  In this work, we describe our contribution to the Purdue-SANDIA-LLNL
\emph{Damage Mechanics Challenge}. The phase field fracture model is adopted to
blindly estimate the failure characteristics of the challenge test, an
unconventional three-point bending experiment on an additively manufactured
rock resembling a type of gypsum. The model is formulated in a variationally
consistent fashion, incorporating a volumetric-deviatoric strain energy
decomposition, and the numerical implementation adopts a monolithic
unconditionally stable solution scheme. Our focus is on providing an efficient
and simple yet rigorous approach capable of delivering accurate predictions
based solely on physical parameters. Model inputs are Young's modulus $E$,
Poisson's ratio $\nu$, toughness $G_c$ and strength $\sigma_c$ (as determined
by the choice of phase field length scale $\ell$). We show that a single mode I
three-point bending test is sufficient to calibrate the model, and that the
calibrated model can then reliably predict the force versus displacement
responses, crack paths and surface crack morphologies of more intricate
three-point bending experiments that are inherently mixed-mode. Importantly,
our peak load, crack trajectory and crack surface morphology predictions for
the challenge test, submitted before the experimental data was released, show a
remarkable agreement with experiments. The characteristics of the challenge,
and how changes in these can impact the predictive abilities of phase field
fracture models, are also discussed.
