This paper presents current research on impedance-based structural health monitoring technique. The basic principle behind this technique is to apply high-frequency structural excitations (typically higher than 30 kHz) through the surface-bonded piezoelectric transducers, and measure the impedance of structures by monitoring the current and voltage applied to the piezoelectric transducers. Changes in impedance indicate changes in the structure, which in turn can indicate that damage has occurred. Three examples, including a bolted joint, gas pipeline, and composite structure, are presented to illustrate the effectiveness of this health monitoring technique to a wide variety of practical field applications. Although many proof-of-concept experiments have been performed using the impedance methods, the impedance-measuring device (HP4194A) is still bulky and expensive. Therefore, an operational amplifier-based turnkey device that can measure and record the electric impedance of a PZT has been developed. The performance of this miniaturized and portable device has been compared to our previous results and its effectiveness has been demonstrated. This paper summarizes the experimental setup, procedures, and considerations needed to implement the device in field applications.

An investigation of self-sensing and self-repairing bolted joints is presented. The goal of the self-sensing and self-repairing joint is to reduce the likelihood of failure due to self-loosening and to reduce the cost of maintenance of critical bolted joints. The concept combines piezoelectric-based health-monitoring techniques with shape memory alloy (SMA) actuators to restore tension in a loose bolt. The objective of this study is to enhance the practicality of the self-healing bolted joint. One of the primary issues is the actuation of the SMA actuators. The relatively large mass of the shape memory washer and low resistance because of the washer’s short length make resistive heating particularly difficult. A series of models was developed to assess the viability of resistive heating and provide an estimate for the power requirements for effective actuation. Modeling and experimental testing have shown that an external heater can be used to actuate a shape memory alloy actuator with conventional power sources. By making the shape memory alloy washer substantially easier to actuate, this method provides a convenient alternative to resistive heating, and aids the practical implementation of the concept of self-sensing, self-repairing joints.

A model is generally not needed for the basic damage identification problem when using the electromechanical impedance-based method of structural health monitoring (SHM). However, modeling becomes necessary when more information is needed for more complex functions of the SHM system, such as estimation of remaining life. In addition, suitable models would aid in more accurately identifying and locating damage and in designing the SHM system. Since impedance-based SHM relies on high frequency excitation of the structure using piezoelectric patches, finite element modeling may not be computationally efficient. In this study, the spectral element method (SEM) is used in combination with electric circuit analysis for impedance modeling. SEM more accurately models higher frequency vibrations than finite element methods since the mass is modeled exactly and it incorporates higher order models more easily. Simulations of sensor multiplexing, high frequency response, and the inclusion of damage are presented. Experimental verification is also included.

Impedance-based structural health monitoring uses collocated piezoelectric transducers to locally excite a structure at high frequencies. The response of the structure is measured by the same transducer. Changes in this response indicate damage. Frequency range selection for monitoring with impedance-based structural health monitoring has, in the past, been done by trial and error methods or has been selected after analysis by engineers familiar with the method. This study aims to determine if, in future applications, it is possible to automatically select preferred frequency ranges based on sensor characteristics, perhaps even before installing the system. In addition, the paper demonstrates a method for determining preferable frequency ranges for monitoring. The study examines the analysis of the measurement change through a damage metric and relates the results of the analysis to characteristics of the measurement. Specifically, outlier detection concepts were used to statistically evaluate the damage detection ability of the transducers at various frequency ranges. The variation in undamaged measurements is compared to the amount of change in the measurement upon various levels of damage. Testing was performed with both solid piezoceramic transducers and macrofiber composite piezoelectric devices of different sizes bonded to aluminum and fiber reinforced composite structures. The results indicate that characteristics of the structure, not the sensor alone, determine the optimal monitoring frequency ranges.