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WLF Coefficients Fitting

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Posts: 124
Topic starter
(@bw_composite)
Estimable Member
Joined: 17 years ago

Good Day All,

I am trying to employ the WLF for an ageing forecast.

We have performed accelrated ageing at 50, 60 and 70 C for 4 months and then measured various properties, one of particular relevance being the elongation at break.

I thimk I understood the principles of the WLF approach, but I am struggling in its application.

Especially how the coefficients should be detrmined is mysterious to me. I am generally conversant with numeric techniques, but I am unclear on what the best fit should be based upon.

I attach the raw data, should anyone be interested.

any help would be the most appreciated.

Thank you very much

Muzialis

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Posts: 37
(@lamvuong84)
Eminent Member
Joined: 16 years ago

WLF versus Arrhenius Approch

Just only one comment to the application of WLF in case aging is involved: I think WLF approach is not the best choice in this case. Better to apply the so called Arrhenius-Approach How this can be done see for example the attachment!

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Posts: 124
Topic starter
(@bw_composite)
Estimable Member
Joined: 17 years ago

Manfred,

thank you for your post and the attachment.

I understand what you say. We have already performed a forecast using an Arrhenius based tecnique, the fact is that we would like to see a comparison with WLF.

You evidently have specificc experience in the procedure, it would be very useful to hear your suggestions on how to calculate / estimate the shift value per temperature.

Thank you

Kind regards

Muzialis

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Posts: 37
(@lamvuong84)
Eminent Member
Joined: 16 years ago

Arrehnius versus WLF Evaluation

Muzialis,

a comparison between an evaluation of aging data according to Arrehnius and WLF-Shift-Function can easily be done formally as you can see in the attachment. Once again to my knowledge there is no need to apply WLF to aging data. Note, WLF-function is originally derived ( see the original derivation by the authors Williams, Landel and Ferry) for polymers that undertake a phase transformation (thermal or dynamic glass transition) and much less for a material that undergoes some degradation processes. Latter ones are usually involved when aging is concerned. Thus WLF-function is expressly not suitable for aging evaluations.With other words applying WLF-Shift-function is not recommented when a long term forecast is wanted.

Manfred

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Posts: 124
Topic starter
(@bw_composite)
Estimable Member
Joined: 17 years ago

Manfred,

many thanks for all this, I appreciate it must have taken you some time.

I understand what you say about the suitability of the two methods, and acknoweldge it.

I t was interesting to see the results. Would it be possible to know though how the master curve in the WLF case is obtained?

It is very difficult to find any literature to how the curve is fitted. I have found a couple of references about shape recognizing software, as in the ideal case the shape of any isothermal curve should be the same, and they are shifted to get the master curve.

In any case, thank you very much for your valuable help.

All the Best

Muzialis

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Posts: 37
(@lamvuong84)
Eminent Member
Joined: 16 years ago

Would it be possible to know though how the master curve in the WLF case is obtained

Muzialis,

[I][COLOR=Blue]It was interesting to see the results. Would it be possible to know though how the master curve in the WLF case is obtained[/COLOR][/I]?

I think with the help of MATHEMATICA or MATLAB in principal it should be possible to build up an appropriate procedure to solve the problem (shifting isotherms in order to receive a master curve by applying a peselected shift-function, may be curve fitting in this case is not as simple). I myself did it by using the customized software ViscoShift ([url] http://www.viscodata.de/ [/url]). On that indicated web site you may also find some basic explanations how to get master curves and applying various shift-functions. Good luck!

Manfred

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Posts: 124
Topic starter
(@bw_composite)
Estimable Member
Joined: 17 years ago

Manfred,

your help was the most appreciated.

Thank you and Kindets Regards

Muzialis

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