Reliability quantification is a critical and necessary process for the evaluation and assessment of any inspection technology that may be classified either as a nondestructive evaluation or structural health monitoring technique. Based on the sensitivity characterization of nondestructive evaluation techniques, appropriate processes have been developed and established for the reliability quantification of their performance with respect to damage/flaw detection in materials or structures. However, in the case of structural health monitoring methods, no such well-defined and general applicable approaches have been established for neither active nor passive sensing techniques that allow for their accurate reliability quantification. The objective of this study is to characterize the sensitivity of active-sensing acousto-ultrasound-based structural health monitoring techniques with respect to damage detection, as well as to identify the parameters that influence their sensitivity. With such an understanding, it is believed that adequate quantitative methods could then be established to enable the practical use of acousto-ultrasound structural health monitoring methods in the aerospace and mechanical engineering communities. In order to evaluate the sensitivity of a pre-selected active-sensing acousto-ultrasound structural health monitoring system, both numerical simulations and experiments were performed on 30 aluminum coupons each outfitted with a pair of lead zirconate titanate piezoelectric sensors/actuators. A damage index versus damage size relationship was investigated numerically and experimentally to assess the applicability of the traditional nondestructive evaluation linear regression framework for probability of detection for an active-sensing structural health monitoring system. The results of the study show that the position of each sensor–actuator pair with respect to a known damage location and the damage growth pattern are the two most critical parameters influencing the reliability of the same structural health monitoring system applied to identical structural components under the same environmental conditions.

A near-field magnetic-dipole probe suitable for noncontact and nondestructive imaging of metals is described and the effects of resonator coupling strength, operation frequency, and the probe wire tip geometry on the conductivity resolution of the probe are experimentally determined. Using a simplified circuit model of the resonator, we were able to interpret the system’s output and predict the magnitude of reflected wave and relate it to the properties of the samples under investigation. Thus, the probe was calibrated to perform quantitative conductivity measurements with the ability to detect metal nonuniformities with 1% accuracy and 5×10−3σ and 2×10−2σ conductivity resolutions at 2GHz operation frequency for both the critical and over-coupling probes, respectively. We also discussed the calibration results of probes with different coupling strengths over a 0.91Ω∕square resistive sample. The calibration results of a critical-coupled resonator probe were also compared with a microstrip transmission line probe. It was observed that the resonator probe has 100 times higher conductivity resolution than that of the transmission line probe. Furthermore, we characterized and compared the calibration results of probes with tip wires of different diameters. Images obtained by an evanescent microwave probe are presented.