  Fracture mechanics is crucial for many fields of engineering, as precisely
predicting failure of structures and parts is required for efficient designs.
The simulation of failure processes is, from a mechanical and a numerical point
of view, challenging, especially for inhomogeneous materials, where the
microstructure influences crack initiation and propagation and leads to complex
crack patterns. The phase field method for fracture is a promising approach to
encounter such materials since it is able to describe complex fracture
phenomena like crack kinking, branching and coalescence. Moreover, it is a
largely mesh independent approach, given that the mesh is homogenous around the
crack. However, the broadly used formulation of the phase field method is
limited to isotropic materials and does not account for preferable fracture
planes defined through the material's microstructure. In this work, the method
is expanded to take orthotropic constitutive behavior and preferable directions
of crack propagation into account. We show that by using a stress-based split
and multiple phase field variables with preferable fracture planes, in
combination with a hybrid phase field approach, a general framework can be
found for simulating anisotropic, inhomogeneous materials. The stress-based
split is based on fictitious crack faces and is, herein, expanded to
anisotropic materials. Furthermore, a novel hybrid approach is used, where the
degradation of the sound material is performed based on a smooth traction free
crack boundary condition, which proves to be the main driving factor for
recovering observed crack patterns. This is shown by means of a detailed
analysis of two examples: a wooden single edge notched plate and a wood board
with a single knot and complex fiber directions. In both cases, the proposed
novel hybrid phase field approach can realistically reproduce complex failure
modes.
