Experimental, numerical and analytical studies on tensile strength of rocks

Abstract

The difficulties associated with performing a direct uniaxial tensile test on a rock specimen have led to a number of indirect methods for assessing the tensile strength. This study compares experimental results of direct and indirect tensile tests carried out on three rock types: Brisbane tuff, granite and sandstone. The standard Brazilian indirect tensile test caused catastrophic crushing failure of the disk specimens, due to the stress concentration produced by the line loading applied and exacerbated by the brittleness of the rock tested, rather than the expected tensile splitting failure initiated by a central crack. This finding led to an investigation of the effect of loading conditions on the failure of Brazilian disk specimens using three steel loading arcs of different angle applied to three different rock types, using numerical modeling and analytical results. Numerical modeling studies were also performed to investigate the effect of a pre-existing crack on the stress distribution within Brazilian disk specimens. It was found that there is substantially higher tensile stress concentration at the center of the disk with a pre-existing crack compared with that for a disk without a pre-existing crack. The maximum stress intensity factor (fracture toughness) values at the tip of the central pre-existing cracks were determined from numerical modeling and compared with fracture toughness values obtained experimentally for the three rock types. It was concluded that a 20° loading arc gives the best estimate of the indirect tensile strength.