I'm trying to measure in house the viscoelastic properties of solithane-113c in its rubbery regime (>20 deg C). It seems that the easiest method to use is a mass-spring resonance technique. The versions I'm looking at are (1) a circular cross section beam with a mass/accelerometer on it (Elastic Modulus) and (2) a cup shape of the material with a mass in the center (Shear Modulus).
The book I'm working from is published by NAFEMS and is called How to Analyze the Static and Dynamic Response of Viscoelastic Components. It says that this resonant type test is useful for about 100 Hz to 10 kHz. However, the ASTM test for this type of test says these tests are rarely useful for above 100 Hz. The ideal frequency range for my characterization is 20 Hz to at least 2000 Hz, but preferably 5000 HZ. It seems that the reason the ASTM makes the statement that the test is only useful up to 100 Hz is because the specimen must be short enough that there is no appreciable variation of stress along its length. This is accomplished by having the beam length to be much less than the wavelength of the stress wave. It is seen that very quickly the beam would shrink to a size that is impossible to mount an accelerometer on.
So I have the following questions (any help is appreciated):
(1) Is there any other reason these tests are not good above 100 Hz?
(2) Is there any way to approximate the error in calculated properties if the sample's length is not much less than the wavelength of the stress wave?
(3) Is it correct that in the rubbery region a viscoelastic solid does not show much change in elastic modulus (as well as shear modulus and loss factor) with increasing frequency?
Thanks for any information or resources you can refer me to.
Lead Mechanical Engineer
Frequency Electronics, Inc.