Tensile Crack Speed in Brittle Rocks

An important issue in rapid brittle fracture is the limiting speed of crack propagation. It is widely believed that brittle mode I crack cannot propagate faster than the Rayleigh wave speed, or the speed of sound on a solid surface. Mode II cracks are also limited by longitudinal speed wave. The origin for this belief stems from the predictions of continuum mechanics. Once the crack speed reaches a theoretical upper limit in a material, which is most often larger than one fifth of the Rayleigh wave velocity, branching of a propagating crack occurs. To verify this hypothesis, this paper presents the results of an experimental program aimed at disclosing the size effect on the crack velocity in the Splitting Tensile Strength indirect test (i.e. the Brazilian Test) using high-speed photography techniques. Over 100 Brazilian tests with more than 10 different rock types at various diameters were prepared and tested according to the ASTM standard recommendations using either a servo hydraulic machine or an electromechanical load frame at a wide ranges of load/displacement rates. By adopting a high frame rate of above 100,000 frames per second (fps), crack initiation, propagation, and coalescence were captured to study the size effect on the crack speed and failure mode on the Brazilian test results.