Piezoelectric wafer active sensors (PWAS) used in structural health monitoring (SHM) applications are able to detect structural damage using Lamb waves. PWAS are small, lightweight, unobtrusive and inexpensive. They achieve direct transduction between electric and elastic wave energies. PWAS are charge mode sensors and can be used as both transmitters and receivers. The focus of this paper is to find a suitable in situ piezoelectric active sensor for sending and receiving Lamb waves to be used in the SHM of structures with a curved surface. Current SHM technology uses brittle piezoceramic (PZT) wafer active sensors. Since piezoceramics are brittle, this approach could only be used on flat surfaces. The motivation of our research was to explore the use of flexible piezoelectric materials, e.g. piezoelastic polymers such as PVDF. However, PVDF stiffness is orders of magnitude lower than the PZT stiffness, and hence PVDF Lamb wave transmitters are much weaker than PZT transmitters. Thus, our research proceeded in two main directions: (a) to model and understand how piezoelectric material properties affect the behaviour of piezoelectric wafer active sensors; and (b) to perform experiments to test the capabilities of the flexible PVDF PWAS in comparison with those of stiffer but brittle PZT PWAS. We have shown that, with appropriate signal amplification, PVDF PWAS can perform the same Lamb wave transmission and reception functions currently performed by PZT PWAS. The experimental results of PZT-PWAS and PVDF-PWAS have been compared with a conventional strain gauge. The theoretical and experimental results in this study gave a basic demonstration of the piezoelectricity of PZT-PWAS and PVDF-PWAS.

Piezoelectric wafer active sensors are small, inexpensive, noninvasive, elastic wave generators/receptors that can be easily affixed to a structure. Piezoelectric wafer active sensor installation on the health-monitored structure is an important step that may have significant bearing on the success of the health monitoring process. The purpose of this paper is to explore the durability and survivability issues associated with various environmental conditions on piezoelectric wafer active sensors for structural health monitoring. The durability and survivability of the piezoelectric wafer active sensor transducers under various exposures (cryogenic and high temperature, temperature cycling, outdoor environment, operational fluids, large strains, fatigue load cycling) were considered over a long period of time. Both free piezoelectric wafer active sensors and bonded piezoelectric wafer active sensors on metallic structural substrates were used. Different adhesives and protective coatings were compared to find the candidate for piezoelectric wafer active sensor application in structural health monitoring. In most cases, piezoelectric wafer active sensors survived the tests successfully. The cases when piezoelectric wafer active sensors did not survive the tests were closely examined and possible causes of failure were discussed. The test results indicate that lead zirconate titanate piezoelectric wafer active sensors can be successfully used in a cryogenic environment; however, it does not seem to be a good candidate for high temperature. Repeated differential thermal expansion and extended environmental attacks can lead to piezoelectric wafer active sensor failure. This emphasizes the importance of achieving the proper design of the adhesive bond between the piezoelectric wafer active sensor and the structure, and of using a protective coating to minimize the ingression of adverse agents. The high-strain tests indicated that the piezoelectric wafer active sensors remained operational up to at least 3000 microstrain and failed beyond 6000 microstrain. In the fatigue cyclic loading, conducted up to 12 millions of cycles, the piezoelectric wafer active sensor transducers sustained at least as many fatigue cycles as the structural coupon specimens on which they were installed. State of the Art Piezoelectric wafer active sensors can send and receive ultrasonic Lamb waves and determine the presence of cracks, delaminations, disbands, and corrosion. In recent years investigators (Chang [1,2],

This paper presents an experimental and analytical study of irreversible change in piezoelectric wafer active sensor (PWAS) electromechanical (E/M) impedance and admittance signature under high temperature exposure. After elevated to high temperatures, change in the material properties of PWAS can be quantified through irreversible changes in its E/M impedance and admittance signature. For the experimental study, circular PWAS transducers were exposed to temperatures between 50°C and 250°C at 50°C intervals. E/M impedance and admittance data were obtained before and after each heating cycle. Irreversible temperature sensitivity of PWAS resonance and anti-resonance frequency was estimated as 0.0246 kHz°C −1 and 0.0327 kHz°C −1 respectively. PWAS transducer material properties relevant to impedance or admittance signature such as dielectric constant, dielectric loss factor, mechanical loss factor, and in plane piezoelectric constant were determined experimentally at room temperature before and after the elevated temperature tests. The in-plane piezoelectric coefficient was measured by using optical-fiber strain transducer system. It was found that the dielectric constant and in-plane piezoelectric coefficient increased linearly with temperature. Dielectric loss also increases with temperature but remains within 0.2% of initial room temperature value. Change in dielectric properties and piezoelectric constant may be explained by depinning of domains or by domain wall motion. The piezoelectric material degradation was investigated microstructurally and crystallographically by using scanning electron microscope and x-ray diffraction method respectively. There were no noticeable changes in microstructure, crystal structure, unit cell dimension, or symmetry. The degraded PWAS material properties were determined by matching impedance and admittance spectrums from experimental results with a closed form circular PWAS analytical model. Analytical results showed that impedance and admittance strongly depend on elastic coefficient, dielectric constant, mechanical loss factor, dielectric loss tangent and in plane piezoelectric constant. These properties were found to be susceptible to change after high temperature exposure.

Ferroelectric BaTiO 3 ͑BTO͒ thin films were deposited on NiO buffered polycrystalline Ni tapes by pulsed laser deposition. Microstructural studies by x-ray diffractometer and transmission electron microscopy reveal that the as-grown BTO films have the nanopillar structures with an average size of approximately 80 nm in diameter and the good interface structures with no interdiffusion or reaction. The dielectric and ferroelectric property measurements exhibit that the BTO films have a relatively large dielectric constant, a small dielectric loss, and an extremely large piezoelectric response with a symmetric hysteresis loop. These excellent properties indicate that the as-fabricated BTO films are promising for the development of the structural health monitoring systems.

A key longstanding objective of the Structural Health Monitoring (SHM) research community is to enable the embedment of SHM systems in high value assets like aircraft to provide on-demand damage detection and evaluation. As against traditional non-destructive inspection hardware, embedded SHM systems must be compact, lightweight, low-power and sufficiently robust to survive exposure to severe in-flight operating conditions. Typical Commercial-Off-The-Shelf (COTS) systems can be bulky, costly and are often inflexible in their configuration and/or scalability, which militates against in-service deployment. Advances in electronics have resulted in ever smaller, cheaper and more reliable components that facilitate the development of compact and robust embedded SHM systems, including for Acousto-Ultrasonics (AU), a guided plate-wave inspection modality that has attracted strong interest due mainly to its capacity to furnish wide-area diagnostic coverage with a relatively low sensor density. This article provides a detailed description of the development, testing and demonstration of a new AU interrogation system called the Acousto Ultrasonic Structural health monitoring Array Module+ (AUSAM+). This system provides independent actuation and sensing on four Piezoelectric Wafer Active Sensor (PWAS) elements with further sensing on four Positive Intrinsic Negative (PIN) photodiodes for intensity-based interrogation of Fiber Bragg Gratings (FBG). The paper details the development of a novel piezoelectric excitation amplifier, which, in conjunction with flexible acquisition-system architecture, seamlessly provides electromechanical impedance spectroscopy for PWAS diagnostics over the full instrument bandwidth of 50 KHz–5 MHz. The AUSAM+ functionality is accessed via a simple hardware object providing a myriad of custom software interfaces that can be adapted to suit the specific requirements of each individual application.