Glenn D. Durell

The cyclic loading of saline ice

Abstract This paper details the results of an experimental programme to investigate the constitutive behaviour of saline ice under reversed direct-stress conditions. The test material was laboratory-grown saline (NaC1) ice. The work explored the effects of temperature (from – 5 to – 50°C), cyclic stress amplitude (0·1–0·8 MPa) and loading frequency (10−3−1 Hz) on the response of the ice. Variations in the ice growth conditions allowed the effects of microstructural variations to be investigated as well, with total porosity in the range 30–104 ppt. The experiments were generally performed by applying a sinusoidally varying uniaxial load, oscillating about zero, to the cylindrical specimens. Several experiments employed cyclic strain control. The material response was typically composed of elastic and anelastic strain, with various degrees of permanent or viscous strain occurring at higher temperatures and lower frequencies, and proved to be very sensitive to variations in loading conditions and to microstr...

A dislocation-based analysis of strain history effects in ice

Abstract The cyclic loading response of ice specimens can be analysed with a dislocation-based model of anelasticity to produce an estimate of the effective mobile dislocation density. Moreover, the cyclic loading response is sufficiently sensitive to track the dislocation density changes that occur during creep straining. A combination of cyclic and creep loading experiments can thus be employed to gain crucial insight regarding the relationship between the dislocation density that evolves during creep straining and the anelastic and viscous components of strain. Creep and cyclic loading experiments have been conducted on laboratory-prepared saline and freshwater ice specimens to shed light on the effects of temperature, creep stress and accumulated strain on the mobile dislocation density and thereby to support the further development of a physically based constitutive model for ice. The findings indicate that, in addition to the expected stress and strain dependence, the dislocation density that develops during straining also depends on the temperature and microstructure. As a consequence of this temperature dependence, the apparent activation energy for creep when dislocation multiplication takes place is higher than that for creep in the absence of dislocation multiplication. Data from the literature are examined and found to support this finding. The results indicate that both the anelastic strain and the viscous strain rate vary with the mobile dislocation density. A preliminary set of results indicates that the power law stress exponent n is 3 when the applied stress is high enough to cause an increase in the dislocation density, but that n ≈ 1 when the dislocation density remains constant during straining. The results show that prior straining increases the stress level associated with the transition from n = 1 to n = 3. The findings provide support for the glide-controlled mechanism of ice creep, and a dislocation glide-based formulation for viscous straining is presented. The model agrees well with the experimental data in the pure flow regime.

Mixing in an Aggregate/Fine-Grained Soil System Subjected to Cyclic Loading with a Geotextile Separator

Repeated-load triaxial tests were conducted to examine mixing of coarse aggregate placed over a layer of silt with and without geotextile separators. A range of silt densities was tested without the separators to examine the mixing conditions. Medium density (i.e., close to optimum dry density), saturated and unsaturated silt specimens were tested with and without geotextile separators to examine the separator function. Deformation gauges mounted directly on the specimens provided strains within each layer and across the interface between the two layers. Mixing occurred due either to downward movement of the aggregate or a combination of downward movement of the aggregate and upward movement of fines. Geotextile separators prevented mixing in saturated, medium-density silt. When the silt was not saturated prior to testing, mixing did not occur and the geotextile separator was not required.