A simple model is developed for the calculation of dynamic stress intensity factors for lengthwise cracked beams subjected to impact or transient loading. The model is based on a Global/Local approach that separates the Global structural dynamics from the Local crack tip zone dominated by singular stresses. The Global model is that of connected waveguides while the Local model is based on a novel application of the J-integral that converts dynamic structural resultants directly into strain energy release rate. The accuracy of this approach is assessed by comparing it to a fully two-dimensional finite element analysis in which the modified crack closure integral is used to calculate the dynamic strain energy release rate. Both mode I and mode II examples are given, and situations with multiple wave reflections are emphasized.