A continuous-wave-based acoustic method to measure viscoelastic properties of materials
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Typical viscoelastic materials do not have a flat frequency response to mechanical vibrations. Viscoelastic property evaluations with a multi-frequency excitation such as a rectangular pulse, therefore, often suffer from extraneous frequency-dependent variations and low signal-to-noise ratio (SNR). As a result, existing acoustic measurement techniques are usually limited to empirical assessments of the medium properties. In this work, the continuous-wave (CW) excitation is used to enhance the output signal strength and consequently the SNR in rheological measurements. An analytical model for the observed sinusoid response of a medium at a transducer-medium interface is derived as a sum of attenuated echoes and the input, modulated by the viscoelastic properties of the medium. Its expression, named as Infinite-echo model, has provided theoretical basis for a novel method to measure sound speed in the medium. An optimization algorithm, fitting this model to the CW response of a medium, is designed to compute the viscoelastic coefficients for that medium. The result is a non-invasive method for in-situ measurement of material viscosity that compares favorably with the conventional techniques.
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