Effect of road quality in structural health monitoring under operational conditions

The effect of unevenness in a bridge deck for the purpose of Structural Health Monitoring (SHM) under operational conditions is studied in this paper. The moving vehicle is modelled as a single degree of freedom system traversing the damaged beam at a constant speed. The bridge is modelled as an Euler-Bernoulli beam with a breathing crack, simply supported at both ends. The breathing crack is treated as a nonlinear system with bilinear stiffness characteristics related to the opening and closing of crack. The unevenness in the bridge deck considered is modelled using road classification according to ISO 8606:1995(E). Numerical simulations are conducted considering the effects of changing road surface classes from class A - very good to class E - very poor. Cumulant based statistical parameters, based on a new algorithm are computed on stochastic responses of the damaged beam due to passages of the load in order to calibrate the damage. Possibilities of damage detection and calibration under benchmarked and non-benchmarked cases are considered. The findings of this paper are important for establishing the expectations from different types of road roughness on a bridge for damage detection purposes using bridge-vehicle interaction where the bridge does not need to be closed for monitoring.

Effects of vehicle speed in structural health monitoring from operational conditions of bridges

The effects of vehicle speed for Structural Health Monitoring (SHM) of bridges under operational conditions are studied in this paper. The moving vehicle is modelled as a single degree oscillator traversing a damaged beam at a constant speed. The bridge is modelled as simply supported Euler-Bernoulli beam with a breathing crack. The breathing crack is treated as a nonlinear system with bilinear stiffness characteristics related to the opening and closing of crack. The unevenness of the bridge deck is modelled using road classification according to ISO 8606:1995(E). The stochastic description of the unevenness of the road surface is used as an aid to monitor the health of the structure in its operational condition. Numerical simulations are conducted considering the effects of changing vehicle speed with regards to cumulant based statistical damage detection parameters. The detection and calibration of damage at different levels is based on an algorithm dependent on responses of the damaged beam due to passages of the load. Possibilities of damage detection and calibration under benchmarked and non-benchmarked cases are considered. Sensitivity of calibration values is studied. The findings of this paper are important for establishing the expectations from different vehicle speeds on a bridge for damage detection purposes using bridge-vehicle interaction where the bridge does not need to be closed for monitoring. The identification of bunching of these speed ranges provides guidelines for using the methodology developed in the paper.

Bridge weigh-in-motion system and Structural Health Monitoring using fiber optic sensors

There have been over 3000 bridge weigh-in-motion (B-WIM) installations in 25 countries worldwide, this has led vast improvements in post processing of B-WIM systems since its introduction in the 1970’s. This paper introduces a new low-power B-WIM system using fibre optic sensors (FOS). The system consisted of a series of FOS which were attached to the soffit of an existing integral bridge with a single span of 19m. The site selection criteria and full installation process has been detailed in the paper. A method of calibration was adopted using live traffic at the bridge site and based on this calibration the accuracy of the system was determined.

Structural health monitoring of storage tanks

In this thesis, Ph.D candidate presents a compact sensor node (SN) designed for long-term and real-time acoustic emission (AE) monitoring of above ground storage tanks (ASTs). Each SN exploits up to three inexpensive low-frequency sensors based on piezoelectric diaphragms for effective leakage detection, and it is capable by means of built-in Digital Signal Processing functionalities to process the acquired time waveforms extracting the AE features usually required by testing protocols. Alternatively, capability to plug three high frequency AE sensors to a SN for corrosion simulated phenomena detection is envisaged and demonstrated. Another innovative aspect that the Ph.D candidate presents in this work is an alternative mathematical model of corrosion location on the bottom of the AST. This approach implies considering the three-dimensional localization model versus the two-dimensional commonly used according to the literature. This approach is aimed at significant optimization in the number of sensors in relation to the standard approach for solving localization problems as well as to allow filtering the false AE events related to the condensate droplets from AST ceiling. The technological implementation of this concept required the solution of a number of technical problems, such as the precise time of arrival (ToA) signal estimation, vertical localization of the AE source and multilaration solution that were discussed in detail in this work. To validate the developed prototype, several experimental campaigns were organized that included the simulation of target phenomena both in laboratory conditions and on a real water storage tank. The presented test results demonstrate the successful application of the developed AE system both for simulated leaks and for corrosion processes on the tank bottom. Mathematical and technological algorithms for localization and characterization of AE signals implemented during the development of the prototype are also confirmed by the test results.

Damage Detection in Flexible Plate Using Lamb Waves

This paper presents an experimental technique for structural health monitoring (SHM) based on Lamb waves approach in an aluminum plate using piezoelectric material as actuators and sensors. Lamb waves are a form of elastic perturbation that remains guided between two parallel free surfaces, such as the upper and lower surfaces of a plate, beam or shelf. Lamb waves are formed when the actuator excites the surface of the structure with a pulse after receiving a signal. Two PZTs were placed in the plate surface and one of them was used to send a predefined wave through the structure. Thus, the other PZT (adjacent) becomes the sensor. Using this methodology, this paper presents one case of damage detection considering the aluminum plate in the free-free-free-free boundary condition. The damage was simulated by adding additional mass on the plate. It is proposed two damage detection indexes obtained from the experimental signal, involving the Fast Fourier Transform (FFT) and the power spectral density (PSD) that were computed using the output signal. The results show the viability of the presented methodology to damage detection in smart structures

Structural health monitoringby using transmissibility

El control del estado en el que se encuentran las estructuras ha experimentado un gran auge desde hace varias décadas, debido a que los costes de rehabilitación de estructuras tales como los oleoductos, los puentes, los edificios y otras más son muy elevados. En las últimas dos décadas, se han desarrollado una gran cantidad de métodos que permiten identificar el estado real de una estructura, basándose en modelos físicos y datos de ensayos. El ensayo modal es el más común; mediante el análisis modal experimental de una estructura se pueden determinar parámetros como la frecuencia, los modos de vibración y la amortiguación y también la función de respuesta en frecuencia de la estructura. Mediante estos parámetros se pueden implementar diferentes indicadores de daño. Sin embargo, para estructuras complejas y grandes, la implementación de metodologías basadas en la función de respuesta en frecuencia requeriría realizar hipótesis sobre la fuerza utilizada para excitar la estructura. Dado que el análisis modal operacional utiliza solamente las señales de respuesta del sistema para extraer los parámetros dinámicos estructurales y, por tanto, para evaluar el estado de una estructura, el uso de la transmisibilidad sería posible. En este sentido, dentro del análisis modal operacional, la transmisibilidad ha concentrado mucha atención en el mundo científico en la última década. Aunque se han publicado muchos trabajos sobre el tema, en esta Tesis se proponen diferentes técnicas para evaluar el estado de una estructura basándose exclusivamente en la transmisibilidad. En primer lugar, se propone un indicador de daño basado en un nuevo parámetro, la coherencia de transmisibilidad; El indicador se ha valido mediante resultados numéricos y experimentales obtenidos sobre un pórtico de tres pisos. En segundo lugar, la distancia de Mahalanobis se aplica sobre la transmisibilidad como procedimiento para detectar variaciones estructurales provocadas por el daño. Este método se ha validado con éxito sobre una viga libre-libre ensayada experimentalmente. En tercer lugar, se ha implementado una red neuronal basada en medidas de transmisibilidad como metodología de predicción de daño sobre una viga simulada numéricamente. ABSTRACT Structural health monitoring has experienced a huge development from several decades ago since the cost of rehabilitation of structures such as oil pipes, bridges and tall buildings is very high. In the last two decades, a lot of methods able to identify the real stage of a structure have been developed basing on both models and experimental data. Modal testing is the most common; by carrying out the experimental modal analysis of a structure, some parameters, such as frequency, mode shapes and damping, as well as the frequency response function of the structure can be obtained. From these parameters, different damage indicators have been proposed. However, for complex and large structures, any frequency domain approach that relies on frequency response function estimation would be of difficult application since an assumption of the input excitations to the system should be carried out. Operational modal analysis uses only output signals to extract the structural dynamic parameters and, therefore, to identify the structural stage. In this sense, within operational modal analysis, transmissibility has attracted a lot of attention in the scientific field in the last decade. In this work new damage detection approaches based on transmissibility are developed. Firstly, a new theory of transmissibility coherence is developed and it is tested with a three-floor-structure both in simulation and in experimental data analysis; secondly, Mahalanobis distance is taken into use with the transmissibility, and a free-free beam is used to test the approach performance; thirdly, neural networks are used in transmissibility for structural health monitoring; a simulated beam is used to validate the proposed method.

Structural health monitoringby using transmissibility

El control del estado en el que se encuentran las estructuras ha experimentado un gran auge desde hace varias décadas, debido a que los costes de rehabilitación de estructuras tales como los oleoductos, los puentes, los edificios y otras más son muy elevados. En las últimas dos décadas, se han desarrollado una gran cantidad de métodos que permiten identificar el estado real de una estructura, basándose en modelos físicos y datos de ensayos. El ensayo modal es el más común; mediante el análisis modal experimental de una estructura se pueden determinar parámetros como la frecuencia, los modos de vibración y la amortiguación y también la función de respuesta en frecuencia de la estructura. Mediante estos parámetros se pueden implementar diferentes indicadores de daño. Sin embargo, para estructuras complejas y grandes, la implementación de metodologías basadas en la función de respuesta en frecuencia requeriría realizar hipótesis sobre la fuerza utilizada para excitar la estructura. Dado que el análisis modal operacional utiliza solamente las señales de respuesta del sistema para extraer los parámetros dinámicos estructurales y, por tanto, para evaluar el estado de una estructura, el uso de la transmisibilidad sería posible. En este sentido, dentro del análisis modal operacional, la transmisibilidad ha concentrado mucha atención en el mundo científico en la última década. Aunque se han publicado muchos trabajos sobre el tema, en esta Tesis se proponen diferentes técnicas para evaluar el estado de una estructura basándose exclusivamente en la transmisibilidad. En primer lugar, se propone un indicador de daño basado en un nuevo parámetro, la coherencia de transmisibilidad; El indicador se ha valido mediante resultados numéricos y experimentales obtenidos sobre un pórtico de tres pisos. En segundo lugar, la distancia de Mahalanobis se aplica sobre la transmisibilidad como procedimiento para detectar variaciones estructurales provocadas por el daño. Este método se ha validado con éxito sobre una viga libre-libre ensayada experimentalmente. En tercer lugar, se ha implementado una red neuronal basada en medidas de transmisibilidad como metodología de predicción de daño sobre una viga simulada numéricamente. ABSTRACT Structural health monitoring has experienced a huge development from several decades ago since the cost of rehabilitation of structures such as oil pipes, bridges and tall buildings is very high. In the last two decades, a lot of methods able to identify the real stage of a structure have been developed basing on both models and experimental data. Modal testing is the most common; by carrying out the experimental modal analysis of a structure, some parameters, such as frequency, mode shapes and damping, as well as the frequency response function of the structure can be obtained. From these parameters, different damage indicators have been proposed. However, for complex and large structures, any frequency domain approach that relies on frequency response function estimation would be of difficult application since an assumption of the input excitations to the system should be carried out. Operational modal analysis uses only output signals to extract the structural dynamic parameters and, therefore, to identify the structural stage. In this sense, within operational modal analysis, transmissibility has attracted a lot of attention in the scientific field in the last decade. In this work new damage detection approaches based on transmissibility are developed. Firstly, a new theory of transmissibility coherence is developed and it is tested with a three-floor-structure both in simulation and in experimental data analysis; secondly, Mahalanobis distance is taken into use with the transmissibility, and a free-free beam is used to test the approach performance; thirdly, neural networks are used in transmissibility for structural health monitoring; a simulated beam is used to validate the proposed method.

Structural health of a road tunnel intersecting a large and active rockslide

This work presents the case of the San Lorenzo road tunnel, a transportation infrastructure located in the northern part of Italy, involved in the so-called Passo della Morte landslide. This tunnel crosses a large rockslide characterized by slow movements. Damages like water seepage inside the tunnel and concrete lining detachments have surfaced through the years, increasing the risk. This work develops the objective of tracing back the landslide-induced stresses directly responsible for the cracks’ pattern on the most damaged segments of the tunnel. The first section of this work gives information about the global framework: site geography and its strategic relevance, geological setting, hydrological and climate conditions will be provided. The road tunnel infrastructure and its interaction with the landslide phenomena will be discussed together with the active monitoring system, which has been working for more than 20 years. In the second part the several steps and tools used to add more details about the road damages are reported. A visualization of the actual state of the most damaged portions of the road has been reached. Then the attention has been addressed to the stresses acting on the road tunnel’s aforesaid portions, developing a FEM model of a section of the tunnel through a selected software. This latter process can be deemed as a beginning for further developments. Some preliminary results are shown to demonstrate the goodness of the assumptions made. The possible future set by this work aims at constant enlargement of information to be provided to the FEM software, and at the validation of the obtained results through the monitoring data interpretative tools.

Artificial Immune Systems with Negative Selection Applied to Health Monitoring of Aeronautical Structures

In this paper we present a system for aircraft structural health monitoring based on artificial immune systems with negative selection. Inspired by a biological process, the principle of discrimination proper/non-proper, identifies and characterizes the signs of structural failure. The main application of this method is to assist in the inspection of aircraft structures, to detect and characterize flaws and decision making in order to avoid disasters. We proposed a model of an aluminum beam to perform the tests of the method. The results obtained by this method are excellent, showing robustness and accuracy.

Health monitoring of web plastifying dampers subjected to cyclic loading through vibration tests

This article investigates experimentally the application of health monitoring techniques to assess the damage on a particular kind of hysteretic (metallic) damper called web plastifying dampers, which are subjected to cyclic loading. In general terms, hysteretic dampers are increasingly used as passive control systems in advanced earthquake-resistant structures. Nonparametric statistical processing of the signals obtained from simple vibration tests of the web plastifying damper is used here to propose an area index damage. This area index damage is compared with an alternative energy-based index of damage proposed in past research that is based on the decomposition of the load?displacement curve experienced by the damper. Index of damage has been proven to accurately predict the level of damage and the proximity to failure of web plastifying damper, but obtaining the load?displacement curve for its direct calculation requires the use of costly instrumentation. For this reason, the aim of this study is to estimate index of damage indirectly from simple vibration tests, calling for much simpler and cheaper instrumentation, through an auxiliary index called area index damage. Web plastifying damper is a particular type of hysteretic damper that uses the out-of-plane plastic deformation of the web of I-section steel segments as a source of energy dissipation. Four I-section steel segments with similar geometry were subjected to the same pattern of cyclic loading, and the damage was evaluated with the index of damage and area index damage indexes at several stages of the loading process. A good correlation was found between area index damage and index of damage. Based on this correlation, simple formulae are proposed to estimate index of damage from the area index damage.

Health monitoring of a new hysteretic damper subjected to earthquakes on a shaking table

This paper presents the experimental results obtained by applying frequency-domain structural health monitoring techniques to assess the damage suffered on a special type of damper called Web Plastifying Damper (WPD). The WPD is a hysteretic type energy dissipator recently developed for the passive control of structures subjected to earthquakes. It consists of several I-section steel segments connected in parallel. The energy is dissipated through plastic deformations of the web of the I-sections, which constitute the dissipative parts of the damper. WPDs were subjected to successive histories of dynamically-imposed cyclic deformations of increasing magnitude with the shaking table of the University of Granada. To assess the damage to the web of the I-section steel segments after each history of loading, a new damage index called Area Index of Damage (AID) was obtained from simple vibration tests. The vibration signals were acquired by means of piezoelectric sensors attached on the I-sections, and non-parametric statistical methods were applied to calculate AID in terms of changes in frequency response functions. The damage index AID was correlated with another energy-based damage index-ID- which past research has proven to accurately characterize the level of mechanical damage. The ID is rooted in the decomposition of the load-displacement curve experienced by the damper into the so-called skeleton and Bauschinger parts. ID predicts the level of damage and the proximity to failure of the damper accurately, but it requires costly instrumentation. The experiments reported in this paper demonstrate a good correlation between AID and ID in a realistic seismic loading scenario consisting of dynamically applied arbitrary cyclic loads. Based on this correlation, it is possible to estimate ID indirectly from the AID, which calls for much simpler and less expensive instrumentation.

Vibration based health monitoring of civil infrastructure using energy harvesting techniques

The use of structural health monitoring of civil structures is ever expanding and by assessing the dynamical condition of structures, informed maintenance management can be conducted at both individual and network levels. With the continued growth of information age technology, the potential arises for smart monitoring systems to be integrated with civil infrastructure to provide efficient information on the condition of a structure. The focus of this thesis is the integration of smart technology with civil infrastructure for the purposes of structural health monitoring. The technology considered in this regard are devices based on energy harvesting materials. While there has been considerable focus on the development and optimisation of such devices using steady state loading conditions, their applications for civil infrastructure are less known. Although research is still in initial stages, studies into the uses associated with such applications are very promising. Through the use of the dynamical response of structures to a variety of loading conditions, the energy harvesting outputs from such devices is established and the potential power output determined. Through a power variance output approach, damage detection of deteriorating structures using the energy harvesting devices is investigated. Further applications of the integration of energy harvesting devices with civil infrastructure investigated by this research includes the use of the power output as a indicator for control. Four approaches are undertaken to determine the potential applications arising from integrating smart technology with civil infrastructure, namely • Theoretical analysis to determine the applications of energy harvesting devices for vibration based health monitoring of civil infrastructure. • Laboratory experimentation to verify the performance of different energy harvesting configurations for civil infrastructure applications. • Scaled model testing as a method to experimentally validate the integration of the energy harvesting devices with civil infrastructure. • Full scale deployment of energy harvesting device with a bridge structure. These four approaches validate the application of energy harvesting technology with civil infrastructure from a theoretical, experimental and practical perspective.

Towards a Workflow Management Approach for Health Monitoring of Bridges

Part 14: Interoperability and Integration

Modelling of Lamb waves in composite laminated plates excited by interdigital transducers

The technique of permanently attaching interdigital transducers (IDT) to either flat or curved structural surfaces to excite single Lamb wave mode has demonstrated great potential for quantitative non-destructive evaluation and smart materials design, In this paper, the acoustic wave field in a composite laminated plate excited by an IDT is investigated. On the basis of discrete layer theory and a multiple integral transform method, an analytical-numerical approach is developed to evaluate the surface velocity response of the plate due to the IDTs excitation. In this approach, the frequency spectrum and wave number spectrum of the output of IDT are obtained directly. The corresponding time domain results are calculated by applying a standard inverse fast Fourier transformation technique. Numerical examples are presented to validate the developed method and show the ability of mode selection and isolation. A new effective way of transfer function estimation and interpretation is presented by considering the input wave number spectrum in addition to the commonly used input frequency spectrum. The new approach enables the simple physical evaluation of the influences of IDT geometrical features such as electrode finger widths and overall dimension and excitation signal properties on the input-output characteristics of IDT. Finally, considering the convenience of Mindlin plate wave theory in numerical computations as well as theoretical analysis, the validity is examined of using this approximate theory to design IDT for the excitation of the first and second anti-symmetric Lamb modes. (C) 2002 Elsevier Science Ltd. All rights reserved.

Single mode Lamb waves in composite laminated plates generated by piezoelectric transducers

The technique of permanently attaching piezoelectric transducers to structural surfaces has demonstrated great potential for quantitative non-destructive evaluation and smart materials design. For thin structural members such as composite laminated plates, it has been well recognized that guided Lamb wave techniques can provide a very sensitive and effective means for large area interrogation. However, since in these applications multiple wave modes are generally generated and the individual modes are usually dispersive, the received signals are very complex and difficult to interpret. An attractive way to deal with this problem has recently been introduced by applying piezoceramic transducer arrays or interdigital transducer (IDT) technologies. In this paper, the acoustic wave field in composite laminated plates excited by piezoceramic transducer arrays or IDT is investigated. Based on dynamic piezoelectricity theory, a discrete layer theory and a multiple integral transform method, an analytical-numerical approach is developed to evaluate the input impedance characteristics of the transducer and the surface velocity response of the plate. The method enables the quantitative evaluation of the influence of the electrical characteristics of the excitation circuit, the geometric and piezoelectric properties of the transducer array, and the mechanical and geometrical features of the laminate. Numerical results are presented to validate the developed method and show the ability of single wave mode selection and isolation. The results show that the interaction between individual elements of the piezoelectric array has a significant influence on the performance of the IDT, and these effects can not be neglected even in the case of low frequency excitation. It is also demonstrated that adding backing materials to the transducer elements can be used to improve the excitability of specific wave modes. (C) 2002 Elsevier Science Ltd. All rights reserved.