ANSYS viscoelastic curve fitting

[juan]

Hello,

I'm trying to modelate a viscoelastic plastic in ansys

In another message, Jorgen_Bergstrom said that the easiest way is to perform a stress relaxation experiments, but in a stress relaxation test you obtain a stress relaxation & time curve, what is the method to transform this curve into a shear & time or bulk & time curve in order to introduce in the Ansys viscoelastic curve fitting ,to obtain the Prony series parameters??

Thanks in advance.

Juan

[Jorgen]

Hi Juan,

Once you have the stress relaxation results you basically are done. Simply take the stress response and divide it with the applied strain amplitude, in order to get a stress realaxation modulus.

Then all you have to do is to find the most appropriate parameters for the prony series that fit the experimental data curve. Many FE programs (for example ABAQUS) have automatic tools to find the prony series from the experimental data. I dont' use ANSYS that often so I cannot give specific direction for how to perform this last step using ANSYS. Also, as an option, you can find the material parameters rather easily using a minimization algorithm in Matlab or Mathematica.

Jorgen

[chublake]

Jorgan,

I am trying to make use of the viscoelastic model in Simulation (used to be called Cosmos). The inputs include the Prony constants for the shear stress relaxation curve and the that for the bulk modulus. There is also a facility for entering the the actual relaxation curves, and the program will do the curve fitting.

The question I have, is this: Does the model generally require both the Shear Stress and Bulk Relaxation curves or will the model work with just the Shear Stress relaxation curve.

I have found some published shear stress relaxation curves, but bulk relaxation data seems more difficult to find.

Also, do you know of a good source for high density PE data that I could use for further investigation. I am particularly trying to build models of high (.960-.965) and medium (.945) density.

One last question--it appears that there more acurate polyethylene models out there including a non-linear viscoeleastic model and a viscoplastic model as outlined by Zhang and Moore. My understanding is that the use of a model not built into Simulation would require producing a subroutine that would interface with the FEA solver--is this typically a very challenging thing to do?

Also, I have the following canned plasticity models in Simulation: Von Mises, Drucker Prager, and Tresca. Would any of these be useful for polythylene in a high stress analysis?

Best regards,

Ron Dickmeyer

[Jorgen]

Hi Ron, good questions.

(1) You can typically get away with using only the shear relaxation data. For HDPE the shear relaxation is significantly more pronounced than the volumetric relaxation response.

(2) There are not too many general references available that contain viscoelastic properties. The best reference that I have found is: "Effect of Creep and Other Time Related Factors on Plastics and Elastomers" from The Plastics Design Library (William Andrew Publishing).

(3) There are certainly many non-linear viscoplastic material models that are significantly more accurate for HDPE than linear viscoelasticity. I often use the Three-Network model or the Parallel Network models (http://polymerfem.com/index.php?pageid=umats) in my own work.

(4) Writing a user-material subroutine is quite challenging and time consuming.

(5) If you are loading the material beyond yield, then you will likely get more accurate results with the Von Mises plasticity model than linear viscoelasticity. Not that both of these very simple models are very poor if you are intersted in both loading and unloading. These topics, examples, and guidelines are presented in my training class (http://polymerfem.com/index.php?pageid=PMclass).

-Jorgen

[chublake]

Jorgen,

Thank you for your advice. I will try to put toether the von mises and visco-elastic models and compare to my experimental results.

Best regards,

Ron D.

[miresan]

I would like to know good sources for MDPE (.940 density) data ir order to modelate a viscoelastic model in ansys,

Thanks