Journal Articles

A novel sensor for in situ monitoring of the body reaction to implants has been developed. A piezoelectric wafer active sensor was adapted for biomedical applications (bio-PWAS). A number of bio-PWAS sensors have been implanted in rats and left in place up to 64 days. The bio-PWAS were able to oscillate in several resonance modes, radial-wise (in-plane) and thickness-wise (out-of-plane). The electromechanical impedance was measured over a wide frequency band, covering several radial vibration modes and the first thickness mode. The recorded data was processed with impedance spectroscopy methods. Preliminary results indicate a correlation between the electromechanical impedance spectrum of the bio-PWAS and the state of implantation. Quantitative studies have shown that the first radial mode amplitude seems to correlate with the short-time inflammatory and immune response, while the thickness mode amplitude seems to correlate with both the short-term inflammatory response and long-term encapsulation and fibrosis response. Since radial vibrations generate shear waves in the surrounding tissue, while thickness vibrations generate pressure waves, it seems that the shear and pressure wave interactions have specificity in detecting the different stages of the body s reaction response to implants. These observations were supported by histological examinations of the tissue surrounding the bio-PWAS. Though these initial results are encouraging, further experiments need to be conducted and more data needs to be collected in correlation with histological determinations. In-depth impedance spectroscopy studies should be conducted on this extensive data and closely correlated with extensive histological studies.

Piezoelectric wafer active sensors can be applied to aging aircraft structures to monitor the onset and progress of structural damage such as fatigue cracks and corrosion. Two main detection strategies are considered: (a) the wave propagation method for far-field damage detection; and (b) the electro-mechanical (E/M) impedance method for near-field damage detection. These methods are developed and verified on simple-geometry specimens, and then tested on realistic aging-aircraft panels with seeded cracks and corrosion. The specimens instrumentation with piezoelectric-wafer active sensors and ancillary apparatus is presented. The experimental methods, signal processing, and damage detection algorithms, tuned to the specific method used for structural interrogation, are discussed. In the wave propagation approach, the pulse-echo and acousto-ultrasonic methods were considered. Reflections from seeded cracks were successfully recorded. In addition, acoustic emission and low-velocity impact were also detected. In the E/M impedance method approach, the high-frequency spectrum is processed using overall-statistics damage metrics. The (1-R2 ) 3 damage metric, where R is the correlation coefficient, was found to yield the best results. The simultaneous use of the E/M impedance method in the near field and of the wave propagation method in the far field opens the way for a comprehensive multifunctional damage detection system for aging aircraft structural health monitoring.

High-field theoretical and experimental analysis of piezoelectric and magneto-strictive actuators is presented. First, the analysis of a piezoelectric stack actuator (PiezoSystems Jena PAHL 120/20) is described. A theoretical model based on the linear theory of piezoelectricity is developed. Extensive experiments were conducted, aimed at low-frequency dynamic electro-mechanical behavior characterization. Curve fitting procedures are used to adjust the model coefficients for various load levels. Through comparison with experimental data, the model is adjusted to include nonlinear terms related to higher losses on the unloading cycle. Second, the impedance analysis of a magnetostrictive actuator (Etrema AA140J025) is described. Linear piezomagnetism is assumed, as an approximation to nonlinear magnetostrictive behavior about a bias point. Low-field and high-field impedance measurements were performed, revealing left shifting of the actuator resonance as the power is increased. Model tuning of the impedance model on the experimental data showed material parameters trends similar with those reported in the literature. Although the numerical values developed during this phenomenological study are particular for the actuators under consideration, the characterization approach can be extended to analysis of other actuators of this type.