Structural integrity monitoring with fibre Bragg grating sensors

Abstract

PurposeThis paper describes a recent collaborative project involving the development of a multiplexed fibre Bragg grating (FBG) sensor system for structural integrity monitoring.Design/methodology/approachThe system is described and field trials on both conventional and novel composite bridges are discussed. A FBG sensor‐based structural monitoring system was developed, based on a fluorescent fibre as the optical source. It used a tuneable, fibre‐coupled, Fabry‐Perot filter, actuated by piezoelectric transducers and operated over the bandwidth of the source at up to 250 scans/second. Light from the source was filtered and reflected back from the Bragg gratings, through optical couplers, to eight photodiode detectors. These detected the resulting time‐domain spectra of the sensors in each of the serially connected sensor arrays. The system was tested at City University and then subjected to trials on the Mjosund road bridge in Norway and on West Mill bridge in Oxfordshire, UK, which is the first bridge to be fabricated from a new type of composite material.FindingsDuring the Norwegian trials the system was arranged with four or five FBG sensors per channel giving a total of 32 measurement points with eight parallel channels. Twelve conventional foil strain gauges and a number of thermocouples were also installed. Different static and dynamic loads were applied over a period of 18 months and the results showed that the thermally compensated strain data obtained optically matched those from the resistive gauges to within <5 με. During the construction stage of the Oxfordshire bridge, sections of the decking and longitudinal composite support beams were instrumented with 40 FBG sensors with temperature compensation, placed at pre‐selected sites of maximum strain. These exhibited a resolution of ±5 με and an operating range of over ±2,000 με.Originality/valueThis research has shown that multiplexed, multi‐point FBG sensor systems can accurately and reliably monitor both static and dynamic strains in large structures over a range of temperatures and for extended periods of time.