Borehole indentor — A tool for assessing in-situ bulk ice strength and micromechanics

National Research Council (NRC) borehole indentor (BHI) is increasingly being used in the Arctic for measuring in-situ bulk strength of ice and its seasonal variation in conjunctions with investigations on climate change. Before NRC-BHI system was taken to the Arctic, its performance was evaluated on columnar-grained S1 ice in Dowʹs Lake, Ottawa. S1 ice with its huge and clear grains is ideal for microstructural analysis of indented ice and understanding the micromechanisms of failures (ductile and brittle) under three-dimensionally confined BHI conditions involving large volumes of ice. The upper yield (UY) strength was shown to exhibit power-law dependence on average stress rate to UY. This power-law is universally applicable to low-salinity (0.3 ppt) brackish-water S2 ice and various types of sea ice — first-year S3, granular and frazil, second-year S3 and multi-year ridge ice. The concept of shift function is introduced for quantifying the effect of temperature on BHI-UY strength. For 0.62 m thick S1 ice sheet at an average temperature of − 3.8 °C (or 0.986 Tm) the failure indentation is constant for strength of 15 to 30 MPa or stress of 1.5 × 10− 3 E to 3.0 × 10− 3 E — analogous to high-temperature creep failure strains under comparable normalized stresses in metal and complex alloys — thus establishing the same physics of deformation and failure processes. For BHI tests, the preferred independent and controllable variable, however, is the indentation rate. Both BHI-UY and BHI-flow strength (for 5 mm penetration, say) also obey power-law dependence on indentation rate and 0.1 mm s− 1 is suggested for standardization. While UY failures are characterized by parabolic zone of deformed, recrystallized and HIPed (hot isostatically pressed) ice, premature brittle fractures occurring at speeds at or higher than 0.2 mm s− 1, are caused by the propagation of cracks nucleated at the ice-indentor contact surface. Two different measures of the degree of maturity in brittle fractures are proposed — one based on failure time and the other on UY stress.