A Review of Distributed Optical Fiber Sensors for Civil Engineering Applications

António Barrias, Joan Casas, Sergi Villalba
2016 Sensors  
The application of structural health monitoring (SHM) systems to civil engineering structures has been a developing studied and practiced topic, that has allowed for a better understanding of structures' conditions and increasingly lead to a more cost-effective management of those infrastructures. In this field, the use of fiber optic sensors has been studied, discussed and practiced with encouraging results. The possibility of understanding and monitor the distributed behavior of extensive
more » ... tches of critical structures it's an enormous advantage that distributed fiber optic sensing provides to SHM systems. In the past decade, several R & D studies have been performed with the goal of improving the knowledge and developing new techniques associated with the application of distributed optical fiber sensors (DOFS) in order to widen the range of applications of these sensors and also to obtain more correct and reliable data. This paper presents, after a brief introduction to the theoretical background of DOFS, the latest developments related with the improvement of these products by presenting a wide range of laboratory experiments as well as an extended review of their diverse applications in civil engineering structures. Sensors 2016, 16, 748 2 of 35 damage a structure. Damage can be defined as alterations that when introduced into a system will have an adverse effect in its current or future performance [3] . The process of employing a damage identification strategy for engineering and aerospace infrastructures is referred to as structural health monitoring (SHM). When dealing with long-term SHM, the goal of the system is to deliver updated information regarding the ability of the structure to continue to serve its intended purpose and function despite the unavoidable effects of the mentioned aging process and the accumulation of damage that results from the operational environment. SHM systems are also able to rapidly assess and screen the structure condition under an accidental or extreme event such as earthquakes, increase of water levels or unexpected loadings. The early detection of structural malfunctions allows the increase of the service life-time of the structure at the same time that decreases the maintenance costs. The act of damage identification has been around probably, in a qualitative manner, since man started to using tools [3] . Notwithstanding, SHM has been recently a fast-developing area in aerospace and engineering disciplines especially in the civil engineering field. The innovation in the SHM technologies as well as the development of large-scale SHM systems has been a great subject of interest within the engineering and academic communities over the last two decades [4] [5] [6] [7] [8] [9] [10] [11] . However, despite its great potential, SHM has not been applied in large scale and in a systematic manner to civil infrastructures. One significant reason for this is the lack of reliable and affordable generic monitoring solutions [12] . Optical Fiber Technology in SHM of Built Environment Currently, assessment of buildings, bridges, dams, tunnels and other vital civil engineering infrastructures are carried out by engineers trained in visual inspection, which sometimes can be inaccurate due to differences in their background for safety condition assessment. In order to improve the inspection accuracy and efficiency, optical fiber sensors (OFS) are one of the fastest growing and most promising researched areas, due to their features of durability, stability, small size and insensitivity to external electromagnetic perturbations, which makes them ideal for the long-term health assessment of built environment [13] . Furthermore, the most regularly practiced SHM approaches are based on electric strain sensors, accelerometers, inclinometers, GNSS-based sensors, acoustic emission, wave propagation, etc. Nevertheless, all of them present genuine challenges when deployed in real world applications [4] . Different kinds of sensors, embedded or attached to the structure, can be used in SHM systems but only those based on fiber technology provide the ability to accomplish integrated, quasi-distributed, and truly distributed measurements on or even inside the structure, along extensive lengths [2] . Standard monitoring practice is normally based on the choice of a limited and relatively small number of points that are supposed to be illustrative of the structural behavior [14] . For a large scale structure, the number of point sensors needed to generate complete strain information can grow rapidly. Discrete short-gauge sensors provide useful and interesting data of the structure related with local behavior but might omit important information in locations where degradation occurs but that is not instrumented. There are ways of covering larger extensions of structures with the application of long-gauge sensors allowing the detection and characterization of phenomenon's that have a global impact on the structure. Notwithstanding, the reliable detection and characterization that occurs far from instrumented areas continues to be challenging since it depends on high-level algorithms whose performance may decrease due to external interferences that can mask the real damage, such as temperature variations, load changes, outliers and missing data in monitoring results [15] . Distributed optical fiber sensors (DOFS) offer an advantage over point sensors for global strain measurements. The thousands of sensing points that the DOFS provides enables mapping of strain distributions in two or even three dimensions. Thus, real measurements can be used to reveal the global behavior of a structure rather than extrapolation from a few point measurements. A truly distributed optical sensor is expected to measure temperature, strain and vibration data at any point along an entire fiber through light scattering. The great challenge has been to develop these Sensors 2016, 16, 748 3 of 35 sensors in a way that they can achieve appropriate sensitivity and spatial resolution [13] . Fortunately, great advances have been made in the last decades in order to improve this area. Several general state of the art papers of FOS have been published [16] [17] [18] [19] and also various where the applications of these sensors on civil engineering structures were reviewed [10, [20] [21] [22] . A comprehensive DOFS state of the art paper was presented in [11] where the theoretical background of the most used DOFS techniques was extensively elaborated and where some civil engineering applications, especially focused on bridges, were presented. In the present paper, a broader and wider range of civil engineering applications are presented (geotechnical structures, pipelines, dams, bridges, laboratory experiments) as some other innovative implementations. Furthermore, and finally, a review of the dynamic capabilities of distributed sensing and the most practical challenges associated with the implementation of these sensors are presented.
doi:10.3390/s16050748 pmid:27223289 pmcid:PMC4883438 fatcat:rpc5yukllvgx5j7wr4zcvnsdj4