FRACTURE-TOUGHNESS OF ICE AND FIRN DETERMINED FROM THE MODIFIED RING TEST MODIFIED RING TEST

The modified ring test is used to determine the fracture toughness of synthetic, granular, fresh-water ice (average density 0.891 Mg m(-3)) and firn (average density 0.605 Mg m(-3)) from depths between 26 and 27.2 m in the E core of the Greenland Ice Sheet Project II. Average fracture toughness is 145.7 kPa m(1/2) for the manufactured ice and 108.6 kPa m(1/2) for the firn. Comparison between the ice and firn suggests that ice-fracture toughness decreases with decreasing density (i.e. increasing porosity), suggesting lateral and vertical variations in the near-surface fracture resistance of glaciers and ice sheets may be related to firn densification. The modified ring test has many advantages over conventional; notch-based specimens in that complications which arise in notched specimens due to crack-length, loading-rate, notch-acuity and specimen-size effects are irrelevant for a modified ring-specimen geometry.