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Abstract: We study a model of interface crack in which contact of the crack faces obeys a Coulomb law of friction. For such cracks, the possibility that the stress has a stronger singularity than $r^{-\honehalf}$ near the tip has been reported. In this paper, we demonstrate that these strong singularities can in fact be discarded, because they would suppose a backward propagation of the crack. In passing, we prove that near-tip slip is possible in one direction only, which is imposed by the sign of the elastic mismatch. The locking of the stress intensity factor during a non-monotonic cycle of loading is pointed out, as well as the formation of a bubble near the tip under certain loading conditions.