Crack kinking from an initially closed crack

Crack kinking in elastic solids in two-dimensional situations is studied in the case where the crack is initially closed (without friction) due to compressive forces but kinking opens it. The ®rst problem which arises, namely the determination of the stress expansion near the crack tip prior to kinking has been studied previously showing that because of contact between the crack lips, the classical singular mode I term of this expansion is replaced by another, bounded one involving a new non-singular stress corresponding to a uniform compressive stress perpendicular to the crack lips. From there, one derives universal formulae for the ®rst two terms of the expansion of the stress intensity factors at the tip of the open, extended crack in powers of the crack extension length. Combining the formulae found with the principle of local symmetry one can then determine ®rst the kink angle, which is found to always amount to precisely277.38, the sign being opposite to that of the initial mode II stress intensity factor, and second the initial curvature of the crack extension, which is found to depend upon both initial non-singular stresses. The problem of whether or not, after the initial kink, the crack tends to come back to its original direction is ®nally investigated