Ice fracture mechanics

Ice Fracture Mechanics

For industries working in arctic and sub arctic waters ice loads on structures and vessels is a major concern. Due to the inherently heterogeneous structure of ice however, this load is very difficult to estimate. The situation is further complicated by the fact that unlike classical materials, ice exhibits a scale effect. The scale effect refers to the fact that failure pressures on ice in field tests have commonly been found to be on the order of 1 MPa. This is significantly lower than the values of 20-30 MPa that have been recorded at the laboratory scale. While a great deal of effort has previously gone into the study of the scale effect, it is not yet fully understood. It is expected that the improved knowledge of the material behavior of ice will help to clarify the scale effect.

During an ice structure interaction, areas of high pressure, called high-pressure zones (HPZs) form. HPZs are randomly distributed across the area of interaction and vary in both intensity and location with time. Their formation is influential in the evolution of the interaction. When HPZs form near the edge of an ice sheet, they can precipitate large-scale fracture and spalling events. This reduces both the load felt by the structure and the contact area between the structure and the ice sheet. As such, an understanding of the mechanics involved in the formation and evolution of HPZs will certainly aid in the understanding of the scale-effect.

Additionally, the formation of a HPZ causes the creation of a damage layer. This layer contains fine-grained material resulting from a combination of processes including recrystallization and micro-cracking. Interestingly, this layer demonstrates remarkable similarity regardless of the scale of observation.

The presence of preexisting flaws within the ice may also play a role in the scale effect. In nature, ice contains many flaws. These flaws may include such things as large cracks and leads, entrained air bubbles or dirt, and irregular grain size and composition. As the area of interaction increases, the probability of stressing a flawed area within the ice increases, perhaps leading to the lower failure pressures that have been found in the field. My current experiments focus on the role that these flaws play in fracture behavior.

Since grain boundaries are a weakness within ice, my experiments have been designed to exploit this weakness. My current work involves a series of indentation tests that are being performed in the cold room facilities of the NRC-IOT in St. John’s, NL. The tests are performed on ice specimens with dimensions of 20 x 20 x 10 cm. These ice specimens are made of polycrystalline ice with a grain size of ~ 4 mm. In order to explore the role of grain boundaries, the test specimens include a 2 cm cubed monocrystals placed at specific locations with in the specimen. We assume that the large grain boundaries provided by the monocrystal will help to precipitate a fracture in the area of the crystal. This should affect both the maximum load attainable during the test as well as the maximum duration of the tests as compared to a specimen that does not contain an embedded crystal.

For industries working in arctic and sub arctic waters ice loads on structures and vessels is a major concern. Due to the inherently heterogeneous structure of ice however, this load is very difficult to estimate. The situation is further complicated by the fact that unlike classical materials, ice exhibits a scale effect. The scale effect refers to the fact that failure pressures on ice in field tests have commonly been found to be on the order of 1 MPa. This is significantly lower than the values of 20-30 MPa that have been recorded at the laboratory scale. While a great deal of effort has previously gone into the study of the scale effect, it is not yet fully understood. It is expected that the improved knowledge of the material behavior of ice will help to clarify the scale effect.
During an ice structure interaction, areas of high pressure, called high-pressure zones (HPZs) form. HPZs are randomly distributed across the area of interaction and vary in both intensity and location with time. Their formation is influential in the evolution of the interaction. When HPZs form near the edge of an ice sheet, they can precipitate large-scale fracture and spalling events. This reduces both the load felt by the structure and the contact area between the structure and the ice sheet. As such, an understanding of the mechanics involved in the formation and evolution of HPZs will certainly aid in the understanding of the scale-effect.
Additionally, the formation of a HPZ causes the creation of a damage layer. This layer contains fine-grained material resulting from a combination of processes including recrystallization and micro-cracking. Interestingly, this layer demonstrates remarkable similarity regardless of the scale of observation.

The presence of preexisting flaws within the ice may also play a role in the scale effect. In nature, ice contains many flaws. These flaws may include such things as large cracks and leads, entrained air bubbles or dirt, and irregular grain size and composition. As the area of interaction increases, the probability of stressing a flawed area within the ice increases, perhaps leading to the lower failure pressures that have been found in the field. My current experiments focus on the role that these flaws play in fracture behavior.
	Since grain boundaries are a weakness within ice, my experiments have been designed to exploit this weakness. My current work involves a series of indentation tests that are being performed in the cold room facilities of the NRC-IOT in St. John’s, NL. The tests are performed on ice specimens with dimensions of 20 x 20 x 10 cm. These ice specimens are made of polycrystalline ice with a grain size of ~ 4 mm. In order to explore the role of grain boundaries, the test specimens include a 2 cm cubed monocrystals placed at specific locations with in the specimen. We assume that the large grain boundaries provided by the monocrystal will help to precipitate a fracture in the area of the crystal. This should affect both the maximum load attainable during the test as well as the maximum duration of the tests as compared to a specimen that does not contain an embedded crystal.