Mode and frequency control always plays an important role in ultrasonic guided wave applications. In this paper,
theoretical understanding of guided wave excitations of axisymmetric sources on plate structures is established. It is
shown that a wave number spectrum can be used to investigate the guided wave excitations of an axisymmetric source.
The wave number spectrum is calculated from a Hankel transform of the axial source loading profile. On the basis of
the theoretical understanding, phased annular array transducers are developed as a powerful tool for guided wave mode
and frequency control. By applying appropriate time delays to phase the multiple elements of an annular array
transducer, guided wave mode and frequency tuning can be achieved fully electronically. The phased annular array
transducers have been successfully used for various applications. Example applications presented in this paper include
phased annular arrays for guided wave beamforming and a novel ultrasonic vibration modal analysis technique for
damage detection.
KEYWORDS: Digital signal processing, Sensors, Signal processing, Tomography, Corrosion, Structural health monitoring, Analog electronics, Wireless communications, Data acquisition, Clocks
Structural Integrity of predetermined critical zones in a structure is of growing interest in the non-destructive
testing (NDT) and structural health monitoring (SHM) communities. Quite often the presence
of defects does not imply the end of life of the underlying structure and it could be economical to continue
using the structure until the damage severity reaches a point where it can no longer be used. For structures
like pipelines and aircraft in which a failure can be catastrophic, it is extremely important to monitor
continuously any defects on the structure. Leave in place sensors provide a convenient way to embed the
sensors permanently on the structure to monitor periodically and to establish its integrity. Wireless sensing
units provide a robust means to regularly monitor a structure and return the data to a central data collection
infrastructure. In this study we explore the design of a wireless tomographic imaging system that uses
Lamb wave propagation characteristics on a structure to map accurately the material loss zones due to corrosion in the area enclosed by the sensors. The wireless unit has an actuator to excite the piezoceramic sensors and the computational capability to interrogate the signals in real time and to communicate the information. The capability of the system is illustrated by testing it on an aluminum plate with simulated corrosion damage and the results are presented.
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