A beneficial feature in guided-wave SHM systems is represented by their additional capability for acoustic data communications. Here, information about the structural integrity might be transferred between sensor nodes across the monitored mechanical waveguide itself without the need for cabling. The transferred information can be given e.g. by a numerical damage indicator which is crucially needed for the diagnostic capability inherent to the respective SHM system. In such application scenarios, the installed piezoelectric transceivers transmit encoded bit sequences which are subsequently reconstructed at the receiving piezoelectric transceiver. This combined inspection and communication approach has been recently presented in a metallic plate, as well as the effectiveness of communication involving an orthotropic composite plate has also been analyzed. The present work extends recent studies demonstrating the effective deployment of elastic guided waves (GWs) for multiple-in and multiple-out (MIMO) data transmission in the framework of an SHM application. Customized and miniaturized low-power communication nodes have been developed for this purpose. They are positioned in a spatially distributed and permanently installed network. Cable-free exchange of encoded information across a stiffened carbon-fiber reinforced plastics (CFRP) panel is studied. A combination of square-wave excitation sequences and frequency-division multiplexing (FDM) is explored for simultaneous communication with multiple nodes aiming at energy-aware application scenarios.
In this work, a small footprint, low power, and light weight sensor node for guided wave detection on laminate composite and metallic structures is presented. This device is meant as a basic building block for smart structure passive sensor networks development. It draws power from a two-wires data-over-power (DoP) network communication interface, which is also used for half-duplex data handling at 200kbps. Each node is roughly 20x24mm, consumes less than 40mW, and weights less than 20 grams, making it attractive for aerospace systems where size, power and weight reduction are crucial. Elastic waves generated from impacts and propagating on the structure are recorded by an innovative, patent-pending, dual-element piezoelectric transducer and processed by an embedded low-voltage 8-bit PIC. A 1Mbit SPI serial SRAM is used for data storage while program instruction are stored in the PIC embedded 7 KB ash. A low-voltage, high-speed, half-duplex RS485 transceiver with an internal, programmable termination resistance is used to interface the PIC to the bus through a filtering mesh of passive components. This mesh also connects to a low-dropout voltage regulator, allowing it to draw power from the DoP bus without interfering with data transmission. A separate gateway device has also been developed: it is capable to simultaneously interface and feed the DoP bus by drawing power either from the USB or from an external power supply. A network counting up to 256 nodes can be implemented and interfaced to a PC for real-time impact detection applications.
In this work, a novel piezoelectric sensor for guided waves detection on laminate composite and metallic structures is presented. The sensor is composed by two electrodes (E1 and E2) on the top surface of the device, plus a common electrode (EC) on the bottom surface, which is bonded to the structure to be inspected. E1 has a circular shape, whereas E2 is shaped as a segment of a logarithmic spiral (or spira mirabilis). Because of this asymmetric shaping, the wavefront of a generic acoustic event (e.g. the one generated by an impact) hits the electrodes in two points whose distance D varies with the Direction of Arrival (DoA) of the wave itself. With a dedicated processing procedure, the information about the distance D first, and then about the DoA can be retrieved from the waveforms acquired and digitized at the two electrodes E1 and E2. In particular, the procedure computes the cross-correlation of the dispersion compensated signals, and extracts the distance D by looking at the position of the maximum of the cross-correlation envelope. Here, a first experimental test is performed to validate the effectiveness of the proposed technology.
This paper illustrates a Human-Machine Interface based on Augmented Reality (AR) conceived to provide to
maintenance operators the results of an impact detection methodology. In particular, the implemented tool
dynamically interacts with a head portable visualization device allowing the inspector to see the estimated
impact position on the structure. The impact detection methodology combines the signals collected by a network
of piezosensors bonded on the structure to be monitored. Then a signal processing algorithm is applied to
compensate for dispersion the acquired guided waves. The compensated waveforms yield to a robust estimation
of guided waves difference in distance of propagation (DDOP), used to feed hyperbolic algorithms for impact
location determination. The output of the impact methodology is passed to an AR visualization technology that
is meant to support the inspector during the on-field inspection/diagnosis as well as the maintenance operations.
The inspector, in fact, can see interactively in real time the impact data directly on the surface of the structure.
Here the proposed approach is tested on the engine cowling of a Cessna 150 general aviation airplane. Preliminary
results confirm the feasibility of the method and its exploitability in maintenance practice.
Delamination faults in composite plates are considered dangerous as they can cause catastrophic failure before being visually assessed. Effects of delaminations are particularly relevant in guided waves scattering, local resonances and mode conversion. Detecting and analyzing these phenomena is relevant for plate characterization. In this work, leaky guided waves are used to detect delamination in composite plates. To such purpose, a hybrid ultrasonic set-up and a dedicated signal processing are proposed. An air-probe with a proper lift-off is used to detect the leakage in terms of air pressure wave over the plate surface. A piezoelectric transducer is used to generate acoustic guided waves in the composite plate. Multiple acquisitions are averaged to increase the SNR for each position of the air-probe. Curvelet Transform (CT) domain processing of the projection coefficients of the acquired elastic wave is exploited to decompose waves that are overlapped both in the time/space and in the frequency/wavenumber domain. In fact, CT is a special member of the family of multiscale and multidimensional transforms whose spatial and temporal localization is very well suited for processing signals which are sparse in the above mentioned domains. In this work this sparsity is exploited to emphasize the information of leaky guided waves scattered by the delamination by removing from the data the information related to the incident wave field. As an application, the presence of a delamination generated by a 21 Joule impact performed on a 4.9 mm thickness composite laminate was detected contactless by exploiting guided wave leakage.
In this work a pulse-echo procedure suitable to locate defect-induced reflections in irregular waveguides is proposed.
In particular, the procedure extracts the distance of propagation of a guided wave scattered from a defect
within the echo signal, revealing thus the source-defect distance. To such purpose, first, a Warped Frequency
Transform (WFT) is used to compensate the signal from the dispersion of the guided wave due to the traveled
distance in a portion of the waveguide that is assumed as reference. Next, a pulse compression procedure is
applied to remove the additional dispersion introduced by the remaining irregular portion of the waveguide.
Thanks to this processing the actual distance traveled by the wave in the regular portion of the irregular
waveguide is revealed. Thus the proposed strategy extends pulse-echo defect localization procedures based on
guided waves to irregular waveguides. Since the processing is based on Fast Fourier Transforms, the algorithm can
be easily implemented in real time applications for structural health monitoring. The potential of the procedure
is numerically demonstrated by processing Lamb waves propagating in an irregular waveguide composed by
aluminum plates with different thicknesses and tapered portions.