The two-dimensional elastodynamic problem of a semi-infinite plate containing an edge crack is considered. A normal velocity is suddenly imposed on the boundary of the plate on one side of the edge crack. A combination of transient mode I and mode II deformation fields is induced near the crack tip. The corresponding stress intensity factor histories are determined exactly by linear superposition of several more readily obtainable stress wave propagation solutions. The stress intensity factor histories are determined for the time interval from initial loading until the first wave scattered at the crack tip is reflected at the plate edge and returns to the crack tip