Explicit transformation between non-adhesive and adhesive contact problems by means of the classical Johnson–Kendall–Roberts formalism

The classic Johnson–Kendall–Roberts (JKR) contact theory was developed for frictionless adhesive contact between two isotropic elastic spheres. The advantage of the classical JKR formalism is the use of the principle of superposition of solutions to non-adhesive axisymmetric contact problems. In the recent years, the JKR formalism has been extended to other cases, including problems of contact between an arbitrary-shaped blunt axisymmetric indenter and a linear elastic half-space obeying rotational symmetry of its elastic properties. Here the most general form of the JKR formalism using the minimal number of a priori conditions is studied. The corresponding condition of energy balance is developed. For the axisymmetric case and a convex indenter, the condition is reduced to a set of expressions allowing explicit transformation of force–displacement curves from non-adhesive to corresponding adhesive cases. The implementation of the developed theory is demonstrated by presentation of a two-term asymptotic adhesive solution of the contact between a thin elastic layer and a rigid punch of arbitrary axisymmetric shape. Some aspects of numerical implementation of the theory by means of Finite-Element Method are also discussed. This article is part of a discussion meeting issue ‘A cracking approach to inventing new tough materials: fracture stranger than friction’.

The solution of Dirac’s equation in Kerr geometry

Dirac’s equation for the electron in Kerr geometry is separated; and the general solution is expressed as a superposition of solutions derived from a purely radial and a purely angular equation.

An Elastic Half Plane Weakened by a Rectangular Trench

The problem of an elastic half plane loaded in tension and weakened by a rectangular trench is set up in a manner leading to a system of simultaneous singular integral equations. The method is to consider the superposition of solutions for two cracks perpendicular and one parallel to the surface of the half plane. The limiting case when the three cracks intersect appropriately leads to the geometry of a rectangular trench. A collocation scheme developed by Erdogan and Gupta leads to numerical solution of the equations to determine the stress intensity factors at the corners and the horizontal displacement of the trench wall.