Improving Distributed Fiber-Optic Sensor Measures by Digital Image Correlation: Two-Stage Structural Health Monitoring

This paper deals with the integrated use of distributed fiber-optic sensors and digital image correlation techniques to develop a two-stage monitoring method for damage detection, localization, and quantification. The proposed methodology was applied in the laboratory on reinforced concrete beam specimens and is suitable for further field developments in concrete structures of large dimensions. The first stage is based on distributed strain monitoring through Brillouin scattering-based fiber-optic sensors to detect and locate potential damage zones within the entire structure, while the second focuses on verification of the critical regions identified by the optical-fiber sensor using the digital image correlation technique

Methods for Structural Health Monitoring based on Distributed Fiber Optic Sensor

Many countries around the world are facing serious problems with the aging of infrastructures. Besides the degradation process that is usually associated with aging, it is also important to consider the design and construction standards that may have led to deterioration of the built environment. Sensor-based Structural Health Monitoring (SHM) has been identified as one of the most important approaches to ensure structural safety, and the resilience of civil constructions. Brillouin scattering-based distributed fiber optic sensors have been distinguished as a great tool for SHM. Their success in sensor application is due to many advantages when compared with their electrical counterparts, including immunity to electrical and magnetic interference, geometrical adaptability, serving both as the sensor and the lead line, and others. Accurate interpretation of substrate strains from the optical measurements may require direct determination of elastic properties of optical fibers from tension tests. Therefore, stress-strain response of optical fibers in direct tension has been investigated in research presented herein. The study involved direct tension tests on two types of optical fibers, namely ribbon and standard single mode fiber (SMF-28). By employing the experimental data and by using the generalized Ramberg-Osgood law, the theoretical stress-strain responses of the optical fibers were established. The impetus for the study was the need for the elastic properties of the optical fibers, such as the modulus of elasticity and the elastic limit for accurate interpretation of strains measured by optical fiber sensors. The mechanical characteristics, and the stress-strain responses of the optical fibers serve as important parameters in numerical modeling, theoretical analysis, and accurate interpretation of sensed strains. For example, interpretation of strain transfer mechanisms, as well as quantification of structural cracks require information about the mechanical characteristics of 2 the optical fibers. Practical implementation of distributed sensor systems in SHM requires monitoring strain distributions, detection and growth of damage, changes in structural stiffness, and others. Hence, there is a need for distributed optical fiber sensors to extend their capabilities beyond strain measurements, especially in terms of the quantification of cracks. Hence, a method for conversion of the distributed strain measurements to crack opening displacements in structural elements has been investigating in this research. A hybrid approach involving theoretical and experimental strain distribution at the crack location was employed to formulate the conversion equation. The viability of the method was accomplished by experiments involving a 15-meter steel beam with prefabricated flaws. FBG-based displacement sensors were also used for direct measurement of crack opening displacements at the locations of the flaws and comparison with the theoretical calculations. Finally, field tests were carried to test the feasiability of the distributed monitoring of operating structures. A bridge monitoring approach was investigated for distributed detection and quantification of crack opening displacements along the lengths of large operating bridges. FBG-crack sensors were also employed for verification of the COD measurements based on direct acquisition of distributed strains. At the same time, a method for the out-put only modal analysis of structures based on distributed strains was proposed. The BOTDA modal shapes were first computed through laboratory experiments on a three-span beam and compared with canonic accelerometers results. The procedure was then applied on an opterating five-span bridge

Theoretical and Experimental Studies of Micro-Surface Crack Detections Based on BOTDA

Micro-surface crack detection is important for the health monitoring of civil structures. The present literature review shows that micro-surface cracks can be detected by the Brillouin scattering process in optical fibers. However, the existing reports focus on experiment research. The comparison between theory and experiment for Brillouin-scattering-based optical sensors is rarely reported. In this paper, a distributed optical fiber sensor for monitoring micro-surface cracks is presented and demonstrated. In the simulation, by using finite element methods, an assemblage of a three-dimensional beam model for Brillouin optical time domain analysis (BOTDA) was built. The change in Brillouin frequency (distributed strain) as a function of different cracks was numerically investigated. Simulation results indicate that the amplitudes of the Brillouin peak increase from 27 με to 140 με when the crack opening displacement (COD) is enlarged from 0.002 mm to 0.009 mm. The experiment program was designed to evaluate the cracks in a beam with the length of 15 m. Experimental results indicate that it is possible to detect the COD in the length of 0.002~0.009 mm, which is consistent with the simulation data. The limitations of the proposed sensing method are discussed, and the future research direction is prospected

Safety of existing infrastructures: the collapse of the Morandi bridge in Genoa

This work is focused on the study of the collapse of the Morandi bridge in Genoa. From the free-available data from literature, a numerical model of the structure has been prepared. Since no monitoring measurements or laboratory tests on materials were available, it was not possible to carry out an effective validation process of the model. However, the refined numerical approach and the collected design conditions allow a reasonable heuristic study to be carried out, to outline potential structural behaviors and responses to external perturbations. The analysis identified two critical aspects that may have affected the structural response. Both are related to the time variable that has likely affected both the operating conditions of the bridge and its loading capacity. Besides, structural solutions for stays (e.g. continuous over saddles) and the essentially static determinate scheme may have also affected the response

Embedded fiber-optic sensors in reinforced concrete elements of bridge structures

Reinforced concrete elements in bridge structures are subjected during their service life to different loading conditions that may affect their durability and efficiency. This can reduce the safety level of the infrastructures over time until their complete degradation. In particular, reinforced concrete elements can develop cracking conditions due to tensile stresses. However, such cracking conditions can propagate leading to the exposure of the steel reinforcement bars to the aggression of external agents such as chlorides. Being able to detect, localize and monitor the development and evolution of cracking can be crucial and this research is aimed at this purpose. In particular, laboratory tests have been performed on reinforced concrete beams that are equipped with embedded and externally applied distributed fiber optics sensors to monitor the state of cracking and its propagation within the thickness

A child with vestibular neuritis. Is adenovirus implicated?

Vertigo in children is relatively under examined in the literature. Among its causes, vestibular neuritis (VN) represents only 2% of cases, with its etiology remaining unknown. We report for the first time a 4-year-old boy with vestibular neuritis and serological results compatible with adenoviral infection. Serological diagnosis was performed on the basis of a rise and consequent normalization of complement fixation (CF) titers of the plasma antibodies. Although we were not able to detect exactly when the infection started, we were able to detect an increased level of adenovirus antibodies by CF titers, followed by a decrease (i.e. 1/16, then 1/8, then <1/4) during the recovery. This is typical of a resolving infection. Furthermore, that this increase in antibodies was specific to an adenovirus infection was suggested by the observation that we did not detect increases in antibodies to other common viruses (i.e. herpes simplex and zoster viruses, Epstein-Barr virus, cytomegalovirus, influenza and parainfluenza viruses). This allows us to exclude the chance of nonspecific antibody activation. We concluded that, although our data do not formally demonstrate an involvement of adenovirus in VN, they suggest such an involvement. This may be of interest, given that a viral etiology for VN has been proposed but not definitively proven

Numerical simulations of collapse tests on RC beams

Recent events of bridge collapses, e.g. in Genoa (Italy) on August 14th 2018 and in Kolkata (India) on September 4th 2018, have focused the public interest on the infrastructures' safety for their consequences in terms of fatalities and injuries, but also of economy and social losses. Indeed, focusing on reinforced concrete structural components such as beams, they are subjected during their service life to different loading conditions that may affect their durability and efficiency. This can reduce the structural safety over time until its complete degradation up to the ultimate limit state. In particular, reinforced concrete elements can develop cracking conditions due to tensile stresses that are normally absorbed by the reinforcement. However, such cracking conditions can develop and propagate leading to the exposure of the reinforced concrete element to the aggression of external agents such as chlorides up to collapse. In this work, collapse tests on reinforced concrete beams are reproduced in laboratory through nonlinear numerical simulations. The numerical outcomes will be also compared to available monitoring data collected by distributed fiber optics sensors and image correlation techniques to monitor the state of cracking and its propagation in the thickness

Perinatal exposure to delta-9-tetrahydrocannabinol causes enduring cognitive deficits associated with alteration of cortical gene expression and neurotransmission in rats

The aim of the present study was to investigate whether perinatal exposure to a moderate dose of delta-9-tetrahydrocannabinol (THC) alters cortical gene expression and neurotransmission, leading to enduring cognitive dysfunctions in rat offspring. To this purpose, rat dams were treated, from gestational day 15 to postnatal day 9, with THC at a daily dose (5 mg/kg, per os) devoid of overt signs of toxicity. THC did not influence reproduction parameters, whereas it caused subtle neurofunctional deficits in the adult offspring. Particularly, perinatal THC induced long-lasting alterations of cortical genes related to glutamatergic and noradrenergic systems, associated with a decrease in the cortical extracellular levels of both neurotransmitters. These alterations may account, at least in part, for the enduring cognitive impairment displayed by THC-exposed offspring. Taken together, the present results highlight how exposure to cannabinoids during early stages of brain development can lead to irreversible, subtle dysfunctions in the offspring

Perinatal exposure to delta-9-tetrahydrocannabinol causes enduring cognitive deficits associated with alteration of cortical gene expression and neurotransmission in rats

The aim of the present study was to investigate whether perinatal exposure to a moderate dose of delta-9-tetrahydrocannabinol (THC) alters cortical gene expression and neurotransmission, leading to enduring cognitive dysfunctions in rat offspring. To this purpose, rat dams were treated, from gestational day 15 to postnatal day 9, with THC at a daily dose (5 mg/kg, per os) devoid of overt signs of toxicity. THC did not influence reproduction parameters, whereas it caused subtle neurofunctional deficits in the adult offspring. Particularly, perinatal THC induced long-lasting alterations of cortical genes related to glutamatergic and noradrenergic systems, associated with a decrease in the cortical extracellular levels of both neurotransmitters. These alterations may account, at least in part, for the enduring cognitive impairment displayed by THC-exposed offspring. Taken together, the present results highlight how exposure to cannabinoids during early stages of brain development can lead to irreversible, subtle dysfunctions in the offspring. © 2007 The Authors