Article

Normalized Curvature Ratio for Damage Detection in Beam-Like Structures

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Abstract

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.

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... The changes in boundary conditions can be due to varied causes including scour from flooding and gradual degradation of bearings due to daily wear. Overall, such damage or changes are typically linked to changes in rotational stiffness at the supports (Kliewer & Glisic, 2017), for which an array of strain sensors is often unfeasible due to practical challenges on site despite numerical or small-scale experimental feasibility studies (Kliewer & Glisic, 2017). In fact, repair or rehabilitation of such damage will also demonstrate a change in rotational stiffness as compared to the damage state or the reference state. ...
... The changes in boundary conditions can be due to varied causes including scour from flooding and gradual degradation of bearings due to daily wear. Overall, such damage or changes are typically linked to changes in rotational stiffness at the supports (Kliewer & Glisic, 2017), for which an array of strain sensors is often unfeasible due to practical challenges on site despite numerical or small-scale experimental feasibility studies (Kliewer & Glisic, 2017). In fact, repair or rehabilitation of such damage will also demonstrate a change in rotational stiffness as compared to the damage state or the reference state. ...
... Deviation of mode shapes due to damage has been characterised through changes in curvature (Kliewer & Glisic, 2017), change in modal energy (Shi et al., 2000) and similar markers. However, the process of measuring the curvature involves the differentiation of bridge accelerations which is prone to errors. ...
Article
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Mode shapes are sensitive to the structural condition of bridges but a reasonable estimate of such changes require several accelerometers, which can be resource intensive. This paper obviates this problem through a novel structure health monitoring (SHM) approach for estimating modal parameters of bridges, including damage-induced changes of boundary conditions by using progressively re-deploying sensors along a monitored bridge. This concept of re-deployable sensors and subsequent use of a series of measurements allow extracting data from different bridge segments and also to get an indication of the condition of the bridge through frequency domain decomposition. Data from different segments are combined to estimate the global mode shape of the bridge and its gradient is observed to be indicative of support stiffness change. The concept is successfully tested through a full-scale field trial on a railway bridge in the Republic of Ireland, before and after the rehabilitation of its supports. The results are expected to guide future on-site measurement of damages due to flooding, scour, and other natural hazards, along with the effectiveness of intervention actions like repair and rehabilitation, providing a clear evidence base for practical value of SHM.
... This study is the world-first to investigate the potential of normalized curvatures and dynamic mode shapes of railway sleepers to detect ballast voids and pockets. Based on a study by Kliewer and Glisic [24], the normalized curvature ratio can be used to detect damages in concrete beams and girders over bridge. The potential of the method has inspired this study with the aim to provide an alternative method to detect ballast voids and pockets in the field. ...
... The simulation results are used to explore the possibility of the normalized curvature ratio method and to develop a novel curvature-based approach capable of detecting ballast voids and pockets using free vibration characteristics of the sleepers. The lowest mode of flexural vibration has been considered in this study since free vibration occurs primarily in the first or lowest mode in most cases in real structures [24][25][26]. The insight into this novel approach will pave itself as a supplement technique that helps rail track engineers evaluate the ballast support conditions, which cannot be visually inspected in practice [26][27][28][29][30][31]. ...
... This study develops and explores a novel curvature-based method capable to detect ballast voids and pockets under railway sleepers using free vibration characteristics that can be predicted by simulations or measured in the field using either experimental modal testing or operational modal analysis. This novel technique has been inspired by a recent work [24] that depicts a potential to use this method for bridge girders. In general, the modeshapes and curvature modes under free vibration are not expected to change unless the structure experiences irregular behavior. ...
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After a railway track experiencing dynamic loading, the track settles and causes ballast to deform, spread and sometime damage. Without appropriate maintenance, void and pocket of ballast underneath railway sleepers can establish overtime and impair the ride quality of train services. In this study, the emphases will be placed on the application of non-destructive vibration-based technology, to investigate and evaluate dynamic characteristics of voided railway concrete sleepers, which are the fundamental element to provide track support to railway systems. The study has developed a curvature-based damage detention method to identify ballast voids under railway track sleepers. This method can be easily deployed in the field by using fibre bragg grating strain sensors to measure strains for curvature analysis. In this study, the assumption is that the time-dependent material degradation negligibly affects the curvature ratios. The dynamic finite element model has been established and validated for railway sleepers in the field. A variety of losses of ballast support have been simulated using the validated model. The dynamic mode shape has been analysed to evaluate curvature ratios under different types of ballast losses. Although the method provides positive outcomes, the advantages, disadvantages and limitation of the method are then identified and discussed.
... This is where measuring displacement indirectly can be beneficial, with methods using sensors such as accelerometers (Park et al., 2005), tilt sensors/inclinometers (Hou et al., 2005), or strain sensors (Foss and Haugse, 1995;Davis et al., 1996;Sigurdardottir et al., 2017). The benefit of using network of strain sensors to determine the displacement of a structure, is that the same sensor network can also be used to calculate other monitoring parameters such as the neutral axis (Sigurdardottir and Glisic, 2014), pre-stressing forces (Abdel-Jaber and , thermal signatures (Reilly and Glisic, 2018), and curvature (Kliewer and Glisic, 2017), and that is the reason why this research focuses on strain-based methods. ...
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Damage detection in civil engineering constructions using the dynamic system parameters has become an important research topic. A direct, fast and inexpensive method is therefore required to evaluate and localize damage using the change in dynamic parameters between the intact and damage states. This paper investigates the application of the change in modal curvatures to detect damage in a prestressed concrete bridge. To establish the method simply supported and continuous beams containing damaged parts at different locations are tested using simulated data. Some important conclusions concerning the computation of the modal curvatures are drawn. A damage indicator called “curvature damage factor” is introduced, in which the difference in curvature mode shape for all modes can be summarized in one number for each measured point. The technique is further applied to a real structure, namely bridge Z24 which lies between the villages Koppigen and Utzenstorf and crosses the highway A1 between Bern and Zurich in Switzerland. In the framework of a Brite-Euram project, the bridge is used as a full-scale specimen and subjected to different damage scenarios in order to introduce damage.
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