Дисертації з теми "Plate-like structure"

A considerable amount of mineral particles are found to have a plate-like shape. The work in this thesis concerns theoretical investigations, using a Monte Carlo method, of the properties of such particles in aqueous solutions. The objectives were first to create a model that could capture the essential physics of clay suspensions and also to understand the role of thermodynamics in certain chemical processes. For all investigations, the results are related to experimental studies. The acid-base behavior of clays have been studied, using the primitive model, and an excellent agreement between simulated and experimental results was found. The formation of gel phases as a function of the charge anisotropy have also been investigated. Liquid-gel and sol-gel transitions are found to occur for high and moderate charge anisotropy, respectively. These transitions were also found to be size and salt dependent. In absence of charge anisotropy, a liquid-glass transition is reported. The formation of smectic and columnar liquid crystals phases with plate-like particles has been found to be favored by a strong charge anisotropy, in opposition to what was observed for nematic phases. New liquid-crystal phases were also reported. The stability and growth of nanoplatelets is discussed. It was found that the internal Coulombic repulsion could be the cause of the limited growth of C-S-H platelets. The influence of thermodynamics on the agregation mode of such platelets was also investigated

In the Structural Health Monitoring (SHM) field, the Acoustic Emission (AE) technique is a passive method by which damage is localized and identified by capturing Lamb Waves (LW) signals propagating in a plate-like structure. The reconstruction of emitted signals from damage at the source location constitutes one of the main challenges faced by the SHM community. Recently, the application of a Frequencies Compensation Transfer Function (FCTF) has been used to reconstruct narrowband and broadband signals through a hybrid experimental and numerical Time Reversal (TR) process on aluminum plates. This study aims to reconstruct through experimental methods different types of narrowband and broadband signals on different plate-like structures making use of FCTF. In particular, Hanning Window (HW) and numerical broadband signals have been reconstructed for aluminum and steel plates. The results obtained in this study show how the FCTF method can be applied to different types of materials in plate-like structures. Moreover, the FCTF method has been applied on real broadband signals emitted by the Pencil Lead Break (PLB) technique and Rock Impact (RI) test. These last results prove that the FCTF method is able to compensate for the frequency changes on a single wave packet. Such results are fundamental, as they open the possibility to reconstruct any type of source signals emitted by any damage type.

Large civil infrastructure systems all over the world have become an integral part of our civilization. The inspection and maintenance of these structures for public safety is a difficult task. The assessment of integrity of such huge structures due to local damages is even more difficult to deal with. The conventional inspections are performed manually, generally by visual examination and sometimes by more advanced techniques like ultrasonic, electromagnetic and fiber optic techniques. These inspections involve human interventions, depend on individual inspector's experience, and are time consuming. Such inspection methods may not be very useful for real time health assessment of a structure in service and as a result are not very helpful in preventing any disastrous situation through early warning. Therefore, it is very important to look for a comprehensive strategy of global integrity monitoring infused with information about local damages in the structure. For local damage assessment the current state of the health monitoring technology lacks a generalized and definitive approach to the identification and localization of damage. In past decades several signal processing tools have been used for solving different health monitoring problems but the commutability of the tools between different problems has been restricted. Fundamental reasons for this shortcoming have never been investigated in detail. In this dissertation an investigation has been carried out employing almost all promising feature extraction tools on a representative problem - a plate with rivet holes. The problem considered has radial cracks around rivet holes in a joint panel of a steel truss bridge. Such defects are very difficult to detect. Although well established, Lamb wave based nondestructive evaluation techniques are revisited and new tools are developed to address this issue. Simulation of the scattered ultrasonic wave field is carried out using the finite element method. This ultrasonic wave field is further analyzed to evaluate the integrity of the structure using various feature extraction (FE) techniques. Joint time-frequency-energy representation is obtained from ultrasonic signals recorded at various locations on the plate (joint panel) and used to extract damage sensitive features. Those features were then used to formulate a new Damage Parameter (DP) for better visualization of the crack. Results are shown to demonstrate the comparative effectiveness of these techniques. It is concluded that any particular FE technique cannot detect all possible sizes and orientations of the crack. It is suggested that the statistical occurrence and pattern of the crack must be visualized through a few selective FE techniques in a sequence. Modeling of the wave scattering phenomenon by conventional numerical techniques such as finite element method requires very fine mesh at high frequencies necessitating heavy computational power. Distributed point source method (DPSM) which is a recently developed semi-analytical technique, is applied to model the scattering of ultrasonic wave field on representative problem geometries and the results are used to diagnose structural damages. DPSM is a newly developed robust mesh-free technique for simulating ultrasonic, electrostatic and electromagnetic field problems. In most of the previous studies the DPSM technique has been applied to model two dimensional surface geometries and relatively simple three dimensional scatterer geometries. It has been very difficult to perform the wave scattering analysis for very complex three-dimensional geometries. This technique has been extended to model wave scattering in an arbitrary geometry. The simulation has been carried out with and without the presence of cracks near the rivet holes.

Aircraft, containers, and storage tanks contain plate-like structures that are safety critical. The structures often undergo non-destructive inspections. The inspection frequency tends to be over-conservatively high, and it may be possible to reduce the intervals between inspections to realize cost savings. This goal can possibly be realized by automated structural health monitoring (SHM) of structures using sparse active guided wave sensor arrays. Guided waves are sensitive to small defects and can propagate long distances across feature dense plates. Thus, a guided wave SHM system that enables reliable detection of critical defects or monitoring of their growth can potentially be used to reduce the frequency of inspections for real structures. Industrial guided wave SHM systems must be reliable throughout prolonged exposure to temperature, humidity, and loading changes encountered in operation. Research at Imperial College shows that temperature compensation and subtraction between monitored guided wave signals and baselines acquired from healthy plates enables detection of 1.5% reflection change over areas ~1 m^2 in the presence of thermal swings and uniform liquid layers. These results and findings from scattering studies indicate it may be possible to detect reflections from hole type defects and notches affecting structures during their operation. An issue is that demonstrations of SHM system capabilities have only been shown in controlled laboratory tests within short periods following baseline acquisition. There is concern whether sustained exposure to service conditions will subject transducer elements to irreversible changes and introduce variability in baseline subtraction results that would mask signals due to slowly growing damage. This thesis studies the reliability of guided wave SHM for monitoring plate-like structures over longer time periods. The theoretical characteristics of the fundamental Lamb waves and their use to monitor and detect damage are reviewed. Strategies for sensing and signal processing are described alongside experimental validation of their performance. The effectiveness of the SHM system is tested in experiments where damage-free plates are exposed to British weather as well as thermal variations in an environmental chamber. The monitoring capabilities of bonded piezoelectric sensors are quantified and compared to the performance achieved using electromagnetic acoustic transducers. Experimental results and findings from simulations of bonded piezoelectric transduction establish that performances achieved with bonded sensors degrade due to variations in the properties of adhesives used to attach sensors to plates. EMATs are relatively stable and capable of enabling detection of 1.5% reflection change at points away from the edges of plates after sustained exposure to thermal cycling loads.

This study presents the investigation and the verification of the modal parameters of a plate-like structure by using different modal analysis methods. A fin-like structure which is generally used in aircraft is selected as a subcategory of a plate-like test structure. In the first part of the thesis, the natural frequencies and the corresponding mode shapes of the fin are extracted by Finite Element Analysis method. Classical Modal Analysis and Testing methods comprising both impact hammer and modal shaker applications are then applied in order to obtain the modal parameters such as
resonance frequencies, mode shapes and damping ratios. In the second part, a recent modal analysis technique, Operational Modal Analysis, is also applied in the laboratory environment. Since Operational Modal Analysis method does not require any information of input forcing, the fin structure is tested under both mechanical and acoustical types of excitations without measuring the given input forces. Finally, Operational Modal Analysis and Testing is also performed under various flow conditions generated in the wind tunnel which may simulate the real operating environment for the fin structure. The modal parameters extracted under these flow conditions are then compared with the previously obtained Finite Element, Classical and Operational Modal Analyses results.

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Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)
Esta tese apresenta o estudo e implementação de técnicas de inspeção de estruturas delgadas tipo placa utilizando arrays de transdutores piezelétricos e ondas acústicas guiadas, associados a métodos de formação e composição de imagens. A detecção de danos em estruturas através de ondas guiadas e arrays de transdutores é um campo de pesquisa de grande importância, principalmente em áreas como a aeroespacial e de geração de energia, que utilizam estruturas delgadas e que exigem inspeção eficaz e completa, visando confiabilidade e segurança. O emprego de ondas acústicas guiadas permite inspecionar grandes áreas utilizando sensores sem a necessidade de realizar varreduras pela peça, de maneira não-destrutiva. Além disso, apresenta sensibilidade a diversos tipos de defeitos, como furos, corrosão, variação de espessura e defeitos superficiais. São estudados aspectos de propagação de ondas, diferentes modos de propagação e dispersão, assim como as principais características dos arrays e a formação de imagens. Os estudos teóricos são acompanhados por simulações usando os softwares MATLAB e PZFlex para obtenção dos modos de propagação das ondas, interação com defeitos e diagramas de radiação dos transdutores, cujos resultados são validados por meio da análise experimental em placas de materiais isotrópicos (alumínio). Montou-se um array linear de transdutores piezelétricos com 16 elementos na borda de uma placa de alumínio com defeitos artificiais. Os sinais provenientes de todas as possíveis combinações dos pares transmissor-receptor foram adquiridos para a utilização de técnicas de abertura sintética. A primeira contribuição deste trabalho é um método para detecção dos defeitos baseado na fase instantânea dos sinais. Substituindo-se a informação de amplitude dos sinais pela fase instantânea (IP) no método de formação de imagem, ...
This thesis presents the study and implementation of non-destructive testing techniques of platelike structures using piezoelectric array transducers and guided acoustic waves, associated with beamforming techniques and image compounding methods. Damage detection in structures using guided waves and arrays is a promising research area, especially in the aerospace and energy fields, where the use of plate-like structures demands thorough inspection due to the high safety levels of operation. Guided acoustic waves techniques can be used to inspect relatively large areas without the necessity of moving the transducers, in a non-destructive way. There are different propagation modes that can be coupled to the plate, and each mode is sensitive to specific types of defects, like notches, delamination and surface defects. Wave propagation, different propagation modes and dispersion characteristics, as well as arrays characteristics and imaging algorithms, are studied. MATLAB and PZFlex simulations are used to get further insight in the theoretical aspects, to obtain dispersion curves, interaction with defects and array response. Experimental results validate the simulations with isotropic materials (aluminum). A 16-elements piezoelectric linear array is mounted at the border of an aluminum plate with artificial defects. The signals related to all combinations of transmit-receive pairs are obtained with burst excitation, for use with synthetic aperture (SA) techniques. The first contribution of this work is a method for defect detection based on the instantaneous phase of the aperture data. The instantaneous phase (IP) image is obtained by replacing the amplitude information by the instantaneous phase in the conventional SA beamforming. From the analysis of the IP image, a threshold level is proposed in terms of the number of signals used for imaging. This threshold is applied to the IP image, resulting ...
FAPESP: 10/02240-0
FAPESP: 10/16400-0
FAPESP: 13/00330-0

Structural Health Monitoring (SHM) is an emerging area of research associated to improvement of maintainability and the safety of aerospace, civil and mechanical infrastructures by means of monitoring and damage detection. Guided wave structural testing method is an approach for health monitoring of plate-like structures using smart material piezoelectric transducers. Among many kinds of transducers, the ones that have beam steering feature can perform more accurate surface interrogation. A frequency steerable acoustic transducer (FSATs) is capable of beam steering by varying the input frequency and consequently can detect and localize damage in structures. Guided wave inspection is typically performed through phased arrays which feature a large number of piezoelectric transducers, complexity and limitations. To overcome the weight penalty, the complex circuity and maintenance concern associated with wiring a large number of transducers, new FSATs are proposed that present inherent directional capabilities when generating and sensing elastic waves. The first generation of Spiral FSAT has two main limitations. First, waves are excited or sensed in one direction and in the opposite one (180 ̊ ambiguity) and second, just a relatively rude approximation of the desired directivity has been attained. Second generation of Spiral FSAT is proposed to overcome the first generation limitations. The importance of simulation tools becomes higher when a new idea is proposed and starts to be developed. The shaped transducer concept, especially the second generation of spiral FSAT is a novel idea in guided waves based of Structural Health Monitoring systems, hence finding a simulation tool is a necessity to develop various design aspects of this innovative transducer. In this work, the numerical simulation of the 1st and 2nd generations of Spiral FSAT has been conducted to prove the directional capability of excited guided waves through a plate-like structure.

Structures made of plate-like components are common in a variety of industries where the impacts of structural failures are severe. In many cases these structures are surrounded and only partially accessible, such as storage tanks and bridges, making them difficult to inspect frequently. The application of ultrasonic Guided Waves (GWs) in the evaluation and monitoring of relatively large plate-like structures is evermore a feasible option with the continuous development of transducer arrays. The use of transducer arrays is however complex due to directional control and the existence of many GW modes. Aimed at the evaluation of plate-like structures, in this research two piezoelectric transducer arrays respectively capable of omnidirectional and unidirectional control of the fundamental GW shear mode in plates (SH0) with above 20 dB mode purity are successfully designed, produced and validated. Omnidirectionality facilitates full structural evaluation coverage and can lead to defect mapping of large volumes with relatively few transducers. A unidirectional beam with relatively high mode purity facilitates evaluation of specific structural locations. Preference to the SH0 mode was given because of its non-dispersive and in-plane propagation properties making it more suitable than other GW modes to propagation in structures surrounded by fluid material. To enable the array development, a number of monolithic piezoelectric thickness-shear transducers of varied area were characterised with respect to GW mode directionality, amplitude and SH0 mode purity. The characterisation of each thickness-shear transducer allows for optimised superposition manipulation for specific applications. A single characterised shear transducer was selected for use in the development of omnidirectional and unidirectional SH0 mode transducer arrays. To aid development a linear superposition analysis model was produced and used to predict for a circular array design the optimum parameters for omnidirectional SH0 mode transmission with significant mode purity. A range of parameter combinations were evaluated and their predicted influence on array performance was characterised. The same method was employed to optimise a dual row linear array design for the unidirectional transducer array. All results were validated by FE models and later with empirical data. Both developed transducer arrays were characterised with respect to GW mode directionality, magnitude and SH0 mode purity. Both their detection sensitivity to pertinent defects and structures was validated, demonstrating relevance to Non Destructive Evaluation (NDE) and Structural Health Monitoring (SHM) applications.

This thesis investigates the characteristics of a close proximity underwater explosion and its effect on a ship-like structure. Finite element model tests are conducted to verify and validate the propagation of a pressure wave generated by an underwater explosion through a fluid medium, and the transmission of the pressure wave in the fluid to a structure using the Multi-Material Arbitrary Lagrangian/Eulerian method. A one dimensional case modeling the detonation of a spherical TNT charge underwater is investigated. Three dimensional cases modeling the detonation of an underwater spherical TNT charge, and US Navy Blast Test cases modeling a shape charge and a circular steel plate, and a shape charge and a Sandwich Plate System (SPS) are also investigated. This thesis provides evidence that existing tools and methodologies have some capability for predicting early-time/close proximity underwater explosion effects, but are insufficient for analyses beyond the arrival of the initial shock wave. This thesis shows that a true infinite boundary condition, a modified Gruneisen equation of state near the charge, and the ability to capture shock without a very small element size is needed in order to provide a sufficient means for predicting early-time/close proximity underwater explosion effects beyond the arrival of the initial shock wave.
Master of Science

Orientador: Ricardo Tokio Higuti
Co-orientador: Oscar Martinez-Graullera
Banca: Claudio kitano
Banca: Samuel da Silva
Banca: Nicolas Leonardo Pérez Alvarez
Banca: Joaquim Miguel Maia
Resumo: Esta tese apresenta o estudo e implementação de técnicas de inspeção de estruturas delgadas tipo placa utilizando arrays de transdutores piezelétricos e ondas acústicas guiadas, associados a métodos de formação e composição de imagens. A detecção de danos em estruturas através de ondas guiadas e arrays de transdutores é um campo de pesquisa de grande importância, principalmente em áreas como a aeroespacial e de geração de energia, que utilizam estruturas delgadas e que exigem inspeção eficaz e completa, visando confiabilidade e segurança. O emprego de ondas acústicas guiadas permite inspecionar grandes áreas utilizando sensores sem a necessidade de realizar varreduras pela peça, de maneira não-destrutiva. Além disso, apresenta sensibilidade a diversos tipos de defeitos, como furos, corrosão, variação de espessura e defeitos superficiais. São estudados aspectos de propagação de ondas, diferentes modos de propagação e dispersão, assim como as principais características dos arrays e a formação de imagens. Os estudos teóricos são acompanhados por simulações usando os softwares MATLAB e PZFlex para obtenção dos modos de propagação das ondas, interação com defeitos e diagramas de radiação dos transdutores, cujos resultados são validados por meio da análise experimental em placas de materiais isotrópicos (alumínio). Montou-se um array linear de transdutores piezelétricos com 16 elementos na borda de uma placa de alumínio com defeitos artificiais. Os sinais provenientes de todas as possíveis combinações dos pares transmissor-receptor foram adquiridos para a utilização de técnicas de abertura sintética. A primeira contribuição deste trabalho é um método para detecção dos defeitos baseado na fase instantânea dos sinais. Substituindo-se a informação de amplitude dos sinais pela fase instantânea (IP) no método de formação de imagem, ...
Abstract: This thesis presents the study and implementation of non-destructive testing techniques of platelike structures using piezoelectric array transducers and guided acoustic waves, associated with beamforming techniques and image compounding methods. Damage detection in structures using guided waves and arrays is a promising research area, especially in the aerospace and energy fields, where the use of plate-like structures demands thorough inspection due to the high safety levels of operation. Guided acoustic waves techniques can be used to inspect relatively large areas without the necessity of moving the transducers, in a non-destructive way. There are different propagation modes that can be coupled to the plate, and each mode is sensitive to specific types of defects, like notches, delamination and surface defects. Wave propagation, different propagation modes and dispersion characteristics, as well as arrays characteristics and imaging algorithms, are studied. MATLAB and PZFlex simulations are used to get further insight in the theoretical aspects, to obtain dispersion curves, interaction with defects and array response. Experimental results validate the simulations with isotropic materials (aluminum). A 16-elements piezoelectric linear array is mounted at the border of an aluminum plate with artificial defects. The signals related to all combinations of transmit-receive pairs are obtained with burst excitation, for use with synthetic aperture (SA) techniques. The first contribution of this work is a method for defect detection based on the instantaneous phase of the aperture data. The instantaneous phase (IP) image is obtained by replacing the amplitude information by the instantaneous phase in the conventional SA beamforming. From the analysis of the IP image, a threshold level is proposed in terms of the number of signals used for imaging. This threshold is applied to the IP image, resulting ...
Doutor

Technique for Impact Localization on Carbon Fiber Laminate Sheets: il problema affrontato è stato quello della localizzazione di un impatto a bassa energia (simulati con sfere in caduta libera) su strutture planari in materiale polimerico rinforzato con fibra di carbonio (CFRP) per mezzo della formula di triangolazione di Tobias sviluppando un nuovo algoritmo “di verosimiglianza” per l’estrazione del tempo di arrivo differenziale (DToA) da una coppia di sensori piezoelettrici flessibili incollati alla superficie del materiale. In particolare, è stata fatta la caratterizzazione della lastra in CFRP in termini di diagramma delle velocità di propagazione delle onde di Lamb. L’algoritmo ha reso indipendente la valutazione del DToA dal tipo di trasduttore utilizzato, ha migliorato la stima l’accuratezza delle coordinate di impatto. I parametri da inserire nel sistema di elaborazione sono minimi e sono stati migliorati i tempi di elaborazione. Sono state sviluppate nuove geometrie per i sensori in PVDF. Infine, è stato popolato un data base con i segnali acquisiti ed estratti dati statistici con i quali è stato possibile valutare l’algoritmo e confrontarlo con il metodo classico a soglia fissa per lastre in CFRP. Analysis of the errors in the estimation of impact positions in plate-like structure through the triangulation formula: l’attività di ricerca si è svolta concentrandosi sullo studio di una sperimentazione che diminuisse i parametri di incertezza. È stata condotta quindi un’analisi degli errori nella stima della posizione di impatto su alluminio attraverso la formula di triangolazione. Inizialmente, al fine di ridurre l’incertezza sulla generazione degli impatti con sfere in caduta libera, è stato realizzato un sistema meccanico, completo di elettronica di pilotaggio, per la generazione di impatti controllati e ripetibili. Il lavoro si è concentrato su una semplice procedura di laboratorio basata su un set-up con una coppia di sensori posizionati simmetricamente rispetto al punto di impatto, per stimare l'incertezza del DToA e la velocità di propagazione. Successivamente dallo studio del modello matematico di triangolazione sono stati individuati ed indagati, in modo simulato, i due fattori che ne influiscono sulla stima: il tempo differenziale di arrivo (DToA) ad ogni coppia di sensori dell’array e l’incertezza sulla stima della velocità di gruppo delle onde guidate di Lamb. Le prove sperimentali per la misura della velocità delle onde di Lamb e per la stima del DToA, sono state fatte su di una lastra di alluminio di spessore 1.4 mm con sensori piezoelettrici commerciali. Poiché l'errore per la stima DToA dipende anche dal tipo di elaborazione del segnale adottato, tra i molti metodi riportati in letteratura per la stima del DToA, abbiamo analizzato e confrontato tre metodi: l'attraversamento di una soglia predeterminata, il metodo di correlazione e l’algoritmo di “verosimiglianza” sviluppato in [A1]. Per il rilevamento dell'incertezza della velocità di propagazione, sono state calcolate le curve di dispersione della piastra di alluminio ed i risultati sono stati poi confrontati con le misurazioni sperimentali. Inoltre, un'analisi teorica ha mostrato come gli errori che influenzano il DToA e la velocità di propagazione agiscano sulla stima del punto di impatto nella formula di triangolazione. L'analisi dell'errore di posizionamento, nell’ottica di un utilizzo di configurazione multisensoriale, è considerata utile per la progettazione di un sistema di monitoraggio strutturale (SHM).

This thesis deals with the topology optimization of plate and shell structures. The shell structures are modeled as plate-like at facets. Firstly, the formulation of the topology optimization problem is presented in an introductory chapter, which introduces two frequently used topology optimization algorithms (being the optimality criterion and the method of moving asymptotes). Examples of applications are shown, and filtering schemes are introduced. Secondly, the derivation of the finite element formulation and interpolation of plates is presented. Both a shear rigid Kirchoff plate element and a shear flexible Mindlin element are considered. The latter element uses substitute shear strains to overcome locking; hence reduced integration is not necessary, as is normally required when shear flexible plate elements are used in topology optimization. The effect of both element formulations on optimal topology is then illustrated. The results reveal the notable effect of through thickness shear on optimal topolog. Thirdly, a at shell element is constructed by combining the plate elements with a membrane element with drilling degrees of freedom. (The membrane is not discussed in any detail.) To illustrate the topology optimization of shell structures, the so-called Scordelis-Lo roof is then selected as an example problem. The analysis includes an assessment of the effect of eccentric stiffeners or ribs on optimum topology.
Dissertation (Master of Engineering)--University of Pretoria, 2007.
Mechanical and Aeronautical Engineering
unrestricted

Quantitative identification and evaluation of damage are of utmost importance for the safety and sustainability of a structure. Identifying early stage damage is also necessary for reducing the maintenance and repair costs of a structure and increasing its service life. Numerous Structural Health Monitoring (SHM) systems have been developed over time to address these issues in various capacities and prevent huge losses to life and economy. Lamb Wave Diffraction Tomography (LWDT) is an advanced SHM process that is used for identifying the location and quantifying the severity of damage. The process consists of interrogating a region of probable damage with Lamb waves and gathering information about the presence and geometry of the damage from the scattered waves. This process can be used in plate-like structures, such as metallic plates and composite laminates. One of the main areas of research in LWDT is investigating the effects of multiple non-central frequency components of data on the image quality and utilizing information from these multiple frequencies to reconstruct the damage. This thesis contains journal papers that 1) study the scattering of Lamb waves at delaminations within a composite laminate, focussing on the mode-conversion effect at the damage boundary 2) study the effects of non-central frequencies on the quality of imaging for damage reconstruction and 3) propose multi-frequency approaches that improve the efficiency of LWDT. The key outcome of this study is that a multi-frequency approach to damage reconstruction greatly improves the resolution of damage in plate-like structures, without increasing the number of transducers. A multi-frequency based reconstruction approach would thus lead to the advancement of LWDT as a feasible technique for the SHM of plate-like structures.
Thesis (Ph.D.) -- University of Adelaide, School of Civil, Environmental and Mining Engineering, 2020

Aerospace structures comprising of metals and composites are exposed to extreme loading and environmental conditions which necessitates regular inspection and maintenance to verify and monitor overall structural integrity. The timely and accurate detection, characterization and monitoring of structural cracking, corrosion, delaminating, material degradation and other types of damage are of major concern in the operational environment. Along with these, stringent requirements of safety and operational reliability have lead to evolutionary methods for evaluation of structural integrity. As a result, conventional nondestructive evaluation methods have moved towards a new concept, Structural Health Monitoring (SHM). SHM provides in-situ information a bout the occurrence of damage if any, location and severity of damage and residual life of the structure and also helps in improving the safety, reliability and confidence levels of critical engineering structures. While the concepts underlying SHM are well understood, development of methods is still in a nascent stage which requires extensive research that is challenging and has been the main motivating factor for undertaking the work reported in the thesis. Under the scope of the investigations carried out in this thesis, an integrated approach using Ultrasonic (active) and Acoustic Emission (passive) methods has been explored for SHM of metallic and composite plate structures using a distributed array of surface bonded circular piezoelectric wafer active sensors(PWAS). In ultrasonic method, PWAS is used for actuation and reception of Lamb waves in plate structures. The damage detection is based on the interaction of waves with defects resulting in reflection, mode conversion and scattering. In acoustic emission (AE) technique, the same sensor is used to pick up the stress waves generated by initiation or growth of defects or damage. Thus, both the active and passive damage detection methods are used in this work for detection, location and characterization of defects and damage in metallic and composite plates with complex geometries and structural discontinuities. And, thus the strategy adopted is to use time-frequency analysis and time reversal technique to extract the information from Lamb wave signals for damage detection and a geodesic based Lamb wave approach for location of the damage in the structure. To start with experiments were conducted on aluminum plates to study the interaction of Lamb waves with cracks oriented at different angles and on a titanium turbine blade of complex geometry with a fine surface crack. Further, the interaction of Lamb wave modes with multiple layer delaminations in glass fiber epoxy composite laminates was studied. The data acquired from these experiments yielded complex sets of signals which were not easily discern able for obtaining the information required regarding the defects and damage. So, to obtain a basic understanding of the wave patterns, Spectral finite element method has been employed for simulation of wave propagation in composite beams with damages like delamination and material degradation. Following this, time-frequency analysis of a number of simulated and experimental signals due to elastic wave scattering from defects and damage were performed using wavelet transform (WT) and Hilbert-Huang transform(HHT).And, a comparison of their performances in the context of quantifying the damages has given detailed insight into the problem of identifying localized damages, dispersion of multi-frequency non-stationary signals after their interaction with different types of defects and damage, finally leading to quantification. Conventional Lamb wave based damage detection methods look for the presence of defects and damage in a structure by comparing the signal obtained with the baseline signal acquired under healthy conditions. The environmental conditions like change in temperature can alter the Lamb wave signals and when compared with baseline signals may lead to false damage prediction. So, in order to make Lamb wave based damage detection baseline free, in the present work, the time reversal technique has been utilized. And, experiments were conducted on metallic and composite plates to study the time reversal behavior ofA0 andS0Lamb wave modes. Damage in the form of a notch was introduced in an aluminum plate to study the changes in the characteristics of the time reversed Lamb wave modes experimentally. This experimental study showed that there is no change in the shape of the time reversed Lamb wave in the presence of defect implying no breakage of time reversibility. Time reversal experiments were further carried out on a carbon/epoxy composite T-pull specimen representing a typical structure. And, the specimen was subjected to a tensile loading in a Universal testing machine. PWAS sensor measurements were carried out at no load as also during different stages of delamination due to tensile loading. Application of time reversed A 0 and S0 modes for both healthy and delaminated specimens and studying the change in shape of the time reversed Lamb wave signals has resulted in successful detection of the presence of delamination. The aim of this study has been to show the effectiveness of Lamb wave time reversal technique for damage detection in health monitoring applications. The next step in SHM is to identify the damage location after the confirmation of presence of damage in the structure. Wave based acoustic damage detection methods (UT and AE) employing triangulation technique are not suitable for locating damage in a structure which has complicated geometry and contains structural discontinuities. And, the problem further gets compounded if the material of the structure is anisotropic warranting complex analytical velocity models. In this work, a novel geodesic approach using Lamb waves is proposed to locate the AE source/damage in plate like structures. The approach is based on the fact that the wave takes minimum energy path to travel from the source to any other point in the connected domain. The geodesics are computed numerically on the meshed surface of the structure using Dijkstra’s algorithm. By propagating the waves in reverse virtually from these sensors along the geodesic path and by locating the first inter section point of these waves, one can get the AE source/damage location. Experiments have been conducted on metallic and composite plate specimens of simple and complex geometry to validate this approach. And, the results obtained using this approach has demonstrated the advantages for a practicable source location solution with arbitrary surfaces containing finite discontinuities. The drawback of Dijkstra’s algorithm is that the geodesics are allowed to travel along the edges of the triangular mesh and not inside them. To overcome this limitation, the simpler Dijkstra’s algorithm has been replaced by a Fast Marching Method (FMM) which allows geodesic path to travel inside the triangular domain. The results obtained using FMM showed that one can accurately compute the geodesic path taken by the elastic waves in composite plates from the AE source/damage to the sensor array, thus obtaining a more accurate damage location. Finally, a new triangulation technique based on geodesic concept is proposed to locate damage in metallic and composite plates. The performances of triangulaton technique are then compared with the geodesic approach in terms of damage location results and their suitability to health monitoring applications is studied.