Laboratory indentation testing of polycrystalline ice: an investigation of fracture

Mackey, Thomas R. (2006) Laboratory indentation testing of polycrystalline ice: an investigation of fracture. Masters thesis, Memorial University of Newfoundland.

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Abstract

For industries working in artic and sub artic waters ice load is a major concern. Ice has an inherently varying mechanical nature related to material randomness caused by such things as internal flaws, grain size variation, varying growth history, varying temperature, among others. Due to theses uncertainties, accurate estimation of ice load is difficult. The objective of this report is to present the results of a laboratory test program designed to increase the knowledge of the material behaviour of ice. Work such as this will eventually allow the high degree of conservatism in ice load estimates to be reduced, leading to more economical design. -- The report provides a relevant review of ice mechanics and ice load design. For design, two pressure area relationships are shown, one for local area and one for the global area. These are probabilistic tools, based on full scale data; this is currently necessary due to an incomplete understanding of the mechanisms occurring during an ice-structure interaction process. Presently there exists an inability to effectively predict the behaviour of ice using physics based numerical models. -- The main objective of the report is to describe and present the results from an indentation test series completed as a joint collaboration between Memorial University and The Institute for Ocean Technology. The series consisted of small scale indentation tests (see test matrix below) designed to provide insight into the changing behaviour of ice under varying loading conditions. It is believed that an indention test can be used as a model of a spatially stationary, single, high pressure zone. It is proposed that a strong understanding of the mechanics occurring during an indention test can be used to further understand full scale interactions of ice with structures, which consist of many high pressure zones randomly distributed across the area of interaction. -- [table omitted] -- A high speed video camera was used in the analysis of the tests; this is a recent development in the study of ice mechanics in the laboratory arising from past observations during load tests in which qualitatively suggested that ejected ice appeared to be linked to load trace spike/drop sequences. The high speed imagery was used to correlate load trace events (i.e. load spikes/drop sequences, etc.) with visual events. It was shown that large fractures or spalls are correlated with load drop spikes. It has also been demonstrated that "spiky" load graphs are related to repetitious spalling throughout the test (effective clearing of material from under the indenter) while smoother load graphs are related to continuous but small extrusion (ineffective clearing of material form the indenter). -- Post test analysis of the tested samples (thin sectioning) provided evidence of damage mechanisms during testing. Evidence for micro-cracking, inter-grain boundary cracking, crushing, and recrystallization were all present in zone beneath the indenter. This is consistent with the current mechanics description of a high pressure zone, providing confidence in the current ice structure interaction models.

Item Type: Thesis (Masters)
URI: http://research.library.mun.ca/id/eprint/10951
Item ID: 10951
Additional Information: Bibliography: leaves 82-85.
Department(s): Engineering and Applied Science, Faculty of
Date: 2006
Date Type: Submission
Library of Congress Subject Heading: Fracture mechanics; Ice mechanics; Load factor design.

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