  In this contribution, a novel framework for simulating mixed-mode failure in
rock is presented. Based on a hybrid phase-field model for mixed-mode fracture,
separate phase-field variables are introduced for tensile (mode I) and shear
(mode II) fracture. The resulting three-field problem features separate length
scale parameters for mode I and mode II cracks. In contrast to the classic
two-field mixed-mode approaches it can thus account for different tensile and
shear strength of rock. The two phase-field equations are implicitly coupled
through the degradation of the material in the elastic equation, and the three
fields are solved using a staggered iteration scheme. For its validation, the
three-field model is calibrated for two types of rock, Solnhofen Limestone and
Pfraundorfer Dolostone. To this end, double-edge notched Brazilian disk (DNBD)
tests are performed to determine the mode II fracture toughness. The numerical
results demonstrate that the proposed phase-field model is able to reproduce
the different crack patterns observed in the DNBD tests. A final example of a
uniaxial compression test on a rare drill core demonstrates, that the proposed
model is able to capture complex, 3D mixed-mode crack patterns when calibrated
with the correct mode I and mode II fracture toughness.
