Session: 07-04-01 Vessels in Ice and model test

Paper Number: 104638

104638 - Effect of Axial Confinement on Flexural Strength of Freshwater Ice 

In the design of bridges, wind turbine towers, offshore structures and ice-class ships for operations in ice-prone regions, sloped structures may be employed to promote flexural failure of level ice to reduce loads on the structure. During such interactions, the ice sheet does not fail in pure bending since a component of the applied force at the sloped interface results in an axial load that induces a compressive stress in the ice. The net effect of this axial component is that the corresponding compressive stresses balance with flexure-induced tensile stresses in the outmost fibres of the ice. As a result, the apparent flexural strength of the ice is expected to increase with increasing axial compression, since larger bending forces would be required to generate sufficient tension to trigger fracture. In ice load prediction models for sloped structures, an in-plane compression (IPC) factor is applied to calculated loads to account for increased flexural strength which is empirically determined to be 1.5. While the method of superposition may be used to assess combined loading effects for elastic structures, assessing such effects in ice is more complex since the behaviour of ice is not purely elastic. In this paper, the relationship between axial confinement and the flexural strength of freshwater ice is studied experimentally to assess how the flexural failure behaviour of the ice changes for different levels of in-plane compression factor. This series of experiments used lab-grown polycrystalline ice that were prepared using a vacuum-mold process to produce bubble-free, freshwater ice with an average grain size of ~4.5 mm. For all experiments in this program, rectangular prismatic specimens with dimensions of 2.54x5.08x20.32 cm have been used. An initial series of simple, unconfined four-point flexural tests were conducted to allow for comparison with values reported in the literature. To study confinement effects, a specially designed confinement frame was designed and built to allow flexural tests to be conducted while axial stresses equal to 12.5%, 25% and 50% of the flexural strength were simultaneously applied to the ends of ice prisms. To assess rate effects, each axial compression level was tested at three different indentation rates: 0.1 mm/s, 1.0 mm/s and 10.0 mm/s. For all tests, 3 repetitions have been conducted. Thin-sectioning of ice samples has been completed to verify grain structure and size. Regular-speed and high-speed videos were taken to observe failure origination and propagation. These results indicate that axial compression has a strong effect on the apparent flexural strength of ice. For freshwater ice samples, a 1.5 IPC produces significant flexural strength increases, with even more significant increases for slow loading rates and high compression. In this paper, a detailed analysis of these new test results is presented, along with a discussion of underpinning mechanisms and potential implications for design. This new testing approach provides a promising direction for further examination of these important effects, including extending the analysis to sea ice.  

Presenting Author: Taha Anwar Memorial University of Newfoundland

Presenting Author Biography: Taha Anwar is a Master of Mechanical Engineering student at Memorial University of Newfoundland where he studies material properties of freshwater and saline ice. He also completed his Bachelor's degree in Mechanical Engineering at Memorial University with a technical focus on Materials Science. He is a recipient of many prestigious awards such as A.G. Hatcher Memorial Scholarship, Shell Canada Engineering Scholarship, American Bureau of Shipping Scholarship and Honeywell Limited Scholarship. His research interests include material characterization of ice, studying combined loading behavior and microstructural analysis.

Authors:

Taha Anwar Memorial University of Newfoundland
Rocky Taylor Memorial University of Newfoundland
Jungyong Wang National Research Council Canada