A model originally developed to characterize the extension and breakage of interatomic bonds at the tip of a propagating brittle crack is used to describe crack extension through a crystalline lattice by kink motion. Magnitudes of the effective kink barriers against crack extension and healing are computed as a function of lattice strain and are found to exhibit a marked asymmetry, relative to each other, in their strain dependences. In addition, decohesion effects associated with the presence of certain foreign atomic species are simulated, and it is shown that, for a broad range of relative bond-weakening, the kink barriers against both crack extension and healing are completely eliminated.