Guillot F., Trivett D., Rogers P.

Highly compliant viscoelastic polymers are widely used in many applications for their vibration isolation and damping properties. The prediction of their performance in such applications requires the knowledge of their dynamic elastic properties as a function of temperature and, in some instances, hydrostatic pressure. In the case of a homogeneous and isotropic material, only two elastic constants (along with the density) are needed to completely characterize that material, and any other constant can then be computed from the pair using well-known relationships. This paper introduces two independent measurement systems that were developed in our laboratory, one for the Young’s modulus, and the other for the bulk modulus. Each system is capable of measuring the dynamic complex modulus as a function of temperature and hydrostatic pressure, and together they constitute a unique tool for the evaluation of viscoeleastic materials. The Young’s modulus system combines a resonance measurement technique for low frequency data and a wave speed measurement technique for higher frequency data. Both techniques are implemented in air, inside a pressure vessel, using laser vibrometers. The bulk modulus system measures the dynamic compressibility of a sample immersed in Castor oil, inside a pressure chamber equipped with two piezoelectric transducers for the transmission and reception of acoustic signals. Data can be obtained at frequencies typically ranging from 50 Hz to 5 kHz, at temperatures comprised between -2°and 50°C and under hydrostatic pressures ranging from 0 to 2 Mpa (Young’s), or 6.5 Mpa (bulk). Typical results will be presented, and the combination of data from both systems to completely characterize the elasticity of the material under investigation will be illustrated. In particular, it will be shown how these data can be used to obtain experimental values of the complex Poisson’s ratio, whose accurate measurement is otherwise quite challenging to perform directly.