Experimental characterization of periodic frequency-steerable arrays for structural health monitoring

Beam steering through phased arrays is a well-established technique, used extensively in ultrasonic imaging for medical, NDE and SHM applications. Phased arrays typically need individual control of their elements, which involves hardware and software complexity. This paper presents the characterization of a novel frequency-steerable array for structural health monitoring. In the considered configuration, beam steering is achieved by exploiting interference phenomena generated by the spatial lay-out of the array elements, and their simultaneous activation at specific frequencies. Such frequencies correspond to wavenumbers which are associated with radiation in determined spatial directions. In essence, the array acts as a spatial filter, which preferentially radiates at wavenumbers defined by the spatial arrangement of the elements. As such, the array is also effective at tuning its radiation to specific wave modes. In this paper, a simple quadrilateral periodic topology illustrates the directional properties of the array and shows its tuning capabilities. The investigations are supported by a preliminary numerical analysis, which is used to design an experimental prototype. Tests successfully validate the numerical predictions and demonstrate the directional and tuning capabilities of the proposed array design.