We investigate the use of impediographic tomography to achieve high sensitivity and high resolution damage identification in plate-like structures. The impediographic approach exploits the coupled piezo-resistive and electrostatic response of the host structure to generate high sensitivity and high resolution maps of its internal electrical conductivity. Focused acoustic waves are used to generate localized electrical conductivity perturbations that allow a drastic improvement in the conditioning of the inverse problem. The localized acoustic perturbations are obtained by exploiting the concept of Frequency Selective Structures (FSS) in which intentional mistuning of periodically distributed structural features, such as thin notches, enables self-focusing and vibration localization by using a single ultrasonic transducer. The impediographic reconstruction is achieved by using two different methods: the 0-Laplacian and the Levenberg-Marquardt. Both methodologies are compared in terms of accuracy of the reconstructed electrical conductivity and of their ability to deal with important practical issues such as limited view and limited perturbation data. Numerical results show that, although both approaches perform well in terms of damage identification, localization, and sizing, the LM technique allows higher flexibility in handling imperfect data.
In this paper, we investigate the use of dynamic structural tailoring via the concept of an Acoustic Black Hole (ABH) to enhance the performance of piezoelectric based energy harvesting from operational mechanical vibrations. The ABH is a variable thickness structural feature that can be embedded in the host structure allowing a smooth reduction of the phase velocity while minimizing the amplitude of reflected waves. The ABH thickness variation is typically designed according to power-law profiles. As a propagating wave enters the ABH, it is progressively slowed down while its wavelength is compressed. This effect results in structural areas with high energy density that can be exploited effectively for energy harvesting. The potential of ABH for energy harvesting is shown via a numerical study based on fully coupled finite element electromechanical models of an ABH tapered plate with surface mounted piezo-transducers. The performances of the novel design are evaluated by direct comparison with a non-tapered structure in terms of energy ratios and attenuation indices. Results show that the tailored structural design allows a drastic increase in the harvested energy both for steady state and transient excitation. Performance dependencies of key design parameters are also investigated.
Due to the heterogeneous nature of the cement-based materials, the ultrasonic waves in concrete exhibit highly scattering and attenuation, leading to the difficulty of concrete damaged detection. This paper presents a dual mode ultrasonic array imaging methodology that can map damage using Rayleigh surface waves and permanently installed piezoelectric sensors. The dual mode sensing integrates passive acoustic emission and active ultrasonic wave inspection. When a crack is developing, acoustic emission (AE) occurs and the disturbance can propagate outwards along the structure surface. A novel AE source imaging algorithm has been developed to detect and locate the AE source. Once the AE source is located, the sensor array switches to its active mode. For active sensing, one sensor in the array is used to generate Rayleigh wave for interrogation, while all the others are used as the wave receivers. All the sensory data are processed by the active ultrasonic array imaging algorithm. The proof-of-concept testing was performed on a grout specimen with representative dimensions. The passive array imaging algorithm was able to locate the AE source simulated by pencil lead break while active sensing imaging was able to detect the damage simulated by a hole. The duel mode imaging method is promising and economically beneficial for solving a key source localization problem in damage detection on large concrete structures.
Monitoring of fatigue cracking in bridges using a combined passive and active scheme has
been approached by the authors. Passive Acoustic Emission (AE) monitoring has shown to be able to
detect crack growth behavior by picking up the stress waves resulting from the breathing of cracks while
active ultrasonic pulsing can quantitatively assess structural integrity by sensing out an interrogating
pulse and receive the structural reflections from the discontinuity. In this paper, we present a
comparative study of active and passive sensing with two types of transducers: (a) AE transducers, and
(b) embeddable piezoelectric wafer active sensors (PWAS). The study was performed experimentally on
steel plates. Both pristine and damaged (notched) conditions were considered. For active sensing, pitchcatch
configuration was examined in which one transducer was the transmitter and another transducer
acted as the receiver. The ping signal was generated by the AE hardware/software package AEwin. For
passive sensing, 0.5-mm lead breaks were executed both on top and on the edge of the plate. The
comparative nature of the study was achieved by having the AE and PWAS transducers placed on the
same location but on the opposite sides of the plate. The paper presents the main findings of this study
in terms of (a) signal strength; (b) signal-to-noise (S/N) ratio; (c) waveform clarity; (d) waveform
Fourier spectrum contents and bandwidth; (e) capability to detect and localize AE source; (f) capability
to detect and localize damage. The paper performs a critical discussion of the two sensing
methodologies, conventional AE transducers vs. PWAS transducers.