A free-rising buoyant sphere can break an ice plate floating above it. The problem is when the light sphere breaks the ice plate most, or the optimal relative density of the sphere which can break the ice plate the most severely. This experimental study was done to answer this problem. A set of experimental devices were designed, and a high-speed camera system was adopted to record the whole dynamic process, including the free-rising of the sphere, the collision between the sphere and the ice plate, the crack initiation and propagation, as well as the breakup of the ice plate. The failure mode of the ice plate under impact load was analyzed. It was found that conical cracks were formed under the reflected tensile wave at the top surface of the ice plate. On this basis, the influences of ice thickness, the initial submergence depth, and the relative density of the sphere on icebreaking were further investigated. An optimal relative density of the sphere was found when the sphere was released at a certain initial submergence depth, at which point the ice was damaged the most severely. For example, when the dimensionless initial submergence depth of the sphere was 2.31, the optimal relative density of the sphere was close to 0.4, with the probability of the ice plate breakup as high as 91.7%. It was also found from the experiments that the degree of damage to the ice plate correlated well with the kinetic energy of the sphere just before collision. Results showed that the optimal relative density can be estimated by theoretical analysis of the kinetic energy of the sphere, which will provide a reference for potential icebreaking applications in the future.

Based on the physical properties of the laboratory experiments, a grain-based discrete element model (GB-DEM) is developed to simulate the fracture behaviour of ice under confined compressive loading conditions. The microscopic contact parameters are successfully calibrated in the iterative framework to match the macroscopic mechanical properties of the single crystals and polycrystalline ice obtained from laboratory tests. The numerical results are in great agreement with the macroscopic mechanical properties obtained from laboratory tests with errors of less than 6%. Then, the GBM model is used to study the multi-scale fracture behaviours and failure modes of the polycrystalline ice under tension and compression under different confined pressures. The numerical results reveal that only tensile cracks are generated on the grain boundary in the direct tension test. More trans-granular contacts are failed with tension or shear when under compression loading. In addition, tension is the main failure mechanism of specimens under tension or compression loading conditions through the evolution of microscopic cracks. With the increase in confined pressure, the macroscopic cracks formed by microscopic cracks and the high-crack-density area induced by the grain breakage and boundary failure become prominent. At the same time, the number of shear cracks increases obviously.

Diametrical compression tests were performed on cylindrical ice single crystals with typical c -axis orientations under nominally constant strain rates. Strain evolution was tracked using digital image correlation, and crack morphology was observed under cross-polarized light to investigate the effects of c -axis orientation and strain rate on plastic deformation and failure behavior of ice single crystals. The results showed that ice single crystals exhibited pronounced anisotropic and rate-dependent mechanical characteristics. Heterogeneous plastic deformation is primarily manifested as localized strain bands aligned with basal dislocation slip, with the morphology, intensity, and width of these bands strongly influenced by c -axis orientation. Rate-sensitivity exponents for basal and non-basal slip were determined from the nearly log-linear relationship between specimen load and strain rate in the ductile regime. As the strain rate increased, the failure mode transitioned from ductile to splitting failure, with crack morphology depending on c -axis orientation: ductile failure occurred mainly via shear cracks along the basal plane in basal slip-favored orientations, while splitting failure appeared as tensile cracks parallel to the c -axis in small-angle specimens and as cleavage cracks parallel to the basal plane in large-angle specimens.