Effect of sliding friction forces on the strength of brittle materials

Abstract

A model is developed for the strength degradation of brittle surfaces in sliding contact with spherical indenters. The loss of strength is associated with the propagation of partial cone cracks in the wake of the indenter. Detailed fracture mechanics calculations are circumvented by working in the limit of ideal point-load contacts, with the key proposition that the crack dimensions remain insensitive to rotations of the cone axis relative to the specimen free surface. In this way the simple Roesler solution for classical, well-developed cone cracks may be retained as a convenient “reference state” for a more general theoretical description, whereby the superposition of a tangential friction force onto the normal loading is accommodated via a straightforward coordinate transformation operation. The major predictions of the model are tested by measuring the strengths of glass surfaces after contact with a sliding steel sphere. In particular, the degraded strengths are not strongly influenced by the coefficient of friction, contrary to expectations from some earlier indentation analyses.