Understanding crashes, explosions and impacts

What happens to fragments of concrete, stone or a granular material after an impact? And what exactly is their residual velocity? These are just some of the complex questions that the Computational Solid Mechanics Laboratory (LSMS) is hoping to resolve with the launch of a new modeling project and the help of a super computer.

What happens when you drop a plate on the floor? Well, it breaks, of course. But how are the pieces propelled through the air? What is the logic behind how they move? And how fast do they go? These questions are far more complicated than they may seem. "At impact, cracks form inside a material. They then spread out like branches and ramify. This causes fragmentation, with the tension eventually becoming so great that the material explodes into pieces," explains Professor Jean-François Molinari, director of LSMS. "For the moment," he adds, "we don’t really understand how this fragmentation works – it’s so complex and random. It’s therefore extremely difficult to study the speed and the way in which the residue from these fractures spreads out. The fact that fragments continue to bump into each other in the air after the initial impact makes things even more complicated." 


Schematic of a simulation and result of
a mesoscale finite element calculation

Fascinated by this problem, doctoral student Marco Vocialta, a new arrival at LSMS, is delving into the subject in his doctoral thesis entitled Residual velocities of fragments after impact loading. His aim is to develop robust algorithms that he can use to model the behavior of fragments after an impact event and to simulate a collision between two objects. "Studying these phenomena experimentally is delicate and dangerous work because the objects travel at such high speed and the impacts are so explosive," explains Marco Vocialta. "That’s why we’re conducting numerical simulations, to establish the laws that can predict what will happen after an impact." To start off, the researcher is looking at the behavior of brittle materials, like concrete. "We excluded ductile metals and materials straight away, as they can deform considerably after an impact." 

 

Marco Vocialta is currently working on developing the necessary codes based on the laws of physics and then checking the results with experimental data already available. Once he’s achieved this, he’ll conduct high-performance calculations on super computers. This is a necessary method, given the complexity of the phenomenon and the random way in which fragments form and then shoot into the air.

Protecting against terrorist attacks 
If the project is successful, it could prove extremely useful in constructing buildings and bridges that are significantly more resistant to an explosion or impact caused by either a terrorist attack or an accident. "It will be possible to know how dangerous the fragments resulting from an impact are, based on their propagation speed and their trajectory," states Marco Vocialta. The method could also be of interest in other fields such as mining: "We’ll be able to conduct precise studies of how best to extract minerals, i.e., by placing explosives in the right place to control the fragmentation of rock," adds Jean-François Molinari.


High performance computer with multiple processors

When asteroids and planets collide
Looking further afield, the team eventually plans to apply its calculations to astrophysics. This would, for instance, involve studying impacts between asteroids and planets to understand how ejecta – the material expelled into the atmosphere after an impact – are formed. "We plan to collaborate with the EPFL’s Earth and Planetary Science Laboratory (EPSL), headed by Philippe Gillet, whose research projects include looking into the characteristics of Martian meteorites that landed on Earth following a collision between Mars and an asteroid," explains Jean-François Molinari.
This simulation project, like all research carried out at the Computational Solid Mechanics Laboratory, is open source.

 

For more information, visit the website of the Computational Solid Mechanics Laboratory (LSMS), which is part of the School of Architecture, Civil and Environmental Engineering (ENAC) and is also linked to the School of Engineering (STI) through the Institute of Materials.