Calibration exercise of a Hardin-type resonant column

This technical note presents the calibration exercise of a Hardin-type resonant column of fixed–partly fixed ends, which is capable of exerting dynamic torsional vibration on the specimen. The apparatus uses a mechanical system connected to the drive mechanism which works as a massless torsional spring–dashpot and can apply axial load of compression or extension to the specimen. Thus, the vibration of this system follows the physics of oscillators (spring-type model), which is highly strain-rate dependent. The calibration exercise led to the estimation of the apparatus rotational inertia (J_A). The calibration exercise was carried out using aluminium bars of known stiffness and of variable dimensions covering a range of frequencies from about 60 to 95 Hz, as well as real soil specimens, which were used as calibration rods covering an additional range of frequencies from about 85 to 170 Hz. This allowed a wide range of frequencies to be captured, which are representative of resonant frequencies of soil specimens of 70 mm dia. and 140 mm long. For the real soil specimens, which were used as calibration rods, the rotational stiffness necessary for the estimation of J_A was estimated from bender elements inserted into the resonant column. It was shown that J_A was frequency dependent, which is in alignment with the vibration of oscillators.