Vertical and inclined edge-indentation of freshwater ice sheets

Finn, David W. (1991) Vertical and inclined edge-indentation of freshwater ice sheets. Masters thesis, Memorial University of Newfoundland.

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Abstract

A series of freshwater ice indentation experiments were performed with stiff vertical and downward-breaking inclined indentors in the ice tank at the Institute for Marine Dynamics in the fall of 1989. Indentor width, indentor angle, indentor velocity and ice sheet thickness were varied for a total of 94 tests. Three forces, one moment, three accelerations, indentor displacement, and carriage speed were measured. High sampling rates were used to capture the dynamics of the indentor response. Crushed ice extruded in front of the indentor was collected and sieved whenever possible. -- Failure modes of crushing, radial cracking, circumferential cracking, spalling and in-plane cleavage were all observed during vertical indentation. The maximum force on the structure was 29.1 kN. Analysis of peak pressures revealed good agreement with data collected by previous researchers. Peak pressures (averaged over the indentor area) ranged from 4 to 11 MPa. A slight decrease in peak pressures with increasing aspect ratio was observed. The small range of contact areas used in the test series produced no evidence of a pressure-area relationship. Peak pressure increased slightly with velocity, up to speeds of 50 mm/s. At velocities higher than 50 mm/s the indentor began to vibrate steadily and peak pressures dipped slightly. Significant vibrations were induced in the structure by crushing failure of the ice. Crushing frequency varied proportionately with velocity at low velocities. At velocities above 50 mm/s the response became locked at the natural frequency of the structure. An unusual "double-beat” force-time trace was sometimes observed during this locked-in response, and spectral analysis of the signal revealed the emergence of a higher second frequency in the vibration of the structure. -- For the inclined indentation tests, mixed failure modes of crushing and flexural failure were observed. For low indentor angles, crushing dominated. For high indentor angles flexural failure dominated, with crushing still occurring during many tests. Flexural failure was characterized by the formation of a principal circumferential crack and one or two "subsidiary" circumferential cracks. The formation of these subsidiary cracks was due to high in-plane forces combined with vertical deflection of the ice sheet to produce shear and buckling failure. The failure mode underwent transition from bending to crushing as velocity increased, ice thickness increased, and indentor angle decreased. The maximum horizontal force on the structure was 32.7 kN, produced with a 10° indentor while crushing failure was occurring. The normalized peak horizontal force exerted on the inclined indentors increased as the indentor approached vertical. The broken ice pieces produced by flexural failure were between 5% and 40% of the characteristic length of the ice sheet. The dynamic effect of decreasing broken piece size with increasing velocity was not clearly demonstrated.

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