Detecting anomalies in beams and plate based on the Hilbert-Huang transform of real signals

This paper illustrates the feasibility of the Hilbert-Huang transform (HHT) as a signal processing tool for locating an anomaly, in the form of a crack, delamination, stiffness loss or boundary in beams and plate, based on physically acquired propagating wave signals. The basis of detection hinges on simple wave propagation concepts using flight times, velocities and frequency changes. The basic concept of HHT is first presented where the empirical mode decomposition must be applied on the signal using a sifting process to obtain intrinsic mode functions before the Hilbert spectral analysis can be meaningfully performed. Some implementation issues are discussed, such as end effects and the criterion to terminate the sifting process, and an alternative criterion is proposed and implemented using MATLAB V6.1. Four examples are used to illustrate the suitability of the technique, namely an aluminum beam with a crack, a sandwiched aluminum beam with an internal delamination, a reinforced concrete (RC) slab with different degrees of damage and a plate with distorted input signal. The results indicate that HHT is able to represent a localized event well and is sensitive to slight distortion in the signal. Crack and delamination in homogeneous beams can be located accurately and damage in a RC slab can be identified if it has been previously loaded beyond first crack. However, the sensitivity of HHT is such that analysis with a distorted signal needs careful interpretation, as illustrated by the aluminum plate example.