  We propose a full 3D benchmark problem for brittle fracture based on
experiments as well as a validation in the context of phase-field models. The
example consists of a series of four-point bending tests on graphite specimens
with sharp V-notches at different inclination angles. This simple setup leads
to a mixed mode (I + II + III) loading which results in complex yet stably
reproducible crack surfaces. The proposed problem is well suited for
benchmarking numerical methods for brittle fracture and allows for a
quantitative comparison of failure loads and propagation paths as well as
initiation angles and the fracture surface. For evaluation of the crack
surfaces image-based 3D models of the fractured specimen are provided along
with experimental and numerical results. In addition, measured failure loads
and computed load-displacement curves are given. To demonstrate the
applicability of the benchmark problem, we show that for a phase-field model
based on the Finite Cell Method and multi-level hp-refinement the complex crack
surface as well as the failure loads can be well reproduced.
