The use of piezoelectric wafer active sensors (PWAS) phased arrays for Lamb wave
damage detection in thin-wall structures is presented. The PWAS capability to
tune into specific Lamb-wave modes (which is an enabling factor for our
approach) is first reviewed. Then, a generic beamforming formulation that does
not require the conventional parallel-ray approximation is developed for
PWAS phased arrays in connection with the delay-and-sum beamforming
principles. This generic formulation is applied to a 1-D linear PWAS phased
array. Particularly, 1-D PWAS array beamforming reduces to the simplified
parallel ray algorithm when the parallel ray approximation is invoked. The
embedded ultrasonic structural radar (EUSR) algorithm is presented. A
couple of simple experiments are used to show that the linear EUSR PWAS
phased array system can successfully detect cracks in large aluminum thin
plates.
To improve the EUSR image quality, advanced signal processing is studied for
possible integration into the EUSR system. The approaches include Hilbert transform
for envelope detection, thresholding techniques for removing background noise,
discrete wavelet transform for denoising, continuous wavelet transform for
single frequency component extraction, and cross-correlation for time-of-flight
detection.
The optimization of linear PWAS arrays is studied next. First we consider the effect
of several parameters affecting the phased-array beamforming: (1) number of elements
; (2) elementary
spacing
; (3) steering
angle
; (4) location
of the target
.
Second, we examine the so-called nonuniform PWAS arrays which are generated by
assigning different excitation weights to each of the array elements. The design of two
nonuniform linear PWAS arrays, the binomial array and the Dolph–Chebyshev array,
is presented. Significant improvement of the EUSR image is observed when using
these nonuniform arrays.
Keywords
phased array, piezoelectric wafer , active sensors,
embedded ultrasonics structural radar, beamforming, Lamb
waves, damage detection, structural health monitoring,
EUSR, PWAS, SHM, NDE
