TABLE OF CONTENTS This chapter has dealt with the use of PWAS transducers to achieve in-situ phased arrays for active structural health monitoring. The chapter started with a review of the phased array concepts used in conventional NDE practice. Particular attention was paid to the phased arrays based on the guided-wave principles. After this general review of the state of the art, the chapter started with the introduction of PWAS phased array principles. The simplest PWAS phased-array, the linear array, was treated first, and the concept of embedded ultrasonics structural radar (EUSR) was presented. The transmitter beamforming, receiver beamforming, and phased-array pulse-echo concepts were developed from first principles. The practical implementation of the EUSR algorithm, and experimental validation and calibration of this method were presented. The chapter continued with an extensive experimental studies of the linear PWAS arrays, such as EUSR detection studies performed on various defect types, detection experiments on curved panel, and in-situ direct imaging of crack growth during a fatigue cyclic loading experiment.
The optimization of PWAS phased array beamforming was studied in detail. The effect of r/d and d/ ? ratios, of the number of elements M , and of the steering angle 
0 was examined. The case of nonuniform PWAS phased arrays, in which the contribution of each element is individually weighted, was discussed. The binomial and the Dolph Chebyshev arrays were studied in comparison with an uniform array, and tradeoffs between directivity and azimuthal coverage were...
