The molecular wedge in a brittle crack: A simulation of mica/water

Abstract

This paper presents an atomic calculation of the wedging effect which occurs in a brittle crack when molecules of a chemisorbing species of molecules of sufficient size enter the crack mouth. A surface tension develops at the tip of the wedge caused by the difference between the covered and vacuum surface energies. This force draws the chemisorbing molecules toward the crack tip and distorts the crack faces, causing, in turn, a compensating elastic force on the molecules which tends to eject the molecules. We calculate the equilibrium penetration of the wedging molecules and the configuration of the crack and wedge by an atomistic calculation. We simulate mica/water chemistry by means of a simplification of the mica lattice and calculate interactions between the water and mica on the basis of Born–Mayer. Water is found to form a wedge tongue of two or three molecular thicknesses and a length of about 20 molecular distances, which penetrates into the crack tip cohesive zone. When strong wedging action occurs at a crack tip, crack advance near threshold loadings will be limited by molecular diffusion through the wedge tongue.