Starting: October 2015
In recent times, much of solid mechanics research has focused around the “micro” theme, or, more precisely, the micro-macro theme, i.e., the way in which some lower scale mechanics informs the macroscopic behaviour of materials.
Various methods of digital imaging are now available in experimental grain scale mechanics, being able to measure quantities such as the void ratio in a shear band and track the motion of grains during mechanical loading (see our research news for details). Although these techniques have led to substantial progress in the understanding of the contribution of micro-scale mechanisms to the behaviour of granular materials, they still lack the ability to probe one crucial aspect of granular materials at the grain scale: contact forces acting between the grains.
The present project aims at developing a novel technique for computing inter-granular forces in an assembly of grains subjected to loading.
Left image:Typical fields of measured grain displacements (superimposed on a photograph) measured by Particle Image Tracking during a simple shear test performed on a two dimensional sample of rods. Right image: A possible solution of force network (only normal forces are shown; the width of the line that connects touching grains is proportional to the normal force)
This new method, adapted from the classical “Non‐Smooth Contact Dynamics (NSCD)” numerical method, should enable to infer particle contact forces from the experimental measurement of particle kinematics (displacements and rotations) and connectivity (contact network). From these accurate measurements, the algorithm will calculate, through inverse analysis, a set of statistically admissible inter granular forces that are compatible with the observed displacements – the method will therefore be referred to as “Forces Inferred from macroscopic Loading and grain Motions, FILM”.
The originality lies in the fact that the computation will only require the knowledge of rigid body-grain kinematics rather than the measurement of the strain inside of each solid particle as it is the case with the classical methods, and would therefore be the only technique capable of dealing with the actual number of grains forming a typical sand sample used in mechanical assays (i.e. tens of thousands of grains).