  The paper addresses the problem of a Mode III interfacial crack advancing
quasi-statically in a heterogeneous composite material, that is a two-phase
material containing elastic inclusions, both soft and stiff, and defects, such
as microcracks, rigid line inclusions and voids. It is assumed that the bonding
between dissimilar elastic materials is weak so that the interface is a
preferential path for the crack. The perturbation analysis is made possible by
means of the fundamental solutions (symmetric and skew-symmetric weight
functions) derived in Piccolroaz et al. (2009). We derive the dipole matrices
of the defects in question and use the corresponding dipole fields to evaluate
effective tractions along the crack faces and interface to describe the
interaction between the main interfacial crack and the defects. For a stable
propagation of the crack, the perturbation of the stress intensity factor
induced by the defects is then balanced by the elongation of the crack along
the interface, thus giving an explicit asymptotic formula for the calculation
of the crack advance. The method is general and applicable to interfacial
cracks with general distributed loading on the crack faces, taking into account
possible asymmetry in the boundary conditions. The analytical results are used
to analyse the shielding and amplification effects of various types of defects
in different configurations. Numerical computations based on the explicit
analytical formulae allows for the analysis of crack propagation and arrest.
