An established principle for detecting cracks in structures is to monitor high frequency acoustic emissions generated by micro-cracking.  In the case of simple uniform plates then the ultrasonic waves that propagate from a crack to a sensor are straightforward to understand using Rayleigh-Lamb theory.  However, for built-up structures such as an aircraft wing the wave types responsible for propagating disturbances are numerous, complicated and travel at differing velocities.

In this paper the Wave Finite Element method (WFE) is applied to predict the free wave propagation characteristics of an I-beam like structure which is adopted to resemble a wing spar.  The resulting wave modes and dispersion curves are presented.  These are discussed in the context of issues relating to crack detection such as sensor placement, wave attenuation and estimation of time of flight.  Extensions of the model to account for cross section non-uniformity and plate attachments are proposed.