Time-dependent aspects of fracture in ice

Kavanagh, Mark B.(Mark Bradley) (2018) Time-dependent aspects of fracture in ice. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

An important aspect of ice fracture that is yet to be fully resolved in determining the loads and pressures during an interaction with ice is the presence and growth of fractures. Practical applications include icebreakers ramming into ice pack or structures operating in ice-prone regions. Existing models often use a simplified view of ice mechanics, often based on only the elastic properties, which tend to overestimate the strength of the ice. It has long been known that ice exhibits time-dependent properties, including its strength. This is known from ship ramming experience as well as field and laboratory experiments. Accounting for these time-dependent aspects of ice behaviour would allow for a more analytical approach to interaction modelling and complement the empirical data that is used currently in design practice. The aim of this thesis is to provide a better understanding of ice fracture that occurs during an interaction. To better understand the viscoelastic properties of ice, an extensive review of the literature was performed. In particular, an in-depth review of linear elastic fracture mechanics was performed, as it it critical to the foundations of the viscoelastic fracture theory. To guide the development of a new viscoelastic fracture model, three sets of experiments were performed. The first was an indentation series, scaled down from similar field experiments, that displayed several fracture properties of ice. These properties were rate-dependent fracture, delayed fracture, and scale effects. The next two series, designed to study a single crack, used ice samples under 4- point bending. The first of these were constant loading rate test to further study the rate-dependent fracture properties of ice, resulting in ice that is weaker under faster loading in a decreasing power law relationship. The second of these applied constant loads just below the breaking point to show that ice undergoes delayed fracture. The data suggests a decreasing power law between applied load and time to failure for these samples. Building on previous works in viscoelastic fracture theory, and making use of the insights gained from the experiments, a new model has been developed to predict the fracture properties of ice. The model accounts for the delayed fracture of ice and provides insights into the time-dependent fracture properties of ice, as was seen in the laboratory and field experiments. The model, based on viscoelastic theory, was shown to reasonably model time to failure for ice beams under constant load, as well as the fracture strength of ice beams under different loading rates. The model, besides expanding the old theories to beam bending geometries, was also shown to work for compact tension specimens with a few minor changes.

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