Fracturing at contact surfaces subjected to normal and tangential loads

Abstract

Abstract A variety of rock engineering problems including drilling, cutting, abrasion, and milling involve rock tool contact and indentation. The pattern of indentation fractures and the role of slip conditions, surface roughness, tool radius and initial flaw size for an arbitrarily loaded contact are not fully known. The present paper aims to identify the elastic stress field for a contact subjected to both normal and tangential loads, and evaluate the condition for the fracture initiation and propagation. Stress fields within two spheres at contact are available when either only normal load is applied or when tangential load causes full-slip conditions. It is shown here that through appropriate superposition of the above two solutions, the stress field under partial-slip conditions, as well as during the unloading of tangential force may be determined. Maximum tensile stress increases significantly under partial-slip conditions as compared to the full-slip case even though the same magnitude of tangential force is applied. The location of maximum tensile stress moves inward from the trailing edge as the tangential force is unloaded. The stress-intensity factors for a penny-shaped crack which initiates at the contact periphery, and follows the minimum principal stress trajectory are obtained and utilized to study indentation fracturing. The dependency of critical loads on initial flaw size, indenter radius and slip conditions is quantified. The predictions of fracture density and spacing under a sliding indenter are achieved through a simple estimate of shielding interaction between adjacent fractures. Relation of these evenly spaced fractures with the formation of wear grooves on sliding surfaces is discussed.