Fiber Optic Methods for Structural Health Monitoring based on Dynamic Curvature and Displacement

With the growing challenge of aging infrastructure and the increasing cost for replacement and repair, structural health monitoring (SHM) offers an approach to address these challenges. SHM is the process of sensing parameters of a structural system over time, either continuously or periodically, with the goal of better understanding a structures behavior and determining the current state of health and performance. The overarching aim of this research is to create and analyze strain-based curvature and displacement SHM methods for beam-like structures. This thesis will focus on the use of long-gage fiber Bragg grating (FBG) sensors as they offer numerous benefits compared to other sensors currently available, such as moderate cost, multiplexing capabilities and the ability for both static and dynamic monitoring. The SHM research field has grown considerably in the last two decades. Taking into account this growth, this thesis first presents an analysis of the structure and evolution of the SHM research field over the past 15 years through a bibliometric analysis, to assess the field and get insights about the position of FBGs in the field. In particular, long-gage FBG strain sensors allow for the instrumentation of large areas of a structure which helps enable its global monitoring. Based on these sensors, a novel, strain-based damage sensitive feature was identified, the normalized curvature ratio (NCR), and its performance in damage detection was evaluated. In addition, a comparative analysis of the accuracy of stain-based displacement methods was carried out, and their performance in serviceability assessment was evaluated. The research included analytical and numerical modeling, small-scale laboratory tests, and applications to and validation on a full-scale in-service structure

Normalized Curvature Ratio for Damage Detection in Beam-Like Structures

Fiber Optic Sensors (FOS) offer numerous advantages for structural health monitoring. In addition to being durable, lightweight, and capable of multiplexing, they offer the ability to monitor strain in both static and dynamic mode. FOS also allow for instrumentation of large areas of a structure with long-gage sensors which helps enable global monitoring of the structure. Drawing upon these benefits, the Normalized Curvature Ratio (NCR), a curvature based damage detection method, has been developed. This method utilizes a series of long-gage Fiber Bragg Grating (FBG) strain sensors for damage detection of a structure through dynamic strain measurements and curvature analysis. The main assumption is that the ratios between cross-sectional curvature amplitudes under free vibration remain unchanged given the state of the structure is unchanged. The theoretical development of this method is presented along with an analytical study of a simply supported beam with two damage cases: a loss of flexural stiffness in the span and a change in rotational stiffness of the support. Validation of the method is then performed through two implementations. First, through a small-scale laboratory test with a simply supported aluminum beam subjected to a change in the rotational stiffness of the support. Second, the method is applied to an existing in-service highway overpass with over 5 years of data collection of dynamic strain events. The advantages and limitations of the method are identified and discussed. This research shows encouraging results and the potential for the NCR to be used as a simplistic metric for damage detection

Multifunctional cementitious composites with structural and damage monitoring capabilities for smart bridges

Aging and overloaded critical civil infrastructure systems such as bridges pose great risks to the hundreds of millions of daily users. Limited resources for their repairs and replacements have necessitated the need for the development of multifunctional composites with both structural and in-situ damage monitoring capabilities. An in-situ triboluminescent optical fiber (ITOF) sensor with an integrated sensing and transmission system has been developed. The ITOF sensor has been incorporated into reinforced triboluminescent multifunctional cementitious composites (TMCC) to allow for real time structural health monitoring of bridges. Results are reported on the performance characterization of the sensor under flexural loading and show that the integrated sensor is able to detect localized damage (cracks) before the failure of the TMCC beams. This will enable early damage detection that will lead to prompt repairs thereby resulting in significant life and cost savings. Copyright 2013 by Aurora Flight Sciences

Getting light through cementitious composites with in situ triboluminescent damage sensor

Triboluminescent damage sensors comprising highly efficient triboluminescent materials could allow simple, real-time monitoring of both the magnitude and location of damage. The inability to effectively capture and transmit the triboluminescent optical signals generated within opaque composites like concrete has, however, limited their damage monitoring applications. The in situ triboluminescent optical fiber sensor has been developed to enable the detection and transmission of damage-provoked triboluminescent emissions without having to position triboluminescent crystals in the host material. Flexural tests were performed on mortar and reinforced concrete beams having the in situ triboluminescent optical fiber sensor integrated into them. The intrinsic triboluminescent signals generated in the beams under loading were successfully transmitted through the optical fibers to the photomultiplier tube by side coupling. Successful side coupling will make a truly distributed in situ triboluminescent optical fiber sensor possible when the entire length of the sensor is mostly covered with the triboluminescent composite coating. The results show the viability of the in situ triboluminescent optical fiber sensor for the structural health monitoring of cementitious composites. Real-time failure detection was demonstrated in unreinforced mortar beams, while real-time damage (crack) detection was demonstrated in reinforced concrete beams. Preliminary work on reinforced concrete beams showed that the integrated in situ triboluminescent optical fiber sensor was able to detect multiple cracks caused by loading, thereby providing early warning of structural degradation before failure. © The Author(s) 2013

Real time failure detection in unreinforced cementitious composites with triboluminescent sensor

The in-situ triboluminescent optical fiber (ITOF) sensor has an integrated sensing and transmission component that converts the energy from damage events like impacts and crack propagation into optical signals that are indicative of the magnitude of damage in composite structures like concrete bridges. Utilizing the triboluminescence (TL) property of ZnS:Mn, the ITOF sensor has been successfully integrated into unreinforced cementitious composite beams to create multifunctional smart structures with in-situ failure detection capabilities. The fabricated beams were tested under flexural loading, and real time failure detection was made by monitoring the TL signals generated by the integrated ITOF sensor. Tested beam samples emitted distinctive TL signals at the instance of failure. In addition, we report herein a new and promising approach to damage characterization using TL emission profiles. Analysis of TL emission profiles indicates that the ITOF sensor responds to crack propagation through the beam even when not in contact with the crack. Scanning electron microscopy analysis indicated that fracto-triboluminescence was responsible for the TL signals observed at the instance of beam failure. © 2013 Elsevier B.V