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Thread: Rheology - non-Newtonian models to fit flow curve data

  1. #1 Rheology - non-Newtonian models to fit flow curve data 
    New Member
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    Mar 2011
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    Hello everyone,

    I have a "conceptual" doubt about rheological models describing non-newtonian (in my case viscoelastic) materials. I am using a rheometer and its ralated software and, after running flow tests, I can choose either fitting the data obtained for shear-rate vs shear stress or for shear rate vs viscosity. I thought I would obtain the same model for both pair of parameters but I haven't (in general I obtain a better correlation through Cross model when choosing viscosity vs rate and Power Law when stress vs rate). What is the reason for that?

    Thank you so much for your time!


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  3. #2  
    Universal Mind John Galt's Avatar
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    Which viscosity are you measuring? I presume it is an apparent viscosity, since viscosity sensuo stricto can only apply to Newtonian fluids. As I understand the Cross model it is assuming a relationship between shear rate and apparent viscosity under any and all circumstances, so that it will readily accomodate data to give a reasonable fit through modification of C and m.


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  4. #3  
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    Thank you for your reply Ophiolite. Yes, I didn't specified the right concept, I'm measuring apparent viscosity. Well, as you say a Cross model should fit my data, no matter if I'm relating stress and rate or viscosity and rate. About the reason why the software shows me different models I've come to the conclusion that it classifies the equations in two groups: those which concern directly to "stress and shear rate" parameters (i.e. power law or Casson for example) and those which contain "viscosity and shear rate" as parameters (Cross or Sisko models). Conclusion: I must keep attention and check both set of models to be sure about the best fit.

    I wanted to take advantage of the conversation to ask also how can I be sure that a sample is stable before performing any test in the rheometer? Could I use a time sweep test to assess the time it takes to the equilibrium? Should I then perform the flow test after finishing any other tests, avoiding too high "loads" in the sample untill the end?

    I'm a beginner in rheology and rheometers, sorry if I ask simple things or I use inappropriate concepts.

    Thank you.
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