  While cracks in isotropic homogeneous materials propagate straight,
perpendicularly to the tensile axis, cracks in natural and synthetic composites
deflect from a straight path, often increasing the toughness of the material.
Here we combine experiments and simulations to identify materials properties
that predict whether cracks propagate straight or kink on a macroscale larger
than the composite microstructure. Those properties include the anisotropy of
the fracture energy, which we vary several folds by increasing the volume
fraction of orientationally ordered alumina platelets inside a polymer matrix,
and a microstructure-dependent process zone size that is found to modulate the
additional stabilizing or destabilizing effect of the non-singular stress
acting parallel to the crack. Those properties predict the existence of an
anisotropy threshold for crack kinking and explain the surprisingly strong
dependence of this threshold on sample geometry and load distribution.
