compression test 1

[camefem]

Hello users,

I need to validate material model based on uniaxial compression test. I have stress-strain curve. I have some basic doubts

1. What is the difference between plastic strain and volumetric plastic strain(is it strain due to hydrostatic compression).

How to find volumetric plastic strain from nominal strain(uni axial)?

2. For a crushable foam material, will there be major difference between uni-axial compression and hydrostatic compression behaviour.

Thanks for your time. Any further data needed to answer my question, please ask me.

Thanks,

camefem

 

[EdR]

camefem:

1. Volumetric strain is the sum of the normal strain components (regardless of whether elastic, plastic, etc.) i.e. e(v)=e(x)+e+e(z)and use the strain type you are concerned with.

2. Yes. In uniaxial compression (only loaded on one axis and free in other directions) the lateral strains are free to occur. In hydrostatic compression the lateral strains are restrained by the hydrostatic pressure in the lateral directions.....again these comments apply to all types of strain.

Ed.R.

 

[camefem]

Thank you EDR.

I did this analysis with abaqus crushable foam model(Zero - Poisson Ratio). From my stress- strain curve, I calculated yield stress and plastic strain(kind of input needed by abaqus) and did the analysis.

Vonmises/nominal strain curve approx. matches with yieldstress/nominal strain.

yield stress is which i caluculated and entered as input.

I expected, vonmises stress will be equal to nominal stress.
Is it right?

What could be the mistake?

Thanks again for your time.

camefem

 

[EdR]

camefem:

I am not familiar with Abacus input so cannot help with that. The VonMises stress is NOT the same as the nominal stress....Check a text or the Abacus manual for the definition of Von Mises stress...It's a combination of the normal stresses and the shear stresses and is related to the second invarient of stress (j2') and Octahedral shearing stress by Von Mises = sqrt(3) j2' and Oct = sqrt(2)/sqrt(3) j2'...If I understand your problem you have a load in only one direction and no shear...for this case j2' = sqrt (sig*sig/3).....so I guess the Von Mises does reduce to the applied stress after all for this special case......VM = sig ... but this is not true for any more general stress situation....

Ed.R.

 

[camefem]

Thanks again Ed.R.

I had some problem with input data and now it is fine. As you said, in my case i have only applied stress which is equivalent to von-mises.

Regards,

camefem

 

[feaplastic]

Hi camefem,

I am alos dealing with same kind of problem. How did you calculate the input for abaqus from your experimental datas.

Please, see my post for detail info.

Thanks,

feaplastic