hysteresis

[student]

hello everybody,

i'm a student from germany and i'm currently writing my diploma thesis.

i've been modeling a compression-test simulation.
my material options are: hyperelastic with test-data (abr) and ogden (n=3); viscoelastic material with 2term time proney series. the values are from the ABAQUS manual (0.1;0.5;0.2;0.1;0.2;0.2)

the simulation works, but as i can see in the stress/strain plot, my hysteresis has not the right direction. how can i manage this problem? is it just a parametrical thing or are there any other mistakes included?

thx for your help!

[sq]

Two questions-

1) The Prony series constants you used- am I reading this right, that you have two relaxations with the same relaxation times?

2) What do you mean by the hysteresis having the wrong direction? Are you saying that energy is gained in the loop? Perhaps a figure would help.

[student]

hello sq, to answer your questions:

1) i made a mistake! the correct prony series constants are:
0.2,0.5,0.1;0.1,0.2,0.2

2) i suppose that energy is gained in the loop! (i'm not able to post a figure)

[sq]

I'm sorry that you can't post any figures. I can't see anything spectacularly wrong in the constants you've defined. If, for example a ki or gi were negative, that could explain things.


A few further questions I can think of that might help lead in the right direction:

What is your bulk modulus?

I assume that your 'loop' comprises loading in compression, then unloading. Are you defining boundary displacements, or applying a pressure boundary condition? What happens if you set the gi and ki values to zero (i.e., turn of viscoelasticity)?

[student]

Hi sq, I've already found the problem.

I'm using boundary displacement. The problem was the interaction property of the upper compression plate. In contrast to the lower compression plate I had to allow seperation after contact otherwise the specimen was streched.

thx for your quick help

[sq]

Awesome. Glad to hear you were able to work it out!