Experimental investigation of oscillation of loads in ice high-pressure zones, part 1: Single indentor system

•Indentation tests were conducted on confined ice using a spherical-cap indentor.•The speed of ice and flexibility of indentation system were varied among tests.•The regular sawtooth loading that occurred may be seen as self-sustained oscillation.•Thin sections of tested samples show damage (microstructural change) in all cases.•Energy expended per penetration depth during loading and unloading are about equal.

The paper commences with a review of ice-induced vibrations in full-scale structures in the field, in medium-scale tests, and particularly, in small-scale indentation tests. The mechanics of compressive ice failure is reviewed, with emphasis on the processes leading to the formation of a layer of microstructurally modified material under the indenter, as well as fracture. Previous tests in the present series using small-scale indentors are reviewed, and a recently conducted series of tests is introduced. This series involved confined ice specimens of cylindrical shape 154 mm in diameter moving at various constant displacement rates against a 20-mm diameter indentor attached to an elastic-compliant structure. The displacement rates (ice speeds) were chosen so as to induce “sawtooth” vibrations. Two beam structures, one stiff and the other flexible, were used. In general, the desired sawtooth pattern was achieved, particularly with the more flexible system, but also with the stiffer one at higher ice speeds. Some results involving “lock-in” were obtained with the stiffer setup. Measurements were made of the distribution of pressure at the ice–indentor interface using pressure-sensitive film, and thin sections were taken from the indented samples. The thin sections in all cases showed the presence of a microstructurally modified layer of ice adjacent to the indentor. Pressure distributions showed that regular vibrations were associated mainly with activity in the layer, whereas the larger spall fractures generally caused irregular load patterns with significant loss of contact area under the indentor. The regular failure can be analysed as a self-sustained oscillation, which is common among dissipative systems. The system in this case comprises the moving ice field and the compliant structure with which the ice interacts, resulting in autonomous vibrations of the sawtooth variety. Regularity was interrupted by the larger spalling fractures.