Elastic modulus change with strain rate

[KenFong1]

Hey guys, I am simulating a compression test on granite rocks. I have obtained the data from experimentation, and I'm using Abaqus to re-simulate the experimentation conditions. Strain rate used were 0.0001mm/s, 0.001mm/s and 0.01mm/s. So this is my question:

I want to specify the variation of the Elastic modulus as a function of the strain rate applied to the specimen. I am told that this cannot be done directly but as has to be done indirectly using field variables. How does one do it?

In the material properties under the keyword *MATERIAL for *ELASTIC I specified the range of values of E and the corresponding strain rate values in the ascending order. Here I use the strain rate values as Field variable 1.
*MATERIAL, NAME=
*ELASTIC, DEPENDENCIES=1
1.2E5, 0.3, , 0.0001
1.5E5, 0.3, , 0.001
2.0E5, 0.3, , 0.01

However, How do I tell Abaqus that field variable 1 is the Strain rate(velocity used in Abaqus) I'm gonna apply to the specimen?

Thank you in advance.

Regards,
Ken

 

[NunoMPG]

Hi KenFong1,

I believe that if you want to specify strain rates you need to be using plastic and not elastic material. In the plastic section you can specify strain rate as well as yield stress and plastic strain...

 

[burningDiesel]

You can do it in 2 ways:
The easiest is if you know the strain rate (if e.g. your loading is easy), then you can do it in the input file, see:

http://www.eng-tips.com/viewthread.cfm?qid=321293

But more likely you do not know the strain rate, or it is not constant for each element.
You can use USDFLD to set the field variables (the ones u defined in *elastic)
in the subroutine, use GETVRM to get the strain rate (?ER? I think is the keyword).
There's an example in the documentation.

good luck

 

[bmejoe]

You can specify the FV in CAE to make sure you're picking off the correct one based on where you're requesting it in the inp file. I end up always doing that because the wrong FV gets used for me quite frequently.

Be careful about using plasticity considering your material option. If I remember correctly, the incrementation procedure is different in plasticity versus elasticity theory.

 

[KenFong1]

Thanks for the help guys.

You can do it in 2 ways:
The easiest is if you know the strain rate (if e.g. your loading is easy), then you can do it in the input file, see:

http://www.eng-tips.com/viewthread.cfm?qid=321293

However, in that case E is changing with time.

However, in my case, I need Abaqus to assume a different E based on the strain rate applied to specimen. (strain rate sensitiviy) I already have the experimental data for strain rate of 0.0001 mm/s and 0.001mm/s. And I need abaqus to "interpolate" the E when i use a strain rate of 0.00055mm/s.

Can I still use the method in the link above?

 

[MechIrl]

Like has been said, only if your strain rate is the same in every element and changes with the applied loading/displacement in a predictable way (is this the case?). Otherwise you should use USDFLD.

 

[soil1999]

KenFong1
Did you solve this problem.
Thanks