  Shear cracks propagation is a basic dynamical process that mediates
interfacial failure. We develop a general weakly nonlinear elastic theory of
shear cracks and show that these experience tensile-mode crack tip deformation,
including possibly opening displacements, in agreement with Stephenson's
prediction. We quantify this nonlinear symmetry breaking effect, under
two-dimensional deformation conditions, by an explicit inequality in terms of
the first and second order elastic constants in the quasi-static regime and
semi-analytic calculations in the fully dynamic regime. Our general results are
applied to various materials. Finally, we discuss available works in the
literature and note the potential relevance of elastic nonlinearities for
frictional cracks.
