  When a thin film moderately adherent to a substrate is subjected to residual
stress, the cooperation between fracture and delamination leads to unusual
fracture patterns, such as spirals, alleys of crescents and various types of
strips, all characterized by a robust characteristic length scale. We focus on
the propagation of a duo of cracks: two fractures in the film connected by a
delamination front and progressively detaching a strip. We show experimentally
that the system selects an equilibrium width on the order of 25 times the
thickness of the coating and independent of both fracture and adhesion
energies. We investigate numerically the selection of the width and the
condition for propagation by considering Griffith's criterion and the principle
of local symmetry. In addition, we propose a simplified model based on the
criterion of maximum of energy release rate, which provides insights of the
physical mechanisms leading to these regular patterns, and predicts the effect
of material properties on the selected width of the detaching strip.
