Abstract

Tuning-Fork Investigation of Shear Stresses Nonlinearity in Thixotropic Media

P.Pyatakov, M.Mironov (Theoretical laboratory, N.N.Andreev Acoustics Institute, Moscow, Russia)

e-mail: pyatakov@akin.ru

The experimental method of shear stress nonlinearity (viscosity, or shear modulus) investigation in thixotropic media is developed. A mechanical resonator (tuning fork) oscillates near the resonance frequency f0 in the medium under investigation. The amplitude of its oscillations depends on a shear modulus of the medium. As an additional large-amplitude low-frequency oscillation with frequency f <<f0 is superimposed on the signal, owing to the thixotropy of the medium (in case of a clean liquid, owing to its non-newtonianness) the medium time-average shear modulus varies with double frequency 2f of the low-frequency oscillation. As a result the amplitude of resonance oscillations of a tuning fork changes, and also oscillations with combination frequencies of f0 and 2f appear. The method is applied for drill fluids researches. The relations of amplitudes at resonance frequency and at combination frequencies with parameters of low frequency oscillations are obtained. The amplitudes of combination harmonics are 20 - 40 dB lower than the amplitude of the resonance frequency signal and they drop with increase of frequency f. It clearly manifests medium relaxation. For interpretation of the results obtained the modified relaxational model is suggested, where the dependence of a relaxation time on instantaneous stress in medium is taken into consideration. The capabilities of the method for investigation of reversible destruction processes in composite media with internal gel frame are discussed.

Section : 3