Journal Articles

A technical note on ultrasonic excitation is transmitted from a PWAS into a thin-wall structure through the adhesive layer and it is distribution into the Lamb-wave modes existing in the structure has been demonstrated. The piezoelectrically induced strain is constant along the PWAS. The total stress in the structure is a superposition of symmetric (axial) and antisymmetric (flexural) stresses that have constant and linear displacement distributions across the thickness. The interfacial shear stress concentrates about the PWAS edges as the bond layer becomes thinner or stiffer. A closed-form solution for the case of a symmetric and an antisymmetric nonlinear mode and two generic guided-wave modes has also been developed. A closed-form solution for parameter Α and observed parameter is no longer constant, but varies with the frequency-thickness product.

PWAS are small and non-invasive transducers that can be easily affixed to a structure to interrogate it at will. Their wideband non-resonant properties make them advantageous candidates for multiple modes structural health monitoring (SHM) and nondestructive evaluation (NDE) simply by changing the driving frequencies. This paper presents research results obtained by using PWAS transducer for SHM/NDE. The results are given in the following directions: (1) PWAS operation principles; (2) PWAS electromechanical impedance measurements; (3) PWAS ultrasonic SHM/NDE; and (4) PWAS multi-mode corrosion detection. The analytical methods and experimental results can be successfully used in the design of active SHM systems to be embedded on operational structures, such as aircraft panels.

Biomedical sensors can yield new information about the microenvironment of soft tissues around the devices. In the current study, wound healing in response to the placement of piezoelectric wafer active sensors (PWAS) was compared between control animals and those fed a diet with a COX-2 inhibitor over 5 months. Animals fed the experimental diet had statistically less dampening of the PWAS wave form over the entire study period compared with those fed standard chow. This finding implied a less viscous wound exudate in animals that have COX-2 inhibition. Histologically, animals fed the COX-2-inhibited diet displayed delayed maturation of the wound-healing response. However, at 5 months there was little difference histologically between the 2 groups. The use of PWAS sensors provides a new tool for investigating the physical properties of wound healing around soft tissue implants.

We report the fabrication of the orientation preferred structures in BaTiO 3 thin films on Ni substrates using pulsed laser deposition. Transmission electron microscopy studies showed that the films consist of crystalline structures of tetragonal BaTiO 3. More than 60% of BaTiO 3 grains in the films exhibit nearly the same crystallographic orientation with their a-axis lying in the film plane and the (011) direction parallel to the growth direction. Such orientation preferred structures were grown on a Ni nanocrystalline buffer layer. This result demonstrated the possibility of approximating an oriented single crystalline ceramic oxide structures on metallic substrates.

Ferroelectric BaTiO3 thin films were fabricated directly on various metallic substrate materials such as Ni and Ti by using the pulsed laser for the development of structural health monitoring systems and energy harvest applications. Microstructure studies from x-ray diffraction and electron microscopy indicate that the as-grown BaTiO3 thin films have pure BaTiO3 crystal phase. The TEM studies indicate that the BaTiO3 films are composed of crystalline assemblage of nanopillars with average cross sections from 100 nm to 200 nm and can be directly integrated on the Ni tapes without the formation of NiO interlayer suggesting that this system can be developed for super-capacitor devices. The BaTiO3films have good interface structures and strong adhesion with respect to Ni and Ti substrates. Dielectric measurements have shown the hysteresis loop at room temperature in the film with a large remanent polarization, indicating that the ferroelectric domains have been created in the as-deposited BTO films. The successful integration of ferroelectric thin films directly on metallic materials has promised for the development of the structural health monitoring systems and energy harvest devices.

This paper presented development work of an in situ method for damage detection in thin-wall structures using embedded two-dimensional ultrasonic phased arrays. Piezoelectric wafer active sensors were used to generate and receive guided Lamb waves propagating in the plate-like structure. The development of a generic beamforming algorithm that does not require parallel ray assumption through using full wave propagation paths is described. A virtual beam steering method and device, the embedded ultrasonic structural radar, was implemented as a signal post-processing procedure. Several two-dimensional configurations were investigated and compared with beamforming simulation. Finally, rectangular shape arrays were developed for verifying the generic formulas and omnidirectionality. The rectangular arrays yield good directionality within the 360° full range and are able to detect damage anywhere in the entire plate.

In situ structural health monitoring aims to perform on-demand interrogation of the structure to determine the presence of service-induced damage and defects using non-destructive evaluation ultrasonic wave methods. Recently emerged piezoelectric wafer active sensors (PWAS) have the potential to significantly improve damage detection and health monitoring. PWAS are low-profile transducers that can be permanently attached onto the structure or inserted in between composite laminates, and can perform structural damage detection in thin-wall structures using guided wave methods (Lamb, Rayleigh, SH, etc.). This paper describes the analytical and experimental work of using PWAS-guided waves for in situ structural damage detection on thin-wall structures. We begin with reviewing the guided wave theory in plate structures and PWAS principles. The mechanisms of Lamb wave excitation and detection using PWAS is presented. Subsequently, we address in turn the use of PWAS to generate Lamb waves for damage (cracks and corrosion) detection in metallic structures. Pulse-echo, pitch-catch, phased array and time reversal methods are illustrated demonstrating that PWAS Lamb-waves techniques are suitable for damage detection and structural health monitoring. The last part of the paper treats analytically and experimentally PWAS excitation and tuning in composite materials. The research results presented in this paper show that in situ SHM methodologies using PWAS transducers hold the promise for more efficient, effective and timely damage detection in thin-wall structures.

Ultrasonic guided waves inspection using Lamb waves is suitable for damage detection in metallic structures. This paper will present experimental results obtained using guided Lamb waves to detect flaws in aluminum specimens with design features applicable to space applications. Two aluminum panels were fabricated from a variable-thickness aluminum top plate, with two bolted I-beams edge stiffeners and four bonded angle stiffeners. Artificial damages were introduced in the two panels: cracks, corrosions, and disbonds. The proposed investigation methods used bonded piezoelectric wafer active sensors to excite and receive Lamb waves. Three wave propagation methods were used: pitch–catch, pulse–echo, and the embedded ultrasonic structural radar. In addition, we also used a standing-wave damage detection technique, the electromechanical impedance method. The paper will present in detail the salient results from using these methods for damage detection and structural health monitoring. Where appropriate, comparison between different methods in detecting the same damage will be performed. The results have demonstrated the ability of piezoelectric wafer active sensors working in conjunction with guided Lamb waves to detect various types of damages present in complex geometry structures typical of space applications.