Fiber optic white light interferometric sensors for structural monitoring

Abstract

Smart structures are constructed by associating the structure with a health monitoring system that can monitor the physical and mechanical properties of the structure such as temperature and strain, respectively, in real-time during the servicing period. Recent research interest is to use optical fiber sensors as embedded strain monitoring devices for examining the temperature variation and strain profile during manufacturing and loading processes for advanced composite and concrete materials. Fiber optic white light interferometric sensor is one of the most exciting technologies in the field of optical fiber sensors in recent years and appears to be ideally suited for structural health monitoring of composite materials and civil engineering applications. The sensor head of the fiber white light interferometric system is merely an optical fiber segment that is ready to be embedded into the material without causing degradations of the structure. The sensor provides an effective means to monitor the manufacturing process and internal health conditions by distributing sensors at different locations inside/on the surface of the structure. The aim of the current study is to investigate and develop effective and practical fiber optic white light interferometric sensing techniques for strain and temperature measurement for smart structures. The scope of the research work includes fiber optic sensor design, interaction analysis between the fiber optic sensor and matrix materials, white light interferometric fiber optic sensor demodulation systems, development of fiber optic sensor multiplexing techniques including a multi-wave generating scheme using a coupler loop-resonator and bi-directional interrogation method based on a ring-network architecture. An extensive review is given first to describe the concept of smart structures and to discuss the present state of the art of the embedded fiber optic sensor techniques. Much attention is placed on the need of structural monitoring system and the white light interferometric fiber optic sensor based smart structure applications. The advantages of white light fiber optic interferometric sensors are also discussed. Following the comprehensive review, basic equations and working principle of white light interferometric sensors are given. These include the fundamental relationship between strain/temperature and the optical path variation, the white light interference process and its mathematical expression. A preliminary strain and temperature sensing system based on a modified white light fiber optic Michelson interferometer is presented. To understand the behaviors of the fiber optic sensor embedded in a structure, the effect of fiber coating and boundary condition between the fiber sensor and the structural materials is studied, Owing to the complex interactions between the sensor and the surrounding material, the relationship between the sensor output and strain/temperature inside the material cannot be determined by simple tests. The relationship that bridges the sensor output and the engineering values of strain and temperature was established via analytical models. In order to deduce the strain existing inside the material from the sensor signal, the relationship between the strain and temperature inside the sensor and inside the material was also established. Development of practical fiber optic sensors for real-time health monitoring of civil concrete structures is a continuing goal with applications in both the construction and in-service periods. Different kinds of fiber optic pre-embedded bar sensors for the applications of practical engineering construction of concrete structures have been designed and developed. The performance of the pre-embedded bar sensors depends on the bonding characteristics between the bar material and the optical fiber. Careful consideration was therefore given to the entire procedure of the pre-embedded bar structure design, the fiber gauge preparation and the sensor installation and integration. The shapes of the pre-embedded bar sensor have been designed as spindle for steel and epoxy bar sensors and the simple cylinder or rectangular shape for the cement based concrete bar sensors. The manufacturing procedures and the issues on installing the fiber optic sensor into host structures were discussed. Based on the white light interferometer principle, a fiber optic extensometer system was designed and constructed. The extensometer has been used for 1-dimensional and 2-dimensional strain measurements. The fiber sensors were mounted on the surface or embedded within the concrete specimens for strain measurement. Temperature and crack-tip opening displacement measurements of concrete beam were conducted. White light interferometers can be configured to perform quasi-distributed measurements by multiplexing a number of sensor signals on to a single fiber. A novel multiplexing technique based on a fiber optic ring resonator has been proposed and investigated. The theory of the ring resonator as a multi-path optical wave generator is presented. The technique has been applied to multiplex a serial and a parallel sensor array and M x N sensor matrix. In many applications the sensor concept becomes increasingly viable if an array of sensors can be implemented using a single fiber optic bus to link the sensors together. White light interferometry can be used as a passive coherence division multiplexing technique for interrogating such a sensor array. However, for the case of all sensors being multiplexed on to a single fiber line and embedded within a large scale smart structure, a very critical issue is that if somewhere the fiber line were broken down due to the local damage or crack in the structure, it would lead to part of the sensor system not working or at the worst case, the failure of the entire sensing system. As an attempt to solve this problem, a novel white light interferometric fiber optic sensor system consisting of serial sensors connected in a loop topology has been proposed. The system uses a scanning Mach-Zehnder interferometer or Michelson interferometer to interrogate individual sensors from two opposite directions. The sensor system has been configured to perform quasi-distributed measurements by multiplexing a number of sensors on to the fiber loop. The looped sensors network not only satisfies the redundancy requirement of a practical sensing system, but also provides a damage diagnosis methodology for large-scale smart structures. In addition, fiber optic ultrasonic sensors based on the Fizeau and Sagnac-like interferometers have been also proposed and experimentally demonstrated. The ultrasonic sensor can be used for real-time instantaneous detection of cracks in structures. They should find applications in acoustic emission monitoring and non-destructive evaluation (NDE) of large structures. Finally, conclusions and recommendations for further improvement of the white light fiber optic interferometric sensors are made. Extension of the technique for practical applications such as soil mechanics including slope landslide monitoring and foundation settlement measurements is discussed.