Resonant Column Calibration and Dynamic Torsional Shear Testing Using Stepped Frequency Sweeps

Abstract

Abstract This article introduces a new apparatus calibration procedure along with testing protocols, data acquisition, and reduction procedures for a fixed-free resonant column test in which a fixed amplitude sinusoidal torque is applied to the specimen at discrete frequencies over a wide bandwidth that encompasses the resonant frequency. Recently developed resonant column theory, consistent with ASTM D4015-21, Standard Test Methods for Modulus and Damping of Soils by Fixed-Base Resonant Column Devices, is shown to hold for all frequencies in this bandwidth and enables determination of shear modulus, damping, and shear strain amplitude at each frequency. For off-resonant frequencies, shear strain amplitudes are much lower than those at resonance; hence, under a constant applied torque, the method measures values over a range of shear strains. These procedures are facilitated using a spectrum analyzer. The new and more robust approach for apparatus calibration relies on conducting stepped frequency sweeps (SFS) on calibration rods and allows concurrent determination of all apparatus calibration factors, which apply to the full frequency bandwidth for testing soils. Test results on sand are used to illustrate the developed procedures and recommendations are provided for determining shear modulus and damping over the full shear strain range of the apparatus. If the fixed amplitude applied torque is low, the test provides shear modulus and damping at very low strains. For sufficiently high values of the applied torque, a single SFS allows semicontinuous characterization of the shear modulus degradation and shear damping increase behavior. The article provides recommendations for selecting frequency bandwidth, frequency step values, and spectrum analyzer settings. For testing with large input torques, SFS should be from high frequency to low frequency to more accurately capture the response function for strain softening behavior. The total time for SFS is on the order of 10 min, and the number of applied loading cycles can be controlled.