The process of failure of columnar-grained ice

Observations are reported on the cracking activity during compressive creep of columnar-grained ice with the axis of hexagonal symmetry of each grain tending to lie in the plane perpendicular to its long direction. Because of the nature of the ice and the direction of the stress relative to the columnar grains, the deformation behaviour and cracking activity were two-dimensional in nature. The cracks that formed were of the cleavage type, long and narrow, and usually involving only one or two grains. Information is presented on the strain, stress and temperature dependence of the crack density, and cracking rate (i.e. number of cracks per cm[2] per unit strain). It was found that crack formation takes place when the compressive stress exceeds about 6 kg/cm[2]. For stress between 6 and about 10 kg/cm[ 2], most of the cracking activity occurs during primary creep. If the stress is greater than about 12 km/cm[2], deterioration of the structure due to the cracking activity causes the primary stage of creep to be transformed directly to the tertiary stage. The strain dependence of the crack density indicates the existence of two independent families of cracks, each obeying Weibull statistics. The implications of this for the ductile to brittle transition in ice are discussed.