WLF shif factor

[Tmssankar]

Dear DR. Jorgen

i am using prony series to represent stress relaxation of FKM.i have obtained visco elastic parameters at 25C.
in order to obtain stress relaxation parameters at 125C.

WLF shift factor log aT (T) = C1(T − T0)/(C2 + T − T0)

where T0-temperature at which we have visco elastic parameters.

where C1 and C2 are constants obtained from formula referred in ABAQUS manual

C1 = C1g/(1+(T-Tg)/C2g)
C2 = C2g+T-Tg
where C1g =17.4,C2g=51.6K
i have put T=125 in above 2 eqns and obtained C1 & C2.
and sub this C1 and C2 in WLF eqn and obtained the shift factor.

by using the following formula i obtained the viscoelastic parameters at temperature T=125

log tT0 − log tT = log aT

where t-relaxation time
whether my approach is correct?
very sorry for disturbing u by posting 2 more threads.
i am waiting for ur reply
thanks in advance

regards and thanks
Tmssankar

[Jorgen]

I think you can proceed as you outlined. I suggest that you validate your WLF approach by experimental tests.

What material, temperature range, and strain/stress range are you interested in?

-Jorgen

[Tmssankar]

Dear Dr.Jorgen
thank u for ur reply. i didnt have the facility to do stress relaxation at high temperature.

material is fluoro carbon shore A hardness 70. gasket component temperature range is -40C t0 125C.
i have visco elastic parameters at 25C. for material characterization i done uniaxial tensile stress relaxation test at 25C room temperature for 3 hrs.
in FE analysis of gasket nominal strain is 25%.


i am waiting for ur reply

with regards
T.Muthu siva sankar

[Jorgen]

Ok, as long as the strains are quite small a linear viscoelastic model should be OK.

-Jorgen

[Tmssankar]

Dear Dr.jorgen

in order to calculate C1 abd C2 in WLF shift factor. any other experimental method or analytical method is available?
how can i calculate viscoelastic parameter?
experimental method means please tell testing standards of specimen.




thanks and Regards
Tmssankar

[Jorgen]

You should be able to get the WLF parameters from DMA data. I would then perform some traditional mechanical testing to validate the accuracy of the linear viscoelasticity model.

-Jorgen

[Tmssankar]

dear Dr jorgen
thank you for your reply. regarding stress relaxation at room temperature
i have done at 2 various strain rate one at10mm/min. and other at 2mm/min.
both shows not a good diff between rates. i want less decay of stress so how much strain rate can apply?
please give your suggestion


with regards
tmssankar

[Tmssankar]

Dear Dr.Jorgen

in DMA test whether the stress/strain range and frequency i s related to stress relxation test

i am waiting for your reply

[Jorgen]

Yes, a DMA experiment at different strain amplitues and frequencies can give equivalent information to a set of stress relaxation experiments.

-Jorgen

[Tmssankar]

Dear DR.JORGEN

while fitting stress relaxation data with prony series in abaqus. we have to assign long term modulus.
i thought long term modulus is relaxed modulus after long time right?
up to what percent of initial modulus fluoro carbon relaxed?


i am waiting for your reply

[Jorgen]

That is correct. One way to determine the long term modulus is to perform a long-term experiment in which you determine the stress-relaxation after long term.

-Jorgen

[Tmssankar]

thank you jorgen

i have done stress relaxation test for 7 hours with 40% initial strain and
2mm/min as cross head speed specimen guage length is 25 mm. it shows 26.5% decay of initial stress.

if i fit test data for 7 hours with prony series means it shows worst result.
if i fit test data for 3 hours with prony series means it shows good result.

please tell me suggestion i am waiting for your reply

[Jorgen]

Your results may indicate that linear viscoelasticity is not very accurate for your application :(

If needed, you can switch to non-linear viscoelasticity ...

-Jorgen

[Tmssankar]

Dear jorgen

the stress relaxation curve is similar to prony only. how i can choose material model in abaqus? please give me guidance

[Jorgen]

Well, if you can fit a Prony series to the experimental data, then you basically have a complete model.

You can then check the validity of that model by additional tests.

-Jorgen

[Tmssankar]

Dear Dr.Jorgen
i need some theories about tyre analysis. pls send

[manfred]

. . . just a remark to FKM question!

Concerning FKM see also the following web site:
http://www.viscodata.de/Short_Introduction.pdf
The example which has been given in this document is devoted for a typical FKM rubber. See also attached ppt where i have made some snap-shots out of the mentioned document.