Métodos de detecção de dano em pontes mediante a utilização de técnicas de monitorização dinâmica : avaliação e aplicação

Tese de doutoramentoIn this thesis, a detailed analysis of the most important vibration-based damage detection methods applied to bridge structures is addressed. Special attention is focused on those methods capable to detect damage only with information provided from a damage stage of the structure. For that purpose, methods based on wavelet analysis, curvature of the mode shapes and changes in the flexibility and stiffness matrices of the structure methods are selected. These methods are easy to implement to bridge structures for its on-line structural evaluation or for its evaluation at different damage stages. The selected damage detection methods are evaluated under three different cases: (1) damage scenarios are simulated on numerical methods for cracked beam structures; (2) experimental tests are carried out in the laboratory with metallic and concrete beams strengthened with CFRP laminates; (3) real-scale bridge structures are tested under different damage scenarios. To do an accurate simulation of the dynamic behaviour of cracked bridges, some of these methods are investigated and evaluated on beams with rectangular cross sections. Later, the methods are generalized to be applicable to more complex structures, like bridges, and to other cross sections, as in composite bridges. From the research done here, it is concluded that the performance of the damage detection methods depends of several factors, for example, the number of sensors located near the damage zones, level of noise present in the acquired dynamic response, location, extension and severity of the damage. Finally, it is recommended to do the process of damage identification in the bridge using the selected group of damage detection methods where successful damage detection is obtained when more than one method clearly indicates damage.Na presente dissertação é apresentada uma análise comparativa detalhada da eficácia de métodos de detecção de dano em pontes. Uma atenção especial é dada aos métodos baseados, exclusivamente, na resposta dinâmica da estrutura danificada. Para este efeito são escolhidos os métodos baseados na análise de “wavelets”, na curvatura das formas modais e na modificação da matriz de flexibilidade ou de rigidez da estrutura. Estes métodos são de fácil implementação em pontes, tanto nos cases em que ser requer uma monitorização contínua do comportamento da estrutura como nos que é suficiente a obtenção de valores em fases discretas da vida útil das mesmas. Os métodos utilizados são avaliados em três situações distintas: (1) a introdução de cenários de dano em modelos numéricos de estruturas fissuradas; (2) a realização de ensaios experimentais em laboratório, em vigas metálicas e em vigas de betão armado reforçadas com lâminas de material compósito (CFRP); (3) a realização de ensaios dinâmicos em pontes de betão e madeira, sob diferentes cenários de dano. Para uma simulação apropriada do comportamento dinâmico de pontes fissuradas, alguns dos métodos existentes são investigados e avaliados em vigas com secções transversais rectangulares. Posteriormente, estes métodos são generalizados para ser aplicáveis a estruturas com secções transversais mais complexas, nomeadamente pontes de secções mistas aço-betão. Dos estudos realizados concluiu-se que a eficácia de detecção de dano dos métodos estudados depende de vários factores, como por exemplo: o número de sensores próximo da zona danificada; o nível de ruído da resposta dinâmica utilizada; a localização, extensão e intensidade do dano. Finalmente, recomenda-se fazer o processo de identificação de dano na ponte usando o grupo escolhido de métodos de detecção de dano onde uma detecção bem sucedida é obtida quando mais de um método detecta claramente o dano.En esta tesis se realiza un análisis detallado de la eficacia de los más importantes métodos de detección de daño aplicados a la respuesta dinámica de puentes. Una atención especial es considerada a los métodos capaces de detectar daño con únicamente la información obtenida de la estructura dañada. Para el análisis de evaluación, son seleccionados los métodos basados en análisis de “wavelets”, en la curvatura de las formas modales y en el cambio de las matrices de flexibilidades y rigideces de la estructura. Estos métodos son elegidos debido a que pueden ser fácilmente implementados en puentes, ya se requiera una monitorización continua ó a través de fases discretas durante la vida útil de las mismas. Los métodos seleccionados son evaluados bajo tres diferentes casos: (1) la introducción de escenarios de daño en modelos numéricos de estructuras agrietadas; (2) la realización de ensayos experimentales en vigas metálicas y de concreto reforzadas con láminas de fibra de carbono (CFRP); (3) la realización de ensayos dinámicos en puentes ante diferentes escenarios de daño. Para una simulación más precisa del comportamiento dinámico de puentes agrietados, se investigan y evalúan algunos de estos métodos en vigas de sección transversal rectangular. Más adelante, estos métodos se generalizan para ser aplicables a estructuras más complejas como puentes y a otro tipo secciones transversales, como en puentes de sección compuesta. De estos estudios realizados, se concluye que la eficacia de los métodos de detección de daño evaluados depende de varios factores, por ejemplo: el número de sensores próximo de la zona dañada; el nivel de ruido de la respuesta dinámica adquirida; la localización, extensión e intensidad del daño. Finalmente, se recomienda hacer el proceso de identificación de daño del puente utilizando el grupo seleccionado de métodos de detección del daño donde una detección adecuada se obtiene cuando más de un método indica claramente el daño.Sustainable Bridges Project in the Sixth Framework ProgrammePortuguese Foundation of Science and Tecnology (FCT) - SRFH/BD/29317/200

Applications of pattern classification to time-domain signals

Many different kinds of physics are used in sensors that produce time-domain signals, such as ultrasonics, acoustics, seismology, and electromagnetics. The waveforms generated by these sensors are used to measure events or detect flaws in applications ranging from industrial to medical and defense-related domains. Interpreting the signals is challenging because of the complicated physics of the interaction of the fields with the materials and structures under study. often the method of interpreting the signal varies by the application, but automatic detection of events in signals is always useful in order to attain results quickly with less human error. One method of automatic interpretation of data is pattern classification, which is a statistical method that assigns predicted labels to raw data associated with known categories. In this work, we use pattern classification techniques to aid automatic detection of events in signals using features extracted by a particular application of the wavelet transform, the Dynamic Wavelet Fingerprint (DWFP), as well as features selected through physical interpretation of the individual applications. The wavelet feature extraction method is general for any time-domain signal, and the classification results can be improved by features drawn for the particular domain. The success of this technique is demonstrated through four applications: the development of an ultrasonographic periodontal probe, the identification of flaw type in Lamb wave tomographic scans of an aluminum pipe, prediction of roof falls in a limestone mine, and automatic identification of individual Radio Frequency Identification (RFID) tags regardless of its programmed code. The method has been shown to achieve high accuracy, sometimes as high as 98%

Load Estimation, Structural Identification and Human Comfort Assessment of Flexible Structures

Stadiums, pedestrian bridges, dance floors, and concert halls are distinct from other civil engineering structures due to several challenges in their design and dynamic behavior. These challenges originate from the flexible inherent nature of these structures coupled with human interactions in the form of loading. The investigations in past literature on this topic clearly state that the design of flexible structures can be improved with better load modeling strategies acquired with reliable load quantification, a deeper understanding of structural response, generation of simple and efficient human-structure interaction models and new measurement and assessment criteria for acceptable vibration levels. In contribution to these possible improvements, this dissertation taps into three specific areas: the load quantification of lively individuals or crowds, the structural identification under non-stationary and narrowband disturbances and the measurement of excessive vibration levels for human comfort. For load quantification, a computer vision based approach capable of tracking both individual and crowd motion is used. For structural identification, a noise-assisted Multivariate Empirical Mode Decomposition (MEMD) algorithm is incorporated into the operational modal analysis. The measurement of excessive vibration levels and the assessment of human comfort are accomplished through computer vision based human and object tracking, which provides a more convenient means for measurement and computation. All the proposed methods are tested in the laboratory environment utilizing a grandstand simulator and in the field on a pedestrian bridge and on a football stadium. Findings and interpretations from the experimental results are presented. The dissertation is concluded by highlighting the critical findings and the possible future work that may be conducted

New non-destructive method for testing the strength of cement mortar material based on vibration frequency of steel bar: Theory and experiment

Timely and accurately obtaining the strength of pouring material, e.g., concrete, cement mortar, is of great significance for engineering construction. In this paper, a non-destructive, economical and accurate strength detection method that suites for on-site using is proposed for the steel bar cement mortar material. The method based on the relationship between the vibration frequency of the steel bar and the properties of the mortar material, which is obtained by solving the Euler-Bernoulli beam problem. Both Particle Flow Code (PFC) software simulation (calibrated) and Split Hopkinson pressure Bar experiment on test samples of cement mortar and steel bar were performed to verify the theoretically obtained relationship. Studies on samples of various aggregate ratio further confirmed such correspondence. Results show that the dynamic stiffness of the cement mortar material dominates the calculation of the vibration frequency of steel bar, while the combined effect of the density, length, elastic modulus, inertia moment of the steel bar can be safely ignored. A single-valued mapping relation exists in between the dynamic stiffness coefficient and the Uniaxial Compressive Strength (UCS) of the cement mortar sample, i.e., increased dynamic stiffness coefficient with increasing UCS. Both experimental and predicted results showed a linear relationship between the vibration frequency of the steel bar and the strength of the mortar material. Fitted linear relations were proposed with coefficients depending on sample size and aggregate ratio and might serve as a good indicator for the strength of the mortar material. Further studies on the effect of internal defects of the mortar materials as well as on samples of more size and aggregate ratio are required to make the proposed method a practical too

Generation and propagation of acoustic emissions in buried steel infrastructure for monitoring soil–structure interactions

Soil–structure systems (e.g. pipelines, pile foundations, retaining structures) deteriorate with time and experience relative deformations between the soil and structural elements. Whether a result of age, working conditions, or environmental conditions, deformations have the potential to cause catastrophic social, economic, and environmental issues, including limit state failure (fatigue, serviceability, ultimate). The UK spends £100s of millions a year spent on infrastructural maintenance; the early detection of deterioration processes could reduce this spend by an order of magnitude.Techniques to monitor ground instability and deterioration are consequently increasing in use, with most conventional approaches providing localised information on deformation at discrete time intervals. Nascent technologies (e.g. ShapeAccelArray, fibre optics) are however beginning to provide continuous measurements, allowing for near real-time observations to be made, although none are without either technical limitation or prohibitive cost.A novel monitoring system is proposed, whereby pre-existing and newly built steel infrastructure (e.g. utility pipes, pile foundations) are employed as waveguides to measure soil-steel interaction-generated AE using piezoelectric sensors. With this, a two-stage quantitative framework for understanding soil-steel interaction-generated AE and its propagation through steel structures is also developed where (stage 1) informs the creation of an adaptable sensor network for a variety of infrastructure systems, and stage (2) informs interpretations of the collected AE data to allow for decision makers to take appropriate action. Timely actions made possible by such a framework is of great significance to practitioners, having the potential to reduce the direct and indirect impacts of deterioration and deformation, whether long- and short-term.Stage 1 used an extensive programme of computational models, alongside small- and large-scale physical models, to enable attenuation coefficients to be quantified for a range of soil types. It was shown that both the structure and bounding materials, i.e. the burial system, significantly influenced propagation and attenuation through steel structures. In free-systems, though, the frequency-thickness product was more influential; propagation distances of 100s of metres are obtained at products Stage 2 used a programme of large direct-shear box tests to allow for relationships between AE and normal effective stress, mobilised shearing resistance, and shearing velocity to be quantified. This enabled for quantitative interpretations of soil-steel interaction behaviours to be made using various AE parameters. Both the magnitude of values, and the rates of change of the parameters, could be used in the interpretation of behaviours. Shearing and stress conditions of sand could also be determined, increasing proportionally with AE activity, whilst the point at which full shear strength mobilisation occurs was also identifiable.</div

Elastic Waves Along a Fracture Intersection

Fractures and fracture networks play a significant role in the subsurface hydraulic connectivity within the Earth. While a significant amount of research has been performed on the seismic response of single fractures and sets of fractures, few studies have examined the effect of fracture intersections on elastic wave propagation. Intersections play a key role in the connectivity of a fracture network that ultimately affects the hydraulic integrity of a rock mass. In this dissertation two new types of coupled waves are examined that propagate along intersections. 1) A coupled wedge wave that propagates along a surface fracture with particle motion highly localized to the intersection of a fracture with a free surface, and 2) fracture intersection waves that propagate along the intersection between two orthogonal fractures. Theoretical formulations were derived to determine the particle motion and velocity of intersection waves. Vibrational modes calculated from the theoretical formulation match those predicted by group theory based on the symmetry of the problem. For the coupled wedge wave, two vibrational modes exist that range in velocity between the wedge wave and Rayleigh wave velocity and exhibit either wagging or breathing motion depending on the Poisson\u27s ratio. For the intersection waves, the observed modes depend on the properties of the fractures forming the intersection. If both fractures have equal stiffness four modes exist, two with wagging and two with breathing motion. If the fractures have unequal stiffness, four modes also exist, but the motion depends on the Poisson\u27s ratio. The velocity of intersection waves depends on the coupling or stiffness of the intersection and frequency of the signal. In general, the different modes travel with speeds between the wedge wave and bulk shear wave velocity. Laboratory experiments were performed on isotropic and anisotropic samples to verify the existence of these waves. For both waves, the observed signals were determined to depend on the applied load, which affects the stiffness of the fractures. These results have significant implications for fracture network characterization using remote techniques in both the laboratory and the field. The coupling parameter used in this discussion, i.e., specific stiffness, is a potential parameter to link the hydraulic properties of the fracture intersections to their seismic response. These results are a first step towards remote characterization to determine the hydraulic connectivity of fracture networks

Advanced signal processing techniques for multimodal ultrasonic guided wave response

This thesis was submitted for the award of Doctor of Philosophy and was awarded by Brunel University LondonUltrasonic technology is commonly used in the eld of Non-Destructive Testing (NDT) of metal structures such as steel, aluminium, etc. Compared to ultrasonic bulk waves that travel in infinite media with no boundary influence, Ultrasonic Guided Waves (UGWs) require a structural boundary for propagation such that they can be used to inspect and monitor long elements of a structure from a single position. The greatest challenges for any UGW system are the plethora of wave modes arising from the geometry of the structural element which propagate with a range of frequency dependent velocities and the interpretation of these combined signals reflected by discontinuities in the structural element. In this thesis, a technique is developed which facilitates the measurement of Time of Arrival (ToA) and group velocity dispersion curves of wave modes for one dimensional structures as far as wave propagation is concerned. A second technique is also presented which employs the dispersion curves to deliver enhanced range measurements in complex multimodal UGW responses. Ultimately, the aforementioned techniques are used as a part of the analysis of previously unreported signals arising from interactions of UGWs with piezoelectric transducers. The first signal processing technique is presented which used a combination of frequency-sweep measurement, sampling rate conversion and the Fourier transform. The technique is applied to synthesized and experimental data in order to identify different wave modes in complex UGW signals. It is demonstrated that the technique has the capability to derive the ToA and group velocity dispersion curve of the wave modes of interest. The second signal processing technique uses broad band excitation, dispersion compensation and cross-correlation. The technique is applied to synthesized and experimental data in order to identify different wave modes in complex UGW signals. It is demonstrated that the technique noticeably improves the Signal to Noise Ratio (SNR) of the UGW response using a priori knowledge of the dispersion curve. It is also able to derive accurate quantitative information about the ToA and the propagation distance. During the development of the aforementioned signal processing techniques, some unwanted wave-packets are identified in the UGW responses which are found to be induced by the coupling of a shear mode piezoelectric transducer at the free edge of the waveguide. Accordingly, the effect of the force on the piezoelectric transducers and the corresponding reflections and mode conversions are studied experimentally. The aforementioned signal processing techniques are also employed as a part of the study. A Finite Element Analysis (FEA) procedure is also presented which can potentially improve the theoretical predictions and converge to results found in experimental routines. The approach enhances the con dence in the FEA models compared to traditional approaches. The outcome of the research conducted in this thesis paves the way to enhance the reliability of UGW inspections by utilizing the signal processing techniques and studying the multimodal responses.The Engineering and Physical Sciences Research Board (EPSRC), The Centre for Electronic Systems Research (CESR) of Brunel University London, The Integrity Management Group (IMG) of TWI and Plant Integrity Ltd

The applications of near infra-red fibre bragg grating sensors for wave propagation based structural health monitoring of thin laminated composite plates

This thesis contributes to the research and development towards achieving better structural health monitoring (SHM) system for composite structures. Composites are widely used in critical engineering applications due to the advantage of higher specific strength and stiffness compared to other conventional materials. However, composite laminates have a very high probability of unexpected damage development during service. This study uses fiber Bragg grating (FBG) sensor to create a practical and robust SHM tool based on monitoring the acoustic emission, in order to provide continuous information of the structure's condition. The remarkable capability of using the FBG sensors for dynamic sensing has been demonstrated, in particular for the wave propagation based SHM. Combined with FBG sensor technologies, the wave propagation based SHM such as acoustic emission (AE), ultrasonic evaluation and acousto-ultrasonics becomes more exciting. The FBG sensor has the ability of acquiring both static and dynamic strains with a single sensor. Besides, the physical size of FBG sensor provides greater access to embed them in composite structures without significantly affecting its structural properties. This study also emphasizes some drawbacks in the use of piezoelectric sensors in the wave propagation based SHM of composite structures, specifically in the AE applications. In most optical fiber based SHM applications to date, people have used only FBG sensors with wavelength 1550 nm. The FBG sensors with this wavelength are commonly used in industries such as telecommunications and health. However, there is an option of using near infra-red (NIR) FBG range which is comparably cheap in terms of total system design, yet offers the same performance of a conventional 1550 nm range FBGs. This research work presents the NIR FBG dynamic sensing system, as a wave propagation-based SHM system for monitoring the damages in thin glass fiber reinforced composite plates. The NIR-FBG sensor system has been validated successfully, in particular for thin composite plate's applications. The sensor system has shown its unique capability whereby it can be applied in the area which cannot be accessed by standard piezoelectric based system. The developed NIR FBG sensor system has shown its competitiveness and ability to replace the piezoelectric sensors in the 'wave propagation based SHM' of laminated composite plates