Viscoelastic characterization of materials using a dynamic hardness tester

Abstract

The Shore or IRHD hardness test has been widely used for many years and has become one of the standards for characterizing materials in the rubber and plastics industries. The obtained hardness scale values, ASTM or IRHD, are often related to the static elastic properties of rubber‐like materials. The classical Hertz equation has also been used to correlate the hardness scale values to the static elastic properties. Recently, progress has been made on the problem of obtaining dynamic viscoelastic properties through the use of a vibrating spherical indentor and modeling the measured input impedance by the radiation impedance of a sphere radiating into a half‐space. In the limiting case where the sphere radius is small, such that ka≪1 for longitudinal waves, it is shown that the measured shear modulus and loss factor are in good agreement with that found using other dynamic methods such as the DMTA or the resonance method. The model is most appropriate for relatively soft materials or very thick hard materials where the thickness of the material can be ignored. The problem of very hard materials and finite thickness where the terminating boundary is important will be discussed.