Fracture toughness interpretation from breakdown pressure

One of the tests used to assess the possible dependence of the fracture toughness on stress relies on injecting fluid into a slotted borehole within a cylindrical rock sample subject to confining pressure and interpreting the toughness from the observed breakdown pressure. There is a wealth of experimental evidence that the so-measured toughness indeed increases with the confining stress. However, the interpretation of the observed breakdown...