EMD and instantaneous phase detection of structural damage
In this chapter, a new structural health-monitoring and damage-detection method is presented. A general time-frequency data analysis technique (empirical mode decomposition and the Hilbert-Huang spectrum) in conjunction with a wave-mechanics-based concept is developed to provide a diagnostic tool for detecting and interpreting adverse changes in a structure. Sets of simple basis function components, known as intrinsic mode functions (IMF), are extracted adaptively from the measured structural response time series data. These IMFs are amplitude- and phase-modulated signals and are used to define the instantaneous phases of structural waves. The state of a structure is evaluated, and damage is identified based on these instantaneous phase features. Furthermore, fundamental relationships are developed connecting the instantaneous phases to a local physics-based structural representation in order to infer damage in terms of physical parameters, such as structural mass, stiffness, and damping. Damage-detection applications are investigated by using numerical simulations and a variety of laboratory experiments with simple structures. Several different types of excitation mechanisms are used for dynamic input to the structures. The time series output of the structural response is then analyzed by using the new method. The instantaneous phase relationships are extracted and examined for changes which may have occurred due to damage. These results are compared to those from other newly developed detection methods, such as an algorithm based on the geometric properties of a chaotic attractor. The studies presented here show that our method, without linear-system or stationary-process assumptions, can identify and locate structural damage and permit the further development of a reliable real-time structural health-monitoring and damage-detection system.
