The controlled failure of rock discs and rings loaded in diametral compression

Abstract

The diametral compression of discs (the Brazilian test) and of rings induces a zone of tensile stress in the specimens. If, as the load is increased, failure occurs in the tensile zone, then these tests can be regarded as indirect methods of measuring the tensile strength. Control of specimen failure, from initial loading to final zero load-bearing capability was achieved with a closed-loop servo-controlled testing machine. In order to determine if failure initiated in the zone of tensile stress, specimens of granite, Solenhofen limestone and Tennessee marble were unloaded just after the peak of the force-lateral displacement curves and visually examined for damage. Some scanning electron photomicrographs of a crack are included. In the Brazilian test, it was found that failure always initiated directly under the loading points if flat steel platens only were used to load the specimens. For ring tests with a small hole, failure initiated at the loading points for flat steel platen loading and at the boundary of the hole on the loaded diameter when a load distributing device was employed. Neither the Brazilian test nor the ring test is recommended as a method of measuring the material property ‘tensile strength’.