Picking up the pieces
EPFL researchers have developed a new, 3D numerical simulation that reveals
the physics of how things break.
A plate that falls on the ground usually suffers the same fate as a glass filled with boiling water – it shatters. “As soon as you have two pieces, it’s a case of fragmentation. It affects our everyday lives, and is a fundamental part of our existence on the planet. An asteroid might have wiped out the dinosaurs, the Earth’s crust continues to fracture via tectonic plates, and glaciers are also breaking apart,” says Sarah Lévy, who has just defended her PhD thesis on the phenomenon and is particularly interested in how cracks propagate. “In dynamics, there are two main kinds of fragmentation: those caused by impacts – a car crashes, you fall down the stairs and break your arm; and those caused by explosions – like what happened with the Quantas A380 when some sections of the turbine disk detached. I’m interested in this latter case.”
How do materials react in a blast and to what extent will they deform before they ultimately break? These are critical issues to consider when building a nuclear reactor or a pipeline. To answer these questions, Lévy, who has been working for three years in EPFL’s Computational Solid Mechanics Laboratory (LSMS), has harnessed the power of the school’s most powerful supercomputers, such as the IBM Blue Gene and the Pleiade cluster. “Thanks to this number-crunching power, we can go much farther than we could with the standard analytical model that was developed about 30 years ago.
It predicted that all the pieces would be the same size, which is incorrect.” Many other parameters have been added to the model, such as elastic wave interactions and defects in the microstructure, which make the calculations even more complicated. “I kept several computers at work simultaneously, doing calculations in parallel,” she admits.
But this kind of number-crunching is necessary in order to accurately reproduce the microfractures of a material in three dimensions. LSMS is the first laboratory to obtain convergent results (in other words, results that are independent of the level of precision of the calculations) in three dimensions. “The numerical model allows us to visualize the fragments - their shape, size, and weight – and to understand the physics that is taking place.” Lévy’s work will stop there, because she is leaving for a job in Paris at Electricité de France (EDF). But the lab will continue the research she started. The next step is to calculate the speed at which the fragments are ejected in an explosion. Stay tuned!