Emergent properties in interface mechanical problems: A paradigm of organized complexity

Organized complexity resides in the non-random, or correlated, interaction between the parts of a system. The understanding of the emergent properties of such systems is very challenging and demands modelling and extensive computational simulations. Modern measurement techniques have shown that both the geometry of typical microdefects and the topography of rough surfaces exhibit fractal qualities over several length scales. Evidence is mounting that this multiscale character is an important factor in the determination of macroscale properties. Examples regard the size-scale effects on the frictional response of rough surfaces, the thermal and electromagnetic properties of heterogeneous materials, as well as the anomalous fatigue behaviour of short-cracks. In this seminar, two interface problems related to contact and fracture mechanics of solids are presented as a paradigm of organized complexity. The first concerns the determination of a model-independent relation between contact conductance and applied pressure of a rough surface. The second shows the condition of flaw-tolerance in hierarchical polycrystals as a result of the interplay of cohesive interfaces across multiple scales. The solution of these problems requires the use of dimensional analysis, fractal geometry, computational mechanics, as well as optimization methods. Finally, the application of the proposed methodologies to the prediction of durability of photovoltaic modules, an engineering problem requiring multiphysics modelling, is outlined.