Fracture toughness and tensile strength are rock parameters that have influence on hydraulic fracture initiation and propagation, and that to various degrees are implemented in fracture modeling tools. Laboratory measurements represent a valuable source of such data. In particular, the relation between the two parameters is of interest, since both parameters are rarely measured with the same rocks. Here, experimental data obtained with weak sedimentary rocks (Castlegate sandstone, Mons chalk and Mancos shale) are presented, including unconfined Mode I fracture toughness and direct tensile strength. The angular dependence of both parameters is explored with the shale samples and compared to previously measured anisotropy of the Brazilian tensile strength. The new data are in accord with previous correlations between tensile strength and fracture toughness. The need for performing such measurement under stress conditions realistic for u fracturing is pointed out.

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