CASCADE

A deeper picture of friction

Frictional interfaces are all around us, in both the natural world and manufactured objects, yet some questions about their behaviour remain unanswered. Dr Julien Scheibert tells us about the Cascade project’s work in developing a comprehensive picture of friction at all scales, work which holds important implications for both science and industry

There are frictional interfaces all

around us in everyday life, in the natural environment, in manufactured objects, and in our own bodies. These interfaces can be in two kinds of states, as Dr Julien Scheibert, the Principal Investigator of the Cascade project, explains. “They’re either stable – the stuck state – and don’t move relative to each other, or they’re in a slipping state where the interface slides,” he says. The transition between these two states formed the primary research focus for the Cascade project.

Frictional interface

This transition does not happen instantaneously, as was previously believed. In a sheared frictional interface, for example a seismic fault, there is always a region which is weaker than another, and hence will start to slip earlier. “This is what we call slip nucleation – it involves a small region of the interface. This region will grow progressively, and eventually it will be able to invade the whole interface. Only at that point will there be macroscopic relative motion between the two bodies,” explains Dr Scheibert. “This growth of the slipping region defines two sub-regions of the interface – the slipping region, which is growing, and the stuck region, which is shrinking. The frontier between these two regions is what we call a micro-slip front.”

A number of questions around the dynamics of these fronts remain unanswered, in particular their speed and direction, and the force at which they nucleate. The project followed a twopronged strategy to address these questions. “One was experimental, using high-speed camera acquisition and a digital image correlation method to monitor the dynamics of the slip field. The other was numerical. There was a series of unexplained observations in the literature which we wanted to understand so we developed new models of friction and tried to reproduce the observations and interpret them,” says Dr Scheibert.

Heterogeneous interface

Front type

This approach was designed to gain new insights into the dynamics of micro-slip fronts, in particular the differences between fast and slow fronts. One well known type of front is quasi-static fronts, meaning that there is mechanical equilibrium at all times, which affects crack propagation. “If you drive your system and you stop shearing it, then the front will stop. Because you have no more energy available for the front to go on,” explains Dr Scheibert. Researchers have since discovered fronts that are highly dynamic, and so behave differently. “The main difference is that when this type of front has nucleated it cannot be stopped. Even if you stop shearing, it has sufficient elastic energy stored in the system to continue propagating,” says Dr Scheibert.

Researchers have also observed differences within the dynamic regime, with both very fast fronts and much slower fronts in the same system. The project developed a new friction model that was able to reproduce these two dynamic fronts – slow and fast – in the same system. “We were able to understand why there can be a transition between slow and fast fronts,” explains Dr Scheibert. The wider objective in this research is to develop a comprehensive picture of friction at all scales, which will hold important implications across a number of disciplines. “In mechanical engineering, it’s very important

Micro-slip Front

to understand the dynamics of shear cracks, while it’s also central to studying earthquakes, so many fields are interested in this topic,” says Dr Scheibert.

CASCADE: Frictional shear crack dynamics along heterogeneous interfaces

The objective of the project CASCADE is to advance knowledge in the field of friction, by providing a comprehensive picture of the onset of sliding and, in particular, of the dynamics of shear cracks along heterogeneous frictional interfaces. The strategy is based on a tight, quantitative dialogue between experiments and numerical/theoretical approaches. Project Collaborators: CNRS, University of Oslo and Academy of Sciences of Ukraine. Project Funding: Marie Curie action, Career Integration Grant, FP7 Dr Julien Scheibert, CNRS scientist Laboratoire de Tribologie et Dynamique des Systèmes Ecole Centrale de Lyon 36, Avenue Guy de Collongue, 69134 Ecully cedex, France T: +33 47 218 6226 E: julien.scheibert@ec-lyon.fr W: http://perso.ec-lyon.fr/scheibert.julien/

Dr Julien Scheibert is Chargé de Recherche (Researcher) at CNRS. He develops experimental and modelling tools to investigate the rupture dynamics of heterogeneous interfaces, in various fields including biomechanics, mechanical engineering and geophysics. In 2009, he received the Branly prize from the French Federation of Scientific Societies.