I would like to build a creep master curve of a composite thermoplastic for a reference temperature below Tg (glassy state). I have seen in several papers that the superposition is not valid anymore when the mechanism responsible of creep changes, this is verified by checking the activation energy related to the shift factor in the Arrhenius equation. The activation energy seems to change when the temperature reach Tg. So my question is: Is it possible to build a valid master curve at a reference temperature below Tg using curves acquired at temperature above Tg ? Or do I have to limit the temperature below the temperature at which the creep mechanism changes ? Is it possible to check the validity of a master curve without long experimental data ?
Best,
Rodolphe
On your main question "Is it possible to build a valid master curve at a reference temperature below Tg using curves acquired at temperature above Tg ?" I would say no. The point is that at temperatures Tg and higher we are dealing with a cooperative process in molecular mobility. From the point of view of the creep mechanism, before and after Tg, we are dealing with two different materials. Therefore, the curves higher Tg can not be extrapolated to low temperatures.
Thank you for your answer Mr Yakushev. So maybe that using stress-time superposition principle would be a better method to assess the long term behavior of a glassy polymer ?
What do you call "master curve" - the temperature time profile of an elastic modulus, dependence of a yield point, or something else? But in general I agree with Pavel N/ Yakushev - it is different objects
Thank you for sharing your opinion on the question Mr Mikheev. By the term master curve I mean the time profile of the elastic modulus or the strain evolution over time.
Hi Rodolphe,
regarding to your question, I understood in such a way that you performed a creep experiment at a specific temperature above Tg and then by using interconversion the relaxation function e.g., E(t) was derived from the creep data. Now you want to know whether the relaxation curve can be obtained at temperatures below Tg by assuming Thermo-Rheological Simple (TRS)?
At this point, I have a different view with Pavel. I wouldn't say no.
The point is the behavior of thermoplastic materials can be considered viscoleasticity even at T Tg and try to obtain the relaxation curve at T
Hi Anh Tuan,
Thank you for this detailed answer. I do not have yet performed any experiment at the moment and I am looking for some informations in order to do it well. The goal is to assess the creep behavior of semi crystalline thermoplastic composite that will be used at room temperature (Tg to build this master curve because it would extend the time scale more than lower T. I think that if I use temperature limited to Tg I will have to perform longer creep tests than with tests at higher T to reach the same time scale.
If I understood well, I would be able to build a master curve at T
Hi Rodolphe,
I have not conducted the thermoplastic composite material yet, but if you are able to peform the stress relaxation (to record force/stress decaying over time under a constant strain) insetad of creep at temperatures slightly above Tg with your experimental setup (for example beam bending test setup), it should work for your case. Afterwards, plot the stress relaxation curve onto the normalized scale (i.e., F(t)/Fmax or sigma(t)/sigma_max) vs. log(t) and then by non-linear fitting of this curve with a specific model (e.g., generalized Maxwell model) you will define the viscoelastic parameters of the master curve. To obtain the shift factors, repeat the stress relaxation at two other temperatures, since I believe the shape of the normalized curves at those two temperatures will be identical to the master curve. Next, using either WLF or VFT equations (both are identical) to fit the gained shift factors. By doing so, and if you assume the shape of the stress relaxation (the viscoelastic parameters derived from the master curve) at the temperature far below Tg is identical to that of the master curve, you can derive the stress relaxation at any temperature T
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