  Finite element calculations of dynamic fracture based on embedding cohesive
surfaces in a continuum indicate that the predictions are sensitive to the
cohesive law used. Simulations were performed on a square block in plane strain
with an initial edge crack loaded at a constant rate of strain. Cohesive laws
that have an initial elastic response were observed to produce spontaneous
branching at high velocity, but to modify the linear elastic properties of the
body. As a consequence the cohesive surface spacing cannot be refined
arbitrarily and becomes an important length scale in the simulations. Cohesive
laws that are initially rigid do not alter the linear elastic response of the
body. However, crack branching behavior was not observed when such a cohesive
relation was implemented using a regular finite element mesh.
