The Fracture boundary condition relies on the linear slip interface theory proposed by Schoenberg in
Ref. 2. The linear slip condition implements the concept of an imperfectly bonded interface between two elastic domains. Imperfect bonding means that:
where kA is the positive definite symmetric boundary stiffness matrix which has dimensions stress/length. For the velocity, which is the dependent variable in the velocity-strain formulation, the linear slip interface boundary condition reads