Friction, Adhesion & Crack Propagation
The main contribution to rubber friction when a rubber block slides on a rough substrate, i.e. a tyre on a road surface, is the viscoelastic energy dissipation in the surface region of the rubber. This results from the pulsating forces acting on the rubber surface from the substrate asperities. Recently we developed a theory that describes this dissipation process accurately, and also predicts the velocity dependence (and the time-history dependence) of the rubber friction coefficient. The results depend only on the (complex) viscoelastic modulus of the rubber and the substrate surface roughness power spectra, and agree well with experimental data.
Adhesive Microstructures of Insects and Lizards
How can a fly or a cricket walk up a smooth glass window, or a lizard crawl vertically up a stone or concrete wall? These fundamental questions have interested scientists for many years, and recently very important work has been performed in order to gain a deeper insight into these puzzling phenomena. The adhesive microstructures of insects and lizards are the result of perhaps millions of years of evolutionary development driven by the principle of natural selection. Hence one expects the adhesive structures to be highly optimized and it is clear that a good understanding of their construction and function may lead to new, improved man-made adhesives. At our institute, we study adhesion relevant to biological systems, e.g., flies, crickets and lizards, where the adhesive microstructures consist of either smooth attachment pads, as shown in the figure of a cricket adhering to smooth vertical window glass, or arrays of thin fibers.
The singular stress region at a crack tip in continuum mechanics can be removed by (1) tip blunting (tip diameter a), or (2) by introducing a lateral region (linear size a) over which bond breaking occurs. The latter is the so-called Barenblatt process zone. The crack tip process zone in most materials is very complex, involving cavity formation, stringing, chain pull-out (for polymers), and bond breaking (3).
Our treatment of crack propagation in viscoelastic solids can be extended to include the crack tip flash temperature, which - because of the low heat conductivity of rubber materials - is likely to be of extreme importance already at relatively low crack tip velocities.