Subcritical crack growth, initiation, and arrest in columnar freshwater and sea ice

Parsons, Bruce L. (1989) Subcritical crack growth, initiation, and arrest in columnar freshwater and sea ice. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

A study was conducted to determine if slow stable sub-critical crack growth may occur in ice. The Double Torsion fracture toughness geometry was used to explore this phenomena in first year large grained columnar sea ice, and fine grained freshwater columnar ice. The sea ice was tested during a field trip to Resolute NWT during April 1987, and the freshwater ice tested in the cold room at the Institute for Marine Dynamics in St. John's Nfld, from July 1987 to Jan 1988. -- No sub-critical crack growth was observed in either type of ice. All crack growth was abrupt. In sea ice loading was up to one hour long, and in lab grown ice deadweight loading was applied for five days. This set the limit below which any sub-critical crack growth may have taken place as approximately 2 x 10⁻⁹ m/s. Quasi-static loading up to five minutes was also applied, as arrest was more likely after quick loading. -- The fracture toughness of the sea ice was 113± 38 kNm⁻³/², for 0.06 < K < 44 kNm⁻³/²S⁻¹, -20°C < temperature < -14°C; and for the fine grained ice 124 ±38 kNm⁻¹/², 0.7 < K < 85 kNm⁻³/², at -20°C. The arrest stress intensity factor was 91 ± 28 kNm⁻³/³ for the sea ice and 89±14 kNm⁻³/² for the freshwater ice, rate independent, and similar to the high loading rate, or creep free fracture toughness of the two ices. The instability of all crack growth in ice was argued to be a consequence of the stability of the ice crystal structure against dislocation emission. The rate dependence of ice toughness is due to the screening of the crack tip by the dislocation mechanism of creep. -- Crack length was not load rate dependent. It was suggested that the switch of failure mode in ice indentation by structures is a consequence of the rate dependent material properties of ice, possibly the modulus. A risk analysis, based on the material properties of creep crack growth, and the probability distribution function for ice strength, was shown to be inapplicable to ice. The arrest stress intensity factor was used to modify the model of indentation in ice, to accommodate this as a new crack length criterion, and to accommodate the stress relief influence of ice micro cracking.

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