Cracking of rocks and rock-like materials exhibits a rich variety of patterns where tensile (mode I) and shear (mode II) fractures are often interwoven. These mixed-mode fractures are usually cohesive (quasi-brittle) and frictional. Although phase-field modeling is increasingly used for rock fracture simulation, no phase-field formulation is available for cohesive and frictional mixed-mode fracture. To address this shortfall, here we develop a double-phase-field formulation that employs two different phase fields to describe cohesive tensile fracture and frictional shear fracture individually. The formulation rigorously combines the two phase fields through three approaches: (i) crack-direction-based decomposition of the strain energy into the tensile, shear, and pure compression parts, (ii) contact-dependent calculation of the potential energy, and (iii) energy-based determination of the dominant fracturing mode in each contact condition. We validate the proposed model, both qualitatively and quantitatively, with experimental data on mixed-mode fracture in rocks. The validation results demonstrate that the double-phase-field model -- a combination of two quasi-brittle phase-field models -- allows one to directly use material strengths measured from experiments, unlike brittle phase-field models for mixed-mode fracture in rocks. Another standout feature of the double-phase-field model is that it can simulate, and naturally distinguish between, tensile and shear fractures without complex algorithms.
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