J.Crassous Homepage

Frictional Discrete Elastic Rods

2023-02-22T00:00:00+00:00

Based on work carried out in 2005 on the modeling of elastic fibers, I have developed a numerical code of the discrete element. None of the various ingredients of this code (Discrete Elastic Rod, explicit integration of the equations of motion, treatment of frictional contacts) are individually original. However, this combination allows us to tackle a variety of problems in a simple way. Writing the code from scratch means that, at the cost of considerable optimization work, the code is very light and compact. In particular, control over the physical ingredients of the model is total, and the contact detection part is specifically optimized to handle a large number of elongated objects. The code has been tested in a wide range of configurations and geometries, with comparisons to analytical solutions where available. https://doi.org/10.1103/PhysRevE.107.025003

Shear bands formation.

2020-01-01T00:00:00+00:00

Deformation localization in granular media and amorphous plasticity Collaboration: J.Weiss and D. Marsan, ISTERRE (Grenoble, Chambéry). E. Clément PMMH.

One of the main aims of our work is to understand the nature and organization of plastic flow in a deforming amorphous medium, and to study the approach to fracture in these media. One of the original features of our experimental approach is the use of an interferometric method for measuring microstrain that we have developed. Using stacks of glass microspheres as a model amorphous material, we have demonstrated that plastic flow, observed well before fracture, has a heterogeneous, intermittent structure that spontaneously organizes itself into ephemeral microbands. This structure confirms the relevance of the theoretical scenarios currently proposed to describe the plasticity of amorphous materials in terms of cascades of plastic events whose interaction is governed by the material’s elasticity. We have shown experimentally and numerically that this description in terms of an effective elastic medium is relevant for an assembly of rigid grains in frictional contact [Houdoux, PRE2018]. As the system approaches fracture, the plastic flow behavior becomes multiscale. We have shown experimentally that the appearance of a shear band at the sample scale corresponds to a symmetry break in the organization of the plasticity field. In collaboration with E. Clément, we have studied creep flows and their activation under the effect of mechanical perturbations.

https://doi.org/10.1103/PhysRevE.98.022905

https://doi.org/10.1103/PhysRevE.94.022907

https://doi.org/10.1103/PhysRevE.92.020201

https://doi.org/10.1103/PhysRevLett.112.246001

https://doi.org/10.1103/PhysRevLett.108.135502

Plasticity fluctuations in shear bands. Analogy with seismology.

2020-01-01T00:00:00+00:00

We were interested in deformation fluctuations within a shear band, i.e. when the granular material is in stationary flow, beyond the precursors to fracture. We were able to demonstrate that deformation takes place in a highly intermittent manner, with spatial and temporal fluctuations in deformation.

This situation suggests an analogy with plate tectonics, where deformations of the Earth’s crust are localized along faults and occur, among other things, as discrete movements generating earthquakes. We wanted to develop this analogy. To this end, we collaborated with geophysicists from the Isterre laboratory (Observatoire des Sciences de l’Univers de Grenoble, INSU) to test this analogy. The results of this study showed a remarkable analogy between the statistics of microstrain in a shear band and the statistical laws of natural earthquakes1. This experimental system is thus probably one of the best laboratory implementations of a device reproducing natural faults. Existing systems either fail to image events, or do not reproduce the complex statistics of natural events.

https://doi.org/10.1038/s43247-021-00147-1

Capillary condensation and its effect on adhesion and friction

2000-01-01T00:00:00+00:00

I was also interested in the effects of capillary condensation (spontaneous formation of a liquid phase in the vicinity of solids from a vapor at sub-saturating pressure) on friction properties. Liquid bridges are formed which modify contact forces, and hence friction. The wide size distribution of nucleation sites, and hence of metastability barriers, has the effect of inducing logarithmic aging of friction coefficients. I have developed experiments to quantify the relationship between the time evolution of solid friction coefficients and the presence of vapor.

Nanorheology

1995-01-01T00:00:00+00:00

From 1995 to 2005, I mainly developed scientific equipment for studying the rheology of liquids confined to the nanometric scale. The principle is that of a surface force machine: 2 macroscopic surfaces are approximated at a distance of a few nanometres, and the liquid film is made to flow by distance modulations of the order of 10 to 100 pm. The nano-rheometer was built in collaboration with Elisabeth Charlaix (ENS Lyon, then University of Lyon 1). It was built from scratch and typically represented 2/3 of my research activity. It is a thankless piece of instrumental work, the great difficulty of which is due to the combination of metrological problems (sensors, measurements, stability, noise density, etc.), mechanical problems (linearity, hysteresis, dissipation, resonance, etc.) and environmental problems (vibrations, pollution, drift, etc.). I was a very active contributor to this project: I developed the sensors (optical interferometry, capacitive), the mechanical characterisation of the elements (mechanical transfer functions, vibration modes, dissipation), a large part of the drift compensation, interfacing,