| Abstract | Experiments were conducted on first year sea ice at Resolute NWT to determine the effect of aging on the strength/toughness of specimens with prepared cracks. Ten centimeter square cross section beams of length 1.0 m were tested in a four point bending apparatus, at calculated strain rate of 10 super minus super 3 s super minus super 1 at the extreme fiber. Three cracks of identical length, half the beam thickness, were cut with equal spacing in the constant moment region of the beam with a hand held circular saw, and sharpened in quick succession with a hand held scalpel. The cracks were numbered in the order in which they were sharpened, so as to determine subsequently which one fractured. The beams were then placed inside plastic bags to protect them from the scouring effect of the wind - the bags were not air tight - and left in the snow in Resolute. The order of crack preparation did not influence crack fracture initiation site. The strength/toughness of the ice was not influenced by aging up to 150 hours, but a set tested 280 hours and another set tested 380 hours after crack preparation showed a reduction in strength/toughness of approximately 30%. This would not occur where a crack tip blunts, as blunting strengthens a crack. This also indicates that any residual stress created in crack preparation may have shielded the crack, and that as time allowed it to relax, this shielding action was removed. This is opposite to the antishielding effect of residual stress created under indentation in other brittle materials. Under indentation the resulting residual stress contributes to the crack driving force instead of attenuating it. The use of the preferred crack tip preparation technique used to form a microcrack for fracture toughness tests in sea ice may create a crack that has associated with it a dislocation distribution that shields it. |
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