The study of the self-damping properties of overhead transmission line conductors subjected to wind-induced oscillations.

Doctoral Degree. University of KwaZulu-Natal, Durban.Conductors are flexible, elastic structural components of power lines. The relatively high flexibility of the conductors, coupled with the long spans and the axial tension, makes conductors to be highly prone to dynamic excitation such as wind loading. The problem of the dynamic behavior of overhead power transmission line conductors under the action of wind and other forms of excitations is very important, since it proffers the optimal design of the line in terms of its dynamic characteristics. Thus, mechanical vibration of power lines needs to be mitigated, especially from aeolian vibration as they can lead to damage of the lines causing power interruptions. The dynamic behaviour of conductors can be influenced by its damping. However, available tools for the analysis of this phenomenon is scarce. The objective of this study is to evaluate the conductor self-damping. The goal is to characterize and ascertain the influence of various conductors’ parameters on the amount of energy dissipation. In this study, a numerically based investigation of the response of conductors was carried out i.e. finite element analysis (FEA or FEM). This was used to model the conductor using a new modeling approach, in which the layers of its discrete structure of helical strands were modelled as a composite structure. Due to the helical structure of the conductor strands, this give rise to inter-strands contacts. During bending caused by external loading, the stick-slip phenomenon does occur around the contact region resulting in damping of energy out of the system. Characterizing the damping mechanism as hysteresis phenomenon, this resulted from coulomb’s dry-friction with the stick-slip regime at contacts points between the conductor strands. Employing contact mechanics to characterize and the use of FEM to discretize these contact regions, parameters such as the contact forces, strain and stress were established. When the conductor experiences a dynamic excitation in a sinusoidal form, a hysteresis loop is formed. The use of contact region parameters, to evaluate the area of the hysteresis loop and the area of the loop determines the amount of self-damping. Experimental studies were conducted to validate the FEM model. Two forms of experiment were done. The first was the sweep test, done at a specified axial tension i.e. as a function of its ultimate tensile strength. This was used to determine the resonance frequencies for the conductors. In the second test, using the determined resonance frequencies from the first test were used to vibrate the conductors at these frequencies to establish the hysteresis loop at the same specified axial tension. The experiment was conducted with four different conductors with different number of layers. This was used to establish the relation between the numbers of layer and the amount of damping from the conductor. The conductors’ vibration experimental results obtained at a defined axial tension (as percentage of its UTS) correlate with that of FEM model. The results obtained showed a general increase in the resonance frequencies of vibration and a decrease in damping as the axial tension of the conductor is increased. The establishment of the hysteretic constitutive behaviour of strands under specific loading conditions as described in the thesis, using this FEM model, an algorithm was developed to evaluate the conductor self-damping. Based on this algorithm, computer programs have been developed to evaluate the conductor’s dynamic behaviour and implemented in MATLAB environment. Due to the very close relation between damping and conductor fatigue, this model can also be extended to investigate fatigue failure of conductors

Modeling, characterization and integration of thin film resonator microsensors

There is an increasing demand by using smart and miniaturized microsensors and microtransducers in the areas of automobile, environment Science and analytical chemistry. Thin film microresonators (TFRs), meriting from their small size, higher sensitivity and compatibility with VLSI process, have being investigated as microsensors. Gas or vapor absorbed species, or contacted liquids may perturb resonator mechanical, electrical and piezoelectric properties. In this dissertation, a coupled wave theory is presented and applied to analyze acoustic wave propagation phenomena both in TFR-solid and TFR-liquid phases. A two-dimensional analysis is implemented to compare with a one-dimensional model and experimental results. The analysis and characterization of polymer-coated TFR sensors are investigated, along with sensitivity and detection limit analysis. The experimental characterization of liquid-coated TFR sensors are also then investigated and discussed. In order to minimize temperature-induced drift and other possible geometrical and material mismatch, the design and implementation of a TFR microsensor array in which a differential approach was proposed have been demonstrated. The major challenge in the array design is to evaluate the wave crosstalks between individual TFRs and to investigate film mechanical strength in order to host multi-TFRs on a single substrate dice. General analytic EM design rules and numerical methods are used to model lateral wave coupling and assist the design of process masks. The processing and electrical measurements of TFR arrays are then investigated. The implementation and experimental characterization of a TFR polymer sensor array are finally presented and discussed

Micro-saw devices based on randomly-oriented PZT films: Design, fabrication and characterization

The main objective of this work was to research and develop a sensor device to monitor gas leakage in cryogenic temperatures and high vacuum. In addition, the sensor device should be small in size, function with low power, and display a short recovery time. The end application for this technology would be to monitor fuel leaks of hydrazine when refueling space satellites. To meet the specified requirements, an acoustic wave device was studied in this work. A micro-surface acoustic wave (muSAW) device was fabricated through a multi-layer thin film technique, where the active sensing material is based on a 300 nm thick PbZr0.52Tii0.48O3 (PZT) piezoelectric film grown via a sol-gel process. The electrodes were based on Si/SiO2/TiOx/Pt and Si/SiO 2/TiOx multi-layered structures and were fabricated by thermal oxidation and sputtering techniques. Improved deposition and crystallization methods for the PZT film were developed in this work which resulted in crack- and porous-free texturized and randomly-oriented PZT films. The phase, chemistry and microstructure of the PZT films were determined by X-ray diffraction (XRD), Energy Dispersive X-Ray Analysis (EDX), Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) techniques. The surface energy of the films was measured by means of a static sessile drop method. The ferroelectric and electrical characterization of the film was completed longitudinally by measuring the polarization-voltage (P-V), capacitance-voltage (C-V) and current-voltage (I-V) curves. The propagation wave velocity for the multi-layered structure was approximated using an electromechanical equivalent model and the results were used to design the interdigitated transducers (IDTs) for the SAW devices. IDTs were later deposited over the PZT via photolithography process. Finally, the electro-acoustic frequency response in |S21| mode was measured using a vector network analyzer (VNA) and Rayleigh mode SAW and non-Rayleigh mode SAW responses were observed

Optimization of indentification of particle impacts using acoustic emission.

Air-borne or liquid-laden solid particle transport is a common phenomenon in various industrial applications. Solid particles transported at severe operating conditions such as high-flow velocity can cause concerns for structural integrity, through wear originating from particle impacts with the structure. To apply Acoustic Emission (AE) in particle-impact monitoring, previous researchers focused primarily on dry particle impacts on dry target plate and/or wet particle impacts on wet or dry target plate. For dry particle impacts on dry target plate, AE events energy - calculated from the recorded free-falling or air-borne particle impact AE signals - was correlated with particle size, concentration, height, target material and thickness. For a given system, once calibrated for a specific particle type and operating condition, this technique might be sufficient to serve the purpose. However, if more than one particle type is present in the system (particularly with similar size, density and impact velocity), calculated AE event energy is not unique for a specific particle type. For wet particle impacts on dry or wet target plate (either submerged or in a flow loop), AE event energy was related to the particle size, concentration, target material, impact velocity and angle between the nozzle and the target plate. In these studies, the experimental arrangements and operating conditions either did not account for any bubble formation in the system or, if they did, did so at least an order of magnitude lower in amplitude than the sand particle impact. In reality, bubble formation can be comparable with particle impacts in terms of AE amplitude in process industries such as sand production during oil and gas transportation away from a reservoir. Current practice is to calibrate an installed AE monitoring system against a range of sand-free flow conditions. In real-time monitoring for a specific calibrated flow, the flow-generated AE amplitude/energy is deducted from the recorded AE amplitude/energy and the difference is attributed to the sand particle impacts. However, if the flow condition changes, which it often does in the process industry, the calibration is no longer valid and AE events from bubbles can be misinterpreted as sand particle impacts, and vice versa. In this thesis, sand particles and glass beads (with similar size, density and impact velocity) have been studied, dropping from 200 mm using a small cylindrical stepped mild steel coupon as a target plate. For signal recording purposes, two identical broadband AE sensors are installed, one at the centre and one 30 mm off-centred, on the opposite side of the impacting surface. Signal analysis has been carried out by evaluating seven standard AE parameters (amplitude, energy, rise time, duration, power spectral density (PSD), peak frequency at PSD, and spectral centroid) in the time and frequency domain, and time-frequency domain analysis has been performed by applying Gabor Wavelet Transformation. The signal interpretation becomes difficult due to reflections, dispersions and mode conversions caused by close proximity of the boundaries. So, a new signal analysis parameter - frequency-band energy ratio - has been proposed. This technique is able to distinguish between the populations of two very similar (in terms of size, mass and energy) groups of sand particles and glass beads impacting on mild steel, based on the coefficient of variation (Cv) of the frequency-band AE energy ratios. To facilitate individual particle impact identification, further analysis has been performed using a Support Vector Machine (SVM)-based classification algorithm with seven standard AE parameters, evaluated in both the time and frequency domain. The available dataset has been segmented into two parts: training set (80%) and test set (20%). The developed model has been applied on the test data for the purpose of model performance evaluation. The overall success rate in individually identifying each category (PLB, Glass bead and Sand particle impacts) at S1 has been found as 86%, and at S2 as 92%. To study wet particle impacts on a wet target surface in the presence of bubbles, the target plate was sealed to a cylindrical perspex tube. Single and multiple sand particles were introduced in the system using a constant-speed blower, to impact the target surface under water-loading. Two sensor locations, the same as those used in the previous sets of experiments, were monitored. From frequency domain analysis, it has been observed that the characteristic frequencies for particle impacts are centred at 300-350 kHz, and the frequencies for bubble formations are centred at 135-150 kHz. Based upon this, two frequency bands - 100-200 kHz (E1) and 300-400 kHz (E3) - and the frequency-band energy ratio (E3/E1) have been identified as optimal for identifying particle impacts for the given system. E3/E1 > 1 has been associated with particle impacts and E3/E1 < 1 has been associated with bubble formations. By applying these frequency-band energy ratios and setting an amplitude threshold, an automatic event identification technique has been developed for identification of sand particle impacts in presence of bubbles. The method developed can be used to optimize the identification of sand particle impacts. The optimal setting of an amplitude threshold is sensitive to the number of particles and the noise levels. For example, a high threshold of 10% will clearly identify sand particle impacts, but for multiparticle tests the same threshold is unlikely to detect about 20% of lower energy particles. On the other hand, a threshold lower than 3% is likely to result in the detection of AE events with poor frequency content and incorrect classification of the weakest events. The optimal setting of the parameters used in the framework - such as thresholds, frequency bands and ratios of AE energy - is therefore likely to make identification of sand particle impacts in a laboratory environment possible within 10%. An additional advantage of this technique is that calibration of the signal levels is not required, once the optimal frequency bands and ratios have been identified

Studies on nonlinear mechanical wave behavior to characterize cement based materials and its durability

[EN] The test for determining the resonance frequencies has traditionally been used to investigate the mechanical integrity of concrete cores, to assess the conformity of concrete constituents in different accelerated durability tests, and to ascertain constitutive properties such as the elastic modulus and the damping factor. This nondestructive technique has been quite appealed for evaluation of mechanical properties in all kinds of durability tests. The damage evolution is commonly assessed from the reduction of dynamic modulus which is produced as a result of any cracking process. However, the mechanical behavior of concrete is intrinsically nonlinear and hysteretic. As a result of a hysteretic stress-strain behavior, the elastic modulus is a function of the strain. In dynamic tests, the nonlinearity of the material is manifested by a decrease of the resonance frequencies, which is inversely proportional to the excitation amplitude. This phenomenon is commonly referred as fast dynamic effect. Once the dynamic excitation ceases, the material undergoes a relaxation process whereby the elastic modulus is restored to that at rest. This phenomenon is termed as slow dynamics. These phenomena (fast and slow dynamics) find their origin in the internal friction of the material. Therefore, in cement-based materials, the presence of microcracks and interfaces between its constituents plays an important role in the material nonlinearity. In the context of the assessment of concrete durability, the damage evolution is based on the increase of hysteresis, as a result of any cracking process. In this thesis three different nondestructive techniques are investigated, which use impacts for exciting the resonant frequencies. The first technique consists in determining the resonance frequencies over a range of impact forces. The technique is termed Nonlinear Impact Resonant Acoustic Spectroscopy (NIRAS). It consists in ascertaining the downward resonant frequency shift that the material undergoes upon increasing excitation amplitude. The second technique consists in investigating the nonlinear behavior by analyzing the signal corresponding to a single impact. This is, to determine the instantaneous frequency, amplitude and attenuation variations corresponding to a single impact event. This technique is termed as Nonlinear Resonant Acoustic Single Impact Spectroscopy (NSIRAS). Two techniques are proposed to extract the nonlinear behavior by analyzing the instantaneous frequency variations and attenuation over the signal ring down. The first technique consists in discretizing the frequency variation with time through a Short-Time Fourier Transform (STFT) based analysis. The second technique consists of a least-squares fit of the vibration signals to a model that considers the frequency and attenuation variations over time. The third technique used in this thesis can be used for on-site evaluation of structures. The technique is based on the Dynamic Acousto- Elastic Test (DAET). The variations of elastic modulus as derived through NIRAS and NSIRAS techniques provide an average behavior and do not allow derivation of the elastic modulus variations over one vibration cycle. Currently, DAET technique is the only one capable to investigate the entire range of nonlinear phenomena in the material. Moreover, unlike other DAET approaches, this study uses a continuous wave source as probe. The use of a continuous wave allows investigation of the relative variations of the elastic modulus, as produced by an impact. Moreover, the experimental configuration allows one-sided inspection.[ES] El ensayo de determinación de las frecuencias de resonancia ha sido tradicionalmente empleado para determinar la integridad mecánica de testigos de hormigón, en la evaluación de la conformidad de mezclas de hormigón en diversos ensayos de durabilidad, y en la terminación de propiedades constitutivas como son el módulo elástico y el factor de amortiguamiento. Esta técnica no destructiva ha sido ampliamente apelada para la evaluación de las propiedades mecánicas en todo tipo de ensayos de durabilidad. La evolución del daño es comúnmente evaluada a partir de la reducción del módulo dinámico, producido como resultado de cualquier proceso de fisuración. Sin embargo, el comportamiento mecánico del hormigón es intrínsecamente no lineal y presenta histéresis. Como resultado de un comportamiento tensión-deformación con histéresis, el módulo elástico depende de la deformación. En ensayos dinámicos, la no linealidad del material se manifiesta por una disminución de las frecuencias de resonancia, la cual es inversamente proporcional a la amplitud de excitación. Este fenómeno es normalmente denominado como dinámica rápida. Una vez la excitación cesa, el material experimenta un proceso de relajación por el cual, el módulo elástico es restaurado a aquel en situación de reposo. Este fenómeno es denominado como dinámica lenta. Estos fenómenos ¿dinámicas rápida y lenta¿ encuentran su origen en la fricción interna del material. Por tanto, en materiales basados en cemento, la presencia de microfisuras y las interfaces entre sus constituyentes juegan un rol importante en la no linealidad mecánica del material. En el contexto de evaluación de la durabilidad del hormigón, la evolución del daño está basada en el incremento de histéresis, como resultado de cualquier proceso de fisuración. En esta tesis se investigan tres técnicas diferentes las cuales utilizan el impacto como medio de excitación de las frecuencias de resonancia. La primera técnica consiste en determinar las frecuencias de resonancia a diferentes energías de impacto. La técnica es denominada en inglés: Nonlinear Impact Resonant Acoustic Spectroscopy (NIRAS). Ésta consiste en relacionar el detrimento que el material experimenta en sus frecuencias de resonancia, con el aumento de la amplitud de la excitación. La segunda técnica consiste en investigar el comportamiento no lineal mediante el análisis de la señal correspondiente a un solo impacto. Ésta consiste en determinar las propiedades instantáneas de frecuencia, atenuación y amplitud. Esta técnica se denomina, en inglés, Nonlinear Single Impact Resonant Acoustic Spectroscopy (NSIRAS). Se proponen dos técnicas de extracción del comportamiento no lineal mediante el análisis de las variaciones instantáneas de frecuencia y atenuación. La primera técnica consiste en la discretización de la variación de la frecuencia con el tiempo, mediante un análisis basado en Short-Time Fourier Transform (STFT). La segunda técnica consiste en un ajuste por mínimos cuadrados de las señales de vibración a un modelo que considera las variaciones de frecuencia y atenuación con el tiempo. La tercera técnica empleada en esta tesis puede ser empleada para la evaluación de estructuras in situ. La técnica se trata de un ensayo acusto-elástico en régimen dinámico. En inglés Dynamic Acousto-Elastic Test (DAET). Las variaciones del módulo elástico obtenidas mediante los métodos NIRAS y NSIRAS proporcionan un comportamiento promedio y no permiten derivar las variaciones del módulo elástico en un solo ciclo de vibración. Actualmente, la técnica DAET es la única que permite investigar todo el rango de fenómenos no lineales en el material. Por otra parte, a diferencia de otras técnicas DAET, en este estudio se emplea como contraste una onda continua. El uso de una onda continua permite investigar las variaciones relativas del módulo elástico, para una señal transito[CA] L'assaig de determinació de les freqüències de ressonància ha sigut tradicionalment empleat per a determinar la integritat mecànica de testimonis de formigó, en l'avaluació de la conformitat de mescles de formigó en diversos assajos de durabilitat, i en la terminació de propietats constitutives com són el mòdul elàstic i el factor d'amortiment. Esta tècnica no destructiva ha sigut àmpliament apel·lada per a l'avaluació de les propietats mecàniques en tot tipus d'assajos de durabilitat. L'evolució del dany és comunament avaluada a partir de la reducció del mòdul dinàmic, produït com resultat de qualsevol procés de fisuración. No obstant això, el comportament mecànic del formigó és intrínsecament no lineal i presenta histèresi. Com resultat d'un comportament tensió-deformació amb histèresi, el mòdul elàstic depén de la deformació. En assajos dinàmics, la no linealitat del material es manifesta per una disminució de les freqüències de ressonància, la qual és inversament proporcional a l'amplitud d'excitació. Este fenomen és normalment denominat com a dinàmica ràpida. Una vegada l'excitació cessa, el material experimenta un procés de relaxació pel qual, el mòdul elàstic és restaurat a aquell en situació de repòs. Este fenomen és denominat com a dinàmica lenta. Estos fenòmens --dinámicas ràpida i lenta troben el seu origen en la fricció interna del material. Per tant, en materials basats en ciment, la presència de microfissures i les interfícies entre els seus constituents juguen un rol important en la no linealitat mecànica del material. En el context d'avaluació de la durabilitat del formigó, l'evolució del dany està basada en l'increment d'histèresi, com resultat de qualsevol procés de fisuración. En esta tesi s'investiguen tres tècniques diferents les quals utilitzen l'impacte com a mitjà d'excitació de les freqüències de ressonància. La primera tècnica consistix a determinar les freqüències de ressonància a diferents energies d'impacte. La tècnica és denominada en anglés: Nonlinear Impact Resonant Acoustic Spectroscopy (NIRAS). Esta consistix a relacionar el detriment que el material experimenta en les seues freqüències de ressonància, amb l'augment de l'amplitud de l'excitació. La segona tècnica consistix a investigar el comportament no lineal per mitjà de l'anàlisi del senyal corresponent a un sol impacte. Esta consistix a determinar les propietats instantànies de freqüència, atenuació i amplitud. Esta tècnica es denomina, en anglés, Nonlinear Single Impact Resonant Acoustic Spectroscopy (NSIRAS). Es proposen dos tècniques d'extracció del comportament no lineal per mitjà de l'anàlisi de les variacions instantànies de freqüència i atenuació. La primera tècnica consistix en la discretización de la variació de la freqüència amb el temps, per mitjà d'una anàlisi basat en Short-Time Fourier Transform (STFT). La segona tècnica consistix en un ajust per mínims quadrats dels senyals de vibració a un model que considera les variacions de freqüència i atenuació amb el temps. La tercera tècnica empleada en esta tesi pot ser empleada per a l'avaluació d'estructures in situ. La tècnica es tracta d'un assaig acusto-elástico en règim dinàmic. En anglés Dynamic Acousto-Elastic Test (DAET). Les variacions del mòdul elàstic obtingudes per mitjà dels mètodes NIRAS i NSIRAS proporcionen un comportament mitjà i no permeten derivar les variacions del mòdul elàstic en un sol cicle de vibració. Actualment, la tècnica DAET és l'única que permet investigar tot el rang de fenòmens no lineals en el material. D'altra banda, a diferència d'altres tècniques DAET, en este estudi s'empra com contrast una ona contínua. L'ús d'una ona contínua permet investigar les variacions relatives del mòdul elàstic, per a un senyal transitori. A més, permet la inspecció d'elements per mitjà de l'accés per una sola cara.Eiras Fernández, JN. (2016). Studies on nonlinear mechanical wave behavior to characterize cement based materials and its durability [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/71439TESISPremios Extraordinarios de tesis doctorale

Applications of TAP-NDE technique to non-contact ultrasonic inspection in tubulars

The possibility and feasibility of experimental detection of localized defects in tubes using laser-induced ultrasonic wave approach through Thermo Acousto Photonic Non Destructive Evaluation (TAP-NDE) and Signal processing through wavelet transform is examined in this research. Guided waves in cylindrical surfaces provide solutions for detection of different defects in the material. Several experiments were conducted to this respect. Wave propagation in both axial and circumferential directions was studied. The dispersive wave propagation of ultrasonic waves in hollow cylinders has been investigated experimentally, primarily for use in non-contact and nondestructive inspections of pipes and tubes. The laser ultrasonic waves propagated in cylindrical waveguides are particularly attractive because of their unique characteristics in the applications of nondestructive evaluation (NDE). Contrary to studies making use of only axially symmetric guided waves in hollow cylinders, here are analyzed also nonaxisymmetric waves. The analysis of data is made by using the Gabor wavelet transform. The capability of modeling the guided wave dispersion in hollow cylinders is used in developing guided wave experimental techniques for flaw detection. Good agreement was obtained when comparing the dispersion spectra between theory and experimentation. Measurement of group velocities of guided waves, which are obtained directly from the wavelet transform coefficients, can be used to determine allocation and sizing of flaws

Effect of the Distance Between Impact Point and Hole Position and Non-Perpendicular Holes on the Impact Strength of Composite Laminates

The effect of the distance between impact point and hole position and the angle of the hole with the vertical axis was studied. In order to understand this effect, flexural tests were also performed to evaluate the bending strength of CFRP. In terms of distance of the hole, a maximum reduction of 29.7% on the bending load for a distance of 0 mm was found. This reduction was 22.3% on the impact load. In terms of angle of the hole, a maximum load reduction of 15.6% on the bending strength was found and for the impact load this value was found to be 7% for 20º. The fatigue resistance was also studied. An average reduction of 68.5% on the fatigue resistance of GFRP was obtained for an impact energy of 12 J, in the presence of a hole.O efeito da distância entre o ponto de impacto e posição de um furo e o ângulo do mesmo com o eixo vertical foi estudado. Para avaliar este efeito, foram também realizados ensaios de flexão. Em termos de distância do furo, uma redução máxima da resistência à flexão de 29,7% foi verificada para uma distância do furo de 0 mm. Esta redução foi de 22,3% no carregamento de impacto. Em termos de ângulo do furo, a redução máxima do carregamento foi de 15,6% para a flexão e em relação ao impacto este valor foi de 7%, para um ângulo de 20º. A resistência à fadiga foi também estudada. Foi verificada uma redução média de 68,5% na resistência à fadiga em compósitos de fibra de vidro para uma energia de impacto de 12 J, na presença de um furo

Pseudospectral collocation method for viscoelastic guided wave problems in generally anisotropic media

In Non-Destructive Evaluation (NDE) applications guided waves are attractive to perform rapid inspections of long lengths and large areas. However, they are complicated, therefore it is important to have as much information and understanding about their physical properties as possible in order to design the most efficient and robust inspection process as well as to draw the correct conclusions from the measurement results. The main piece of information to gain insight into the guided wave's properties is dispersion curves which, for isotropic structures such as plates and cylinders, have been available for many years. There are many robust algorithms which are currently used to compute them: finite element simulations, partial wave based root finding routines (PWRF) and semi-analytical finite element simulations (SAFE). These methodologies have been generalized and also used to study and compute dispersion curves of more complicated anisotropic materials though the range of tractable cases was limited. Although robust, all these approaches present several challenges, mostly computational, such as missing modes (PWRF), the so called "large-fd" problem (PWRF), artificially increased stiffness (FE, SAFE) or improvement of dispersion curve tracing routines (FE, PWRF, SAFE). In addition, when studying complicated anisotropic materials with a low degree of symmetry or unusual axes configurations where propagation does not take place along any of the principal axes, PWRF routines are frequently unreliable and one must resort to specific SAFE simulations which also present their own challenges and, depending on the SAFE scheme used, can yield spurious modes which need to be carefully filtered. Recently, Pseudospectral Methods (Galerkin and Collocation schemes), were introduced in the field of elastic guided waves, providing a powerful, yet strikingly and conceptually simple alternative to the above algorithms by successfully finding the dispersion curves in isotropic structures and in some simple anisotropic problems. However, a systematic and general approach for accurately and robustly computing dispersion curves of guided waves in anisotropic media, up to the most general case of triclinic symmetry, has not yet been developed. The goal of the work presented in this thesis is to develop such a tool by means of the Pseudospectral Collocation Method (SCM) and to take advantage of its particular features to make it as robust as possible. Firstly, a PSCM scheme is developed for computing dispersion curves of guided waves in anisotropic elastic media by finding all the frequencies for a given value of the real wavenumber. The results are validated with the existing literature as well as with the results provided by the software DISPERSE developed in the NDT group at Imperial College London. Many of the most remarkable features of the PSCM (spectral accuracy, speed, and its failure to miss modes for instance) are already observed in this simple, yet important, class of problems in elastic media. Secondly, guided waves in viscoelastic anisotropic media are studied. In this case, modes present attenuation due to material damping which is reflected in the wavenumber being complex. In order to handle complex wavenumbers the PSCM schemes developed for elastic materials are appropriately extended by means of the Companion Matrix Method. It will be seen that, apart from lowly attenuated propagating modes, all the other highly attenuated modes are found, yielding the full three-dimensional spectrum of the problem under consideration. Moreover, when the PSCM schemes for viscoelastic media are used to compute the dispersion curves of guided waves in an elastic medium, all the remaining, imaginary as well as complex, roots of the elastic problem which were not computed by the simpler PSCM elastic schemes are found, providing the full three-dimensional picture of the dispersion curves. These PSCM schemes, as any other of the aforementioned approaches, only find pairs (\omega,k). If dispersion curves are to be plotted, those pairs must be linked correctly in order to plot the desired dispersion curves, which is non-trivial when crossings amongst modes occur. Motivated by this, an investigation of the parity and coupling properties of guided wave solutions is carried out in detail for all crystal classes. This investigation provides a robust alternative to conventional tracing routines and avoids the problem of mode crossings by exploiting the parity and coupling properties of the solutions. Finally, the most complicated problems involving embedded structures are investigated by including a Perfectly Matched Layer (PML) in the previously developed PSCM schemes for viscoelastic media. The dispersion curves for leaky and trapped modes in an isotropic elastic plate and in a similar cylinder immersed in an infinite ideal fluid are found, showing very good agreement with the results given by PWRF routines in a large range of frequencies. Last, but not least, an illustration of a two-dimensional PSCM scheme is presented to study a vibrating membrane. The results are compared with the available analytical solution showing again excellent agreement.Open Acces

Geometric and scale effects on energy absorption of structural composites

PhDThe challenge faced by structural designers is becoming increasingly difficult as the imposed design criteria of energy absorbing structures requires weight reduction of structures without compromising cost and crushing performance. The current research is thus aimed at investigating the energy absorption of fibre reinforced composites measured as a function of geometry and scale within weight-critical structures. At the first stage, an innovative structure composed of four intersecting composite plates was tested. It was found that the structural stability played a crucial role in this intersecting structure. In order to avoid generating buckling failure before turning to a progressive crushing regime, Finite Element Method (FEM) was used on composite structures as a technical tool. At the second stage, three geometric structures containing corrugated composite laminates and possessing better structural stability were designed and examined. To increase the interlaminar fracture toughness properties of composite materials, through-thickness stitching methods were introduced. Fracture toughness (Mode-I and Mode-II) and flexure tests were performed on composite materials for comparing the effectiveness of different crushing mechanisms. Fracture toughness results presented a significant improvement of using stitching methods on Mode-I properties, while slight reduction on Mode-II properties was also detected. They also indicated the flexural properties of structural composites can significantly affect their energy absorption capabilities. At the final stage, six different factors including resin type, fibre architecture, crushing speed and stitching parameters were scaled in several levels in a modified geometric structure. An optimization approach based on Taguchi methods was utilised in order to statistically determine the relationship and assist in evaluating the contribution of each factor on crushing properties. It showed that by selecting the combinations of these factors with correct levels, the energy absorbed can be improved remarkably. It found that the crushing performance of this structural composite was mainly dominated by resin and fibre architecture, which contributed 71% capability of energy absorption. The other 29% capability was dominated by trigger, beam web length, edge stitching density and the crushing speed