Assessment of structural reliability: a dynamic monitoring approach

The subject of this thesis is framed in the field of vibration based monitoring. In particular the work is focused on: implementing techniques of extraction of features, the use of collected data to recognize damages and the combined application of knowledge coming from monitoring systems with the classical structural safety formulations to a real case stud

Non-linear system identification in structural dynamics: advances in characterisation of non-linearities and non-linear modal analysis

Many new methods for theoretical modelling, numerical analysis and experimental testing have been developed in non-linear dynamics in recent years. Although the computational power has greatly improved our ability to predict non-linear behaviour, non-linear system identification, a central topic of this thesis, still plays a key role in obtaining and quantifying structural models from experimental data. The first part of the thesis is motivated by the industrial needs for fast and reliable detection and characterisation of structural non-linearities. For this purpose a method based on the Hilbert transform in the frequency domain is proposed. The method detects and characterises structural non-linearities from a single frequency response function and does not require a priori knowledge of the system. The second part of the thesis is driven by current research trends and advances in non-linear modal analysis and adaptive time series processing using the Hilbert-Huang transform. Firstly, the alternatives of the Hilbert transform, which is commonly used in structural dynamics for the estimation of the instantaneous frequency and amplitude despite suffering from a number of numerical issues, are compared to assess their potential for non-linear system identification. Then, a possible relation between the Hilbert-Huang transform and complex non-linear modes of mechanical systems is investigated. Based on this relation, an approach to experimental non-linear modal analysis is proposed. Since this approach integrates the Hilbert-Huang transform and non-linear modes, it allows not only to detect and characterise structural non-linearities in a non-parametric manner, but also to quantify the parameters of a selected model using extracted non-linear modes. Lastly, a new method for the identification of systems with asymmetric non-linear restoring forces is proposed. The application of all proposed methods is demonstrated on simulated and experimental data.Open Acces

Active thermography for the investigation of corrosion in steel surfaces

The present work aims at developing an experimental methodology for the analysis of corrosion phenomena of steel surfaces by means of Active Thermography (AT), in reflexion configuration (RC). The peculiarity of this AT approach consists in exciting by means of a laser source the sound surface of the specimens and acquiring the thermal signal on the same surface, instead of the corroded one: the thermal signal is then composed by the reflection of the thermal wave reflected by the corroded surface. This procedure aims at investigating internal corroded surfaces like in vessels, piping, carters etc. Thermal tests were performed in Step Heating and Lock-In conditions, by varying excitation parameters (power, time, number of pulse, ….) to improve the experimental set up. Surface thermal profiles were acquired by an IR thermocamera and means of salt spray testing; at set time intervals the specimens were investigated by means of AT. Each duration corresponded to a surface damage entity and to a variation in the thermal response. Thermal responses of corroded specimens were related to the corresponding corrosion level, referring to a reference specimen without corrosion. The entity of corrosion was also verified by a metallographic optical microscope to measure the thickness variation of the specimens

Feasibility of vibration-based damage detection for pinned turbine blades

Turbine blades are subjected to various damage mechanisms with fatigue as the primary contributor. During operation, damage accumulates in the form of crack initiation and propagation. This may lead to catastrophic failure, which is cause for concern in terms of availability and safety of the turbine. To optimize the maintenance schedule and to provide operational exibility of the turbine, the state of health of the blades is monitored. This is usually accomplished through non-destructive testing (NDT) during outages. Conventional NDT techniques for in-situ inspection of turbine blade and disk assemblies is di cult and often ine ective, due to limited access to areas of concern, as well as the complex geometries of blade roots. O -site inspection can be costly if the blades are still assembled in the turbine disk since the process of removing and reinstalling these blades is critical and labour-intensive, increasing the turbine downtime and overall costs. These problems could potentially be overcome by employing inspection techniques that o er the prospect of assessing obstructed areas through monitoring the global dynamic characteristics of the structure, which provide relatively easily interpretable data. With global inspection methods, a degree of measurement sensitivity is forfeited but the potential to detect more severe damage, without prior knowledge of the precise damage area location, exists. In this dissertation, the feasibility of a vibration-based structural damage identi cation technique that could be usable in support of conventional NDT to detect cracks in pinned turbine blades during o -line in-situ inspection, is evaluated. The investigation was limited to considering uninstalled single blades only, and thus o -site inspection of this component is regarded above the turbine disk assembly. This is clearly a simpli ed case and does not address the critical case from a practical perspective of having a large number of blades mounted onto a disk with pins, which is really the circumstance under which the technique could become useful. This study must thus be considered as a rst step towards addressing the real practical problem. In this simpli ed problem, the following questions are answered: Is it possible to detect damage in an unconstrained and isolated blade using vibration response, and if so, can di erent damage scenarios be identi ed? The proposed vibration-based damage detection method entails a multi-class support vector machine classi cation procedure in which the natural frequencies are employed as the discriminatory feature for damage detection and identi cation of di erent single-location damage scenarios. The natural frequencies were acquired from accurate experimental modal analysis of freely supported individual pinned turbine blades through impact testing. To con rm and predict the expected behaviour of the blades, a healthy numerical model was built and validated whereafter defects and damage were introduced. This includes geometrical variability at the root, observed in the procured blades, and the anticipated worstcase single-location damage at the most probable locations near or on the root, obtained from literature and discussions with experts in the industry. Arti cial damage, i.e. a uniform 1mm notch, was introduced in the root at the upper pinhole on the leading edge pressure side; and just above the root at the aerofoil base on the trailing- and the leading edge. To establish the discriminative quality of the modal property natural frequency, it was necessary to determine its sensitivity to geometrical variability and damage. It was also required to establish the damage-speci c behaviour or damage trend in the experimental data of i Executive Summary Feasibility of Vibration-based Damage Detection for Pinned Turbine Blades these damage scenarios to conclude their distinctiveness. This analysis was extended to outlining the feature quality by exploring the separability of class clusters for the healthy and damage scenario(s). The feasibility of the proposed method is assessed using experimental data through simple hypothesis testing regarding the detection and identi cation of both geometrical variability in healthy blades, and damage. It was found that healthy blades are very similar, as geometrical variability cannot be detected. This is because the distributions of natural frequencies fall within a range about a mean value in an ambiguous cluster. In contrast to this, the damage scenarios were found to be distinct, and thus discernible from the healthy blades. These classes formed discrete clusters, each with a similar distribution than the healthy blades. The conclusion of the feasibility study serves as proof of concept.Dissertation (MEng)--University of Pretoria, 2018.TM2019Mechanical and Aeronautical EngineeringMEngUnrestricte

Novel Approaches for Structural Health Monitoring

The thirty-plus years of progress in the field of structural health monitoring (SHM) have left a paramount impact on our everyday lives. Be it for the monitoring of fixed- and rotary-wing aircrafts, for the preservation of the cultural and architectural heritage, or for the predictive maintenance of long-span bridges or wind farms, SHM has shaped the framework of many engineering fields. Given the current state of quantitative and principled methodologies, it is nowadays possible to rapidly and consistently evaluate the structural safety of industrial machines, modern concrete buildings, historical masonry complexes, etc., to test their capability and to serve their intended purpose. However, old unsolved problematics as well as new challenges exist. Furthermore, unprecedented conditions, such as stricter safety requirements and ageing civil infrastructure, pose new challenges for confrontation. Therefore, this Special Issue gathers the main contributions of academics and practitioners in civil, aerospace, and mechanical engineering to provide a common ground for structural health monitoring in dealing with old and new aspects of this ever-growing research field

Dynamic characterization of high performance materials for application to cultural heritage

Natural hazards, such as earthquakes, can compromise the integrity of the cultural heritage with potentially devastating effects. The reduction of the seismic vulnerability of the cultural heritage constitutes a question of maximum importance especially in countries where vast cultural heritage combines with a medium or high seismic risk, such as in Italy. From the second half of the last century, the scientific community edited a number of important documents and charts for the conservation, reinforcement and restoration of the cultural heritage. The aim is to mitigate the seismic vulnerability of the cultural heritage. This research focused on high performance materials for applications aimed to structural and seismic protection of cultural heritage, with a special focus on historical masonry structures. In particular, the final aim is to define a self-diagnosis strategy for fibres, yarns and ties in view of efficient, non-invasive and reversible interventions on cultural heritage buildings. In order to set up the scene, the present thesis starts by introducing the reader to the seismic protection of cultural heritage thorough an extensive review on high performance materials, strengthening techniques and systems, taking care to highlight real world applications and limitations of their use. The second step of this work concerns in the mechanical and rheological characterization of high performance material fibres. The materials investigated are essentially Kevlar® 29 (para-aramid), Carbon and Silicon Carbide. To reach this goal, an extensive experimental testing campaign was conducted on fibres and yarns in accordance with specific protocols. A further step was defining appropriate damage indices for different materials, with a special focus on Kevlar® 29. Within the same research programme, a novel testing machine was also designed in cooperation with the Laboratory of Electronic Measurements of the Politecnico di Torino. A prototype-testing machine for dynamic testing on high resistance fibres was built using recycled materials and components. A distinctive feature of this machine is that it can apply to the sample any kind of dynamic excitation (random, impulse, harmonic etc.). A second testing campaign concerned the durability of Kevlar® 29 fibres, which are known to be sensitive to long-term exposure to UV radiation. Accordingly, for this campaign, the samples were artificially damaged by using UV lamps. The analysis of the resonance profiles allowed for the extraction of parameters such as the elastic moduli, quality factors, and non-linear coefficient for a set of fibres. In particular, non-linearity parameters derived from the Krylov-Bogoliubov method demonstrated to be consistent with the damage affecting the fibres. The final chapter of the dissertation concerns a new concept for a tie endowed with self-diagnosis properties, which are obtained by integrating a low cost testing device into the tie model. The self-diagnosis properties system of existing structures has an important role in the preservation of the cultural heritage because the best therapy is preventive maintenance. Specifically, the para-aramid tie system proposed for the reinforcement of historic building constitutes a non-invasive, reversible and repeatable intervention, as required by the main guidelines on preservation of cultural heritage

Assessing seismic collapse of structures using digital cloning techniques.

Rapid, reliable information on earthquake-affected structures' current damage/health conditions and predicting what would happen to these structures under future seismic events play a vital role in accelerating post-event evaluations, leading to optimized on-time decisions. Such rapid and informative post-event evaluations are crucial for earthquake-prone areas, where each earthquake can potentially trigger a series of significant aftershocks, endangering the community's health and wealth by further damaging the already-affected structures. Such reliable post-earthquake evaluations can provide information to decide whether an affected structure is safe to stay in operation, thus saving many lives. Furthermore, they can lead to more optimal recovery plans, thus saving costs and time. The inherent deficiency of visual-based post-earthquake evaluations and the importance of structural health monitoring (SHM) methods and SHM instrumentation have been highlighted within this thesis, using two earthquake-affected structures in New Zealand: 1) the Canterbury Television (CTV) building, Christchurch; 2) the Bank of New Zealand (BNZ) building, Wellington. For the first time, this thesis verifies the theoretically- and experimentally validated hysteresis loop analysis (HLA) SHM method for the real-world instrumented structure of the BNZ building, which was damaged severely due to three earthquakes. Results indicate the HLA-SHM method can accurately estimate elastic stiffness degradation for this reinforced concrete (RC) pinched structure across the three earthquakes, which remained unseen until after the third seismic event. Furthermore, the HLA results help investigate the pinching effects on the BNZ building's seismic response. This thesis introduces a novel digital clone modelling method based on the robust and accurate SHM results delivered by the HLA method for physical parameters of the monitored structure and basis functions predicting the changes of these physical parameters due to future earthquake excitations. Contrary to artificial intelligence (AI) based predictive methods with black-box designs, the proposed predictive method is entirely mechanics-based with an explicitly-understandable design, making them more trusted and explicable to stakeholders engaging in post-earthquake evaluations, such as building owners and insurance firms. The proposed digital clone modelling framework is validated using the BNZ building and an experimental RC test structure damaged severely due to three successive shake-table excitations. In both structures, structural damage intensifies the pinching effects in hysteresis responses. Results show the basis functions identified from the HLA-SHM results for both structures under Event 1 can online estimate structural damage due to subsequent Events 2-3 from the measured structural responses, making them valuable tool for rapid warning systems. Moreover, the digital twins derived for these two structures under Event 1 can successfully predict structural responses and damage under Events 2-3, which can be integrated with the incremental dynamic analysis (IDA) method to assess structural collapse and its financial risks. Furthermore, it enables multi-step IDA to evaluate earthquake series' impacts on structures. Overall, this thesis develops an efficient method for providing reliable information on earthquake-affected structures' current and future status during or immediately after an earthquake, considerably guaranteeing safety. Significant validation is implemented against both experimental and real data of RC structures, which thus clearly indicate the accurate predictive performance of this HLA-based method

Investigation of IBMQ quantum device hardware calibration with Markovian master equation.

Masters Degree. University of KwaZulu-Natal. Durban.In the design of quantum technology, it is crucial to account for the quantum system interacting with its environment to understand the influence of thermal processes and design the devices to avoid e↵ects of relaxation and decoherence of quantum states deteriorating the system beyond use. To accomplish this, a broadening of ideal isolated quantum mechanics is required, namely the theory of open quantum systems. This is most prevalent in the research of quantum error correction, which ensures that the initial quantum state remains intact when it is received and doesn’t decay into a di↵erent state which would change the information carried by the qubit. To investigate the intersection of all these phenomena, open-access cloud-computing services o↵er the ideal experimental environment. One such test-bed is o↵ered by IBM in their Quantum Experience platform which allows for remote access to quantum devices. The IBMQ quantum processors, which make use of superconducting qubit technology, are openly accessible through a cloud service. As such, they have been the focus of a lot of research into the evolution of quantum states while interacting with the environment. In the study of open quantum systems, an assumption is often made that the system and environment share no memory of the interaction of individual quantum states, which simplifies the analysis of the system’s evolution while also being e↵ectively true for large enough systems. Systems that obey this assumption are known as Markovian. New research has devised methods of error correction and tomography of quantum processors when this assumption no longer holds. Additionally, the calibration of the IBMQ processors performed by IBM to provide hardware parameters is performed through a set of techniques that are not guaranteed to yield cohesive results. These primary factors, among others, give rise to the research discussed in this dissertation, and pose the question of how accurate the hardware calibrations are when compared to results obtained through experiments performed on the devices. Furthermore, the approach uses the theory of open quantum systems to assess the hardware calibration while also testing whether the Markovian assumption of a memoryless system holds for the IBMQ quantum devices. This gives insight into the current state of superconducting quantum computers while providing a possible new avenue for quantum error correction from the perspective of the theory of open quantum systems.Publications listed on page iv

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