Dear researchers,
Here comes a puzzling question. I am having difficulty in explaining a difference in the data of two subcontractors in a scientific way.
as we all know, polymers properties are time temperature and environment dependant. Without giving away names, the two subcontractors are working for the same goal - to extrapolate the master curves of specific medical polymers. When I say master curve, I mean long term behaviour such as yield strength vs. time, so that I can foresee how my material will behave in 10 years time for example...
One subcontractor extrapolates data through testing dog bones in temperature chambers, at standard loads - so various load cases and elevated temperatures. load cases way below the yield strength of the material - the samples take a while to rupture.
They collect data - such as 40C, 200 hours, 14MPa
and 50C, 10 hours, 16MPa
then use TTS (time temperature superposition technique) to draw the final master curve.
Well then I have the other subcontractor, who only ages the dog bones in temperature chambers at different temperatures for various times.
they then take these samples out and do an instantaneous test where they measure initial modulus. so they collect data again such as
1000 hours, 40C, 10MPa
again use TTS method and draw the final master curve
I then take these two mastercurves of the same material and study the difference - and the difference is huge, it is noticeable...
which one do you think is more reliable ?
cause in the first case you need to wait until the material ruptures, which extends the testing time considerably..
Thank you very much for your answers.
Nazli
Hi Nazli, I wonder if in both tests they are measuring the same mechanical property, because you say that in the first method they are measuring the rupture module, and in the second procedure they are measuring the initial modulus?. As far I understood initial modulus is an elastic property, rupture module is an strenght property.
Good point Richard, I forgot to mention that.. One is plotting yield strength vs. time whereas the other is plotting modulus vs. time.
however, modulus is worked out from the linear bit and yield strength sits on the junction between the linear and the plastic parts.
What I mean is, the rate in decay in the modulus must be same with the decay in the yield strength. these two properties provided the material is the same must decay to the same extent in time..
Nazli,
So labo 1 ages the dog bones under given loads and temperatures up to rupture in temperature chambers. This is similar to creep testing. Labo 2 ages the bones only under given temperature in temperature chambers and stops the ageing after different times to perform mechanical testing. The two tests are obviously different but you seem to consider that they actually are equivalent i.e. that both kinds of tests should give same master curves.
I'm not expert in accelerated ageing of dog bones so I can't decide if the tests are equivalent or not. As being familiar with accelerated creep testing of metals, I can just tell you that the 2 described tests would most probably not give the same master curve in case of metals. No similar rate of decay in yield strength and in modulus. As I understand your description, there is cumulative degradation by two parameters in the first case (load and temperature). While in the second case the degradation is only due to temperature. But metals are not comparable to polymers of course.
Now, you speak of "standard loads". So I'm convinced that there must be some kind of published standards in paper format giving "standard plots" for such dog bones or polymers under various load and temperature conditions. You should then first of all compare the results of both laboratories with the standard plots.
Another point is that the creep-like tests as performed by labo 1 seem to be much more time consuming than the tests of labo 2 (and they are probably also more expensive). So reading your question gives the feeling that you would like to favor labo 2. And if labo 2 had provided you similar results as labo 1, you would have decided in favor of labo 2 without need to ask the question on RG !
But this makes it even more difficult for your readers to give an advice as we don't know what is the "huge difference" beween the results of both labo's. For instance, labo 1 performed a test at 14 MPa / 40 C and obtained a rupture time of 200 hours. If the test are equivalent, did labo 2 perform a test at 14 MPa after 200 hours ageing at 40 C ? What was then the result ? Shouldn't the latter also show exhaustion ?
In summary, if you wish to get an advice, there are only 2 possibilities :
1) Either an expert in dog bone or polymer testing knowing the standards and the two methods of testing reads your question and can provide you an answer (this can take a long time)
2) Or you are ready to read the advice of experts in material testing in general, but then you should provide some more information on : the standards, the references of documents giving evidence of the equivalence between both kinds of tests (under which limits, based on which formula's, etc....), the kind of load (tensile, compression, ...) , the way the load is applied, ..... Providing the figures for the full (or comparable) test results of both labo's here on RG in this thread would anyway be useful to allow to proceed with the discussion. For instance, providing the diagrams of the 2 master curves (without the names of the subcontractors of course)..
You will certainly understand that more substantial information is needed to allow a correct analysis of your problem.
Guibert Crevecoeur,
I bet you are asking whether if I can post the two curves here.. I am sorry but i cannot. Thank you for your answer.
Nazli
I can confirm that the first subcontractor is testing the samples per ASTM D 2990-01. whereas the second subcontractor's specimen geometry is per ISO standards, would geometry play a role as well ?
Hey Nazli,
It would depend on what polymers you are using but thermoplastics above glass transition temperature will undergo alignment of polymer chains when placed under load. This alignment strengthens and stiffens the material (in the direction of the load) and is used with some polymer threads to improve their mechanical properties. This process may be occuring with your samples with the first contractor.
Mat
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