  We experimentally demonstrate the feasibility of visualizing stress waves
propagating in plates using air-coupled acoustic emission sensors.
Specifically, we employ a device that embeds arrays of microphones around an
optical lens in a helical pattern. By implementing a beamforming technique,
this remote sensing system allows us to record wave propagation events in situ
via a single-shot and full-field measurement. This is a significant improvement
over the conventional wave propagation tracking approaches based on laser
doppler vibrometry or digital image correlation techniques. In this paper, we
focus on demonstrating the feasibility and efficacy of this air-coupled
acoustic emission technique using large metallic plates exposed to external
impacts. The visualization results of stress wave propagation will be shown
under various impact scenarios. Such wave visualization capability is of
tremendous importance from a structural health monitoring and nondestructive
evaluation (SHM/NDE) standpoint. The proposed technique can be used to
characterize and localize damage by detecting the attenuation, reflection, and
scattering of stress waves that occurs at damage locations. This can ultimately
lead to the development of new SHM/NDE methods for identifying hidden cracks or
delaminations in metallic or composite plate structures simultaneously negating
the need for mounted contact sensors.
