Raised Cosine Pulse Method as a New Tool for Viscoelasticity Measurements

Abstract

A new method (Raised Cosine Pulse Method) of measurements of viscoelasticity is presented, which is based on the Fourier analysis of the after-effect function related to a shear strain pulse of cosine type. According to the Boltzmann superposition principle, the shear stress is represented by a convolution integral of the rate of strain and the after-effect function, and the complex shear modulus can be determined from the Fourier transform of the after-effect function. Thus, the Fourier transform of the stress obtained as the response to a given strain function allows us to evaluate the complex shear modulus over a wide range of frequency. In the presented method, a small shear strain pulse of cosine type is imposed on samples, and the measured stress is analyzed following the above principle.
Tests of the Raised Cosine Pulse method were made on a 20% polystyrene solution in diethylphthalate and a 20% glass beads suspension in the polystyrene solution. In the former system, the viscoelastic functions obtained by this method were in close agreement with those obtained by the conventional dynamic measurement. In the latter system, the conventional method was not applicable because of the structural change of suspension occuring in the measurement, while the presented pulse method enabled us to evaluate the viscoelastic function at various phases in the course of the structural change.