  In earlier papers (Leblond et.al., 2011, 2019), we presented linear stability
analyses of the coplanar propagation of a crack loaded in mixed-mode I+III,
based on a "double'' propagation criterion combining Griffith (1920)'s
energetic condition and Goldstein and Salganik (1974)'s principle of local
symmetry. The difference between the two papers was that in the more recent
one, the local value of the critical energy-release-rate was no longer
considered as a constant, but heuristically allowed to depend upon the ratio of
the local mode III to mode I stress intensity factors. This led to a much
improved, qualitatively acceptable agreement of theory and experiments, for the
"threshold'' value of the ratio of the unperturbed mode III to mode I stress
intensity factors, above which coplanar propagation becomes unstable. In this
paper, the analysis is extended to the case where a small additional mode II
loading component is present in the initially planar configuration of the
crack, generating a small, general kink of this crack from the moment it is
applied. The main new effect resulting from presence of such a loading
component is that the instability modes present above the threshold must drift
along the crack front during its propagation. This prediction may be useful for
future theoretical interpretations of a number of experiments where such a
drifting motion was indeed observed.
