Novel Methods for the Computation of Small-Strain Damping Ratios of Soils from Cyclic Torsional Shear and Free-Vibration Decay Testing

This paper illustrates two novel methods for computing the small-strain hysteretic material damping ratio, lambda(min), of soils from the cyclic torsional shear (TS) and computing the small-strain viscous material damping ratio, D-min, from the free-vibration decay (FVD) testing. Both lambda(min) and D-min are challenging to measure, due to the significant level of ambient noise at small strains (<10(-4)%). A two-step method is proposed combining the Fourier Transform and a phase-based data fitting method for torsional shear testing, and this method can effectively eliminate the ambient noise at small strains. A Hilbert Transform-based method is proposed for the free-vibration decay testing in order to achieve a more accurate measurement of the viscous material damping ratio, D, at different strain levels, especially at small strains. The improved lambda(min) and D-min at small strains are compared to data available in the literature. The two novel methods are shown to be accurate in computing the small-strain damping ratios.