Determination of the Fracture Toughness of Rock Materials Using the Modified Hoop Test

ABSTRACT: Hoop test was originally introduced as a potentially useful method for measuring tensile strength of rock materials. In the hoop test, a tensile load is applied through a pair of half-cylindrical platens against the inner surface of a thick hollow cylinder of rock specimen. Failure of the specimen is achieved by separation of the loading platens, and the tensile strength of rock material is determined. The location where the tensile fracturing starts, and the direction of the crack propagation is predetermined in the hoop test. With initial notches on both sides of the inner surface, perpendicular to the loading direction, fracture toughness of the rock specimen can be calculated from the maximum load and the specimen geometry. An experimental program was undertaken to determine the fracture toughness of aggregative materials by the modified hoop test. The crack-tip stress intensity factor was numerically calculated using a fit of the nodal displacements in the vicinity of the initial crack, and an equation for the mode I fracture toughness was derived.