Abstract
In this paper we review the state of the art in an emerging new technology: embedded ultrasonic non-destruc-tive evaluation (NDE). Embedded ultrasonic NDE permits active structural health monitoring, i.e. the on-demand inter-rogation of the structure to determine its current state of structural health. The enabling element of embedded ultra-sonic NDE is the piezoelectric wafer active sensor (PWAS). We begin by reviewing the guided wave theory in plate, tube, and shell structures, with special attention to Lamb waves. The mechanisms of Lamb wave excitation and detection with embeddable PWAS transducers is presented. It is shown analytically and verified experimentally that Lamb wave mode tuning can be achieved by the judicious combination of PWAS dimensions, frequency value, and Lamb mode characteris-tics. Subsequently, we address in turn the use of pitch-catch, pulse-echo, and phased array ultrasonic methods for Lamb-wave damage detection. In each case, the conventional ultra-sonic NDE results are contrasted with embedded NDE results. Detection of cracks, disbonds, delaminations, and dif-fuse damage in metallic and composite structures are exem-plified. Other techniques, such as the time reversal method and the migration technique, are also presented. The paper ends with conclusions and suggestions for further work.