  Fracture growth in a material is strongly influenced by the presence of
inhomogeneities, which deviate crack trajectories from rectilinearity and
deeply affect failure. Increasing crack tortuosity is connected to enhancement
of fracture toughness, while often a crack may even be stopped when it impinges
a void, which releases the stress concentration. Therefore, the determination
of crack trajectories is important in the design against failure of materials
and mechanical pieces. The recently developed phase-field approach (AT1 and AT2
models), based on a variational approach to damage localization, is believed to
be particularly suited to describe complex crack trajectories. This belief is
examined through a comparison between simulations and photoelastic experiments
on PMMA plates, which have been designed in a new way, to highlight the effects
of notches and circular holes on fracture propagation. The latter is shown to
initiate from a notch and to be strongly attracted by voids. When a void is
hit, fracture is arrested, unless the void contains a notch on its internal
surface, from which a new crack nucleates and propagates. Different mechanical
models are tested where fracture initiates and grows (i.) under Mode I compact
tension, (ii.) four-point bending and (iii.) a tensile stress indirectly
generated during compression of samples containing a circular hole. The
experiments show that the fracture propagation may be designed to develop in
different tortuous paths, involving multiple arrests and secondary nucleation.
Simulations performed with an ad hoc implemented version of the AT1 and AT2
phase-field methods (equipped with spectral decomposition, in which a crack is
simulated as a highly localized zone of damage accumulation) are shown to be in
close agreement with experiments and therefore confirm the validity of the
approach and its potentialities for mechanical design.
