INSTITUTE OF PHYSICS PUBLISHING SMART MATERIALS AND STRUCTURES Smart Mater. Struct. 10 (2001) 518–527 www.iop.org/Journals/sm PII: S0964-1726(01)22801-8 Vibration-based damage detection in civil engineering: excitation sources and temperature effects Bart Peeters 1 , Johan Maeck and Guido De Roeck Department of Civil Engineering 2 , K U Leuven, W. de Croylaan 2, B-3001 Heverlee, Belgium E-mail: bart.peeters@lms.be Received 15 September 2000 Abstract This paper discusses two very relevant practical issues in the application of vibration-based health monitoring to civil engineering structures: the excitation source and the effect of temperature. The idea of vibration-based damage detection is to measure dynamic characteristics such as eigenfrequencies, damping ratios and mode shapes on a regular basis. The state, and eventually degradation, of the structure is reflected in the evolution of these characteristics. Unfortunately, it is not only the health of a structure that influences its measurable dynamics, but also the applied excitation and the changing temperature are important factors and may erode the damage detection potential. In the first part, the results of different excitation types are compared: band-limited noise generated by shakers, an impact from a drop weight and ambient sources such as wind and traffic. In the second part, the undeniable effect of temperature on measured eigenfrequencies is demonstrated and a methodology is proposed to distinguish these temperature effects from real damage events. The method could be validated on a unique data set from a bridge that was artificially damaged after a one-year monitoring period. (Some figures in this article are in colour only in the electronic version; see www.iop.org) 1. Introduction For many years people have been performing vibration tests on large civil engineering structures. The dynamic characteristics of the structure contain useful information about its state. Therefore, vibration testing can be considered as a global non-destructive health monitoring technique. It can be used in a continuous or intermittent way. In a continuous monitoring system typically a few sensors are installed on the structure whereas a large number of sensors can be used in intermittent monitoring, for example to obtain detailed mode shape information [1]. An important issue is the excitation of large structures. The extrapolation to civil engineering of traditional input devices used in mechanical engineering leads to huge reaction mass shakers or impact testing based on a falling weight. In the literature, other, sometimes creative, solutions are also proposed to excite large structures. Gentile 1 Present address: LMS International, Interleuvenlaan 68, B-3001, Heverlee, Belgium. 2 Web site: http://www.bwk.kuleuven.ac.be/bwm/ et al [2] have described vibration tests on a cable-stayed bridge that was excited in the vertical direction by a heavy truck that drove over a plank and in the horizontal direction by sudden braking of the truck. Another way to vertically excite a bridge is a sudden release of a heavy mass that was suspended from the bridge. This technique was applied to the Vasco Da Gama Bridge in Lisbon, Portugal as reported by Cunha et al [3]. Delaunay et al [4] have described tests where the Normandie Bridge, France, was horizontally excited by the sudden release of a tension cable that connected the bridge to a tug-boat. Finally, Deger et al [5] used rocket engines to excite a composite steel/concrete bridge both horizontally and vertically. All these excitation methods are also referred to as free vibration testing. The input is not necessarily measured but it is impact-like and the responses are free vibrations. In the last ten years or so, more attention was paid to so- called ambient excitation. The structural response is measured to freely available ‘natural’ sources such as traffic, wind, waves and micro-earthquakes. Obviously these sources cannot be measured exactly. The advantage of using ambient sources is 0964-1726/01/030518+10$30.00 © 2001 IOP Publishing Ltd Printed in the UK 518