A careful examination of a dynamic mode I crack problem leads to the conclusion
that the commonly used boundary conditions do not always hold in the case of an
applied crack face loading, so that a modification is required to satisfy the equations.
In particular, a transient compressive stress wave travels along the crack faces,
moving outward from the loading region on the crack face. This does not occur in the
quasistatic or steady state problems, and is a special feature of the transient
dynamic problem that is important during the time interval immediately
following the application of crack face loading. We demonstrate why the usual
boundary conditions lead to a prediction of crack face interpenetration,
and then examine how to modify the boundary condition for a semi-infinite
crack with a cohesive zone. Numerical simulations illustrate the resulting
approach.