Impedance-based Structural Health Monitoring

1. Introduction The development of a real-time, in-service structural health monitoring and damage detection technique has recently attracted a large number of academic and industrial researchers. The goal of this research is to allow systems and structures to monitor their own integrity while in operation and throughout their lives in order to prevent catastrophic failures and to reduce the costs by minimizing explicit preemptory maintenance and inspection tasks. Impedance-based structural health monitoring techniques have been developed by utilizing the electromechanical coupling property of piezoelectric materials (Sun, et al. 1995) and form a new nondestructive evaluation (NDE) method. The basic concept of this approach is to monitor the variations in structural mechanical impedance caused by the presence of damage. Since structural mechanical impedance measurements are difficult to obtain, impedance methods utilize the electrical impedance of piezoelectric materials, which is directly related to the mechanical impedance of the host structure, and will be affected by the presence of structural damage. Through monitoring the measured electrical impedance and comparing it to a baseline measurement, one can qualitatively determine that structural damage has occurred or is imminent. In order to ensure high sensitivity to incipient damage, the electrical impedance is measured at high frequencies (typically greater than 30 kHz). At such high frequencies, the wavelength of the excitation is small and is sensitive enough to detect minor changes in the structural integrity. More importantly, high-frequency (kilohertz) signals require very low voltage (less than 1 volt at micro Watts) to produce a useful impedance excitation in the host structure. By integrating the impedance technique with self-sensing smart materials (Dosch et al. 1992), it has been demonstrated that the impedance-based method is suitable for use in a wide variety of structural health monitoring applications.