Testing at the World’s Largest Indoor Ice Tank Shows Warm Ice May Fracture Differently Than Cold Ice

Close-Up of Final Ice Crack

The displacement measuring instruments at Aalto University’s Ice Tank, the largest of its kind in the world, detect the crack opening to the level of microns. In this image the crack has split the ice completely into two pieces. Credit: Iman El Gharamti/Aalto University

New study suggests old rules on how ice breaks may not always hold up.

Researchers at Aalto University have found strong evidence that warm ice – that is, ice very close in temperature to zero degrees Celsius – may fracture differently than the kinds of ice typically studied in laboratories or nature. A new study published in The Cryosphere takes a closer look at the phenomenon, studied at the world’s largest indoor ice tank on Aalto’s campus.

Understanding how ice breaks is crucial for ensuring safe harbors and bridges in cool climates, as well as transportation through historically ice-heavy regions. As global warming brings changes to once-predictable seasonal conditions, the rules underpinning infrastructure engineering are being tested across borders and continents.

“We need to study warm ice because it’s what we’re seeing in nature; global warming is happening. The mechanical properties of ice and how it responds to force may be fundamentally different when it’s warm rather than cold, as we traditionally study it,” says Iman El Gharamti, lead author of the paper and doctoral student at Aalto University.

To study how warm ice responds to repeated rounds of force – known in the field as cyclical mechanical loading, which simulates conditions in nature – the team made use of Aalto University’s Ice Tank. Measuring 40 meters (130 feet) wide by 40 meters (130 feet) long, the 2.8m-deep (9ft-deep) basin is considered to be the largest of its kind in the world.

Ice Sheet Under Study at Aalto's Ice Tank

The hydraulic loading device hangs by the hook of the carriage, which can move both vertically and horizontally. In this image it is located at the lower left corner of the 3x6m ice sheet being studied. Aalto University’s Ice Tank is considered the largest of its kind in the world. Credit: Iman El Gharamti/Aalto University

Typically ice fractures are studied in small scales, often just 10-20 centimeters (4-8 inches) in length, at temperatures of -10 degrees Celsius (14 degrees Fahrenheit) or colder. In this study, the team used more than one-foot-thick (30-centimeter-thick) ice sheets of fresh water measuring 3 by 6 meters (10 by 20 feet). They also precisely controlled the ambient air temperature, and the ice was, in frozen terms, warm at a balmy -0.3 degrees Celsius (31.5 degrees Fahrenheit). 

With a hydraulic loading device, the team applied multiple rounds of loading and unloading on the ice. Current understanding in the field suggests that ice will show viscoelastic recovery – separate from the immediate elastic response, it is a time-related, delayed elastic response – between loads, at least until the device is told to exert enough force to completely split the ice.

Under the conditions provided, however, the ice behaved in an unexpected way: it showed some elastic recovery but no significant viscoelastic recovery at all. In fact, the ice was permanently deformed.

Ice Grains Polarized Light

Polarized light makes each grain of ice visible, allowing the researchers to see where the crack runs. The results show that the crack ran through the grain rather than along grain boundaries. Credit: Iman El Gharamti/Aalto University, originally published in Acta Materialia (CC BY-NC-ND 4.0)

“What we typically see between mechanical loads is that the ice recovers – it springs back to normal formation until we intentionally apply so much force that it permanently cracks. In our research, the ice was increasingly deformed after each load and we detected no significant delayed elastic recovery,” explains El Gharamti.

The main contributing factor seems to be the temperature of the ice. This research is the first to show warm ice may behave in a fundamentally different way than the cold ice normally studied.

“The fact that the ice didn’t show delayed elastic response doesn’t fit our conventional understanding of how ice copes with repeated rounds of force. We believe that this is because of how the granular level of ice behaves when warm, but we still need to do more research to find out what’s going on,” says Jukka Tuhkuri, professor of solid mechanics at Aalto University.

As warmer conditions are increasingly expected in previously frigid regions like the Great Lakes or Baltic Sea – one of the world’s busiest marine areas – Tuhkuri says it’s crucial to understand the mechanics of warm ice.

“A long-term ice load measurement on an icebreaker in the Baltic Sea has previously shown, surprisingly, that the largest ice load occurred during spring when the weather warms up. If our ships and infrastructure like bridges and wind turbines have been designed for fairly predictable seasons, we need to know what happens when global warming brings new conditions. It looks like the old rules may not hold up,” Tuhkuri says.

The findings were published in The Cryosphere on Thursday, May 27, 2021.

Reference: “Creep and fracture of warm columnar freshwater ice” by Iman E. Gharamti, John P. Dempsey, Arttu Polojärvi and Jukka Tuhkuri, 27 May 2021, The Cryosphere.
DOI: 10.5194/tc-15-2401-2021

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