the deformed body.
FE model description:
Description of the body: A simple rectangular solid (40 x 40 x 0.3)
Initial Bcs: the Bottom plane is fixed in direction 3. (Please see attached pic for the nodal information)
Node 3 - is fixed in 1& 2, Node - 7 is fixed only in direction 2.
I have two steps in my analysis on a simple rectangular solid.
Step - 1: Normal displacement of 0.9 is prescribed on the top plane in direction +3.
Here due to the poisons ratio of (0.499), as the height of the body increases to 1.2, the length and breadth of it decrease to 20, to maintain a constant volume.
Displacements of the nodes on the top plane in direction 1
Node 1: 0
Node 2: 0
Node 5: -20
Node 6: -20
Step - 2: I want to give a simple shear deformation on the top plane of the solid.
BCs: Bottom plane is fixed in 1 2 3 directions.
The nodes on the top plane (1,2,5,6) have to be deformed by a displacement of 1.2 in direction 1.
I was able to specify the displacement of 1.2 on the nodes 1, 2 by using an amplitude function as,
*AMPLITUDE, NAME=A2;AMP-2, DEFINITION=TABULAR, SMOOTH=0.25, VALUE=RELATIVE, TIME=STEP TIME
0., 2., 2., 1., 10., 1.
Problem: I tried to specify a displacement of 1.2 on the nodes of 5, 6, which already have a deformation of -20 from STEP-1, as (-20 +1.2 = -18.8)
*BOUNDARY, TYPE=DISPLACEMENT, AMPLITUDE=AMP-3
5, 1, , -18.8.
*BOUNDARY, TYPE=DISPLACEMENT, AMPLITUDE=AMP-3
6, 1, , -18.8.
But in order to specify the amplitude, I had to give it as follows, (20/18.8 = 1.60382978723.....)
*AMPLITUDE, NAME=AMP-3, DEFINITION=TABULAR, SMOOTH=0.25, VALUE=RELATIVE, TIME=STEP TIME
0., 1.603829., 2., 1., 10., 1.
But the problem is that 20/18.8 is an irrational number, So I would never be able to give exact deformations...
Question: Is there any better alternative way where I can specify a displacement of 1.2 directly on the top plane nodes, of the deformed body?
Prescribed boundary condition magnitude (-18.8) is multiplied by magnitude of the amplitude (because of VALUE=RELATIVE ), so write
*AMPLITUDE, NAME=AMP-3, DEFINITION=TABULAR, SMOOTH=0.25, VALUE=RELATIVE, TIME=STEP TIME
0., 1., 2., 1., 10., 1.
Thank you for your answer Mr. Adlucky.
I will try it and get back to you.
Dear Dr.Adlucky,
When i have used the idea which you have given, the displacement was going to -18.8 (from -20) in one time increment, rather than in 2 sec.
I found an another way, by assigning VALUE = ABSOLUTE, I was able to mention the displacements directly in amplitude function. It is given as,
*AMPLITUDE, NAME=AMP-3, DEFINITION=TABULAR, SMOOTH=0.25, VALUE=ABSOLUTE, TIME=STEP TIME
0., -20., 2., -18.8, 10., -18.8.
In this way the displacement is increasing from -20 to -18.8 over the span of 2 sec.
Congratulation!
One more variant is
*AMPLITUDE, NAME=AMP-3, DEFINITION=TABULAR, SMOOTH=0.25, VALUE=RELATIVE, TIME=STEP TIME
0., 0., 2., 1., 10., 1.
I have already tried the variant of,
*AMPLITUDE, NAME=AMP-3, DEFINITION=TABULAR, SMOOTH=0.25, VALUE=RELATIVE, TIME=STEP TIME
0., 0., 2., 1., 10., 1.
but it is not working exactly. It is taking the displacement of the node initially to zero and then increasing it according to the specified displacement.
You said nothing about the type of your simulation. If it is linear statics, then total result must not depend on how we reach desired displacement -18.2 at 2 sec.
Dear Mr.Adlucky,
I am doing a viscoelastic analysis, and the i need the displacement to reach the prescribed value over the span of 2 sec (constant velocity). The load rate is important for me as i am doing viscous analysis. Sorry i have forgot to mention that in the question.
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