Polymer Material definition 1

[AdekunleAdeniyi]

Hi,
My name is Ade and I am trying to understand a problem I am having with my investigation into Energy absorption of polymers in an impact test.

I have specimens (Polycarbonate and ABS) and have carried out accurate tensile tests on the material.

I have model the experiment and given the following material properties using the Density, Elastic and Plastic properties:

PC
Density: 1200
Poison ratio: 0.36
Young's modulus: 2300MPa

ABS
Density: 1060
Poison ratio: 0.39
Young's modulus: 2400MPa

and some plastic strain values up until the yield point i.e. before necking and drawing, that I obtained from the tensile test.

Now, I run the simulation and the impactor just continues to expand the plates until the step time is over and causes element distortion and the energy absorbed graph looks wrong because of the element distortion.

The questions are
Is there anything I have left out in material definition?
Do I have to define a failure criteria like johnson cook and how would I go about doing this?

Thank You in Advance.

 

[vumat721]

Your problem isn't very clear. What are you expecting to see? Any analysis will likely continue even after elements are distorted. Are the elements distorted immediately? Or only after large deformations?

 

[AdekunleAdeniyi]

Only after large deformations but i have done the experiments and know thaty the peak in the energy graph is at about 18s. I do the simulation for twice that and it still has not peaked but it the material continues to stretch.

P.s At about 3seconds, the max stress value entered in the plastic strain has been reached.

 

[vumat721]

The polymers are highly rate sensitive. Is your tensile test data from the same strain rate as your simulation?

 

[AdekunleAdeniyi]

probably not! Its a group project and the tensile test was carried out by another member of the group and he is adamant that it will not be thesame. If i get the strain rate from the experiment or carry out another one... I am guessing the strain rate is adjusted in the material definition but just to be sure, How can I edit the simulation strain rate? And also, my university technician is away and thus no experiments are been carried out at th moment so do you know were I can get johnson cook model values for PC and ABS for the short term.

 

[vumat721]

You change the simulation strain rate by changing the rate of your loading in your simulation. You can look at the log strain rate contours with field variable ER. If this is several orders of magnitude higher than your tensile test, then your simulation results are meaningless. I would try to keep these simulation strain rates within less than an order of magnitude of your experimental data when dealing with polymers. You have more room with metals.

For example, if your tensile data is at a log strain rate of 0.01 /s, then you can't reliably simulate anything that has strain rates outside the range of .005/s to .05/s.