Preface

Abstract

This special issue on Fiber Optic Structural Sensing contains a rich selection of papers that clearly demonstrate both the diversity of the technology and its broad range of application. Although a number of different types of fiber optic sensor are discussed in the 13 papers that constitute this issue, nine of the papers involve fiber Bragg gratings (FBG). This is consistent with the growing interest in this sensor due to its versatility, spectral encoding, ease of multiplexing, small profile, potential for automated production with the resulting low-cost, and consistent performance. Although most of the papers are concerned with short-gauge length fiber optic strain sensors, two papers discuss the merits of long-gauge length sensors. Two papers describe truly distributed strain sensing based on Bragg integrating measurements, and the important issue of temperature and strain decoupling is addressed in one paper. A significant feature of this special issue is the number of papers that describe the use of fiber optic structural sensors in real structures, like bridges. This suggests that we are not at the end of the development of this sensing technology, but rather at the beginning of its broad implementation phase. Bronniman et al report on the use of a multiplexed string of FBGs to monitor the first use of carbon-fiber-reinforced-polymer wires in place of steel in a `stay cable bridge'. Idriss et al demonstrate the use of a multiplexed array of 48 FBGs, integrated into an experimental full-scale (40 ft span) laboratory bridge, to track load redistribution resulting from weakening of the structure. Jones et al show that a single strand of optical fiber with a string of 16 FBG sensors are able to map the two-dimensional strain-displacements of a cantilever honeycomb plate. This same measurement with conventional technology would be very complex requiring two leads to each of the 16 strain gauges. The simplification made possible with FBGs could greatly increase the frequency of structural monitoring in various fields. Grossman and Huang suggest that the three normal strain components might be measured by a planar array of three fiber optic sensors. Huang et al describe the development of the first truly distributed fiber optic strain sensing system that permits continuous, arbitrary strain profiles to be measured. Applications include: free edge effects of composite materials, bond strength evaluation for bonded structures, the development of new anchorage systems for fiber composite tendons and cables, and the detection of debonding between a FBG strain sensor and its host structure - a common mode of failure for strain sensors. Since corrosion of the steel reinforcement and prestressing tendons within concrete structures, like bridges, represent a serious threat to the integrity and functional capacity of such structures, measurement of the state of corrosion represents an important component of structural monitoring. The paper by Fuhr and Huston describes a possible approach to assessing the state of corrosion by means of fiber optic technology and spectroscopic analysis. Fiber optic strain sensors of arbitrary long gauge length are particularly well suited for monitoring hoop strains in cylindrical structures damaged by corrosion or earthquakes. Fan et al show that a localized, low-coherent, long-gauge sensor made from two FBGs can be used to monitor corrosion induced changes in the hoop strains within FRP wraps used for rehabilitation and strengthening of damaged concrete columns. Inaudi et al demonstrate that a set of long-gauge, low-coherence fiber optic strain sensors can be used to monitor the deformations of a bridge and thereby evaluate its state of health. Microminiaturization promises to increase reliability and ruggedness, and greatly reduce the cost of systems. An important first step in this direction is reported by Coroy and Measures, who used a mesa quantum well electroabsorption filtering detector to demodulate a FBG sensor. Measures et al demonstrate that a rapidly tuned semiconductor laser can demodulate each of the different sensing modalities possible with FBGs. A preliminary form of this universal demodulation architecture was developed and tested on: short-gauge length sensors and a string of FBG sensors. This architecture was also shown to be capable of extracting strain profiles from within a single extended FBG. In summary this special issue reflects on some of the most exciting advances being made in the field of fiber optic structural sensing and illustrates the application potential for this technology. Raymond M Measures Special Issue Editor