The study on Brittle-ductile transition mechanism and failure mode of sea ice under uniaxial compression

Under various compressive strain rates, sea ice fails in different modes and behaves a brittle-ductile transition. To investigate the mechanism behind the transition, a fieldwork of uniaxial compression tests were performed on the level ice collected from Northeast of the Bohai Sea. The ice owns a typical h2 columnar structure, whose salinity varies from 5.5% to 7.4% while temperature ranges from -15 degrees C to -18.3 degrees C. In the experiments, the samples with uniform dimension of 50 mmx50 mmx107 mm were applied compressive load across to the grain columns. The data shows that the maximum failing stress is 4.7MPa appears at 2.8x10(-3) s(-1) while the lowest value is 1.5 MPa at 4.7x10(-5) s(-1). During the compression tests, the samples failed in three different modes over a wide range of strain rate (4.5x10(-5)-7.5x10(-3) s(-1)). When the failing stress is further categorized by failure mode, the stain rate-stress relation shows a consistent trend in each mode. Under low strain rate, the compressive strength has positive correlation with strain rate. The crack propagation is limited due to the stress concentration at crack tip is relaxed by creep, which leads to a ductile failure. When stain rate is between 9x10(-4) and 5x10(-3) s(-1), the crack was able to develop stably through the specimen along the loading direction and formed a splitting failure. The crack development became unstable and inclined cracks were preferred under high strain rate, where ice failed in a spalling mode. From ductile failure to splitting, the transition is terms of the competition between crack-tip creep and propagation. With sufficient confinement, the spalling dominates the global failure as splitting is suppressed. For both failure mode transitions, the condition is triggered by stain rate. From the measurements, it shows that each failure mode owns an according stress-strain rate relation, which is believed to explain the Brittle-ductile transition. Moreover, the influence of stain rate on the effective elastic modulus is discussed.