Nanocracks in nature and industry.

Keywords: nanocracks EN nanocracks 1 6 6 08/12/22 20220919 NES 220919 This theme issue is a celebration of the paper published half a century ago in I Proceedings of the Royal Society i on "Surface energy and the contact of elastic solids" [1]. In particular, it is important that the original Griffith argument is extended [26] to the case where potential energies of applied forces are added to the Griffith elastic and surface energy terms to define a wider equation for equilibrium. This observation goes against the stress theory of cracking first postulated by Galileo in 1638 and also conflicts with the Griffith [2] equation for crack equilibrium that suggests brittle strength should be independent of sample size, but dependent on intrinsic flaw size. Comparison of JKR equilibrium crack strength equation (1.1) on results for gelatin spherical surfaces of different diameters, showing adhesion strength rises for small sphere diameters as predicted. [Extracted from the article]