International team reveals secrets of fractures

The international team of researchers believe their findings provide a deeper understanding of the lifecycle of fractures and could also improve the production of geothermal energy, oil, and gas. The research is detailed in Nature Physics.

Supported by the US National Science Foundation through the Harvard Research Science and Engineering Centre (MRSEC), the collaboration between material scientists, engineers, geophysicists, and seismologists featured academics from Nottingham University, Harvard, the China University of Petroleum-Beijing, Tufts University, the University of Washington, and the Hebrew University of Jerusalem.

In a statement, said David A. Weitz, senior author and Professor of Physics and Applied Physics at Harvard said: “Fracturing is well understood in two dimensions but more realistic fractures in complex, three-dimensional materials present a plethora of complex behaviours which are widely studied yet remain poorly understood at a fundamental level,”

To understand fractures in three dimensions, the team introduced a crack in a transparent material, and then injected liquids of varying viscosities. Using a high-speed camera that can capture 100,000 images per second and acoustic emission sensors, the team visualised and listened to the dynamics of fractures as they spread through the material.

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The team found that, rather than moving through a material like a continuous wave, fractures move in starts and stops, propagating from their origin in a material outward through a series of high-speed jumps. The amplitude and the time between these jumps depend on the viscosity of the liquid.

With low viscosity liquids, like water, the time between jumps is miniscule as the fluid penetrates the crack almost instantaneously. With higher viscosity fluids like glycerol, the lag between the so-called fracture front (where the crack is) and the fluid front (where the liquid tip is) increases as it takes longer for the high-viscosity fluid to penetrate the crack and expand it.

“We also developed a numerical model that builds on the same mathematical equations and assumptions of fracture theory, but is fully three-dimensional. We discovered that the simulation was able to reproduce the experimental data in a quantitative manner, with no fitting parameters. This emphasises the generality of our finding, which is applicable to fractures that arise in a wide range of scenarios and not just in the specific case of a fluid-driven crack,” said Gabriele Albertini, co-author and Assistant Professor at Nottingham University.