FOM archive's video: Physics@Veldhoven 2017 Masterclass Elisabeth Bouchaud

Prior to Physics@Veldhoven 2017, the programme committee organised four masterclasses. These classes offer PhDs and young postdocs a unique opportunity to receive an introduction to their discipline from top researchers. Masterclass abstract: Fracture of heterogeneous media To establish a quantitative interrelation between the microstructure of a material and its mechanical properties has been a long term goal in materials science, since the macroscale mechanical response is controlled, to a large extent, by the structure at a relatively fine scale. Although considerable progress has been made in the prediction of elastic moduli, little is yet understood when it comes to the stress or the elongation to failure, or to the fracture toughness (ability to resist the propagation of a pre-existing crack). The difficulty resides in the fact that, close to the crack tip, the heterogeneous material cannot be simply replaced by an 'effective equivalent' homogeneous material, mainly for two reasons: (i) there are very strong stress and strain gradients in the vicinity of a crack tip, which makes it very difficult to perform averages and (ii) the most brittle components of the heterogeneous medium are likely to break first, hence the statistics is controlled by rare events, which makes the problem both conceptually and experimentally challenging. In order to circumvent these intrinsic difficulties, we have tried to establish 'simple' models able to predict, a minima, two consequences of disorder: the roughness of fracture surfaces, and the intermittency of crack propagation. Experiments indeed show that fracture surfaces are self-affine, scale-invariant structures, characterized by Hurst (or roughness) exponents. Crack dynamics also involves power-law distributions (local velocities, waiting times etc.). Considering that crack fronts are slightly distorted by the presence of microstructural obstacles with contrasted toughness, we have suggested models which consider the propagation of an elastic line through an array of randomly distributed obstacles. These models depict in fact the vicinity of a dynamic phase transition, the so-called depinning transition. It will be shown that these models are actually applicable in the linear elastic case, i.e. at large enough length scales. At smaller scales, i.e. at scales where, due to the presence of dissipative processes such as plasticity, microcracking etc., a departure from linear elasticity is observed, no such model is available. However, experiments performed on very different materials in very different conditions, show that this small scale behaviour also constitutes a universality class. Recent experiments and simulations may open the way to further modelling in this complex regime. About the conference: Physics@Veldhoven is a large congress that provides a topical overview of physics in the Netherlands. It is organised by NWO, the Netherlands Organisation for Scientific Research , and takes place each year in January. Traditionally, young researchers are given the chance to present themselves and their work alongside renowned names from the Dutch and international physics community. The programme covers Light and matter, Atomic, molecular and optical physics, Nanoscience and nanotechnology, Statistical physics and Soft condensed matter, Surfaces and interfaces, Physics of fluids, Subatomic physics, Plasma and fusion physics, and Strongly correlated systems.