  Understanding the role played by the microstructure of materials on their
macroscopic failure properties is an important challenge in solid mechanics.
Indeed, when a crack propagates at a heterogeneous brittle interface, the front
is trapped by tougher regions and deforms. This pinning induces non-linearities
in the crack propagation problem, even within Linear Elastic Fracture Mechanics
theory, and modifies the overall failure properties of the material. For
example crack front pinning by tougher places could increase the fracture
resistance of multilayer structures, with interesting technological
applications. Analytical perturbation approaches, based on Bueckner-Rice
elastic line models, focus on the crack front perturbations, hence allow for a
description of these phenomena. Here, they are applied to experiments
investigating the propagation of a purely interfacial crack in a simple
toughness pattern: a single defect strip surrounded by homogeneous interface.
We show that by taking into account the finite size of the body, quantitative
agreement with experimental and finite elements results is achieved. In
particular this method allows to predict the toughness contrast, i.e. the
toughness difference between the single defect strip and its homogeneous
surrounding medium. This opens the way to a more accurate use of the
perturbation method to study more disordered heterogeneous materials, where the
finite elements method is less adequate. From our results, we also propose a
simple method to determine the adhesion energy of tough interfaces by measuring
the crack front deformation induced by known interface patterns.
