Lamb wave based Structural health monitoring (SHM techniques are widely used for real-time damage detection in critical structural components. For Lamb-based SHM, piezoelectric lead zirconate titanate (PZT) transducers are employed for actuation and sensing. The choice to use a SHM system as a maintenance approach for a structure is directly associated with the incremental cost and reliability of the system in comparison to traditional schedule-based maintenance. The installed sensors are typically unprotected from the aircraft's operational environment, rendering PZT sensor failure a significant risk. The malfunction of any PZT sensor will diminish diagnostic reliability, leading to missed detections and false alarms, thereby undermining system performance. So, it is necessary that the sensors withstand the operating environmental conditions and that there is a provision for easy replacement of the faulty transducer. In recent years, inkjet printing, particularly drop-on-demand (DoD) inkjet printing, has been utilized as an efficient and economical method for the manufacturing of electronic devices. Based on current technologies industry-scale transducer layers are available, like the integrated SHM layer and the SMART Layer. Studies have shown that the insulating coating of wires is mainly based on polymers. The polyamide, known as Kapton, was found suitable as a coating material because of its good bonding properties to composites and its good electrical insulation. Recent studies have also shown the use of polyethylene terephthalate foil (PET foil), thermoplastic polyurethane foil (TPU foil), as support materials for flexible sensors. However, there is no study available on the suitability of these materials as support materials for piezoelectric-based SHM systems for ultrasonic-based damage detection

The present work aims to study the suitability of polyethylene terephthalate (PET) foil, thermoplastic polyurethane (TPU) foil, and Polypropylene (PP) laminated nonwoven fabric as support materials for PZT-based flexible sensors for SHM applications. Specifically, the aim is to find the effect of these materials on Lamb wave propagation in composite structures. Kapton is also used as one support material for comparison. The experimental setup consists of five glass fiber-reinforced polymers (GFRP). Each plate was surface mounted with four DuraAct-made piezoelectric transducers. Experimental studies are conducted at room temperature (20 °C), low temperature (-20 °C), and high-temperature (60 °C) environments to simulate actual operating environments of the SHM system. Lamb wave generation propagation and sensing experiments are conducted without the support materials and also after pasting the support materials. The wave field analysis is also conducted using the plate without support material and with support material. The study reveals that the use of TPU foil and PP laminated nonwoven fabric as support material has a very low effect on the Lamb wave propagation in the structure in comparison to the Kapton and PET foil. The presence of Kapton and PET also significantly influences the Lamb wave propagation in varying thermal environments. This makes TPU foil, and PP laminated nonwoven fabric two suitable candidates as support materials for flexible PZT-based sensor systems for Lamb wave-based SHM applications.

Piezoelectric (PZT) transducers are widely used in structural health monitoring (SHM) and nondestructive evaluation (NDE) for ultrasonic wave generation and sensing. However, their electromechanical coupling, especially when embedded in concrete, is often assumed ideal, with little attention to how the input electrical waveform translates into mechanical output. This assumption can lead to misinterpretation of advanced ultrasonic techniques, such as Coda Wave Interferometry, if source distortions are not accounted for.

In this study, we experimentally characterize wave generation in free and embedded PZT transducers, focusing on the effects of excitation waveform, input voltage, and boundary conditions. The drive voltage of the PZT transducer is monitored via an oscilloscope, and surface displacement is measured using a laser Doppler vibrometer (LDV). Tests included harmonic sine waves, Gaussian-modulated signals, chirps, and broadband pulses across varying voltages and frequencies, applied to transducers in air and embedded in concrete.

Our results reveal that the applied electrical input and mechanical output deviate from the nominal input signal in both configurations, but with distinct patterns. Increasing voltage consistently introduces waveform distortion and spectral redistribution. In the air, these changes are irregular and less predictable, with variable harmonic content and transient behavior. When embedded in concrete, distortions follow a more systematic trend: dominant frequency downshifts, selective emergence or suppression of spectral peaks, and altered damping linked to boundary conditions (sensor boundary) and impedance changes. Oscilloscope and impedance measurements confirm that transducer–host interaction significantly modifies the electromechanical transfer function, introducing amplitude-dependent nonlinearities that cannot be inferred from the generator signal alone.

These findings underscore the need to explicitly characterize embedded transducer behavior before interpreting ultrasonic wavefields in SHM and NDE. Ignoring source distortions risks conflating material changes with transducer-induced effects. By combining electrical and optical measurements, the proposed methodology offers a framework for understanding wave generation in concrete and highlights that the output of embedded PZT depends strongly on excitation parameters and host conditions. Future work will extend this approach to different transducer geometries and concrete compositions to develop practical guidelines for experiment design and data interpretation.

Abstract. Flexible composite marine propeller blades can have the potential for reduced greenhouse gas emissions as well as a reduction in underwater radiated noise. Unfortunately, their application in the real world is hindered, among others, by unknowns in the fatigue behaviour of these blades.

This research assesses the structural health measurements of an experimental campaign wherein a glass-fibre reinforced plastic marine propeller blade was tested under a fatigue load (Fig. 1). In this paper acoustic emissions were investigated that were recorded underwater by hydrophones that were placed close by the blade. This includes signal processing based on time-difference of arrival and frequency content. The results give an insight into the fatigue behaviour of the blade, alongside considerations regarding placement of hydrophones and comparison to other methods of structural health monitoring such as the use of embedded piezoelectric sensors and CT-scanning.

Keywords: Acoustic Emission, Hydrophone, Composite marine propeller, Fatigue

Fig. 1. Assessment of a composite marine propeller blade. The left picture visualises CT-scanning of the blade. In the middle a photograph is shown of the blade in the fatigue set-up, including the hydrophones mounted underneath the blade. The right picture shows the full experimental setup.

The ability to monitor variations in seismic or acoustic wave velocity is of major interest in many applications like Structural Health Monitoring (SHM), near surface geophysics, the security in an underground context or oil industry.

The use of buried sensors is particularly suited to this goal, as it provides information about the internal properties of the probed medium. Fiber Optic Sensors (FOS) offer some advantages in buried contexts, such as their low intrusiveness (due to their small diameter) and their immunity to electromagnetic interferences. In particular, Fiber Bragg Gratings (FBGs) sensors can be used to measure dynamic strain fields caused by mechanical waves. The primary information that can be gathered from these measurements is the travel time of a P-wave between a source and a buried receiver.

Here we attempt to measure P-wave travel time variations in order to image a local change in the mechanical properties of the probed medium. A differential inversion approach is used to convert travel time differences into an image of the relative velocity variations.

To validate this method, a combination of numerical studies and laboratory experiments is used. To this end, a polyurethane resin mock-up is made, in which FBGs sensors are embedded and used as ultrasonic sensors. Then, the local changes in P-wave velocity between a reference state and two disturbed states of the mock-up is map, based on the inversion of P-wave travel time differences. The results of differential inversions demonstrate that the imaging method developed can be used to monitor a local P-wave velocity contrast using embedded FBGs as ultrasonic sensors.

Structural Health Monitoring (SHM) techniques based on wave propagation demands high performance interrogators with high bandwidth, high accuracy, and, simultaneously, low weight, size and price. This combination of challenging requirements for traditional off-chip solutions motivates the development of integrated photonic interrogators, which can deliver high performance in a compact, scalable and cost-effective platform.

It is proposed a photonic-integrated-circuit (PIC) interrogator tailored for Fiber Bragg Grating (FBG) for SHM requirements, as following: (i) interrogation bandwidth (kHz–MHz), (ii) accuracy and sensitivity at least of 0.01-pm wavelength-shift resolution (≈ few-to-10 nε), and (iii) capable of multiplexing several FBG sensors on a single fiber. The architecture uses edge-filter demodulation: FBG-reflected power is converted to intensity by a designed on-chip spectral transfer function whose local slope around each operating point sets the sensitivity, while the linear span of that slope sets dynamic range. We formalize the intrinsic trade-off between steep slope (sensitivity) and broad linearity (range).

For that purpose, a modular synthesis approach cascades and/or parallels basic building blocks—Mach Zender interferometers (MZI), ring resonators, and Bragg sections—and uses numerical optimization (fitting, correlation-based objectives) over path-length differences, coupling coefficients, resonance orders, and grating periods. While this approach increases die area, it automates re-targeting to different FBGs and simplifies tolerance management at the component level. We analyse sensitivity, linear-range and outline multiplexed operation to distinct FBG channels for WDM-style simultaneous readout.

Attention is also devoted to the impact of temperature variations and fabrication tolerances on the spectral response of PIC-based interrogators, with focus on interferometric or resonant filtering structures. The thermal dependence and fabrication robustness of these architectures is analysed, highlighting its impact on interrogation stability and accuracy. Based on this analysis, different design-level strategies to reduce thermal sensitivity and widen fabrication tolerances will be discussed, providing solutions towards integrated FBG interrogation systems and minimizing the needs for complex isolation/active temperature control.

Finally, experimental measurements of Lamb waves in composite plates with hybrid system FBG/PZT will be presented, in order to compare with traditional PZT based SHM systems. The test campaign will take into account relevant performance metrics for SHM applications such as wavelength-shift resolution, sensitivity, usable linear range, dynamic strain measurement bandwidth, and robustness to thermal drift will be discussed.