G. Zumpano

Optimal sensor placement on a large-scale civil structure

Health monitoring systems set up to assure the safe operation of structures require linking sensors with computational tools able to interpret sensor data in terms of structural performance. Although intensive development continues on innovative sensor systems, there is still considerable uncertainty in deciding on the number of sensors required and their location in order to obtain adequate information on structural behavior. This paper considers the problem of locating sensors on a bridge structure with the aim of maximizing the data information so that structural dynamic behavior can be fully characterised. Four different optimal sensor placement techniques, two based on the maximisation of the Fisher Information Matrix and two on energetic approaches, have been investigated. Mode shape displacements are taken as the measured data set and two comparison criteria were employed. The first was based on the mean square error between the FE model and the cubic spline interpolated mode shapes. The second criterion measured the information content of each sensor location to investigate on the strength of the acquired signals and their ability to withstand the noise pollution keeping intact the information relative to the structure properties. The results highlight that the Effective Independence Driving-Point Residue (EFI-DPR) method provides an effective method for optimal sensor placement to identify vibration characteristics of the studied bridge.

Corrosion identification on an aluminium plate-like structure by monitoring the wave propagation phenomena

In order to reduce the problems related to the detection of corrosion damage in aircraft structures, it is vital to develop new robust, accurate and reliable damage detection methods. A possible answer to this problem is offered by an evolution of the Non linear Elastic Wave Spectroscopy (NEWS), a wave propagation methodology developed for geophysics applications. The NEWS damage detection based technique consists in the analysis of the frequency spectrum generated by a bi-harmonic signal. In pristine condition, the signal spectrum presents two picks at the excitation frequencies. In presence of damage, the material starts to behave non-linearly around the damage location, and this behaviour shows up in the bi-harmonic excited signal spectrum as side bands and harmonics of the excited frequencies. The magnitude and the number of the side bands and harmonics are related to damage size and magnitude. In this study, numerical findings on a welded aluminium plate-like structure are reported in order to understand the sensitivity of the NEWS technique to detect corrosion damages. The results showed that the proposed methodology appear to be highly sensitive to the presence of corrosion damage, but the methodology has yet to be developed and applied to aircraft structures and much work is needed to demonstrate the effectiveness of the methods and the ease-ofimplementation in a structural health monitoring system. 1 Phone: +441234750111 ext.5220 Fax: +441234752149 email: m.meo@cranfield.ac.uk

Identification of Nottingham Wilford Bridge modal parameters using wavelet transforms

Studies have been conducted on the identification of modal parameters including natural frequencies, damping coefficients and mode shapes of the Nottingham Wilford Bridge using ambient excitation. An approach to estimate modal parameters, from only output data (in the time domain) using the wavelet transform, is presented. Displacements responses of this structural system are used in the wavelet transform to identify its dynamic characteristics. To measure real-time displacement of the bridge a global positioning system (GPS) sensor network was designed and installed on the bridge. The modal properties were extracted using a two-step methodology. In the first step, the random decrement method was used to transform random signals in free vibration responses. Finally, the Eigensystem Realization Algorithm and a Wavelets based technique were used to extract natural frequencies and to determine the mode shapes of the structure.

A new damage detection technique for rails based on wave propagation

Rails maintenance is becoming a critical issue because of the increase of the traffic and the train speed, which amplify the risk of catastrophic failures. A methodology, based on wave propagation theory, aimed at detecting damage in rails is presented in this paper. The damage detection algorithm is based on the assumption that the rails behave as a wave guides and stress waves may travel alongside the rail length without being reflected unless they meet discontinuities (damages). The Wave Propagation Based Damage Detection (WPBDD) algorithm proposed is a two steps technique. In the first step the travel time of a perturbation wave, generated by a train passage, from its arrival to the sensor locations to the discontinuity and back to the sensor, is measured by a Time Correlation Function (TCF) evaluated using the wavelet transform. The second algorithm locates the damage sites using a Ray-Tracing (RT) algorithm. The WPDDD algorithm has been designed to use indifferently either body waves (P and S waves) or surface waves (Rayleigh waves). The technique proposed aimed at the identification of single and multi-site rolling contact fatigue damages was tested on a numerical test case.