Flexural Strengthening of Ice

Natural bodies of ice such as the cover on the Arctic Ocean and the crust of Europa and Encelladus are subjected to episodes of cyclic loading. The question is: how does the ice respond? To that end, new experiments have been carried out on plates of columnar-grained saline ice and fresh-water ice cyclically loaded across the columns by flexing at temperatures from -25 to -3^o C at frequencies from 0.01 to 1 Hz, using a servo-hydraulic 4-point loading system. The experiments revealed that reversed cyclic loading increases flexural strength, by as much as a factor of two or more, and that the increase is directly proportional to the amplitude of the outer-fiber stress once the ice has been cycled a few hundred times. Strengthening also occurs under non-reverse cycling, but only when the ice is bent in the same sense as when cycled. Over the ranges explored, neither temperature nor frequency has a significant effect. Strengthening, it is suggested, is caused by the development of an internal back-stress that opposes the applied stress, thereby raising the level of the applied stress that is needed to nucleate cracks.