Ductile compressive failure of columnar saline ice under triaxial loading

Abstract

Abstract. Experiments and analysis have shown that the ductile compressive yield stress (at -10øC at 4 x 10 -5 s -1) of S2 saline ice, an orthotropic material whose crystallographic c axes are randomly oriented in the plane perpendicular to the long axis of the columnar grains [Michel, 1978], loaded triaxially both across and along the columnar-shaped grains obeys Hill's [1950] criterion under low to intermediate levels of confinement. Correspondingly, the ratio of inelastic strains can be described by the attendant associated flow rule. IntroductionSeveral factors affect the failure of a sheet of first-year sea ice. These include wind and ocean currents and hence the strain rate of the ice, the temperature profile through the sheet, the microstructure (as well as macrostructure) of the material, the spatial variation in thickness, plus stress concentrators such as refrozen leads and other flaws. Experimental ProcedureThe saline ice that was used in the investigation was prepared in the manner described by Schulson and Nickolayev [1995]. In short, salt water of salinity 23%0 was unidirectionally solidified in the laboratory, leading to the growth of sheets of S2 columnar ice of around 300 mm thickness. Cubes were cut from the sheets, with care being taken to remove the upper granular and the lower skeletal layers before sampling. The growth direction was parallel to one set of edges of the specimens. Figure 1 shows the ice and defines the coordinate system to which reference is made; X1 and X2 are perpendicular to the columns (i.e., define the horizontal plane within a natural sheet), and X 3 is parallel to the long axis of the columns. . The material appears to be a good facsimile of the first-year sea ice described by Weeks and Ackley [1982].The triaxial compression experiments were carried out at -10øC _+ 0.2øC using the same apparatus as in the previous studies, namely, a true triaxial, servohydraulic loading system. The loads were applied proportionally through brass brushtype platens [Gies, 1988] Results General ObservationsUnder all conditions the ice exhibited macroscopically ductile behavior. Faulting was not observed, and the ice continued to support load up to the point that the load was removed.Three kinds of response (o-• versus •) were observed. Under low confinement, strain hardening was followed by strain softening once the stress reached a maximum at a small strain (• • 0.005), as commonly observed under uniaxial compression (Figure 3a). Under intermediate confinement the flow stress generally did not reach a maximum, but it did reach a more or less steady state value at around • = 0.01 ( Figures 3b, 3c, 3e, and 3g). Under high confinement, only strain hardening was seen (Figures 3f and 3h).

Brittle Failure of Ice

“…From the effects of confinement it is clear that the brittle compressive failure stress of ice, like rock (Jaeger and Cook 1979) and concrete (Hobbs et al 1977), depends upon the hydrostatic component of the applied stress tensor. This contrasts with the ductile compressive failure stress which is independent of that component (Melton and Schulson 1998). Different failure criteria are thus at play.…”

“…Different failure criteria are thus at play. Ductile failure (Melton and Schulson 1998) obeys Hill's (Hill 1950) criterion and is rooted in dislocation slip and in the fact that the critical resolved shear stress for basal glide does not depend upon the component of stress normal to the slip plane (Trickett et al 2000). Brittle failure, on the other hand, is based upon crack mechanics and upon frictional sliding, as discussed below.…”

The fracture of water ice Ih: A short overview

“…Within the ductile regime, Jones (1982) found from compression experiments on equiaxed and randomly oriented polycrystals (termed granular ice) of finely grained (d~1 mm) ice loaded triaxially at −11 °C that the differential stress at yield (i.e., the difference between the greatest and the least principal stress) is independent of confining pressure, implying conformity with the classical von Mises criterion for plastic yielding, as discussed by Nadreau and Michel (1986). Similarly, Schulson and Nickolayev (1995) and Melton and Schulson (1998) found from compression experiments on both fresh-water and salt-water S2-textured columnar-grained material of ~6 mm column diameter loaded at −10 °C under both biaxial (across-column) and triaxial compressive stresses that textured ice obeys Hill's criterion (1950); correspondingly, the associated flow rule applies, which means that the "strain vector" is essentially normal to the failure envelope. Hill's criterion extends von Mises' criterion by incorporating the fact that, owing to crystallographic texture and to the preference for glide on basal [0001] planes, the yield strength of S2 ice is an anisotropic property.…”

The structure and mechanical behavior of ice

“…The confinement raises the failure stress, as noted above, with the effect being greater within the regime of brittle behavior. Envelopes and surfaces describing both ductile and brittle failure under both biaxial 32, 34,35,42 and triaxial 30, 31,36,[43][44][45][46][47]49,50 loading have now been obtained and can be understood within the context of the mechanisms that are described herein. The challenge is to incorporate them in models of ice loads.…”