The effect of crack-tip point loads on fracture

The placement of a point load at a crack tip does not contribute to the magnitude of the stress intensity factors (SIF) at that tip. It nevertheless can couple with other SIF-producing loads and lead to a change in the potential energy of the system. As a result, the configurational equilibrium condition or the fracture criterion of linear elastic fracture mechanics is altered. The inclusion of surface stress leads to yet another point load with similar effect. In the context of linear elasticity, however, the change produced by surface stress is of second order, while that affected by a point force is of first order, in that the. surface tension is actually reduced by an amount equal to the applied force. This is a situation in which the applied mechanical force combines with the so-called fictitious force in bringing about configurational changes. The splitting of a double-cantilever-beam by a wedging force, which is not the same as a crack-tip point load, is used to demonstrate that the wedging force at fracture is of the order of magnitude of the surface tension. (C) 2000 Elsevier Science Ltd. All rights reserved.