An inter-disciplinary group of researchers from across the globe has discovered the reason why cracks start, spread and end in such an unpredictable way.
Almost everyone has experienced the moment when you drop your phone and hear that dreaded "crack". Until you survey the damage, you never know just how bad it's going to be – a small fracture in the corner, or a line that stretches across the entirety of your screen.
The project's findings, published in Nature Physics, provide a deeper understanding of the lifecycle of fractures, and could improve our understanding of material science, earthquakes, and production of geothermal energy, oil, and gas.
Supported by the National Science Foundation through the Harvard Research Science and Engineering Centre (MRSEC), the collaboration between material scientists, engineers, geophysicists, and seismologists featured academics from the University of Nottingham, Harvard, the China University of Petroleum (Beijing), Tufts University, the University of Washington, and the Hebrew University of Jerusalem.
Hydraulic fracturing, known as fracking, is the process of creating fractures in rocks by injecting pressurised fluids into the ground to generate a network of connected cracks. This process, widely used for oil and gas recovery or geothermal energy, is also observed in nature, for example, in the formation of magmatic dikes.
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 with a spatial resolution of a couple of micrometres and cutting-edge acoustic emission sensors, the team was able to visualise and listen to the dynamics of fractures as they spread through the material.
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, which has viscosity like honey, 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.
To read the paper in Nature Physics, please click here. To watch a video of the experiment, click here.
