Dear colleagues,
I have a need to compare (not very precisely) mechanical properties of two materials. The first one is solid-like material (fibers mat) and the second one is a weak gel. I have a rheometer and a simple tensile machine working under creep mode only. What I was going to do is to determine Young’s modulus, E of the mat on the base of stress-strain curve. Then, to determine shear modulus, G of the gel and to compare them using a well-known relationship E=2G(1+u). Could anyone recommend me a suitable testing mode for G determination? Which settings should be applied (I believe they should correlate with the ones used for tensile test)? Can the complex dynamic modulus in LVE region be used for this purpose? All suggestions are appreciated.
I would suggest to measure the yield stress of the weak gel and compare to the yield strength of the fibers mat.
With the rheometer, under pure shear, you can try to measure a flow curve of the gel. You should get information regarding any change of property as a function of shear stress. From the pure shear the principal directions are at 3 degrees. So, you can diagonalize the stress tensor and make a direct estimation of the Young´s module, which is a normal stress property. I hope this helps.
Dear Misters Ernest Ong and Sergio Quinones Cisneros,
Thank you very much for your suggestions.
It is important that the rheological quantities measured for the solid and the gel are comparable. For example, a modulus (hardness) is something completely different from yield stress or breaking strength (strength): a cookie may be hard but have low strength, i.e. it breaks or crumbles easily. Modulus is measured in a small deformation test (linear range) whereas strength is evaluated in a large deformation test (non-linear range).
Your tensile tester operates in the creep mode (deformation vs. time at imposed constant stress). This gives the tensile compliance roughly equal to 1/E (Young's modulus). Ideally, for the gel this should be compared to the output of a stress-controlled rheometer, operating at imposed constant (small) stress. This gives the shear creep compliance J roughly equal to 1/G (shear modulus). Please note that E and G are generally time dependent. Equivalent results on the gel can be obtained in an oscillatory rheometer. If only a steady shear rheometer is available, the best experiment is to measure shear stress of the gel as a function of time at a given constant shear rate. G can be approximately calculated as the ratio of shear stress and total shear (= shear rate x time) at a range of as short as possible times.
A classical textbook on polymer rheology (linear behaviour) is J.D. Ferry, Viscoelastic properties of polymers, Wiley (1980).
Dear Dr. Steeneken
I’m thankful to you for your answer and advises. I understand the points you mentioned. I was not constrained in the choice of characteristics for the comparison and the choice of E vs. G was my own as the most acceptable and well-known. I think comparison of yield stresses of the two materials would be acceptable too, although not the best and the correlation equation is different in this case. My preference was to compare the results obtained when operating at similar deformation mode (continuous deformation under control stress mode, for example). However, to my experience it is very difficult to get stable values of shear rate if constant stress below yield point is applied. This is why, finally, stress amplitude sweep test has been chosen. Oscillation stress vs. oscillation strain curves were plotted basing on the results of the tests and the oscillation shear modulus was determined at small strains. Then, this modulus was compared with the Young’s modulus determined as a slope of stress-strain curves at small strains under uniaxial extension.
Sincerely,
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