Repulsion and Attraction between a Pair of Cracks in a Plastic Sheet.

We study the interaction of two collinear cracks in polymer sheets slowly growing towards each other, when submitted to uniaxial stress at a constant loading velocity. Depending on the sample's geometry-specifically, the initial distances d between the two cracks' axes and L between the cracks' tips-we observe different crack paths with, in particular, a regime where the cracks repel each other prior to being attracted. We show that the angle θ characterizing the amplitude of the repulsion-and specifically its evolution with d-depends strongly on the microscopic behavior of the material.

Debonding energy of PDMS: A new analysis of a classic adhesion scenario.

We investigated the debonding energy between confined layers of a soft elastic solid (PDMS) and a circular steel indenter in a flat punch geometry. PDMS is extensively used in applications, but also a widespread model system for fundamental research. Varying systematically the pulling speed and the viscoelastic properties, notably the modulus, we determined scaling laws for the debonding energy.

Softening induced instability of a stretched cohesive granular layer.

We report on a cellular pattern which spontaneously forms at the surface of a thin layer of a cohesive granular material submitted to in-plane stretching. We present a simple model in which the mechanism responsible of the instability is the "strain softening" exhibited by humid granular materials above a typical strain. Our analysis indicates that such a type of instability should be observed in any system presenting a negative stress sensitivity to strain perturbations.