Hi guys,
I am facing problems to interpret a DMA results.
Material: Composite Bisphenol A + curaua fiber
What is being tested: the influence of fiber volume on the composites properties (Matrix:Fibre tested proportions are 70:30, 60:40, 50:50)
No treatment were given to the fibers before incorporation.
Frequency: 1Hz
As one can see on the image, Tg clearly increase with the increasing fiber volume (dot-line curve). The first graphic (on top left) is pure epoxy. But, the E' (and E'', but I am more interested in the first) decreases: the continuos line represents the behaviour of E'. For instance, when starts the Tg rage, at 50ºC, for 70:30, one could say is prox. 6000MPa, 60:50 is 5000MPa and 50:50 is 4000MPa. I thought this was a bit strange. Does anyone has a clue? I think maybe the poor adhesion between fiber and matrix could explain this, but I am not sure. I know storage moduli and Young moduli are NOT THE SAME. But, the last one increases with the incresing fiber volume. That's why I am confused.
Thanks in advance for your help.
THE STORAGE MODULUS REPRESENT THE CAPACITY OF A MATERIAL TO "STORAGE" ENERGY, IN THIS CASE REPRESENT THE CAPACITY TO SUPPORT A OSCILATORY STRESS, IF MODULUS DECREASE MEANS THAT THE MATERIAL HAS LESS CAPACITY TO STORAGE ENERGY, THAT ALSO REPRESENT THE LESS ELASTIC CAPACITY.
Kui Wang and Jose LUIS Rivera-Armenta thanks so much for the answer. It really helped me to visualize better my results!
I have another question: I also performed tensile test on this material and the Young modulus increase with the amount of fiber. Is this smth possible, even when E' decrease? I thought they were intrinsic connected.
BOTH TEST WERE CARRIED OUT UNDER DIFFERNT CONDITIONS, ONE IT IS UNDER STATIC STRESS AND OTHER ONE (DMA) IN OSCILLATING STRESS
Proper analysis of the dynamic test results and their comparison with the tensile test results requires more details about the samples and the tests. The relevant questions include dynamic test deformation mode (tensile, bending, rotational etc.) and anisotropy of the composite. Unidirectional composite deformed in longitudinal (parallel to the fibers alignment) and transversal direction will behave differently, manifesting different properties. Also of relevance are the curing conditions and the possible effect of the fibers on the resin hardening: there are indications in the graphs that the beta transition of the matrix (E” and tan d peaks at about 10C) is depressed more in 40% sample than both in 30% and 50% ones, indicating possibly some differences in the matrix cross linking density.
Besides, maybe, it is better to correlate the modulus in tensile test with the complex (rather than storage) modulus.
In describing your conditions you haven't mentioned the %strain under which you carried out the DMA experiment. If your strain is for instance larger than the strain span that you use to calculate the young modulus, this difference can be easily explained.
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