Strain rate control during deformation of ice: An assessment of the performance of a new servo-controlled triaxial testing system

Abstract

A new ice triaxial testing system has been developed that necessitates the use of jacketed specimens which have prevented the use of a direct strain feedback signal from the specimen during servo-controlled experiments. An alternative arrangement has therefore been devised for which a thorough analysis is provided here, complementing the full apparatus description given in a companion paper. During testing, displacement is recorded using a remote transducer which measures deformation of both loading ram and ice specimen. The ram deformation component, which is elastic and proportional to the applied load, is continuously subtracted from the displacement signal within the servo-loop and thereby eliminated from the control signal. Because of the high strain rate sensitivity of ice deformation it is essential that proper control is maintained and consequently a series of calibration tests has been conducted to check the system performance. Results show close agreement between nominal test strain rates and those measured directly on polyethylene and polycrystalline ice specimens for strain rates in the range 10 −2–10 −5/s. Control was lost immediately after failure during brittle fracture, however, implying a limit to the response of the system when dealing with very severe changes in loading.