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# How to obtain Viscosity vs. strain rate curve??

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Posts: 3
Topic starter
(@mikejwg)
New Member
Joined: 17 years ago

i subjected a thermoset to oscillatory rheometry testing, strain sweep test to find the limits of linear viscoelasticity and frequency sweeptest.

how can i determine a viscosity vs. strain rate curve from these two results?

currently i divided the strain output by the time output from the freq. sweeptest and plot this vs. the viscosity. not sure if it is right.

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Posts: 3998
(@jorgen)
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Joined: 5 years ago

Thats an uncommon request. What makes you interested in viscosity instead of the storage and loss moduli?

I plugged in the Maxwell model into the Boltzmann Integral equation for linear viscoelasticity and got the following implicit equation for the viscosity as a function of frequency:

[TeX:42907fd9dc]E_{loss} \omega^2 \eta^2 - E_0^2 \omega \eta + E_{loss} E_0^2 = 0[/TeX:42907fd9dc]

where:

âž¡ [TeX:42907fd9dc]\omega[/TeX:42907fd9dc] is the frequency

âž¡ [TeX:42907fd9dc]E_{loss}(\omega)[/TeX:42907fd9dc] is the loss modulus

âž¡ [TeX:42907fd9dc]\eta[/TeX:42907fd9dc] is the viscosity

âž¡ [TeX:42907fd9dc]E_0[/TeX:42907fd9dc] is the storage modulus at low frequency

- Jorgen

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Posts: 3
Topic starter
(@mikejwg)
New Member
Joined: 17 years ago

i am trying to get the consistency and power index for a particular material.

with the information i get from rheometry can i input does into ABAQUS as UMat?

Posts: 3998
(@jorgen)
Member
Joined: 5 years ago

I am not quite sure I understand what material model you have in mind, but if you know what model you want to use then you should be able to implement that model as an ABAQUS UMAT and use the viscosity data to calibrate the model.

Note, that the ABAQUS manual states that: it takes considerable experties to write a user material model (UMAT). In other words, be prepared to spend significant time to write the material subroutine, or get expert help...

Best of luck,

Jorgen

Posts: 3
Topic starter
(@mikejwg)
New Member
Joined: 17 years ago

Im actually trying to model PMMA. and how it flows into a mold. however, not too sure which would be better for me a pure CFD program or ABAQUS (which just came to my attention).

the viscosity is given by, eta = consistency index * shearRate^power index

so purely from the viscosity data im able to calibrate UMAT? i do not need to conduct compression tests?

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(@jorgen)
Member
Joined: 5 years ago

The type of testing that you would need to perform is determined by the details of the material model that you want to use. If you want to consider the PMMA as a pure fluid model that does not have any long term memory and is simply characterized by a rate dependent viscosity, then it should not be necessary to perform comprression experiments.

In your case, the choice of material model will be very important for the accuracy of the simulation results. The application that you describe can be modeled either using CFD or a traditional solids program such as ABAQUS.

- Jorgen

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