Tension-Compression Behavior 3

[karabiber]

Hi,

I am trying to simulate a plastic with a hyperelastic model.
Input: Uniaxial test data

Output 1: stress-strain curve under tension
Output 2: stress-strain curve under compression

When I compare these two curves, I have different behavior. does anyone know what can be the cause of this difference?
regards

 

[GBor]

What type of plastic and how significant is the difference in the data?

 

[karabiber]

It is a fluoropolymer.
Red line is under tension
Blue one is under compression
In order to make it easy, I put compression into the first quarter of the coordinate system.

 

[GBor]

Actually, your tension and compression moduli don't look that different...pretty typical. It is the post-failure behavior that varies, and that is also not unusual.

Your tension specimen probably yielded locally and then ultimately failed in net section failure. Your compression sample probably failed in buckling, but that is purely a guess.

Use the curve that is appropriate for your anticipated failure mode...time to be an engineer

 

[karabiber]

what you are saying is totally right. Obviously, the change in response is caused by a difference in deformation mechanism btw tension and compression.

Actually, I could understand it if it was from experiment. What I do not understand is how simulation generates this.

 

[GBor]

It depends on the software, the material model that is being used, and how closely that material model maps reality, but the information you have is basically the input for the software, and it came from some test data.

What software are you using and what "hyperelastic" model does it follow? Learn the "black box" and you will understand how the software uses this information to perform its calculations.

 

[MechEng2005]

I would guess that the "black box" is checking the cross-sectional area after yielding. In tension, in decreases, causing more elongation. In compression, the cross-sectional area increase, reducing the rate of delongation. Just a guess though.

Is delongation a word and the right word?

-- MechEng2005

 

[GBor]

Highly unlikely that the software has the sophistication to check a cross-section. It will apply the material model on an element-by-element basis. If the strain is negative, it would apply the stress information from the compressive section of the curve. If the strain is positive, it would follow the tension curve. In the software, they are probably the same curve input, but, again, how this is applied depends on which "hyperelastic" model your particular software is using. There are a few different models for this type of behaviour.

 

[ThomasH]

Actually I think there are softwares that include the change in crossection. I think I've heard that Marc does just that but I may be wrong.

I think it requires the model to be based on solids but then it should work just as described in the results.

Regards

Thomas

 

[karabiber]

I am using Abaqus/cae 6.7 right now. Strain energy potential is marlow.

I do not have any stress vs. strain data for compression(from experiment), and I do not know the behavior of the material under compression.

obviously, it is implemented in the model that way. Thanks for this valuable answers.