Stretching Method-Based Operational Modal Analysis of An Old Masonry Lighthouse.

We present in this paper a structural health monitoring study of the Egyptian lighthouse of Rethymnon in Crete, Greece. Using structural vibration data collected on a limited number of sensors during a 3-month period, we illustrate the potential of the stretching method for monitoring variations in the natural frequencies of the structure. The stretching method compares two signals, the current that refers to the actual state of the structure, with the reference one that characterizes the structure at a reference healthy condition. For the structure under study, an 8-day time interval is used for the reference quantity while the current quantity is computed using a time window of 24 h. Our results indicate that frequency shifts of 1% can be detected with high accuracy allowing for early damage assessment. We also provide a simple numerical model that is calibrated to match the natural frequencies estimated using the stretching method. The model is used to produce possible damage scenarios that correspond to 1% shift in the first natural frequencies. Although simple in nature, this model seems to deliver a realistic response of the structure. This is shown by comparing the response at the top of the structure to the actual measurement during a small earthquake. This is a preliminary study indicating the potential of the stretching method for structural health monitoring of historical monuments. The results are very promising. Further analysis is necessary requiring the deployment of the instrumentation (possibly with additional instruments) for a longer period of time

Monitoreo sísmico continuo de una construcción vertical: Edificio Crisanto Luque, en Bogotá, Colombia

ilustraciones a color, fotografías, mapas, tablasEl monitoreo continuo del estado de cualquier estructura es actualmente un tema de investigación y desarrollo. En geofísica es común el uso del estudio que, por sus siglas en inglés, recibe el nombre de Seismic Structural Health Monitoring (S2HM), el cual evalúa de manera continua estructuras civiles para estimar su seguridad y hacer recomendaciones de mejora a través del análisis de datos y modelos matemáticos. Por primera vez en Colombia, se ha desplegado una red de monitoreo permanente y continuo, con propósitos académicos, en un edificio de 14 pisos en el centro de Bogotá. Se instalaron 6 acelerómetros ETNA-2 de tres componentes, los cuales iniciaron el registro de datos en junio de 2019, permitiendo usar diferentes grupos de datos para este estudio. Inicialmente, 25 días de datos continuos registrados basados en vibraciones ambientales, fueron analizados para comprender la respuesta del edificio. Se realizó un análisis espectral preliminar que permitió identificar un modo muy claro a 1,25 Hz, para el componente longitudinal (X) de los acelerómetros. Otros modos de vibración en frecuencias más alta también se notaron alrededor de 1.5 - 2.5 Hz y 3.5 - 4 Hz, incluso por encima de 5 Hz, particularmente visto en los pisos superiores; esta información permitió seleccionar diferentes bandas de frecuencia de 0.5 - 2 Hz, 2 - 5 Hz, 6 - 10 Hz y 0.5 - 10 Hz para un análisis más detallado. Siguiendo el enfoque de Interferometría Sísmica basada en deconvolución propuesto por Prieto, y otros, 2010, para una campaña de monitoreo de 225 días (3 de julio de 2019 a 14 de febrero de 2020), las funciones de respuesta al impulso (IRF) fueron estimadas a partir de 2 fuentes diferentes de datos: 49 terremotos registrados (IRF basados en terremotos) y 225 días de datos registrados continuamente (IRF basados en vibración ambiental), ambos conjuntos de datos fueron utilizados como datos de entrada en las mediciones de variación de velocidad, utilizando la técnica de estiramiento. Un notable terremoto de Magnitud 6 ocurrió el 24 de diciembre de 2019 en Mesetas, Meta, produciendo un cambio significativo en la respuesta del edificio, notado en ambos conjuntos de datos (basados en vibraciones ambientales y basados en terremotos), para el componente longitudinal de los sensores.Seismic Structural Health Monitoring (S2HM) allows the continuous evaluation of engineering structures to estimate their safety and making recommendations for improvement through data analysis and mathematical models. For the first time in Colombia, a permanent and continuous monitoring network for engineering structures with an academic purpose has been deployed in a 14-story ecofriendly steel-frame building combined with a reinforced concrete structure in the downtown of Bogota. The six 3-component ETNA-2 accelerometers started recording on June 2019, and different sets of data were used for this study. As an initial attempt to understand the building’s response, with only 25 days of continuous recorded data, the anthropogenic behavior from the ambient vibrations-based data was analyzed. A preliminary spectral analysis was performed, allowing to identify a very clear mode at 1.25 Hz, in the longitudinal (X) component. Higher frequency modes were also noticed around 1.5 – 2.5 Hz and 3.5 – 4 Hz, even above 5 Hz, particularly seen in the top floors; this information leaded on the selection of particular frequency bands at 0.5 – 2 Hz, 2 – 5 Hz, 6 – 10 Hz and 0.5 – 10 Hz for further analysis. Following the deconvolution-based seismic interferometry approach proposed by Prieto, et al., 2010, for a 225 daylong monitoring campaign (from July 3rd 2019 to February 14th 2020), the Impulse Response Function (IRF) was estimated, from 2 different sources of data: 49 registered earthquakes (IRFs based on earthquakes) and 225 days of continuously recorded data (IRFs based on ambient vibration), both used as an input in the velocity variation measurements, using a stretching technique. A remarkable M6 earthquake occurred on December 24, 2019, in Mesetas, Meta, yielding a significant change in the building’s response, noticed in both sets of data (ambient vibration-based data and earthquake-base data), for the longitudinal component.MaestríaMagíster en Ciencias - Geofísic

Monitoring the cementitious materials subjected to sulfate attack with optical fiber excitation Raman spectroscopy

Formation of ettringite and gypsum from sulfate attack together with carbonation and chloride ingress have been considered as the most serious deterioration mechanisms of concrete structures. Although electrical resistance sensors and fiber optic chemical sensors could be used to monitor the latter two mechanisms on site, currently there is no system for monitoring the deterioration mechanisms of sulfate attack. In this paper, a preliminary study was carried out to investigate the feasibility of monitoring sulfate attack with optical fiber excitation Raman spectroscopy through characterizing the ettringite and gypsum formed in deteriorated cementitious materials under an optical fiber excitation + objective collection configuration. Bench-mounted Raman spectroscopy analysis was also conducted to validate the spectrum obtained from the fiber-objective configuration. The results showed that the expected Raman bands of ettringite and gypsum in the sulfate-attacked cement paste can be clearly identified by the optical fiber excitation Raman spectrometer and are in good agreement with those identified from bench-mounted Raman spectrometer. Therefore, based on these preliminary results, it is considered that there is a good potential for developing an optical fiber-based Raman system to monitor the deterioration mechanisms of concrete subjected to sulfate attack in the future

Continuous Monitoring of High‐Rise Buildings Using Seismic Interferometry

The linear seismic response of a building is commonly extracted from ambient vibration measurements. Seismic deconvolution interferometry performed on ambient vibrations can be used to estimate the dynamic characteristics of a building, such as its shear-wave velocity and its damping. The continuous nature of the ambient vibrations allows us to measure these parameters repeatedly and to observe their temporal variations. We used 2 weeks of ambient vibrations, recorded by 36 accelerometers that were installed in the Green Building at the Massachusetts Institute of Technology campus, to monitor the shear wavespeed and the apparent attenuation factor of the building. Because of the low strain of the ambient vibrations, we observed small speed changes followed by recoveries. We showed that measuring the velocity variations for the deconvolution functions, filtered around the fundamental-mode frequency, is equivalent to measuring the wandering of the fundamental frequency in the raw ambient vibration data. By comparing these results with local weather parameters, we showed that the air humidity is the dominating factor in the velocity variations of the waves in the Green Building, as well as the main force behind the wandering of the fundamental mode. The one-day periodic variations are affected by both the temperature and the humidity. The apparent attenuation, measured as the exponential decay of the fundamental-mode waveforms, is strongly biased due to the amplitude of the raw vibrations and shows a more complex behavior with respect to the weather measurements. We have also detected normal-mode nonlinear interaction for the Green Building, likely due to heterogeneity or anisotropy of its structure. We found that the temporal behavior of the frequency singlets may be used for monitoring.Royal Dutch-Shell Group (through MIT Energy Initiative)National Science Foundation (U. S.) (Grant Grant EAR-1415907

Polymer-based Piezoelectric Material and Device for Energy Harvesting/Health Monitoring in Civil Infrastructure

Recent studies on piezoelectric materials have resulted in the development of a wide verity of piezoelectric devices such as nanogenerators and sensors. This technology is prevalently dominated by the ceramic materials which are brittle and have a very limited strain level. Moreover, despite a wide working frequency range, the ceramic-based piezoelectric devices can only work under tiny forces to avoid damage to the device. As such, due to inherent brittleness, the piezoelectric technology has not been widely explored in civil engineering applications due to the aforementioned drawbacks of ceramic materials. This thesis aims to develop an efficient piezoelectric polyvinylidene fluoride (PVDF) nanofiber device which can be used in both energy harvesting and sensing civil infrastructure applications. The β-phase of PVDF is responsible for its electroactive properties such as ferroelectric, piezoelectric and pyroelectric properties. In spite of several efforts to improve the β-phase content, it is still a challenge to fabricate a PVDF sensor with high efficiency due to the complication of the required post-treatment process which mainly includes electrical poling and mechanical stretching. The electrospinning method was used in this study to synthesize the cost-effective and large-scale piezoelectric nanofiber composite, making it feasible for commercial, industrial and civil engineering applications. The process-structure-property relations of electrospun PVDF nanofiber has been systematically studied. As a result, a reliable model was developed that enables an accurate prediction of PVDF structure properties, particularly morphological and a fraction of the β-phase content. It was found that the fraction of β-phase is considerably affected by evaporation rate so that the high concentration of PVDF and DMF/acetone decreases the evaporation rate of the solution resulting in a formation of a high fraction β-phase content. The electrospinning method was found to be very effective to promote the β-phase formation in PVDF nanofiber. Additionally, electrospun PVDF nanofibers were experienced high electrical field and mechanical stretching during the fabrication which eliminates a need for the post-treatment process. This study proposes a core-shell structured PVDF-graphene oxide (GO) nanofiber composite, in which the polar phase content and piezoelectric properties are considerably improved. The results indicate that only 0.2 wt. % of GO is enough to nucleate most of the PVDF polymer chain. It was found that the β-phase content in core-shell structured PVDF-GO nanofiber composite can reach up to 92 % for which is 23% and 73% higher that of electrospun PVDF and spin coated PVDF, respectively. This suggests that the core-shell structure of PVDF-GO is effective in improving the phase transformation of α-phase to β-phase, even at a low content of GO. As an interior core-shell, the GO is solidified into nanofiber form which increases the number of heterogeneous nucleation sites to interact with the PVDF polymer chain. The d33 piezoelectric coefficient of PVDF-GO was found to be 61 pm/V which is almost two times higher than PVDF nanofiber. The enhancement of the piezoelectric coefficient can be attributed to the higher β-phase content which can induce a stronger displacement in the sample as a result of the applied electrical field. This might be because of the interaction between the π-bond in GO with the fluorine atoms and hydrogen atoms on adjacent carbon atoms in PVDF polymer chains. It was found that the efficiency of the PVDF sensor in detecting the signal is not sensitive to the amplitude of the transmitted signal. Also, the transmitted signal\u27s amplitude has an insignificant effect on the attenuation rate of the transmitted signal over the distance. It means that the efficiency of the PVDF sensor in detecting the Lamb wave signal is not affected by the amplitude of the transmitted signal. However, the efficiency of the PVDF sensor to detect the transmitted signal is highly affected by the distance between the transducer and receiver. The results indicate that the PVDF device is less efficient in detecting the transmitted signal either at a low-frequency range (\u3c1 \u3ekHz) or the higher range of frequency (\u3e 100 kHz). The optimized frequency was found to be in the range of 1 kHz to 100 kHz to enhance the efficiency of the PVDF sensor. The efficiency of PVDF sensor for detecting the acoustic wave was also studied by hammer impact testing. These results clearly indicate that the sensor is able to detect different magnitudes of surface acoustic waves propagating on the surface. The higher of the impact energy applied to the concrete, the higher the voltage generated by electrospun PVDF AE sensor. The results of this thesis can assist in adopting the electrospun PVDF piezoelectric sensor in a variety of sensing and energy harvesting applications in civil engineering infrastructure

Structural health monitoring of concrete structures using diffuse waves

The work presented in this thesis has aimed to investigate and implement techniques for ultrasonic measurements in structural health monitoring applications for civil structures. The focus of the work has been to make these systems practical in real applications, where the large size of the structures, and the changing environments they are exposed to, pose problems for many methods which otherwise fare well in laboratory settings.There is an increasing demand on the safety and reliability of the civil structures that make up our cities and infrastructure. The field of structural health monitoring aims to provide continuous non-destructive evaluation of such structures. Large concrete structures, such as nuclear power plants or bridges, provide a challenge when implementing such systems. Especially if minor damage is to be detected and even located. Methods based on propagating mechanical waves are known to be useful for detecting structural changes, due to the coupling between the properties of such waves and the mechanical properties of the material. The sensitivity of such measurements generally increase with higher frequencies, and ultrasonic waves can be used to detect minor cracks and early signs of damage. Unfortunately, concrete is a complex material, with aggregates and reinforcement bars on the same order of size as the wavelengths of ultrasonic waves. Ultrasonic waves are quickly scattered and attenuated, which makes traditional pitch-catch measurements difficult over long distances. However, multiply scattered waves contain much information on the material in the structure, and have been shown to be very sensitive to material changes.In this project continuous wave excitation has been used when creating the multiply scattered wave fields. This enables narrow-band detection, which is shown to enable the detection of significantly weaker signals, and thus increase the maximum distance between transducers. Techniques for localizing damage using such continuous wave fields, as well as methods for compensating for effects of changing environmental conditions, are demonstrated. Recommendations are also given for future designers of structural health monitoring systems, as to the choice of frequency, when using multiply scattered wave fields

Smart FRP Composite Sandwich Bridge Decks in Cold Regions

INE/AUTC 12.0

Carbon Nano Tubes (CNTS) for the development of high-performance and smart composites.

Los nanotubos de carbono han atraído una enorme atención en los últimos años debido a sus propiedades multifuncionales sobresalientes. Un número cada vez mayor de trabajos de investigación de primera línea centran su interés en la búsqueda de aplicaciones prácticas que den uso de las notables propiedades de los nanotubos de carbono, incluyendo una elevada resistencia mecánica, propiedades piezorestivas, alta conductividad eléctrica, ligereza, excelente estabilidad química y térmica. En concreto, los estudios más recientes plantean dos grandes ramas de aplicación: fabricación de estructuras aligeradas de alta resistencia, y desarrollo de estructuras inteligentes. Con respecto a la primera línea de aplicación, el desarrollo de materiales compuestos ligeros de alta resistencia conecta con la creciente tendencia de la ingeniería estructural a incorporar materiales compuestos innovadores. Ejemplos recientes como el avión comercial Boeing 787, en el que la mitad del peso fue diseñado con materiales compuestos, predicen un futuro auspicioso para los nanotubos de carbono en la ingeniería aeronáutica. Sin embargo, aún resulta más interesante el comportamiento piezorresistivo de los compuestos reforzados con nanotubos de carbono, ya que posibilita la creación de estructuras que no sólo presentan altas capacidades portantes y reducido peso específico, sino que también ofrecen capacidades de auto-detección de deformaciones. Cuando el material se ve sometido a una deformación externa, en virtud de dicha propiedad piezoresistiva, la conductividad eléctrica varía de modo que es posible correlacionar su respuesta eléctrica con el campo deformacional aplicado. Estas propiedades multifuncionales entroncan con el nuevo paradigma de la Vigilancia de la Salud Estructural el cual aboga por el uso de materiales/estructuras inteligentes para resolver el problema de escalabilidad. En este contexto, la estructura o parte de ella presenta capacidades de auto-detección de tal manera que el mantenimiento basado en la condición puede llevarse a cabo sin necesidad de incluir sensores externos. En ambas líneas, la mayoría de las investigaciones han centrado el estudio en la experimentación, siendo mucho menor el número de trabajos que plantean modelos teóricos capaces de simular las propiedades mecánicas, eléctricas y electromecánicas de estos compuestos. Desde un punto de vista mecánico, existen estudios experimentales que informan acerca de los efectos perjudiciales sobre la respuesta macroscópica de aspectos micromecánicos tales como la tendencia a formar aglomerados, así como la curvatura de los nanotubos de carbono. Es por ello esencial desarrollar modelos teóricos que incorporen estos efectos y asistan al diseño de elementos estructurales reforzados con nanotubos de carbono. Respecto al estudio de las propiedades de conductividad y piezoresistividad, es esencial desarrollar formulaciones teóricas capaces de abordar la optimización de las propiedades de autodetección de deformaciones. Asimismo, es crucial comprender los diferentes mecanismos físicos que rigen la conductividad eléctrica de estos compuestos, de modo que sea posible incorporar su efecto diferencial dentro de un marco teórico. Por último, también es fundamental avanzar hacia el dominio del tiempo con el fin de desarrollar aplicaciones de vigilancia de la salud estructural basada en vibraciones. Con todo ello, los esfuerzos de esta tesis se han centrado en el modelado de las propiedades mecánicas, conductivas y electromecánicas de los compuestos reforzados con nanotubos de carbono para el desarrollo de estructuras inteligentes y de alta resistencia. Estas dos aplicaciones, a saber, compuestos de alta resistencia e inteligentes, han sido enmarcadas en el ámbito de los materiales poliméricos y de cemento, respectivamente. La razón de esta distinción se debe a la presunción de que los compuestos poliméricos pueden encontrar aplicaciones directas como paneles de fuselaje para estructuras de aeronaves, así como refuerzos mecánicos sobre estructuras pre-existentes. En cuanto al uso de nanotubos de carbono como inclusiones multifuncionales para compuestos inteligentes, tanto los materiales poliméricos como los de base cemento ofrecen una amplia gama de aplicaciones potenciales. Sin embargo, la similitud entre los compuestos de base cemento y el hormigón estructural convencional sugiere la idea de desarrollar sensores embebidos que ofrezcan una monitorización continua integrada sin comprometer a priori la durabilidad de la estructura huésped. Tanto las propiedades mecánicas como las conductivas han sido estudiadas mediante métodos de homogeneización de campo medio. Aspectos micromecánicos tales como la relación de aspecto, el contenido, la distribución de la orientación, la ondulación o la aglomeración de los nanotubos se han estudiado en detalle e incorporado al análisis de diferentes elementos estructurales. De manera similar, se han estudiado las propiedades de conductividad eléctrica y auto-detección de deformaciones bajo cargas cuasi-estáticas mediante modelos mixtos de homogenización micromecánica de Mori-Tanaka. Los principales mecanismos que gobiernan las propiedades de transporte eléctrico de estos compuestos, a saber, los efectos de túnel cuántico y la formación de canales conductores, se han incorporado por separado en las simulaciones a través de la teoría de percolación de fibras conductoras. Los resultados teóricos han sido validados con éxito mediante experimentos en condiciones de laboratorio. Finalmente, se ha desarrollado un nuevo circuito equivalente piezorresistivo/piezoeléctrico para el modelado electromecánico de materiales de base cemento reforzado con nanotubos de carbono en el dominio del tiempo. Con los experimentos como base de validación, se ha demostrado que el enfoque propuesto proporciona resultados precisos y ofrece un marco teórico apto para aplicaciones de procesamiento de señales y monitorización de la salud estructural. Se espera que el trabajo desarrollado en esta tesis pueda proporcionar herramientas valiosas que permitan profundizar en la comprensión de los principales aspectos físicos que controlan las propiedades mecánicas, eléctricas y electromecánicas de los compuestos reforzados con nanotubos de carbono. Además, se espera que los resultados presentados en esta tesis impulsen el desarrollo de materiales compuestos auto-sensibles embebidos para aplicaciones de vigilancia de la salud estructural.Carbon nanotubes have drawn enormous attention in recent years due to their outstanding multifunctional properties. A constantly growing number of works at the front line of research pursue potential applications of their remarkable physical properties, including elevated load-bearing capacity, piezoresistive properties, high electrical conductivity, lightness, and excellent chemical and thermal stability. In particular, most recent works contemplate two different application branches: manufacture of light-weight high-strength structures, and development of smart structures. With regard to the first line of application, the development of high-strength lightweight composites connects with the growing tendency of structural engineering to incorporate advanced composite materials. Recent noticeable examples such as the commercial aircraft Boeing 787, in which half of the total weight was designed with composite materials, predict an auspicious future for carbon nanotubes in aircraft structures. Nonetheless, what is even more interesting is the piezoresistive behavior of carbon nanotube-reinforced composites, which allows us to create structures that are not only high-strength and lightweight but also strain-sensitive. When the composites are subjected to external strain fields, in virtue of such piezoresistive properties, the overall electrical conductivity varies in such a way that it is possible to correlate the electrical response with the deformational state of the material. These multifunctional properties are in line with the new paradigm of Structural Health Monitoring which advocates the use of smart materials/structures to solve the scalability issue. In this context, the structure or part of it presents self-sensing capabilities in such a way that the condition-based maintenance can be conducted without necessitating external off-the-shelf sensors. In both lines, most investigations have focused on experimentation. Conversely, the number of theoretical models capable of simulating the mechanical, electrical, and electromechanical properties of these composites is still scarce. From a mechanical point of view, experiments have reported about the detrimental effects of micromechanical aspects such as agglomeration of fillers and curviness on the macroscopic properties. Hence, it is essential to develop theoretical models that allow us to include these effects and assist the design of composite structural elements. With regard to the study of the conductivity and piezoresistivity of carbon nanotube-reinforced composites, it is essential to develop theoretical formulations capable of tackling the optimization of their strain sensitivity. In addition, it is crucial to understand the different physical mechanisms that govern the electrical conductivity of these composites and include them separately in the theoretical framework. Finally, it is also fundamental to move towards the time domain in order to develop applications for vibration-based structural health monitoring. Overall, all the efforts of this thesis have been put into the modeling of the mechanical, conductive and electromechanical properties of carbon nanotube-reinforced composites for the development of high-strength and smart structures. These two applications, namely high-strength and smart composites, have been framed in the realm of polymeric and cement-based materials, respectively. The reason for this distinction is the idea that polymer composites with high load-bearing capacity can find direct applications as fuselage panels for aircraft structures, as well as mechanical reinforcements attached to pre-existing structures. With regard to the use of carbon nanotubes as fillers for smart composites, both polymer and cement-based materials offer an enormous range of potential applications. Nonetheless, the similarity between cement-based composites and regular structural concrete suggests the idea of developing continuous embedded monitoring systems without compromising the durability of the hosting structure a priori. Both mechanical and conductive properties have been studied by means of mean-field homogenization methods. Micromechanical aspects such as filler aspect ratio, content, orientation distribution, waviness or agglomeration have been studied in detail and incorporated to the analysis of different structural elements. Similarly, the electrical conductivity and strain-sensing properties of these composites under quasi-static loadings have been studied by means of mixed Mori-Tanaka micromechanics models. The main mechanisms that underlie the electrical conduction of these composites, namely quantum tunneling effects and conductive networks, have been distinguished by a percolative-type behavior. The theoretical results have been successfully validated by means of experiments under laboratory conditions. Finally, a novel piezoresistive/piezoelectric equivalent lumped circuit has been developed for the electromechanical modeling of carbon nanotube-reinforced cement-based materials in the time domain. With experiments as validating basis, the proposed approach has been shown to provide accurate results and offers a theoretical framework readily applicable to signal processing applications and structural health monitoring. The work developed in this thesis is envisaged to provide valuable tools to further the understanding of the main physical aspects that control the mechanical, electrical and electromechanical properties of composites doped with carbon nanotubes. Furthermore, it is expected to boost the development of embedded self-sensing carbon nanotube-reinforced composites for structural health monitoring applications.Premio Extraordinario de Doctorado U

Dynamic structural health monitoring of slender structures using optical sensors

In this paper we summarize the research activities at the Instituto de Telecomunicações—Pólo de Aveiro and University of Aveiro, in the field of fiber Bragg grating based sensors and their applications in dynamic measurements for Structural Health Monitoring of slender structures such as towers. In this work we describe the implementation of an optical biaxial accelerometer based on fiber Bragg gratings inscribed on optical fibers. The proof-of-concept was done with the dynamic monitoring of a reinforced concrete structure and a slender metallic telecommunication tower. Those structures were found to be suitable to demonstrate the feasibility of FBG accelerometers to obtain the structures’ natural frequencies, which are the key parameters in Structural Health Monitoring and in the calibration of numerical models used to simulate the structure behavior