Forward Modelling and Inversion of the Ultrasonic Wave Propagation Through a Homogeneous and Porous Rock

The aim of my work is to estimate viscoelastic parameters of rock samples from waveforms of ultrasonic waves propagating through these samples. To this end, I develop an automated Python modules in Finite Element Modelling software Abaqus, and tailored it specifically for a controlled transmission experiment using ultrasonic source and receiver. The approach is verified using test Aluminium samples, and then applied to real rocks to estimate ultrasonic attenuation using Prony formulation of viscoelasticity

Oil transmissions pipelines condition monitoring using wavelet analysis and ultrasonic techniques

Proper and sensitive monitoring capability to determine the condition of pipelines is desirable to predict leakage and other failure modes, such as flaws and cracks. Currently methods used for detecting pipeline damage rely on visual inspection or localized measurements and thus, can only be used for the detection of damage that is on or near the surface of the structure. This thesis offers reliable, inexpensive and non-destructive technique, based on ultrasonic measurements, to detect faults within Carbon steel pipes and to evaluate the severity of these faults. The proposed technique allows inspections in areas where conventionally used inspection techniques are costly and/or difficult to apply. This work started by developing 3D Finite Elements Modelling (FEM) to describe the dynamic behaviour of ultrasonic wave propagations into the pipe’s structure and to identify the resonance modes. Consequently, the effects of quantified seeded faults, a 1-mm diameter hole of different depths in the pipe wall, on these resonance modes were examined using the developed model. An experimental test rig was designed and implemented for verifying the outcomes of the finite element model. Conventional analysis techniques were applied to detect and evaluate the severity of those quantified faults. However, those signal processing methods were found ineffective for such analysis. Therefore, a more capable signal processing technique, using continuous wavelet techniques (CWT), was developed. The energy contents of certain frequency bands of the CWT were found to be in good agreement with the model predicted responses and show important information on pipe’s defects. The developed technique is found to be sensitive for minor pipe structural related deficiencies and offers a reliable and inexpensive tool for pipeline integrity management programs

Baseline free structural health monitoring using modified time reversal method and wavelet spectral finite element models

The Lamb wave based, non-contact damage detection techniques are developed using the Modified Time Reversal (MTR) method and the model based inverse problem approach. In the first part of this work, the Lamb wave-based MTR method along with the non-contacting sensors is used for structural damage detection. The use of non-contact measurements for MTR method is validated through experimental results and finite element simulations. A novel technique in frequency-time domain is developed to detect linear damages using the MTR method. The technique is highly suitable for the detection of damages in large metallic structures, even when the damage is superficial, and the severity is low. In this technique, no baseline data are used, and all the wave motion measurements are made remotely using a laser vibrometer. Additionally, this novel MTR based technique is not affected due to changes in the material properties of a structure, environmental conditions, or structural loading conditions. Further, the MTR method is improved for two-dimensional damage imaging. The damage imaging technique is successfully tested through experimental results and finite element simulations. In the second part of this work, an inverse problem approach is developed for the detection and estimation of major damage types experienced in adhesive joints. The inverse problem solution is obtained through an optimization algorithm wherein the objective function is formulated using the Lamb wave propagation data. The technique is successfully used for the detection/estimation of cohesive damages, micro-voids, debonds, and weak bonds. Further, the inverse problem solution is separately obtained through a fully connected artificial neural network. The neural network is trained using the Lamb wave propagation data generated from Wavelet Spectral Finite Element (WSFE) model which is computationally much faster than a conventional finite element model. This inverse problem approach technique requires a single point measurement for the inspection of the entire width of the adhesive joint. The proposed technique can be used as an automated quality assurance tool during the manufacturing process, and as an inspection tool during the operational life of adhesively bonded structures

Defect detection and condition assessment of adhesively-bonded joints using acoustic emission techniques.

The aim of this study is to investigate the application of acoustic emission (AE) techniques to the defect detection and monitoring of adhesively-bonded joints. Pencil Lead Breaks (PLBs) have been used as a simulated AE source to experimentally investigate the characteristics of AE wave-propagation in adhesively-bonded joints, and have been combined with Artificial Neural Networks (ANNs) to provide a novel method of defect detection and sizing. Modal AE analysis has been applied to destructive testing of adhesively-bonded specimens as a novel method to differentiate between fracture modes. Dynamic Finite Element Analysis (FEA) has been utilised to simulate the AE generation and propagation to further investigate the findings of the experimental studies and to assess the applicability of the findings over a broader range of conditions than could be achieved experimentally. PLB tests have been conducted on large (500mm x 500mm x 1mm) aluminium sheet specimens to identify the effects of an adhesive layer on AE wave-propagation. Three specimens were considered: a single sheet; two sheets placed together without adhesive; and an adhesively-bonded specimen. The simulated AE source was applied to the specimens at varying propagation distances and orientations. The acquired signals were processed using wavelet transforms to explore time-frequency features, and compared with modified group velocity curves based on the Rayleigh-Lamb equations to allow identification of wave modes and edge reflections. The effects of propagation distance and source orientation were investigated, while comparison has been made between the three specimens. PLB tests were also used to detect, size and investigate the effect of void-type adhesive defects. Defect-free specimens were used for reference, and specimens with two different void sizes were tested. The PLB source was used to generate simulated AE which would propagate through the defect region and then be recorded with the AE system. Four configurations were tested to assess the effects of source-sensor propagation distance, and source and sensor proximity to the defect. Typical AE parameters of peak amplitude, rise time, decay time, duration, number of counts and AE energy were investigated. Frequency analyses by Fast Fourier Transformation (FFT), partial powers and wavelet transform (WT) were also implemented. ANNs, using the raw Time-Domain signal as an input, were successfully trained and tested to differentiate between the tested specimen types and to estimate the defect sizes. AE-instrumented Double Cantilever-Beam (Mode-I fracture) and Lap-Shear (Mode-II fracture) tests were conducted on similar adhesively-bonded aluminium specimens. Linear source location was used to identify the source-to-sensor propagation distance of each recorded hit. Theoretical dispersion curves were used to identify regions of the signal corresponding to the symmetric and asymmetric wave-modes. Additionally, peak wavelet transform coefficients for the wave modes were compared between the two fracture modes and assessed as an indicator of fracture mode. It was concluded that there is a relationship between the fracture mode and the generated wave modes, with Mode-II fracture typically generating a relatively greater symmetric wave mode than Mode-I fracture. Dynamic FEA was used to replicate both the PLB tests and the destructive tests, and to investigate the effects of a range of parameters that could not all be practically varied in experimental work. Adhesive Young's modulus (representative of different adhesive types), adhesive layer thickness and adhesive void size were varied in the simulated PLB tests. FEA was also used to investigate the effects of fracture mode on the generated acoustic emissions in simulated mixed mode-bending tests, conducted over a range of mode mixities. The FEA results were found to corroborate the results of the experimental work and support a relationship between fracture mode and generated wave modes. It was also identified that a variety of other parameters may also affect the wave modes, and thus need to be considered to achieve effective use of modal analysis to differentiate between fracture modes

Optimal Location of Distributed Generation Sources and Capacitance of Distribution Network to Reduce Losses, Improve Voltage Profile, and Minimizing the Costs Using Genetic and Harmonic Search Algorithm

International audienceReducing losses and improving the voltage profile have been the main objectives of electrical power system designers. One of the suggested solutions for achieving these goals is the use of parallel capacitors and distributed generation sources in distribution systems. A location that is optimized for DG installation may not be the best place to minimize losses in improving the system voltage profile. In this paper, determining the optimal location of the dispersed generation unit and the capacitive bank with the goal of optimizing a target function, including losses, improving the voltage profile, and the cost of investment in capacitors and dispersed production. In this paper, IEEE standard 33 buses is considered for simulation, and the results are obtained by using genetic and harmonic search algorithm indicate that DG optimization and capacitor with a target function in which the loss reduction and improvement of the voltage profile is considered to reduce costs, reduce losses, and improve the voltage profile, which are remarkable improvements

Improvement of acoustic emission technology for stress corrosion cracking

This thesis was submitted for the award of Doctor of Philosophy and was awarded by Brunel University LondonThis thesis investigates the advancement of Acoustic emission (AE) monitoring techniques and analysis methods that could be applied in the field of Structural Health Monitoring (SHM), with an emphasis on the use of advanced AE techniques to detect and locate damage and their application on the monitoring of corrosion and Stress corrosion cracking (SCC) in complex metallic structures. The work was divided into three main areas of research, which are an investigation into using FE (Finite Element) generated delta-T mapping to locate experimental AE signals on complex structures, source location of AE signals generated by corrosion and an experimental investigation into SCC damage locations on a metallic specimen.Lloyd's Register Foundation, Brunel University London and the National Structural Integrity Research Centre (NSIRC

Non-destructive quality control of carbon anodes using modal analysis, acousto-ultrasonic and latent variable methods

La performance des cuves d’électrolyse utilisées dans la production d’aluminium primaire par le procédé Hall-Héroult est fortement influencée par la qualité des anodes de carbone. Celles-ci sont de plus en plus variables en raison de la qualité décroissante des matières premières (coke et braie) et des changements de fournisseurs qui deviennent de plus en plus fréquents afin de réduire le coût d’achat et de rencontrer les spécifications des usines. En effet, les défauts des anodes, tels les fissures, les pores et les hétérogénéités, causés par cette variabilité, doivent être détectés le plus tôt possible afin d’éviter d’utiliser des anodes défectueuses dans les cuves et/ou d’apporter des ajustements au niveau du procédé de fabrication des anodes. Cependant, les fabricants d’anodes ne sont pas préparés pour réagir à cette situation afin de maintenir une qualité d'anode stable. Par conséquent, il devient prioritaire de développer des techniques permettant d’inspecter le volume complet de chaque anode individuelle afin d’améliorer le contrôle de la qualité des anodes et de compenser la variabilité provenant des matières premières. Un système d’inspection basé sur les techniques d’analyse modale et d’acousto-ultrasonique est proposé pour contrôler la qualité des anodes de manière rapide et non destructive. Les données massives (modes de vibration et signaux acoustiques) ont été analysées à l'aide de méthodes statistiques à variables latentes, telles que l'Analyse en Composantes Principales (ACP) et la Projection sur les Structures Latentes (PSL), afin de regrouper les anodes testées en fonction de leurs signatures vibratoires et acousto-ultrasoniques. Le système d'inspection a été premièrement investigué sur des tranches d'anodes industrielles et ensuite testé sur plusieurs anodes pleine grandeur produites sous différentes conditions à l’usine de Alcoa Deschambault au Québec (ADQ). La méthode proposée a permis de distinguer les anodes saines de celles contenant des défauts ainsi que d’identifier le type et la sévérité des défauts, et de les localiser. La méthode acousto-ultrasonique a été validée qualitativement par la tomographie à rayon-X, pour les analyses des tranches d’anodes. Pour les tests réalisés sur les blocs d’anode, la validation a été réalisée au moyen de photos recueillies après avoir coupé certaines anodes parmi celles testées.The performance of the Hall-Héroult electrolysis reduction process used for the industrial aluminium smelting is strongly influenced by the quality of carbon anodes, particularly by the presence of defects in their internal structure, such as cracks, pores and heterogeneities. This is partly due to the decreasing quality and increasing variability of the raw materials available on the market as well as the frequent suppliers changes made in order to meet the smelter’s specifications and to reduce purchasing costs. However, the anode producers are not prepared to cope with these variations and in order to maintain consistent anode quality. Consequently, it becomes a priority to develop alternative methods for inspecting each anode block to improve quality control and maintain consistent anode quality in spite of the variability of incoming raw materials.A rapid and non-destructive inspection system for anode quality control is proposed based on modal analysis and acousto-ultrasonic techniques. The large set of vibration and acousto-ultrasonic data collected from baked anode materials was analyzed using multivariate latent variable methods, such as Principal Component Analysis (PCA) and Partial Least Squares (PLS), in order to cluster the tested anodes based on vibration and their acousto-ultrasonic signatures. The inspection system was investigated first using slices collected from industrial anodes and then on several full size anodes produced under different conditions at the Alcoa Deschambault in Québec (ADQ). It is shown that the proposed method allows discriminating defect-free anodes from those containing various types of defects. In addition, the acousto-ultrasonic features obtained in different frequency ranges were found to be sensitive to the defects severities and were able to locate them in anode blocks. The acousto-ultrasonic method was validated qualitatively using X-ray computed tomography, when studying the anode slices. The results obtained on the full size anode blocks were validated by means of images collected after cutting some tested anodes

Acoustic emission monitoring of pipes; combining finite element simulation and experiment for advanced source location and identification

Impact is a common source of damage in pipes and pipeline systems, detecting the location and nature of damage is vital for reliability and safety of these systems. This work sets out to assess the capacity of Acoustic Emission (AE) to monitor pipes and pipelines for externally applied mechanical damage. AE is a non-destructive testing and monitoring technique that relies on the propagation of elastic (stress) waves generated by impulsive events such as particle impingement, cracking or fluid flow. These waves are recorded at one or more sensors mounted on the surface of the object to be monitored. The key scientific question was to determine the extent to which the structure of a non-impulsive event could be reconstructed using sensors located on the external surface of a pipe. The aim was to combine Finite Element simulations with a series of experiments in order that the relationship between the generating event (source) and the resulting stress-time history at a given point on the surface could be elucidated. Experiments and simulations were carried out with impulsive sources (pencil-lead breaks) and dropped objects, the latter being used to represent a non-impulsive event with a reproducible structure lasting around one second. The AE resulting from these sources was recorded over a period of around 2 seconds for both experiments and simulations. Two test objects, a solid cylindrical steel block of diameter 307mm and length 166mm and various lengths of pipe of diameter 100mm and wall thickness 10mm were used, the former to provide a relatively simple and well-studied platform to examine a number of essential principles. The work on the solid cylinder first validated the simulation of the stress wave from an impulsive source and identified the main modes present, by comparing with analytical solutions. Then it was possible to identify the part of the experimental time series record at a given sensor which is uncontaminated by reflections from the edges and surfaces of the cylinder. The dropped object measurements on the solid cylinder provided clear records of the first and subsequent impacts as the dropped steel balls recoiled and returned back to the surface. There was a clear relationship between the measured AE energy and the estimated incident energy of the dropped objects at a range of timescales irrespective of contamination by reflections. The work on the pipe sections formed the main series of systematic experiments. First it was established that an unloading time in the simulations of around 10-8 seconds gave a reasonable representation of the frequency structure of experimentally observed stress waves. It was also observed from both experiments and simulations that a low amplitude wave travelling at around 5500ms-1 was the first to arrive at any surface sensor. The structure thereafter was complex, probably involving reflections from the inner wall of the cylinder and geometric interference as the wave spreads around the circumference of the pipe. The key finding of this aspect of the work is that the AE line structure of an impulsive source can be reproduced by simulation for short times, for longer times, the damping associated with reflections would require to be measured and introduced into the simulations in order to fully represent the real practical simulation. The degree of damping is important in making a cumulative assessment of multiple impulsive sources. The dropped objects on the pipe confirmed that a mechanical disturbance which is extended in time can be identified from its energy-time imprint carried on the stress wave. The analysis was carried out at three different timescales; short (initial interactions free of reflections), medium (first contact including recoil) and long (involving several bounces). Generally, for medium and short timescales, the AE energy varied with drop height and mass consistently with existing models for balls on plate. For multiple bounces, the behaviour was more erratic probably due to the imprecise control of ball contact point. The simulations of AE worked well at medium and long timescales, providing an idealised framework unto which could be added effects of restitution and damping. At the short timescale, the twin challenges of time and spatial resolution meant that a solution could not be obtained within the limitations of the computing power available. It is generally concluded that AE monitoring can be used to identify the nature of a mechanical disturbance on the surface of a pipe. Suggestions for future work include improvements to the simulations to include attenuation and to better simulate the dynamics of mechanical interactions at the surface, and extensions to the experiments to cover the effect of internal and external pipe environment and the use of mechanical sources which involve actual pipe damage

Wave-based numerical methods for damage identification in components and structures

Components and structures accumulate damage during operation, which degrades their load bearing capacity and is prone to causing catastrophic failure. The demand for fuel efficiency and reduction of pollutant emissions has shifted the design of many structures, predominantly aerospace, to incorporate more composite materials. Composite materials are especially susceptible to critical failure due to operation-induced and accidental damage modes, that have adverse impact on the material strength. Timely detection and identification of damage is important in ensuring structural integrity and safety. Continuous and reliable condition monitoring of components is even more important in lightweight structures that have lower loadbearing redundancy. Recent advances in sensors and signal processing, along with the availability of computational power, have rendered model-based monitoring and damage identification solutions attractive. Computational models for wave simulation remain, however, too heavy for conventional use. Robust and efficient modelling of certain damage modes, such as cracks, introduces additional complexities in numerical models for solids. Computational cost for inverse schemes, where multiple solutions for the unknown and sought damage parameters are required, even becomes prohibitive. This work introduces mesh-independent modelling of damage through XFEM, in wave analysis. The behaviour of damage is investigated with the developed method, and validated by established explicit Finite Element models. A signal processing methodology with wavelet transform is also implemented to further investigate the feasibility of wave scattering as means of damage identification, with a view over available wave actuation and measurement methods. The proposed methodology can achieve significant model reduction calculating wave scattering. Furthermore, identification of cracks is feasible, provided multiple wavemodes can be identified and measured

Hybrid BEM-FEM for 2D and 3D dynamic soil-structure interaction considering arbitrary layered half-space and nonlinearities

Experiences and studies have shown that soil-structure interaction (SSI) effect has a vital role in the dynamic behaviour of a soil-structure system. Despite this, analyses involving dynamic SSI are still challenging for practicing engineers due to their complexity and accessibility. In this thesis, the hybrid BEM-FEM implementation is aimed at practicality by combining commercial software and an in-house code. The pre-processing task can be performed under one graphical environment, and it is enhanced with the capability to compute different types of dynamic sources and other improvements to increase its efficiency, accuracy, and modeling flexibility. Further, the underlying soil is commonly a layered profile with arbitrary geometries. Most existing solutions solve the problem through simplification of the geometry and pattern. One of the main contributions in this thesis is the development of layer-wise condensation method to solve these cases using hybrid BEM-FEM. The method significantly reduces the computational memory requirement. Another challenge in the dynamic SSI addressed in this work is the consideration of secondary nonlinearities. Existing solutions using the time domain BEM and iterative hybrid method are computationally costly, and implementation of such a hybrid method on commercial software is tedious. The solution to address this case using a sequential frequency-time domain procedure is presented. The relatively simple approach makes it possible to consider the nonlinearities in the simulation without using the time domain BEM and without requiring additional iterations. Case studies demonstrating the application of the enhanced hybrid method are presented including cases of bridges, containment structures, and a 3D multi-storey structure under point source and double-couple sources. These case studies illustrate the role of following critical factors such as the site effect, inhomogeneity, and nonlinearities