Crater shapes, fragments, ejecta : patterns resulting from impacts – CraSh

The objective of the project is to conduct experiments to understand the fundamental aspects of the formation of patterns by impacts. Our approach will be to simplify the problem to focus on some of its fundamental features. Thus, though it definitely serves as a motivation, our goal is not to reproduce meteorite impact craters in all their complexity but rather to identify model situations in which some specific aspects of the patterns can be reliably related to physical mechanisms. For this we will conduct experiments devised to study the complete dynamics from the short time response to the final pattern.

We have studied the transverse impact of a rigid object on a thin brittle plate. After impact, the plate exhibits radial cracks whose number varies with the speed of the impactor. We have exhibited a scaling law between the number of radial cracks and the impact speed.
To study the dynamics of crater formation in model yield-stress fluids (size of the transient cavity, shape of the final crater), we have designed an experiment where a Carbopol drop (a transparent micro-gel) is falling under its own weight into a container filled with the same fluid. The cavity dynamics was visualized using a high-speed camera and the deformation of the substrate was measured using Particle Tracking and PIV. By changing the fluid rheological properties (elasticity, yield-stress, viscosity) and the effective gravity (the container can be accelerate uniformly using a magnetic translation stage), we have shown that the transient cavity is mainly controlled by the fluid elasticity even above the yield-stress, because the impact time is much shorter than the fluid relaxation time (large Deborah number). Scaling laws are found for the size and growth time of the cavity as function of the elastic Mach number.

After a violent impact on a cohesive substrate, fragments are ejected. The circumferential distribution of fragments is in general non-uniform. A natural question, which we plan to address at least in model configurations, is the selection of this distribution and in particular the relation between ejecta and radial structures (cracks, wrinkles) mentioned above.
In the selection of the final crater shape, the role of the short-time dynamics remains barely understood. A third objective is to relate short-time dynamics as it appears in different types of materials to the final pattern. For this purpose we propose the use of model complex fluids (not only fluids with a complex rheology but also Newtonian fluids coated with elastic or brittle membranes), which have been hardly used in impact experiments.

N. Vandenberghe, R. Vermorel, E. Villermaux. Star-Shaped Crack Pattern of Broken Windows. Phys. Rev. Lett., vol. 110, 174302 (2013)

L.-H. Luu, Y. Forterre. Giant drag reduction in complex fluid drops on rough hydrophobic surfaces, Phys. Rev. Lett., vol 110, 18450 (2013)

The proposal aims at studying shapes resulting from an impact on cohesive matter. The project is based on different model experiments, intending to relate the transient dynamics, including at short-times, to the final patterns. The different situations we shall consider concern various scientific fields (elasticity, fracture mechanics, hydrodynamics, rheology of complex fluids). We shall study different aspects such as fragmentation after impact and the formation of crack patterns in brittle plates, the different instabilities that affect an impacted membrane lying on a substrate, the spatial structure that may appear when ejecta are excavated from the crater, the extension and shrinking dynamics of a crater in a yield-stress fluid, and the behaviour under impact of fluids of complex rheology (clays, wet granular...).

The main objective is to increase the fundamental knowledge on the behaviour of matter under the effect of an external violent disturbance involving a wide range of time-scales, as well as large deformations. The configurations we plan to study will be close to situations encountered in geo- and astrophysics (impact cratering), in industrial processes (welding, material forming, crushing and grinding, etc...), as well as applications related to security (armor penetration …).