Brittle and ductile behavior at high strain rate : ultra high-speed imaging of halite deformation

Laboratoire de rattachement : ISTerre

Encadrants : Mai-Linh Doan (Mai-Linh.Doan ujf-grenoble.fr) et Frans Aben

Téléphone : 04 76 63 52 09

Mots clés : High strain rate testing of rock, brittle-ductile transition, coseismic damage, petrophysics, microstructures

Subject :
In the last 10 years, several spectacular examples of coseismic damage have been found. One of the most striking examples is pulverized rock that has been found near the San Andreas Fault (Dor et al, 2006) and other large strike-slip faults. This damage is thought to be the witness of former major earthquakes along these faults. Indeed, laboratory experiments have shown that pulverized rocks can be reproduced by very high strain rate damage of rock (Doan and Gary, 2009).

This raises the more general issue of coseismic damage of rocks : a possible sink of coseismic energy whose physics is currently poorly known. Especially, high strain rate damage influences the competition between brittle and ductile behavior. An example is the behavior of carbonate rocks that behave purely brittle at high strain rate, with little calcite twinning (Doan and Billi, 2011).

To decipher the competition between ductile and brittle behavior, we plan to perform high strain rate experiments with the in-house Split Hopkinson Pressure Bar on rock samples, particularly halite. This device launches a striker with several meters per second so that the small rock sample is smashed. It allows the retrieval of strain-stress curves at strain rates of 10/s up to 1000/s, about 10^15 larger than tectonic strain rates. The experiments are filmed with a high-speed camera ( 300.000 fps).

Preliminary video results show spectacular behavior of the rock during the experiments with a large ductile deformation of the halite and the later apparition of fractures. We want to investigate the interplay between the two processes and how they affect the mechanical behavior of the rock.

The student will be involved in the high strain rate experiments (a lot of fun), as well as in the processing of the video images and the microstructural characterization of the post-mortem samples.

Bibliography
Doan, M.-L., and Gary, G. (2009). Rock pulverization at high strain rate near the San Andreas fault. Nature Geoscience, 2(10), 709–712. http://doi.org/10.1038/ngeo640
Doan, M.-L., and Billi, A. (2011). High strain rate damage of Carrara marble. Geophysical Research Letters, 38(L19302), 1–6. http://doi.org/10.1029/2011GL049169
Dor, O., Ben-Zion, Y., Rockwell, T. K., & Brune, J. (2006). Pulverized rocks in the Mojave section of the San Andreas Fault Zone. Earth and Planetary Science Letters, 245(3-4), 642–654. http://doi.org/10.1016/j.epsl.2006.03.034

Illustrating figure :

Prerequisites : Geomechanics module in M1, motivation and curiosity.