Dissertations / Theses on the topic 'Dynamic damage'

To acknowledge and account for the uncertainties present in civil engineering applications is an area of major importance and of continuing research interest. This Thesis focuses on an application of Bayes' inference rule to evaluate the probability of damage in structures, using measured modal parameters and a set of possible damage states. The hypothesis is that observed changes in dynamic characteristics are due to damage accumulation over time. The main objective is to identify the most likely damage scenario from a set of previously defined damage states. These are characterized in terms of vectors, θi, the components of which are the parameters, θij, that are associated with the stiffness contribution, Kj, from each substructure undergoing damage. These stiffness matrices are uncertain as a result of random geometric and material properties. For different combinations of the damage parameters and realizations of the random variables, the modal parameters are calculated solving the basic eigenvalue problem. The results are used to calculate the statistics of the parameters given a specific damage state, the likelihood functions, as these are needed to calculate the probability of a given a set of measurements given a damage state. Each damage state Di is associated with a prior probability P(Di). In order to calculate its posterior probability, given a set of measurements, a Bayesian updating is implemented, in which the prior probability is updated by means of the likelihood functions, f(r|Di), which represent the probability density function of the modal parameter, r, given the damage state, Di. This Thesis discusses the effectiveness of the approach in identifying a particular damage state referred to as damage scenario. It is shown that measurement of multiple modal parameters is required to identify, quickly and with confidence, a given damage state. The discussion also considers the effect of error in the measurements, and the number of repeated measurements that are required to achieve a substantial confidence as to the presence of a particular damage state. Ranking of the estimated probabilities, after a set of measurements, offers guidance to the engineer as when and where to conduct a direct inspection of the structure.

This thesis studies structural dynamic system identification in a frequency-based framework. The basic consideration stems from the fact that frequencies may generally be measured with higher accuracy than other pertinent modal data such as mode shapes; however only a limited number of frequencies may be measured in the conventional context of natural frequencies. Being able to measure extra frequencies is a key to the success of a frequency-based method. The main part of the thesis is therefore organised around the involvement of the so-called artificial boundary condition (ABC) frequencies to augment the frequency dataset for general structural damage identification. In essence, the ABC frequencies correspond to the natural frequencies of the system with additional pin supports, but may be extracted from specially configured incomplete frequency response function matrix of the original structure without the need of physically imposing the additional supports. In the first part of the research, a particular focus is placed on the actual extraction of these ABC frequencies from physical experiments through effective modal testing, data collection, data processing and analysis. The influences of key processes involved in a typical modal experimental procedure, including high-fidelity measurement of the (impact) excitation input, averaging, windowing, and an effective use of post-processing techniques, particularly the Singular Value Decomposition (SVD) technique, are scrutinised in relation to the extraction of the ABC frequencies. With appropriate implementation of testing and data processing procedures, results demonstrate that all one-pin and two-pin ABC frequencies from the first few modes can be extracted with good quality in a laboratory setting, and the accuracy of extracted ABC frequencies is comparable to natural frequencies of corresponding orders. A comprehensive study is then carried out to investigate the sensitivities of ABC frequencies to damages. Two-pin ABC frequency sensitivity is formulated by extending the expression of anti-resonance sensitivity. On this basis, the mode shape contribution is adopted as a criterion for the selection of more sensitive ABC frequencies to be employed in detailed parameter identification or finite element model updating procedures. The soundness of using ABC frequencies in structural parameter identification and the effectiveness of the above ABC frequency selection method are subsequently examined through case studies involving laboratory experiments and the corresponding FE model updating. Furthermore, a preliminary study is carried out to examine the possibility of formulating ABC frequency-based damage indicator, herein with an analogy to the mode shape curvature, for direct damage assessment. As an extended investigation in the general framework of frequency-based dynamic identification, in the last part of the thesis, a complex dynamic system, namely a railway bridge under moving loads & masses, is evaluated with regard to the various frequency characteristics involved. The variation of the natural frequencies of the bridge-moving mass system, as well as the presence of the apparent frequencies from the trainloads, are analysed in detail. Besides simplified theoretical analysis, a computational model is developed to simulate the combined bridge-moving vehicle/train system, where the vehicle mass is coupled with the bridge via surface contact. The model is verified by comparison with field measurement data and theoretical predictions. Parametric studies enable a clear identification of the correlation of the frequency contents between the response and the trainload, and provide new insight into the significance of the so-called driving and dominant frequencies. It is found that much of the dynamic response phenomena, including the resonance effect, may be explained from the view point of the frequency characteristics of the trainload pattern, which is governed primarily by the ratio between the carriage length and the bridge length. Finally, a resonance severity indicator (the Z-factor) is developed for the assessment of the resonance effect in the railway bridge response when the trainload moves at a resonance speed. Numerical results demonstrate that the proposed methods are effective for the determination of the critical speed and the resonance effects, including the situations where a significant carriage mass is incorporated.

Laminated composite materials are used in different applications, for example mechanical, civil and aerospace structures, due to their light weight and excellent mechanical properties. However, fibre breakage and delamination are among the more serious damage that often initiate and propagate due to a number of mechanical and, specifically, dynamic loads during the operational life. Also, these damages destroy design functionality of these structures. To address this issue, damage detection is required in time to provide a good understanding of structure state in advance of any potential failure. There are a number of damage detection approaches reported in the literature and reviewed herein. Some of these are base-line free, whilst others use the intact data as a reference for the detection of damaged sections. However, currently there are a very limited number of experimental studies in the literature that use vibration-based damage detection to detect the delaminated areas, and are almost non-existent for fibre breakage; the majority of simulated studies consider delamination only. Defects in laminated structures are quite complicated and in most cases are hidden. Frequency-based damage detection is considered to be a global approach and is not useful when dealing with complex structures. There has been extensive research to develop the curvature mode shape as a reference for damage detection because it is highly sensitive at show the effects of damage. This sensitivity is tested in this research, as it is extremely difficult to detect damaged sections within composite materials, even with an active approach. Hence, the main objective of this research is to develop the curvature damage index by calculating the irregularity curvature index, and the proposal of a novel index, called the Haar index, to support the damage detection process. Both these indexes are used to detect delamination and fibre breakage on high modulus CFRP plate structures under condition of free vibration. Using these indexes gives an efficient method by which to quantify and localize damaged areas in both theoretical and experimental considerations of different lay-ups. In the modelling section, two finite element software programs, COMSOL Multiphysics 5.1 (Licence No. 7074366) and ABAQUS 6.14-1 (Licence No. 200000000008515), are used. This thesis includes development procedure of the curvature index, calculates the Haar index, gives details of the theoretical and experimental analysis, and reports the consequent results and discussion.

This thesis focuses on the development of a new method for the continuous

monitoring of civil engineering structures in order to locate small damages automatically. A

review of the very wide literature on Structural Health Monitoring (SHM) points first out that

the methods can be grouped in four categories based on their need or not of a numerical model,

as well as their need or not of information of the damaged structure to be applied. This state

of the art of the SHM methods highlights the requirement to reach each levels of SHM, which

is in particular for the localization of small damages in civil engineering structures the needs

for a non-model based output-only damage sensitive feature extraction technique. The origin of

the local sensitivity of strains to damages is also analyzed, which justifies their use for damage

localization.

A new method based on the modal filtering technique which consists in combining linearly

the sensor responses in a specific way to mimic a single degree of freedom system and which

was previously developed for damage detection is proposed. A very large network of dynamic

strain sensors is deployed on the structure and split into several independent local sensor networks.

Low computational cost and fast signal processing techniques are coupled to statistical

control charts for robust and fully automated damage localization.

The efficiency of the method is demonstrated using time-domain simulated data on a simply

supported beam and a three-dimensional bridge structure. The method is able to detect and

locate very small damages even in the presence of noise on the measurements and variability

of the baseline structure if strain sensors are used. The difficulty to locate damages from acceleration

sensors is also clearly illustrated. The most common classical methods for damage

localization are applied on the simply supported beam and the results show that the modal filtering

technique presents much better performances for an accurate localization of small damages

and is easier to automate.

An improvement of the modal filters method referred to as adaptive modal filters is next

proposed in order to enhance the ability to localize small damages, as well as to follow their

evolution through modal filters updating. Based on this study, a new damage sensitive feature

is proposed and is compared with other damage sensitive features to detect the damages with

modal filters to demonstrate its interest. These expectations are verified numerically with the

three-dimensional bridge structure, and the results show that the adaptation of the modal filters

increases the sensitivity of local filters to damages.

Experimental tests have been led first to check the feasibility of modal filters to detect damages

when they are used with accelerometers. Two case studies are considered. The first work

investigates the experimental damage detection of a small aircraft wing equipped with a network

of 15 accelerometers, one force transducer and excited with an electro-dynamic shaker. A

damage is introduced by replacing inspection panels with damaged panels. A modified version

of the modal filtering technique is applied and compared with the damage detection based principal

component analysis of FRFs as well as of transmissibilities. The three approaches succeed

in the damage detection but we illustrate the advantage of using the modal filtering algorithm as

well as of the new damage sensitive feature. The second experimental application aims at detecting

both linear and nonlinear damage scenarios using the responses of four accelerometers

installed on the three-storey frame structure previously developed and studied at Los Alamos

National Labs. In particular, modal filters are shown to be sensitive to both types of damages,

but cannot make the distinction between linear and nonlinear damages.

Finally, the new method is tested experimentally to locate damages by considering cheap

piezoelectric patches (PVDF) for dynamic strain measurements. Again, two case studies are investigated.

The first work investigates a small clamped-free steel plate equipped with 8 PVDFs sensors, and excited with a PZT patch. A small damage is introduced at different locations by

fixing a stiffener. The modal filters are applied on three local filters in order to locate damage.

Univariate control charts allow to locate automatically all the damage positions correctly.

The last experimental investigation is devoted to a 3.78m long I-steel beam equipped with 20

PVDFs sensors and excited with an electro-dynamic shaker. Again, a small stiffener is added to

mimic the effect of a small damage and five local filters are defined to locate the damage. The

damage is correctly located for several positions, and the interest of including measurements

under different environmental conditions for the baseline as well as overlapping the local filters

is illustrated.

The very nice results obtained with these first experimental applications of modal filters

based on strains show the real interest of this very low computational cost method for outputonly

non-model based automated damage localization of real structures.
Doctorat en Sciences de l'ingénieur
info:eu-repo/semantics/nonPublished

Structural Health Monitoring (SHM) is a non-destructive evaluation tool that assesses the functionality of structural systems that are used in the civil, mechanical and aerospace engineering practices. A much desirable objective of a SHM system is to provide a continuous monitoring service at a minimal cost with ability to identify problems even in inaccessible structural components. In this dissertation, several such approaches that utilize the measured dynamic response of structural systems are presented to detect, locate, and quantify the damages that are likely to occur in structures. In this study, the structural damage is identified as a reduction in the stiffness characteristics of the structural elements. The primary focus of this study is on the utilization of measured dynamic strains for damage identification in the framed structures which are composed of interconnected beam elements. Although linear accelerations, being more convenient to measure, are commonly used in most SHM practices, herein the strains being more sensitive to elemental damage are considered. Two different approaches are investigated and proposed to identify the structural element stiffness properties. Both approaches are mode-based, requiring first the identification of system modes from the measured strain responses followed by the identification of the element stiffness coefficients. The first approach utilizes the Eigen equation of the finite element model of the structure, while the second approach utilizes the changes caused by the damage in the structural curvature flexibilities. To reduce size of the system which is primarily determined by the number of sensors deployed for the dynamic data collection, measurement sensitivity-based sensor selection criterion is observed to be effective and thus used. The mean square values of the measurements with respect to the stiffness coefficients of the structural elements are used as the effective measures of the measurement sensitivities at different sensor locations. Numerical simulations are used to evaluate the proposed identification approaches as well as to validate the sensitivity-based optimal sensor deployment approach.
Ph. D.
All modern societies depend heavily on civil infrastructure systems such as transportation systems, power generation and transmission systems, and data communication systems for their day-to-day activities and survival. It has become extremely important that these systems are constantly watched and maintained to ensure their functionality. All these infrastructure systems utilize structural systems of different forms such as buildings, bridges, airplanes, data communication towers, etc. that carry the service and environmental loads that are imposed on them. These structural systems deteriorate over time because of natural material degradation. They can also get damaged due to excessive load demands and unknown construction deficiencies. It is necessary that condition of these structural systems is known at all times to maintain their functionality and to avoid sudden breakdowns and associated ensuing problems. This condition assessment of structural systems, now commonly known as structural health monitoring, is commonly done by visual onsite inspections manually performed at pre-decided time intervals such as on monthly and yearly basis. The length of this inspection time interval usually depends on the relative importance of the structure towards the functionality of the larger infrastructure system. This manual inspection can be highly time and resource consuming, and often ineffective in catching structural defects that are inaccessible and those that occur in between the scheduled inspection times and dates. However, the development of new sensors, new instrumentation techniques, and large data transfer and processing methods now make it possible to do this structural health monitoring on a continuous basis. The primary objective of this study is to utilize the measured dynamic or time varying strains on structural components such as beams, columns and other structural members to detect the location and level of a damage in one or more structural elements before they become serious. This detection can be done on a continuous basis by analyzing the available strain response data. This approach is expected to be especially helpful in alerting the owner of a structure by identifying the iv occurrence of a damage, if any, immediately after an unanticipated occurrence of a natural event such as a strong earthquake or a damaging wind storm.

Contact fatigue damages resulting either from static or dynamic contact are of interest for understanding the failure modes and mechanisms leading to improvement of the components’ performances in tribological applications. The objective of this research was to ascertain how and to what extent the counterface materials, loading conditions, contact configuration, lubrication, and the environment affect the failure behaviours of material under static and dynamic contact fatigue loading. An experimental ball-on-flat test configuration was employed for both static and dynamic contact fatigue testing. In house designed test rig was used to study static cyclic loading contact fatigue behaviours of brittle polymethylmethacrylate (PMMA) in contact with balls made of different materials, i.e. Si3N4, steel, aluminium, bronze and PMMA in dry and oil-lubricated conditions. A modified four ball test machine was used to study dynamic rolling contact fatigue behaviours of thermally sprayed molybdenum and titanium coatings in contact with steel balls in dry and seawater conditions. The static contact fatigue and the dynamic contact fatigue test results revealed that counterface material, loading magnitude, lubricant and the environment play a vital role in controlling failure modes and the extent of damage. In static contact fatigue, adhesive strength of the interface was the key factor controlling damage of the PMMA plate in both dry and oil-lubricated conditions. In dry conditions, three failure modes, i.e. adhesive wear, ring cracks, and radial cracks controlled the damage of PMMA to a different degree for each combination of materials. Whereas, the damage of each combination in oil-lubricated conditions was affected by the extent of three failure modes, i.e. adhesive wear, radial cracks and abrasive wear. In dynamic contact fatigue tests, adhesive wear and inter-lamellar cracking were the major failure modes controlling damage of molybdenum coating and titanium coating in dry contact conditions while abrasive wear, corrosion and lubrication controlled damage processes in seawater conditions.

This research work deals with damage detection of engineering machines and structures. This topic, developed in particular for bearing diagnostics in the first part of the work, is strictly related to dynamic identification when structures are considered. Thus, subspace-based methods are investigated in the second part of the work, with particular attention to nonlinear system identification. Changes in operational and environmental conditions for structures (such as air temperature, temperature gradients, humidity, wind, etc.) or machines (such as oil temperature, loads, rotating regimes, etc.) are known to have considerable effects on signal features and, consequently, on the reliability of diagnostics. Useful tools for eliminating this influence are provided by a Principal Component Analysis (PCA)-based method for damage detection. The same way as many published works applied PCA-based diagnostics of structures, in this research work a bearing diagnostic application is considered. After a detailed description of the test rig, the huge amount of acquired data, on several different damaged bearings, is investigated. Results are useful for giving an overview on how the PCA-based method for damage detection can be applied on a complicated real-life machine. In general cases of real structures, the application of efficient identification techniques is crucial for correctly exploiting the capabilities of the PCA-based method for damage detection. Moreover, in many cases damage causes a structure that initially behaves in a predominantly linear manner to exhibit nonlinear response: the application of nonlinear system identification methods to the feature-extraction process can also be used as a direct detection of damage. For these reasons, a detailed study of the nonlinear subspace-based identification methods is presented in the second part of this work. Since the classical data-driven subspace method can in some cases be affected by memory limitation problems, two alternative techniques are developed and demonstrated on numerical and experimental applications. Moreover, a modal counterpart of the nonlinear subspace identification method is introduced, to extend its relevance also to realistic large engineering structures. In a conclusive application, two of the main sources of non-stationary dynamics, namely the time-variability and the presence of nonlinearity, are analysed through the analytical and experimental study of a time-varying inertia pendulum, having a nonlinear equation of motion due to its large swinging amplitudes.

The field of bridge monitoring is one of rapid development. Advances in sensor technologies, in data communication and processing algorithms all affect the possibilities of Structural Monitoring in Bridges. Bridges are a very critical part of a country’s infrastructure, they are expensive to build and maintain, and many uncertainties surround important factors determining their serviceability and deterioration state. As such, bridges are good candidates for monitoring. Monitoring can extend the service life and avoid or postpone replacement, repair or strengthening works. The amount of resources saved, both to the owner and the users, by reducing the amount of non-operational time can easily justify the extra investment in monitoring. This thesis consists of an extended summary and five appended papers. The thesis presents advances in sensor technology, damage identification algorithms, Bridge Weigh-In-Motion systems, and other techniques used in bridge monitoring. Four case studies are presented. In the first paper, a fully operational Bridge Weigh-In-Motion system is developed and deployed in a steel railway bridge. The gathered data was studied to obtain a characterization of the site specific traffic. In the second paper, the seasonal variability of a ballasted railway bridge is studied and characterized in its natural variability. In the third, the non-linear characteristic of a ballasted railway bridge is studied and described stochastically. In the fourth, a novel damage detection algorithm based in Bridge Weigh-In-Motion data and machine learning algorithms is presented and tested on a numerical experiment. In the fifth, a bridge and traffic monitoring system is implemented in a suspension bridge to study the cause of unexpected wear in the bridge bearings. Some of the major scientific contributions of this work are: 1) the development of a B-WIM for railway traffic capable of estimating the load on individual axles; 2) the characterization of in-situ measured railway traffic in Stockholm, with axle weights and train configuration; 3) the quantification of a hitherto unreported environmental behaviour in ballasted bridges and possible mechanisms for its explanation (this behaviour was shown to be of great importance for monitoring of bridges located in colder climate) 4) the statistical quantification of the nonlinearities of a railway bridge and its yearly variations and 5) the integration of B-WIM data into damage detection techniques.

QC 20140910

The need for low order models capable of performing damage identification has become apparent in many structural dynamics applications where structural health monitoring and damage prognosis programs are implemented. These programs require that damage identification routines have low computational requirements and be reliable with some quantifiable degree of accuracy. Response surface metamodels (RSMs) are proposed to fill this need. Popular in the fields of chemical and industrial engineering, RSMs have only recently been applied in the field of structural dynamics and to date there have been no studies which fully demonstrate the potential of these methods. In this thesis, several RSMs are developed in order to demonstrate the potential of the methodology. They are shown to be robust to noise (experimental variability) and have success in solving the damage identification problem, both locating and quantifying damage with some degree of accuracy, for both linear and nonlinear systems. A very important characteristic of the RSMs developed in this thesis is that they require very little information about the system in order to generate relationships between damage indicators and measureable system responses for both linear and nonlinear structures. As such, the potential of these methods for damage identification has been demonstrated and it is recommended that these methods be developed further.
Master of Science

Due to the increase in demand on freight transportation it becomes necessary to avoid delays to ensure that the goods reach its destination on time. The main factors causing disturbances in the traffic on the mainline is the breakdown of vehicles mainly due to damaged wheels. The damaged wheels are identified through the wheel-rail impact force measurements provided by the wheel impact load detectors (WILD). This calls for the optimal schedule of maintenance of wheelsets and wagons in general. During the maintenance, the officials manually check for defective wheels and the exchange of wheelsets is performed based on the type of damage. The classification of wheel damages plays a vital role in providing ease of damage identification and insights to deduce a strategy for wheelset exchange. In this study, an attempt to classify the damaged wheelsets is done by analysing the wheel-rail impact force data from the wayside detectors. The data from the detectors are acquired from PredgeAB, a Luleå based startup pioneering in providing decision support on optimal maintenance schedules and predictive maintenance of rail wheels. Through their detection and prediction solutions it was observed that of all the wheels marked as damaged by the detectors 10% were undamaged. The source of the deviation in the impact force readings could help Predge make better estimations in damage detection and prediction. In this study, the sources contributing to the deviation is studied using multi-body simulations in GENSYS. A new method for modelling wheel damage is developed to overcome the shortcomings of the software. The findings of this study can then be used appropriately to make classifications of wheel damages.
På grund av den ökade efterfrågan på godstransporter är det nödvändigt attundvika förseningar för att säkerställa att varorna når sin destination i tid. Deviktigaste faktorerna som orsakar störningar i trafiken på huvudlinjen är stopp ispår av fordon, främst på grund av skadade hjul. De skadade hjulen identifierasmed hjälp av mätningar av slagkraft mellan hjul och spår som tillhandahållsav hjulbelastningsdetektorer (WILD). Detta kräver ett optimerat schema förunderhåll av hjulsatser och vagnar i allmänhet. Under underhållet kontrollerartjänstemännen manuellt för defekta hjul och utbytet av hjulsatser utförs baserat på typen av skada. Klassificeringen av hjulskador spelar en viktig roll närdet gäller att underlätta identifiering av skador och ge insikt för att bedöma enstrategi för hjulbyte. I den här studien görs ett försök att klassificera de skadade hjulsatserna genom att analysera data från slagkraften mellan spår ochhjul från detektorer. Uppgifterna från detektorerna hämtas från PredgeAB, enLuleå-baserad uppstartsbolag som är pionjärer för att ge beslutsstöd om optimala underhållsscheman och prediktivt underhåll av järnvägshjul. Genom sinadetekterings- och prediktiva lösningar observerades att 10% av alla hjul märktasom skadade av detektorerna var oskadade. Källan till avvikelsen i slagkraftavläsningarna kan hjälpa Predge att göra bättre uppskattningar när det gällerupptäckning och förutsägelse av skador. I den här studien studeras de källorsom bidrar till avvikelsen med simuleringar av flera kroppar i GENSYS. En nymetod för modellering av hjulskador har utvecklats för att övervinna programvarans brister. Resultaten av denna studie skulle kunna användas på lämpligtsätt för att göra klassificeringar av hjulskador.

This research studies the effects of a damage arrestment device embedded between a carbon fiber facesheet and foam core to find whether there is an increase in the structural integrity of the sandwich composites. Experimental and theoretical finite element analyses are implemented for two different composite sandwich geometries; plates and beams. Each structure consisted of the same loading criteria and was restricted to the same vibration fixture during the experiment. An accelerometer was placed on the composite plate to record the amplitude and the natural frequencies of the composite structure. Each composite specimen is then fixed to the surface of the Cal Poly Shake Table by two aluminum block fixtures. The mechanical properties of LTM45/CF1803 pre-impregnated carbon fiber and Last-A-foam FR 6710 polyvinylchloride foam were experimentally analyzed using ASTM D3039 and ASTM D1621 standards respectively to determine the material’s mechanical properties. By using the finite element program COSMOS with the pre-software GeoStar, accuracy representation were created to compare numerical, analytical, and theoretical results.

In recent years, the use of composite structures in engineering application has increased. This is mainly due to their special advantages such as high structural performance, high corrosion resistance, tolerance of temperature; extreme fatigue resistance and high strength/weight ratio. However, some disorders like fibre breakage, matrix cracking and delaminations could be caused by operational loading, aging, chemical attack, mechanical vibration, changing of ambient conditions and shock etc. during the service. Although these disorders are hardly visible, they can severely reduce the mechanical properties and the load carrying capability of the composite structure. The aim of this research project is to develop a Vibration-based Structural Health Monitoring (SHM) method for carbon/epoxy composite beam specimens with the embedded artificial delaminations. The Laser Vibrometer Machine was used to excite the beams and gather the responses of the structure to the excitations. The physical properties such as frequency, velocity, mode shapes, and damping of the defective beams were measured. By using a C-SCAN machine, the accuracy of the positions of the delaminations was verified to be about 95% is accurate. Curvature mode shapes as a scalable damage detection parameter is calculated using an analytical model based on the Heaviside step function and the Central Difference Approximation (CDA) technique. The vibration-based damage detection method is then obtained using the difference between curvature mode shapes of the intact and damaged carbon/epoxy beams. An accurate prediction of 90% was attained. These results are proposed and discussed in detail in this study. Finally, the Fatigue Crack Propagation Test was applied on Samples with embedded delamination to extend the crack. The ASTM E399-90 standard is used for the experiment and a careful fatigue crack growth routine was designed and implemented to advance the delamination in a controlled manner. The total extension of 17 mm was observed with Microscope. The total propagation as determined by the curvature mode plots was 17.84 mm.

A three-dimensional finite element code to analyze coupled thermomechanical deformations of composites has been developed. It incorporates geometric nonlinearities, delamination between adjoining layers, and damage due to fiber breakage, fiber/matrix debonding, and matrix cracking. The three damage modes are modeled using the theory of internal variables and the delamination by postulating a failure envelope in terms of the transverse stresses; the damage degrades elastic moduli. The delamination of adjoining layers is simulated by the nodal release technique. Coupled nonlinear partial differential equations governing deformations of a composite, and the pertinent initial and boundary conditions are first reduced to coupled ordinary differential equations (ODEs) by the Galerkin method. These are integrated with respect to time with the Livermore solver for ODEs. After each time step, the damage in an element is computed, and material properties modified. The code has been used to analyze several static and transient problems; computed results have been found to compare well with the corresponding test results. The effect of various factors such as the fiber orientation, ply stacking sequence, and laminate thickness on composite's resistance to shock loads induced by underwater explosions has been delineated.
Ph. D.

The purpose of this thesis is to assess and develop a non-destructive evaluation (NDE) procedure for evaluating the integrity of rectangular and square reinforced concrete (RC) slabs. This procedure employs both dynamic frequency and deformation response signatures to track changes in the slab following dynamic excitation. Such a procedure could provide a good basis for practising engineers to conduct nondestructive testing (NDT) and evaluation of general RC structures. The response of RC floor slabs to dynamic excitation have been experimentally studied at 1/3rd scale for two aspect ratios (square and rectangular), three concrete grades, and with and without cement replacement under clamped edge conditions. The model slabs were subjected to series of quasi-static loading and unloading sequences, to increasing load levels until failure was reached. At the unloaded part of each load cycle, the slabs were subjected to dynamic excitations, alternately using a hand-held, Bruel and Kjaer (B and K) impact hammer, and broad-band burst chirp shaker excitation. For the larger square slabs, at each unloaded part of the load cycle, a 265 gm steel ball bearing dropped from a fixed, standard height to provide more robust impulse excitation. All of the slabs were instrumented with optimally located accelerometers and strain gauges to capture the slab responses. The acceleration, deflection and strain readings resulting from the dynamic excitation were recorded at incremental load steps, from the initial unloaded state up to failure, and subsequently evaluated and analysed. The results show that the changes resulting from damage are readily observable, in the fundamental and higher modes of vibration and in the load-deflection and strain responses. These changes have been examined and analysed in both the time and frequency domains, and using other techniques, to establish the viability of this approach in evaluating the integrity of RC and other complex structures.

Une bonne tenue mécanique des structures du génie civil en béton armé sous chargements dynamiques sévères est primordiale pour la sécurité et nécessite une évaluation précise de leur comportement en présence de propagation dynamique de fissures. Dans ce travail, on se focalise sur la modélisation constitutive du béton assimilé à un matériau élastique-fragile endommageable. La localisation des déformations sera régie par un modèle d'endommagement à gradient où un champ scalaire réalise une description régularisée des phénomènes de rupture dynamique. La contribution de cette étude est à la fois théorique et numérique. On propose une formulation variationnelle des modèles d'endommagement à gradient en dynamique. Une définition rigoureuse de plusieurs taux de restitution d'énergie dans le modèle d'endommagement est donnée et on démontre que la propagation dynamique de fissures est régie par un critère de Griffith généralisé. On décrit ensuite une implémentation numérique efficace basée sur une discrétisation par éléments finis standards en espace et la méthode de Newmark en temps dans un cadre de calcul parallèle. Les résultats de simulation de plusieurs problèmes modèles sont discutés d'un point de vue numérique et physique. Les lois constitutives d'endommagement et les formulations d'asymétrie en traction et compression sont comparées par rapport à leur aptitude à modéliser la rupture fragile. Les propriétés spécifiques du modèle d'endommagement à gradient en dynamique sont analysées pour différentes phases de l'évolution de fissures : nucléation, initiation, propagation, arrêt, branchement et bifurcation. Des comparaisons avec les résultats expérimentaux sont aussi réalisées afin de valider le modèle et proposer des axes d'amélioration
In civil engineering, mechanical integrity of the reinforced concrete structures under severe transient dynamic loading conditions is of paramount importance for safety and calls for an accurate assessment of structural behaviors in presence of dynamic crack propagation. In this work, we focus on the constitutive modeling of concrete regarded as an elastic-damage brittle material. The strain localization evolution is governed by a gradient-damage approach where a scalar field achieves a smeared description of dynamic fracture phenomena. The contribution of the present work is both theoretical and numerical. We propose a variationally consistent formulation of dynamic gradient damage models. A formal definition of several energy release rate concepts in the gradient damage model is given and we show that the dynamic crack tip equation of motion is governed by a generalized Griffith criterion. We then give an efficient numerical implementation of the model based on a standard finite-element spatial discretization and the Newmark time-stepping methods in a parallel computing framework. Simulation results of several problems are discussed both from a computational and physical point of view. Different damage constitutive laws and tension-compression asymmetry formulations are compared with respect to their aptitude to approximate brittle fracture. Specific properties of the dynamic gradient damage model are investigated for different phases of the crack evolution: nucleation, initiation, propagation, arrest, kinking and branching. Comparisons with experimental results are also performed in order to validate the model and indicate its further improvement

The model updating technique allows the understanding of the dynamic behavior of a system and its damage state. In the last years, the structural monitoring has increased its applicability thanks to the decrease of the cost of sensors and improvements in the computational power. More and more structures are today instrumented in order to assess the intervention of progressive damages, understand their structural behavior and safety in almost real time. Nowadays, the real time identification of structural parameters and damage assessment is no longer unachievable. Moreover, the uncertainties evaluation is another important task required by the model updating procedures. Combining real time assessment and uncertainties evaluation, the algorithms can drive to a judgment about unsafety conditions in the buildings, with possible evacuation and securing of the structures, which is more and more required to structural health monitoring systems. The algorithms developed in this work are focused on these topics, especially on very quick model updating procedure, with uncertainties evaluation, which allows to estimate the structural parameters along with an error assessment. The quickness of the algorithm enables for its use in real time monitoring of actual structures. The algorithm itself is based on an innovative two steps procedure, with uncertainties evaluation, solving the inverse eigenvalues problem. The first step is achieved with closed form solution (without considering the determinant equations). If the solution does not satisfy the fixed thresholds, the second iterative step should be performed in order to improve the agreement between experimental outcomes and numerical ones. This procedure allows us to write the partial derivatives of the problem itself, with respect to the experimental outcomes, in closed form. Therefore, the parameters uncertainties are computed using the errors propagation. A second procedure is developed facing the complete problem entirely in iterative way, using a genetic algorithm with response surfaces.
La procedura di model updating è una tecnica alquanto datata che permette di comprendere il comportamento dinamico di un sistema e il suo stato di danno. Negli ultimo anni, il monitoraggio strutturale ha incrementato la sua applicabilità grazie al ridotto costo dei sensori e al miglioramento della potenza computazionale. Sempre più strutture sono oggi strumentate per valutare i loro danni e capire il comportamento dinamico stesso. La valutazione in tempo reale dei parametri strutturali e dello stato di danno è oggigiorno non più irraggiungibile. La valutazione delle incertezze sui parametri è, inoltre, richiesta ai moderni algoritmi di model updating. La combinazione della valutazione in tempo reale e dell'incertezza possono portare a un giudizio di situazioni potenzialmente pericolose in strutture esistenti con possibile evacuazione e messa in sicurezza delle strutture stesse. Questa valutazione è sempre più richiesta ai sistemi di monitoraggio strutturale. L'algoritmo sviluppato in questo lavoro è incentrato su questi aspetti, in particolare sulla rapida valutazione dei parametri strutturali e delle relative incertezze. La velocità dell'algoritmo permette l'uso dello stesso per il monitoraggio in tempo reale delle strutture. L'algoritmo è basato su una procedura innovativa a due fasi, con valutazione dell'incertezza, risolvendo un problema inverso agli autovalori. La prima fase è risolta con formulazione chiusa del problema (senza considerare le equazioni ai determinanti). Se la soluzione non soddisfa delle soglie prefissate per i parametri di controllo, la seconda fase, iterativa, deve essere eseguita in modo da migliorare la corrispondenza tra risultati sperimentali e numerici. La procedura permette, inoltre, di scrivere le derivate parziali del problema stesso, rispetto ai risultati sperimentali, in formulazione chiusa; pertanto le incertezze sui parametri sono calcolate mediante la teoria della propagazione degli errori. Una seconda procedura è sviluppata affrontando il problema completamente in forma iterativa, usando un algoritmo genetico con superfici di risposta.

Sous l’effet des impacts mécaniques, les structures constituées de matériaux fragiles peuvent être exposés à la rupture dynamique. La modélisation appropriée des mécanismes de rupture à différentes échelles d’observation et la prédiction de l’endommagement thermomécanique dans ces matériaux sont essentielles pour la conception de structures fiables. Des observations expérimentales sur la rupture dynamique des matériaux fragiles montrent des effets de refroidissement et d’échauffement importants à proximité d’une pointe de fissure. La modélisation du couplage thermomécanique lors de la rupture fragile a été entreprise, en général, sans tenir compte des aspects microstructuraux. L’objectif de cette thèse est de développer une procédure pour obtenir des lois d’endommagement thermomécaniques dans lesquelles l’évolution de l’endommagement est déduite à partir de la propagation des microfissures et des effets thermiques associés à l’échelle petite du matériau. Nous utilisons la méthode d’homogénéisation asymptotique pour obtenir la réponse macroscopique thermomécanique et d’endommagement du solide. Pour la propagation des microfissures, en mode I ou II, un critère de type Griffith est adopté. Des sources de chaleur sont considérés aux pointes des microfissures en mouvement, en lien avec l’énergie dissipée pendant la propagation. Nous considérons aussi des sources de chaleur représentant la dissipation par frottement sur les lèvres des microfissures qui se propagent en mode de cisaillement. Grâce à une analyse énergétique combinée avec la méthode d’homogénéisation nous obtenons des lois d’endommagement macroscopiques. Dans le système thermoélastique et d’endommagement ainsi obtenu, on identifie de forts couplages entre les champs mécaniques et thermiques. Le calcul des coefficients effectifs nous a permis d’étudier la réponse locale prédite par les nouveaux modèles. Cette réponse montre des effets de vitesse de déformation, de taille de la microstructure, de dégradation des propriétés thermoélastiques et des évolutions thermiques spécifiques engendrées par la microfissuration et le frottement à l’échelle petite du matériau. Dans l’équation macroscopique de la température, on retrouve des termes sources de chaleur distribuées en lien avec les dissipations d’endommagement et de frottement. L’implémentation de modèles d’endommagement dans un logiciel d’éléments finis nous a permis d’effectuer des simulations numériques à l’échelle des structures. Nous avons reproduit numériquement certains tests expérimentaux publiés dans la littérature concernant la rupture rapide d’échantillons de PMMA sous sollicitation d’impact. Les résultats des simulations obtenus sont en bon accord avec les observations expérimentales
Under impact mechanical loadings, structural components made of brittle materials may be exposed to dynamic failure. The appropriate modeling of the failure mechanisms at different scales of observation and the prediction of the corresponding thermomechanical damage evolution in such materials is essential for structural reliability predictions. Experimental observations on dynamic failure in brittle materials report important cooling and heating effects in the vicinity of the crack tip. Theoretical modeling of the thermo-mechanical coupling during fracture have been generally undertaken without accounting for microstructural aspects. The objective of the present thesis is to develop a procedure to obtain macroscopic thermo-mechanical damage laws in which the damage evolution is deduced from the propagation of microcracks and the associated small-scale thermal effects in the material. We use the asymptotic homogenization method to obtain the macroscopic thermo-mechanical and damage response of the solid. A Griffith type criterion is assumed for microcracks propagating in modes I or II. Heat sources at the tips of microcracks are considered as a consequence of the energy dissipated during propagation. Frictional heating effects are also considered on the lips of microcracks evolving in the shear mode. An energy approach is developed in combination with the homogenization procedure to obtain macroscopic damage laws. The resulting thermoelastic and damage system involves strong couplings between mechanical and thermal fields. Computation of the effective coefficients allowed us to study the local response predicted by the new models. The macroscopic response exhibits strain-rate sensitivity, microstructural size effects, degradation of thermoelastic properties and specific thermal evolutions due to microcracking and frictional effects at the small scale. Distributed heat sources are present in the macroscopic temperature equation linked to damage and frictional dissipations. The implementation of the proposed damage models in a FEM software allowed us to perform numerical simulations at the structural level. We reproduced numerically experimental tests reported in the literature concerning the rapid failure of PMMA samples impact. The results obtained in the simulations are in good agreement with the experimental observations

Composite materials are supplanting conventional metals in aerospace, automotive, civil and marine industries in recent times. This is mainly due to their high strength and light weight characteristics. But with all the advantages they have, they are prone to delamination or matrix cracking. These types of damage are often invisible and if undetected, could lead to appalling failures of structures. Although there are systems to detect such damage, the criticality assessment and prognosis of the damage is often more difficult to achieve. The research study conducted here primarily deals with the structural health monitoring of composite materials by analysing vibration signatures acquired from a laser vibrometer. The primary aim of the project is to develop a vibration based structural health monitoring (SHM) method for detecting flaws such as delamination within the composite beams. Secondly, the project emphasises on the method's ability to recognise the locatio n and severity of the damage within the structure. The system proposed relies on the examination of the displacement mode shapes acquired from the composite beams using the laser vibrometer and later processing them to curvature mode shapes for damage identification and characterization. Other identification techniques such as a C-scan has been applied to validate the location and size of the defects with the structures tested. The output from these plots enabled the successful identification of both the location and extent of damage within the structure with an accuracy of 96.5%. In addition to this, this project also introduces a method to experimentally compute the critical stress intensity factor, KIC for the composite beam. Based on this, a technique for extending the defect has been proposed and validated using concepts of fatigue and fracture mechanics. A composite specimen with a 40 mm wide delamination embedded within was loaded under fatigue conditions and extension of the defect by 4mm on either s ide of the specimen's loading axis was achieved satisfactorily. The experimental procedure to extend the defect using fatigue was validated using the SLV system. Displacement and Curvature mode shapes were acquired post-fatigue crack extension. Upon analysing and comparing the displacement and curvature mode shapes before and after crack extension, the extended delamination was identified satisfactorily.

Most engineering structures include nonlinearity to some degree. Depending on the dynamic conditions and level of external forcing, sometimes a linear structure assumption may be justified. However, design requirements of sophisticated structures such as satellites, stabilized weapon systems and radars may require nonlinear behavior to be considered for better performance. Therefore, it is very important to successfully detect, localize and parametrically identify nonlinearity in such cases. In engineering applications, the location of nonlinearity and its type may not be always known in advance. Furthermore, as the structure will be excited from only a few coordinates, the frequency response function matrices will not be complete. In order to parametrically identify more than one type of nonlinearity which may co-exist at the same location with the above mentioned limitations, a method is proposed where restoring force surface plots are used which are evaluated by describing function inversion. Then, by reformulating this method, a second method is proposed which can directly evaluate the total describing function of more than one type of nonlinearity which may co-exist at the same location without using any linear frequency response function matrix. It is also aimed in this study to use the nonlinearity localization formulations for damage localization purposes. The validation of the methods developed in this study is demonstrated with case studies based on simulated experiments, as well as real experiments with nonlinear structures and it is concluded that the methods are very promising to be used in engineering structures.

In the classical continuum theory of solid mechanics, the mathematical framework involves partial derivatives to represent the state of deformation of a solid body. A significant drawback due to derivatives is related to the unphysical results given near the discontinuities, because they are undefined wherever a continuous field of displacements is not verified, such as in the presence of dislocations, voids, cracks, interfaces between different phases within the same body and grain boundaries. Various techniques were employed for overcoming this incapability of the classical theory in describing material behavior in such conditions; in fact, spontaneous formation and growth of discontinuities are of great importance in solid mechanics: they lead to fractures and failures of systems that must be avoided, especially in aerospace structures, primarily, for safety reasons and, secondly, for economic purposes. One of these new approaches concerns employing nonlocal theories, based on integral formulations (more precisely integro-differential formulations), defined even when non-derivable displacement fields are involved. Peridynamics is one of these theories: it was suggested by Stewart Silling in 2000 [1] in order to adopt a consistent formulation describing material behavior not only when a continuous displacement field is provided, but also whenever discontinuities are present, avoiding partial differential equations or pre-setting of conditions which can influence the results. There are two versions of peridynamic models: bond-based, which was introduced first (see [1, 2]) and state-based. In the bond-based version, forces between two material points depend solely on their relative displacement, their relative initial position, and material properties. Due to its simplicity compared to the state-based version, most of the peridynamic applications have employed bond-based Peridynamics. However, bond-based models result in several limitations (the same of other atomistic or molecular dynamics models [3], although this is a continuum theory, not a discrete one), the most important of these is the fixed value of Poisson’s ratio: 1/4 in 3D or 2D plane strain, and 1/3 in 2D plane stress (see e.g. [1, 4]). This peculiarity implies other restrictions, such as the impossibility of reproducing plastic incompressibility in an accurate way. Nevertheless, for many purposes, bond-based Peridynamics fits the requirements and gives satisfying results. State-based peridynamic models remove these restrictions by allowing the interaction (“bond”) between a pair of points to potentially depend on all other bonds connected to the two points. Moreover, there are two types of state-based peridynamic formulations: ordi- nary and non-ordinary [2, 5, 6]. In the former, the forces between two material points act along the vector connecting the points in the deformed configuration. In the latter, such characteristic is not present. The ordinary state-based formulation requires specific derivation of constitutive models, see examples of viscoelasticity and plasticity models in [7, 8]. For non-ordinary state-based formulation, two approaches have been proposed: the development of an explicit model for the peridynamic force state [2] and the development of a map thanks to which classical mechanics constitutive relations are incorporated to indirectly establish the relationship between the interaction force and the deformation. The latter approach is called correspondence model [2]. The purpose of this thesis has been the investigation of possible advantages and drawbacks of this new and unexplored theory, so to identify some guidelines for choosing parameters fundamental for the analyses and the development of models for particular structural analyses. In the first year of the PhD course, the state of the art of this theory was studied and the bond-based linear and nonlinear static solvers developed in Matlabr were analyzed, employed and improved. During the second year of PhD course, the author of this thesis has focused her attention on the second version of the theory, based on concepts of advanced mathematics. She has become familiar with it, thanks to the functional analysis course that she had attended in the first year. One of the main original contributions of the present work to the existing literature is the development of the 2D linearization of the state-based “linear peridynamic solid” model in the state-based formulation. These models are useful whenever simplifying assumptions of plane stress and plane strain can be adopted for the simulation of a system, which, otherwise, would be described by a 3D model requiring high computational resources (time and memory). Particular attention is paid to this aspect, because, being a nonlocal model, implementing a peridynamic code is, in general, more computationally expensive than a code based on a local approach. The study of the state-based version started before going abroad and the development of the 2D models was completed during the six month stay at the University of Nebraska-Lincoln in USA. Both static and dynamic codes have been developed and the relevant parameters of these models have been analyzed. These linearized models are described in chapter 1.2.2. The study of failure criteria in state-based Peridynamics and the improvement of the algorithms in Matlabr to accelerate the codes and to optimize memory resources have been the main issues of the third year research. Some failure criteria, presented in section 1.2.3, have been proposed for brittle homogeneous linear elastic materials. They are criteria based on the maximum admissible stretch: a given bond fails at a critical stretch obtained by the work required to break that bond and this work is related to the fracture energy of the material. The results are compared to experimental data both for static and for dynamic cases, in bondbased and in state-based formulations. The detailed description of the algorithms can be found in chapter 3, while the results are illustrated in chapters 4 and 5.
Nella teoria classica della meccanica dei solidi, la formulazione matematica include derivate parziali, grazie alle quali si possono rappresentare stati di deformazione come funzioni degli spostamenti relativi dei nodi in cui è discretizzato il sistema continuo. Una carenza rilevante dovuto all’utilizzo delle derivate è legato ai risultati privi di significato fisico ottenuti in prossimità delle discontinuità perché le derivate non sono definite laddove manca un campo di spostamenti continuo, come può capitare in presenza di dislocazioni, vuoti, cricche, interfacce tra fasi differenti nello stesso corpo e bordi dei grani. Dato che la formazione spontanea e la crescita di discontinuità sono di grande importanza in meccanica dei solidi, diverse tecniche sono state utilizzate per superare questa incapacità della teoria di descrivere il comportamento dei materiali in tali condizioni, perché situazioni in cui le strutture sono incapaci di continuare a svolgere la propria funzione devono essere evitate, specialmente per strutture aerospaziali, in primo luogo, per ragioni di sicurezza ed, in secondo luogo, per motivi economici. Uno di questi nuovi approcci riguarda l’utilizzo di teorie non locali basate su formulazioni integrali (più precisamente formulazioni integro-differenziali), definite anche quando campi di spostamento non derivabili sono presenti. La teoria “Peridynamics” è una di queste teorie: è stata proposta da Stewart Silling nel 2000 [1] così da adottare una formulazione unica e coerente capace di descrivere i comportamenti dei materiali in corpi sia continui che discontinui, evitando l’uso di equazioni alle derivate parziali o la definizione a priori di alcune condizioni che possono influenzare (e in un certo senso favorire) dei risultati. Ci sono due versioni di modelli peridinamici: la state-based, e un suo caso particolare, la bond-based, che è stata introdotta per prima (vedi [1, 2]). Nella versione bond-based, le forze tra due punti materiali dependono unicamente dal loro spostamento relativo e dalla loro posizione relativa iniziale, oltre che dalle proprietà del materiale. Vista la sua semplicità a confronto con la seconda versione, la maggior parte delle applicazioni e degli articoli sulla Peridynamica ha adottato la formulazione bond-based. Tuttavia, i modelli nella formulazione bond-based sono caratterizzati da alcune limitazioni (le stesse dei modelli di altre teorie atomistiche e dei modelli di dinamica molecolare [3], anche se la Peridinamica è una teoria del continuo, non discreta), la più notevole di queste è il modulo di Poisson fisso: 1/4 nelle simulazioni 3D oppure in caso di deformazione piana 2D, e 1/3 nelle simulazioni in stato di tensione piana 2D (si veda per esempio [1, 4]). Questa particolarità implica altre restrizioni, come l’impossibilità di riprodurre la condizione di incomprimibilità plastica in maniera accurata. Tuttavia, per la maggior parte degli scopi, la formulazione bond-based è sufficiente e fornisce risultati approssimati soddisfacenti. I modelli della versione state-based rimuovono queste restrizioni, permettendo che le interazioni tra due punti possano dipendere da tutte le interazioni (i “bond”) connessi ad almeno uno dei due punti, tramite delle mappe avanzate chiamate “states”. Inoltre, ci sono due tipi di formulazioni state-based: la ordinary e la non-ordinary [2, 5, 6]. Nella formulazione ordinary, le forze tra due punti materiali agiscono lungo la congiungente i due punti nella configurazione deformata, mentre nella formulazione non-ordinary, questa caratteristica non è più vera. La formulazione ordinary della state-based necessita di modelli costitutivi appositamente derivati, come per esempio i modelli di viscoelasticità e platicità in [7, 8]. Per la formulazione non-ordinary della state-based, due approcci sono stati proposti: lo sviluppo di un modello esplicito per l’espressione dello state della forza peridinamica [2] e lo sviluppo di una mappa grazie alla quale le relazioni costitutive della meccanica classica sono incorporate per stabilire indirettamente la relazione tra la forza d’interazione e la deformazione. I modelli derivanti dal secondo approccio sono chiamati modelli correspondence [2]. L’argomento di questa tesi è lo sviluppo di modelli per particolari tipi di analisi e la ricerca di possibili vantaggi e inconvenienti di questa teoria nuova ed inesplorata, così da identificare alcune linee guida per la scelta di parametri fondamentali per le analisi. Durante il primo anno del corso di dottorato, lo stato dell’arte relativo a questa teoria è stato studiato e i solutori statici lineari e non lineari nella formulazione bond-based sviluppati precedentemente in ambiente Matlabr sono stati analizzati, usati e migliorati. Durante il secondo anno, l’autrice di questa tesi si è concentrata sulla seconda versione, basata su concetti di matematica avanzata con cui ha preso dimestichezza grazie al corso di analisi funzionale seguito il primo anno. Uno dei principali contributi originali alla letteratura esistente presenti in questa tesi è lo sviluppo dei modelli linearizzati 2D del modello solido lineare nella formulazione state-based. Questi modelli sono particolarmente utili quando semplificazioni di stato piano di tensione o di deformazione possono essere assunte per la simulazione di un sistema tridimensionale, che altrimenti verrebbe descritto da un modello 3D che necessiterebbe di risorse computazionali più elevate (in termini di tempo e memoria). Una particolare attenzione è richiesta per quest’aspetto, perché, essendo un approccio non locale, implementare un codice basato sulla teoria peridinamica richiede in generale più risorse computazionali di un codice basato su un approccio locale. Lo studio della versione state-based è iniziato prima di andare all’estero e lo sviluppo dei modelli 2D si è poi completato durante il soggiorno di sei mesi alla University of Nebraska-Lincoln negli Stati Uniti. Sono stati sviluppati sia un codice dinamico che uno statico. I parametri principali di questi modelli sono stati analizzati e i modelli linearizzati si possono trovare descritti nel capitolo 1.2.2. Lo studio dei criteri di frattura adottabili nella formulazione state-based e il miglioramento degli algoritmi in Matlabr per accelerare i codici e ottimizzare le risorse di memoria e gestione dei dati sono stati gli argomenti principali del terzo anno. Alcuni criteri di frattura, presentati nel capitolo 1.2.3, sono stati proposti per materiali lineari elastici omogenei e caratterizzati da frattura fragile. Sono criteri basati sul massimo allungamento: un’interazione non locale (“bond”) viene meno quando un valore critico di allungamento è raggiunto; questo valore di allungamento critico è calcolato dal lavoro richiesto per rompere il bond e questo lavoro è a sua volta legato all’energia di frattura. I risultati ottenuti sono stati confrontati con dati sperimentali per casi sia statici che dinamici, sia nella formulazione bondbased che in quella state-based. La descrizione dettagliata degli algoritmi si trova nel capitolo 3, mentre i risultati sono riportati nei capitoli 4 e 5.

The key to producing gas from tight gas reservoirs is to create a long, highly conductive flow path, via the placement of a hydraulic fracture, to stimulate flow from the reservoir to the wellbore. Viscous fluid is used to transport proppant into the fracture. However, these same viscous fluids need to break to a thin fluid after the treatment is over so that the fracture fluid can be cleaned up. In shallower, lower temperature (less than 250°F) reservoirs, the choice of a fracture fluid is very critical to the success of the treatment. Current hydraulic fracturing methods in unconventional tight gas reservoirs have been developed largely through ad-hoc application of low-cost water fracs, with little optimization of the process. It seems clear that some of the standard tests and models are missing some of the physics of the fracturing process in low-permeability environments. A series of the extensive laboratory "dynamic fracture conductivity" tests have been conducted. Dynamic fracture conductivity is created when proppant slurry is pumped into a hydraulic fracture in low permeability rock. Unlike conventional fracture conductivity tests in which proppant is loaded into the fracture artificially, we pump proppant/ fracturing fluid slurries into a fracture cell, dynamically placing the proppant just as it occurs in the field. Test results indicate that increasing gel concentration decreases retained fracture conductivity for a constant gas flow rate and decreasing gas flow rate decreases retained fracture conductivity. Without breaker, the damaging effect of viscous hydraulic fracturing fluids on the conductivity of proppant packs is significant at temperature of 150°F. Static conductivity testing results in higher retained fracture conductivity when compared to dynamic conductivity testing.

Herein, we study the contact damage performance of two armour ceramics, alumina and silicon carbide, with varying microstructures and one particle-reinforced ceramic nanocomposite, alumina/silicon carbide, in an attempt to understand the microstructural mechanisms that affect plasticity and cracking under quasi-static and dynamic conditions. Quasi-static contact damage was imitated using Vickers indentation over a varying load regime. Numerical analysis of the indentation size effect, performed using the proportional specimen resistance model, allowed the contributions of plastic deformation and cracking to be separated into two individual values. In all three samples, higher levels of surface energy were found to correlate with increased amounts of cracking per unit area of indentation impression. Analytical modelling of crack initiation during Vickers indentation together with quantitative measurements of surface flaw populations revealed that such an increase in cracking damage was the result of higher densities of larger flaws. The hardness of the monolithic ceramics was found vary based on grain size and porosity levels, a smaller average grain size and lower porosity levels resulting in higher hardness values. In the nanocomposite materials, hardening was found to occur with further additions of silicon carbide nanoparticles. Such an effect has been attributed to the increased dislocation densities, as measured using Cr3+/Al2O3 fluorescence spectroscopy, and the impedance of dislocation movement within the lattice due to the presence of silicon carbide nanoparticles. In order to simulate dynamic contact damage, a low velocity, scaled-down drop-weight test was designed and developed. The dynamic contact damage resistance was determined based on the depth of penetration of a blunt indenter. In the monolithic ceramics, the indenter penetration was found to be shallower in materials of higher hardness. However, the nanocomposite materials displayed an opposing trend, the indenter penetration becoming deeper in the samples of higher hardness. The macro-scale fracture patterns produced during drop-weight impacts were seen to vary based on flaw populations and indenter penetration. In certain microstructures, extensive micro-cracking was also observed.

Hydropower plays a very important role in the world electricity generation nowadays. Hydropower is one type of renewable energy and is the only renewable energy source that can provide a wide range of power regulation with fast response, which is very important for the electricity grid stability. Hydraulic turbines are the key equipment of hydropower plants. The power concentration in hydraulic turbines is increasing very fast in the past years. As a consequence, heads and fluid velocities are higher, and the hydraulic excitation forces on the turbine runner increase. On the other hand, to improve the efficiency of hydraulic turbines, the thickness and weight of the runner have been decreased as much as possible, which also increases the stresses in the runner. Furthermore, the operation range of hydraulic turbines is widened in order to satisfy the end-users’ demand of larger regulation capacity. This operation at extreme off-design conditions leads to even larger forces. Due to these reasons, many fatigue failure cases have been reported in the literature. Some fatigue failure cases showed very large cracks, which also indicates the challenge of crack monitoring during operations. To monitor the cracks in hydraulic turbines, it is imperative to study the effect of a crack on the dynamic behavior of hydraulic turbines. The dynamic behavior of hydraulic turbines has been studied extensively during the past decade. However, most of these studies were focused on Francis turbines and pump turbines, and the dynamic behavior of other types of hydraulic turbines, e.g., Kaplan turbines, have still been studied limitedly. Moreover, all of these studies were conducted on runners without cracks, and the effect of a crack on the dynamic behavior of hydraulic turbines has still not been studied before. In the present thesis, the effect of a crack on the dynamic behavior of Kaplan turbines and Francis turbines has been studied in detail. The research emphasis is laid on Kaplan turbines. This is divided into two steps. First, the dynamic behavior of an intact Kaplan turbine runner is studied. Then, based on the dynamic behavior of intact turbine runners, the effect of a crack on one blade is investigated. A systematic approach has been used for study. The research start from numerical models, and then, the numerical results are validated by experiments. The studies on the numerical models are conducted step by step from simplified blade models to single blade models and continuously to whole turbine models. The knowledge obtained on Kaplan turbines is also applied to a Francis turbine runner, whose dynamic behavior was previously studied
La energía hidroeléctrica juega un papel muy importante en la generación de electricidad hoy en día. La energía hidroeléctrica es la única fuente de energía renovable que puede proporcionar gran regulacion de potencia con una respuesta rápida, que es precisamente lo que demanda la red eléctrica. El elemento más importante en plantas hidroeléctricas es la turbina hidráulica. La concentración de potencia en turbinas hidráulicas está aumentando muy rápido hoy en día. Como consecuencia, las presiones y velocidades son mayores, y por lo tanto las fuerzas de excitación aumentan. Por otro lado, para mejorar la eficiencia de las turbinas hidráulicas, así como para lograr mayores aceleraciones durante las variaciones de carga, el grosor / peso del rodete se disminuye tanto como es posible, lo que también aumenta es estrés que recibe el rodete. Además, el rango de operación de las turbinas hidráulicas se está viendo ampliado para satisfacer la demanda de los usuarios proporcionando una mayor capacidad de regulación. La operación de las turbinas en condiciones fuera de diseño conlleva todavía a mayores fuerzas y esfuerzos en el rodete. Debido a estas razones, han habido muchos fallos por fatiga de componentes de turbinas hidráulicas en los últimos años. En algunos casos se encontraron grandes fisuras en la estructura, lo que indica que son difíciles de detectar con los actuales sistema de monitoreo de estas máquinas. Para controlar la aparición de fisuras en turbinas hidráulicas, es imprescindible estudiar el efecto de estas fisuras en el comportamiento dinámico de turbinas hidráulicas. El comportamiento dinámico de turbinas hidráulicas se ha estudiado en detenimiento durante la última década.. Sin embargo, la mayoría de estos estudios se centraron en turbinas Francis y bomba turbinas, mientras que el comportamiento dinámico de otros tipos de turbinas hidráulicas, como por ejemplo, las turbinas Kaplan, no ha sido estudiado todavía con detalle. Además, todos estos estudios se realizaron en rodetes sin fisuras, con lo que el efecto de una fisura en el comportamiento dinámico de turbinas hidraulicas todavía no se conoce. En esta tesis se ha estudiado el efecto de una fisura en el comportamiento dinámico de turbinas Kaplan y Francis. El énfasis de la investigación está puesto sobre todo en turbinas Kaplan. Primero se ha estudiado el comportamiento dinámico de un rodete de turbina Kaplan intacto. Luego, basándose en el comportamiento dinámico de los rodetes intactos, se ha investigado el efecto de una fisura en una pala. Para realizar la investigación se ha realizado un estudio sistemático: las investigaciones parten de modelos numéricos, y luego los resultados se han validado con experimentos. Los estudios con los modelos numéricos se han llevado a cabo paso a paso, desde modelos de álabes simplificados hasta un modelo de un álabe de turbina Kaplan o del rodete entero. El conocimiento obtenido en las turbinas Kaplan también se ha aplicado a un rodete de turbina Francis, cuyo comportamiento dinámico se había estudiado previamente.

My dissertation focuses primarily on the following three aspects associated with passing seismic waves in the field of earthquake seismology: temporal changes of fault zone properties, nonlinear site response, and dynamic triggering. Quantifying the temporal changes of material properties within and around active fault zones (FZ) is important for better understanding of rock rheology and estimating the strong ground motion that can be generated by large earthquakes. As high-amplitude seismic waves propagate through damaged FZ rocks and/or shallow surface layers, they may produce additional damage leading to nonlinear wave propagation effects and temporal changes of material properties (e.g., seismic velocity, attenuation). Previous studies have found several types of temporal changes in material properties with time scales of tens of seconds to several years. Here I systematically analyze temporal changes of fault zone (FZ) site response along the Karadere-Düzce branch of the North Anatolian fault that ruptured during the 1999 İzmit and Düzce earthquake sequences. The coseismic changes are on the order of 20-40%, and are followed by a logarithmic recovery over an apparent time scale of ~1 day. These results provide a bridge between the large-amplitude near-instantaneous changes and the lower-amplitude longer-duration variations observed in previous studies. The temporal changes measured from this high-resolution spectral ratio analysis also provide a refinement for the beginning of the longer more gradual process typically observed by analyzing repeating earthquakes. An improved knowledge on nonlinear site response is critical for better understanding strong ground motions and predicting shaking induced damages. I use the same sliding-window spectral ratio technique to analyze temporal changes in site response associated with the strong ground motion of the Mw6.6 2004 Mid-Niigata earthquake sequence recorded by the borehole stations in Japanese Digital Strong-Motion Seismograph Network (KiK-Net). The coseismic peak frequency drop, peak spectral ratio drop, and the postseismic recovery time roughly scale with the input ground motions when the peak ground velocity (PGV) is larger than ~5 cm/s, or the peak ground acceleration (PGA) is larger than ~100 Gal. The results suggest that at a given site the input ground motion plays an important role in controlling both the coseismic change and postseismic recovery in site response. In a follow-up study, I apply the same sliding-window spectral ratio technique to surface and borehole strong motion records at 6 KiK-Net sites, and stack results associated with different earthquakes that produce similar PGAs. In some cases I observe a weak coseismic drop in the peak frequency when the PGA is as small as ~20-30 Gal, and near instantaneous recovery after the passage of the direct S waves. The percentage of drop in the peak frequency starts to increase with increasing PGA values. A coseismic drop in the peak spectral ratio is also observed at 2 sites. When the PGA is larger than ~60 Gal to more than 100 Gal, considerably stronger coseismic drops of the peak frequencies are observed, followed by a logarithmic recovery with time. The observed weak reductions of peak frequencies with near instantaneous recovery likely reflect nonlinear response with essentially fixed level of damage, while the larger drops followed by logarithmic recovery reflect the generation (and then recovery) of additional rock damage. The results indicate clearly that nonlinear site response may occur during medium-size earthquakes, and that the PGA threshold for in situ nonlinear site response is lower than the previously thought value of ~100-200 Gal. The recent Mw9.0 off the Pacific coast of Tohoku earthquake and its aftershocks generated widespread strong shakings as large as ~3000 Gal along the east coast of Japan. I systematically analyze temporal changes of material properties and nonlinear site response in the shallow crust associated with the Tohoku main shock, using seismic data recorded by the Japanese Strong Motion Network KIK-Net. I compute the spectral ratios of windowed records from a pair of surface and borehole stations, and then use the sliding-window spectral ratios to track the temporal changes in the site response of various sites at different levels of PGA The preliminary results show clear drop of resonant frequency of up to 70% during the Tohoku main shock at 6 sites with PGA from 600 to 1300 Gal. In the site MYGH04 where two distinct groups of strong ground motions were recorded, the resonant frequency briefly recovers in between, and then followed by an apparent logarithmic recovery. I investigate the percentage drop of peak frequency and peak spectral ratio during the Tohoku main shock at different PGA levels, and find that at most sites they are correlated. The third part of my thesis mostly focuses on how seismic waves trigger additional earthquakes at long-range distance, also known as dynamic triggering. Previous studies have shown that dynamic triggering in intraplate regions is typically not as common as at plate-boundary regions. Here I perform a comprehensive analysis of dynamic triggering around the Babaoshan and Huangzhuang-Gaoliying faults southwest of Beijing, China. The triggered earthquakes are identified as impulsive seismic arrivals with clear P- and S-waves in 5 Hz high-pass-filtered three-component velocity seismograms during the passage of large amplitude body and surface waves of large teleseismic earthquakes. I find that this region was repeatedly triggered by at least four earthquakes in East Asia, including the 2001 Mw7.8 Kunlun, 2003 Mw8.3 Tokachi-oki, 2004 Mw9.2 Sumatra, and 2008 Mw7.9 Wenchuan earthquakes. In most instances, the microearthquakes coincide with the first few cycles of the Love waves, and more are triggered during the large-amplitude Rayleigh waves. Such an instantaneous triggering by both the Love and Rayleigh waves is similar to recent observations of remotely triggered 'non-volcanic' tremor along major plate-boundary faults, and can be explained by a simple Coulomb failure criterion. Five earthquakes triggered by the Kunlun and Tokachi-oki earthquakes were recorded by multiple stations and could be located. These events occurred at shallow depth (< 5 km) above the background seismicity near the boundary between NW-striking Babaoshan and Huangzhuang-Gaoliying faults and the Fangshan Pluton. These results suggest that triggered earthquakes in this region likely occur near the transition between the velocity strengthening and weakening zones in the top few kms of the crust, and are likely driven by relatively large dynamic stresses on the order of few tens of KPa.

Damage models are developed within the continuum damage mechanics framework which allows the description of material degeneration with general constitutive equations. The difficulty in the description of damage behaviour increases with increasing complexity of the material behaviour. This is especially true when it comes to composite materials which have an orthotropic material behaviour. The conventional description of damage, i.e. the local continuum damage mechanics description, leads to strain-softening behaviour which is characterised by a decline in stress with simultaneously increasing strain. Due to strain-softening the tangent stiffness becomes negative which forces the wave speed to become imaginary in dynamic problems. Consequently the partial differential equations governing the dynamic problem change from hyperbolic to elliptic and, therefore, the initial boundary value problem no longer has a unique solution. Due to this the physical meaning becomes unrealistic. Strain-softening is limited to an infinitely small area in which waves are not able to propagate in a process called wave trapping. A displacement discontinuity in an area of width zero (localisation zone) develops. The strain becomes infinite in this zone and is accompanied with a zero stress. Areas outside the softening zone are not able to interact with the strain-softening domain. As a consequence the strain-softening domain acts similar to a free boundary at which waves reflect. The implementation of local continua with strain-softening behaviour in finite element codes leads to additional numerical problems. Strain-softening behaviour manifests itself in the smallest area possible which is a single point in analytical considerations. This area is defined by the element discretisation in finite element codes. Therefore, strain-softening leads to a pronounced mesh sensitivity of results in addition to mathematical and physical issues. This work aims to find a solution which removes problems associated to strain- softening. Its aim is to represent material behaviour due to damage realistically and enable numerical results to convergence to a unique solution. The strain-softening problem is the focus of this work. It was investigated using a 1D wave propagation problem described by Bažant and Belytschko [1]. This simple experiment allows for an easy comparison of analytical and numerical results and therefore gives an insight into the problems connected to strain-softening. Furthermore, regularisation methods, specifically nonlocal and viscous methods, were investigated. Regularisation methods add additional terms to constitutive equations which keep the initial boundary value problem well-posed and enable a unique solution independent of the element discretisation. It was found that these methods are indeed capable of regulating the softening problem; however, they add additional difficulties in the description of material behaviour. A new approach to the strain-softening issues, unique at this point of time, was developed in this work which implements damage as an equivalent damage force. This approach is able to keep the initial boundary value problem stable and converge to a unique solution without adding additional terms in the constitutive equations, such as regularisation methods. This new approach to strain-softening was implemented for an isotropic material with scalar damage variable in DYNA3D successfully. Numerical results converged to a unique solution and were physically reasonable. The concept of an equivalent damage force was further developed to orthotropic material behaviour. This made an advanced representation, using an 8th rank damage tensor, necessary. The 8th rank damage tensor is able to represent anisotropic damage and it is also the most general damage representation possible.

Elastomers are important engineering materials that have contributed to the different technical developments and applications since the 19th century. The study of crack growth mechanics for elastomers is of great importance to produce reliable products and therefore costly failures can be prevented. On the other hand, it is fundamental in some applications such as adhesion technology, elastomers wear, etc. In this thesis work, crack propagation in rubber under quasi-static and dynamic conditions is investigated. In Paper A, theoretical and computational frameworks for dynamic crack propagation in rubber have been developed. The fracture separation process is presumed to be described by a cohesive zone model and the bulk behavior is assumed to be determined by viscoelasticity theory. The numerical model is able to predict the dynamic crack growth. Further, the viscous dissipation in the continuum is found to be negligible and the strength and the surface energy vary with the crack speed. Hence, the viscous contribution in the innermost of the crack tip has been investigated in Paper B. This contribution is incorporated using a rate-dependent cohesive model. The results suggest that the viscosity varies with the crack speed. Moreover, the estimation of the total work of fracture shows that the fracture-related processes contribute to the total work of fracture in a contradictory manner. A multiscale continuum model of strain-induced cavitation damage and crystallization in rubber-like materials is proposed in Paper C. The model adopts the network decomposition concept and assumes the interaction between the filler particles and long-chain molecules results in two networks between cross-links and between the filler aggregates. The network between the crosslinks is assumed to be semi-crystalline, and the network between the filler aggregates is assumed to be amorphous with the possibility of debonding. Moreover, the material is assumed to be initially non-cavitated and the cavitation may take place as a result from the debonding process. The cavities are assumed to exhibit growth phase that may lead to complete damage. The comparison with the experimental data from the literature shows that the model is capable to predict accurately the experimental data. Papers D and E are dedicated to experimental studies of the crack propagation in rubber. A new method for determining the critical tearing energy in rubber-like materials is proposed in Paper D. The method attempts to provide an accurate prediction of the tearing energy by accounting for the dissipated energy due to different inelastic processes. The experimental results show that classical method overestimates the critical tearing energy by approximately 15%. In Paper E, the fracture behavior of carbon-black natural rubber material is experimentally studied over a range of loading rates varying from quasi-static to dynamic, different temperatures, and fracture modes. The tearing behavior shows a stick-slip pattern in low velocities with a size dependent on the loading rate, temperature and the fracture mode. Smooth propagation results at high velocities. The critical tearing depends strongly on the loading rate as well as the temperature.

QC 20150203

Aluminum alloys have increasing applications in construction and transportation industries. Both 5xxx-series (Al-Mg) and 6xxx-series (Al-Mg) alloys are frequently used in marine construction because of their light weight, high strength, and corrosion resistance. One of the major concerns regarding the marine application of aluminum alloys is their mechanical performance in fire scenarios. The material strength may be degraded due to both thermal and mechanical damage during fire exposure. This work emphasizes the stress-induced mechanical (physical) damage and its impact on the residual (post-fire) performance of 5083-H116 and 6061-T651 aluminum alloy. Thermo-mechanical tests were performed at various temperatures and stresses to study the stress-induced damage at induced plastic creep strain levels. Unstressed thermally exposed and thermo-mechanically damaged samples were examined to separate the stress-induced microstructural damage. The stress-induced microstructural damage primarily manifests itself as dynamic recovery at low creep temperatures, while cavitation, dynamic recrystallization and dynamic precipitation (in Al6061) are the types of damage developed in the high creep strains at high exposure temperatures. Different creep mechanisms are also studied for both Al5083 and Al6061. The post-fire mechanical response at room temperature after thermo-mechanical damage was investigated with reference to the damaged microstructure present in the material. Residual material strengths based on deformed cross sectional area after the creep test were calculated to provide insight into how microstructural damage affects the post-fire material performance. The competing effects of strength degradation caused by cavitation and strengthening due to grain elongation and subgrain refinement were investigated. Engineering residual material strengths calculated based on the original cross sectional area prior to creep tests were also studied to provide guidance for structural design.
Ph. D.

Exercise-induced muscle damage (EIMD) is a commonly experienced phenomenon, yet its effect on the human response to dynamic exercise is poorly understood. Therefore the intention of this thesis was to provide empirical evidence to advance the scientific knowledge and understanding of the phenomenon of EIMD; principally by investigating the physiological, perceived exertion and metabolic responses to the performance of dynamic exercise with EIMD. The eccentric, muscle-damaging exercise protocol employed for all four studies involved participants completing 100 squats performed as 10 sets of 10 repetitions with the load on the bar corresponding to 70% of the individual’s body mass. Measures of markers of muscle damage were taken before and after the eccentric exercise protocol in each of the four studies. The markers used were plasma creatine kinase activity, isokinetic peak torque and perceived muscle soreness. Cycling rather than running was used as the dynamic exercise mode in studies 1, 2 and 4 in order to avoid the confounding influence of alterations in gait subsequent to EIMD. The dynamic exercise in study 3 was performed inside a whole body scanner and was therefore limited to knee extension and flexion. These four studies have provided novel insights into the influence of eccentric, muscle-damaging exercise on the human response to the performance of dynamic exercise. We have demonstrated for the first time that following EIMD, the enhanced ventilatory response to dynamic exercise is provoked by stimuli unrelated to the blood lactate response, and that this enhanced ventilation may provide an important cue to inform the perception of effort. Furthermore, we have shown that the reduced time to exhaustion observed following EIMD is associated with an elevated perception of exertion and increases in [Pi] during dynamic exercise. Finally, we have demonstrated that the kinetic response is unaltered during the transition to high intensity dynamic exercise. Changes in [HHb] kinetics indicate that compensatory mechanisms act to preserve blood-myocyte O2 flux in the face of microvascular dysfunction, resulting in the unaltered observed across the rest-to-exercise transition.

One of the functions of graphite is as a moderator in several nuclear reactor designs, including the Advanced Gas-cooled Reactor (AGR). In the reactor graphite is used to thermalise the neutrons produced in the fission reaction thus allowing a self-sustained reaction to occur. The graphite blocks, acting as the moderator, are constantly irradiated and consequently suffer damage. This thesis examines the types of damage caused using molecular dynamic (MD) simulations and ab intio calculations. Neutron damage starts with a primary knock-on atom (PKA), which is travelling so fast that it creates damage through electronic and thermal excitation (this is addressed with thermal spike simulations). When the PKA has lost energy the subsequent cascade is based on ballistic atomic displacement. These two types of simulations were performed on single crystal graphite and other carbon structures such as diamond and amorphous carbon as a comparison. The thermal spike in single crystal graphite produced results which varied from no defects to a small number of permanent defects in the structure. It is only at the high energy range that more damage is seen but these energies are less likely to occur in the nuclear reactor. The thermal spike does not create damage but it is possible that it can heal damaged sections of the graphite, which can be demonstrated with the motion of the defects when a thermal spike is applied. The cascade simulations create more damage than the thermal spike even though less energy is applied to the system. A new damage function is found with a threshold region that varies with the square root of energy in excess of the energy threshold. This is further broken down in to contributions from primary and subsequent knock-on atoms. The threshold displacement energy (TDE) is found to be Ed=25eV at 300K. In both these types of simulation graphite acts very differently to the other carbon structures. There are two types of polycrystalline graphite structures which simulations have been performed on. The difference between the two is at the grain boundaries with one having dangling bonds and the other one being bonded. The cascade showed the grain boundaries acting as a trap for the knock-on atoms which produces more damage compared with the single crystal. Finally the effects of turbostratic disorder on damage is considered. Density functional theory (DFT) was used to look at interstitials in (002) twist boundaries and how they act compared to AB stacked graphite. The results of these calculations show that the spiro interstitial is more stable in these grain boundaries, so at temperatures where the interstitial can migrate along the c direction they will segregate to (002) twist boundaries.

Moisture-induced damage is one the major causes of deterioration of asphalt pavements and extensive research has been conducted on this topic. Theoretical and experimental results have led the researchers to believe that moisture-induced damages are caused mainly by the generation of pore water pressure in asphalt mixtures when traffic passes over a pavement. The Moisture Induced Sensitivity Tester (MIST) has been recently developed to simulate the phenomenon of repeated pore pressure generation and deterioration in the laboratory. The objective of this study was to evaluate moisture-induced damage in typical Maine Department of Transportation (DOT) asphalt mixes, with the use of MIST, pre and post testing, and analysis of data. The MIST was used to condition Hot Mix Asphalt (HMA) samples that were compacted from eight typical Maine DOT mixes, with different types of aggregates and asphalt binder. A modified Dynamic modulus test in Indirect Tensile Mode was used for the determination of damage. A layered elastic model, along with a fatigue-cracking criterion, was utilized to assess the total impact on the pavement lives. Monte Carlo analysis was conducted to determine the distribution of number of repetitions to failure of pavements that are subjected to moisture damage. The major conclusions are that most of the mixes are likely to experience a reduction in their life due to the effect of moisture and that the Micro-Deval and the fine aggregate absorption test results can be related to such damage. A composite factor, consisting of both of these test results, is recommended for regular use by the DOT to screen mixes with high moisture damage potential.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Biological Engineering, 2014.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references.
The work presented in this thesis explores two novel and complementary facets of endogenous DNA damage: the development of biomarkers of inflammation based on metabolites of DNA damage products and the formation of DNA adducts by electrophilic products of intermediary metabolism. From the first perspective, endogenous DNA damage generated by reactive oxygen and nitrogen species from inflammation and oxidative stress has shown strong mechanistic links to the pathophysiology of cancer and other human diseases, with the damage products reflecting all types of damage chemistries including oxidation, deamination, halogenation, nitration and alkylation. However, the use of DNA damage products as biomarkers has been limited by poor understanding of the damage actually arising in tissues and a lack of appreciation of the fate of DNA damage products from the moment of formation at the site of damage to release from cells to final excretion from the body. The goal of the work presented in the first part of this thesis was to investigate the metabolic fates of the base propenal products arising from 4'-oxidation of 2'-deoxyribose in DNA, one of the most common products of DNA oxidation, and to define base propenal metabolites as potential biomarkers of oxidative stress. This project was approached with systematic metabolite profiling, starting with prediction of potential base propenal metabolites based on a priori knowledge of its chemical reactivity as an [alpha],[beta]-unsaturated aldehyde toward glutathione (GSH) in non-enzymatic reactions and in rat liver cell extracts. Of 15 potential candidates predicted and identified from these in vitro studies, analysis of urine samples from rats given intravenous doses (IV) of thymine propenal revealed three major metabolites: thymine propenoic acid and two mercapturic acid derivatives, which accounted for ~6% of the injected dose. An additional four metabolites, including conjugates with GSH, cysteinylglycine and cysteine, were observed in bile and accounted for ~22% of the dose. One of the major metabolites detected in urine and bile, a bis-mercapturic acid adduct of reduced thymine propenal was detected as a background excretory product in saline-treated rats and was significantly elevated after oxidative stress caused by treatment with bleomycin and CCl₄. Our observations suggest that metabolism and disposition of damaged biomolecules should be considered as crucial factors in the development of biomarkers relevant to inflammation and oxidative stress. The second part of this thesis addresses the complementary hypothesis that electrophilic metabolites generated endogenously from intermediary metabolism can react with DNA to form adducts. This concept is illustrated here with glyoxylate from the glyoxylate metabolic cycle, whicvh plays a key role as an alternative to the TCA cycle in plants, bacteria, protists and fungi under changing conditions of environmental nutrients. The goal of this project was to characterize DNA adducts caused by glyoxylate in the mycobacterium M. smegmatis, with the studies motivated by the higher-than-expected mutation rate of mycobacteria during dormancy induced by nutrient deprivation and a shift to utilization of the glyoxylate cycle. Initially, in vitro reactions of 2'-deoxyguanosine (dG) with glyoxylate yielded N²-carboxyhydroxymethyl dG (N²-CHMdG) as the only adduct. However, the adduct proved to be unstable, so a reduction-based analytical method was developed to yield the stable amine derivative, N2-carboxymethyl dG (N²-CMdG). This stable adduct was used to develop an isotope-dilution chromatography-coupled tandem mass spectrometry method to quantify N²-CHMdG as N²-CMdG in calf thymus DNA treated with glyoxylate in vitro. This analytical method was then applied to quantify and compare the level N2-CMdG in (1) wild-type M. smegmatis grown in rich medium (7H9) or in minimal M9 medium supplemented with acetate, the latter inducing a switch from the TCA cycle to the glyoxylate cycle; and (2) the isocitrate dehydrogenase (ICD)-deficient mutant of M. smegmatis. Mycobacteria grown in the acetate medium experienced a 2-fold increase in the adduct compared to those grown in 7H9. Similarly, the adduct increased 2-fold in the ICD mutant compared to wild-type M. smegmatis grown in 7H9. The results support the idea that shifts in intermediary metabolism can lead to DNA damage that may cause mutations associated with nutrient deprivation in mycobacteria, with implications for the genetic toxicology of other metabolism-derived electrophiles.
by Watthanachai Jumpathong.
Ph. D.

The emphasis on seismic design and assessment of reinforced concrete (RC) frame structure has shifted from force-based to performance-based design and assessment to accommodate strength and ductility for required performance of building. RC frame structure may suffer different levels of damage under seismic-induced ground motions, with potentials for formation of hinges in structural elements, depending on the level of stringency in design. Thus it is required to monitor the seismic behaviour and performance of buildings, which depend on the structural system, year of construction and the level of irregularities in the structural system. It is the objective of the current research project to assess seismic performance of RC frame buildings in Canada, while developing fragility curves as analytical tools for such assessment. This was done through dynamic inelastic analysis by modelling selected building structures and using PERFORM-3D as analysis software, while employing incremental dynamic analysis to generate performance data under incrementally increasing seismic intensity of selected earthquake records. The results lead to probabilistic tools to assess the performance of buildings designed following the National Building Code of Canada in different years of construction with and without irregularities. The research consists of three phases; i) regular buildings designed after 1975, ii) regular buildings designed prior to 1975, and iii) irregular buildings designed prior to 1975. The latter two phases address older buildings prior to the development of modern seismic building codes. All three phases were carried out by selecting and designing buildings in Ottawa, representing the seismic region in eastern Canada, as well as buildings in Vancouver, representing the seismic region in western Canada. Buildings had three heights (2; 5; and 10-stories) to cover a wide range of building periods encountered in practice. The resulting fragility curves indicated that the older buildings showed higher probabilities of exceeding life safety and/or collapse prevention performance levels. Newer buildings showed higher probabilities of exceeding target performance levels in western Canada than those located in the east.

2017 - 2018
One of the most widespread structural systems is represented by Moment Resisting Frames (MRFs). resistant seismic frames. This structural system is made up of frames capable of resisting seismic actions through predominantly flexural tension states. The stiffness and lateral resistance of the system depend on the flexural strength of the members and the type of connection, while the development of the plastic hinges guarantee the dissipation of the seismic input energy. The location of the dissipative zones varies according to the design approach adopted, typically they develop in beams, columns and connections. The most widespread design philosophy is to have strong columns, weak beams and full-strength rigid connections with complete resistance restoration, in this way all the seismic energy tends to be dissipated by the plastic hinges at the ends of the beams and at the base of the columns of the first level. In order to overcome the traditional design approach, the present research work introduces a new type of beam-column connection capable of exhibiting a remarkable rigidity in service conditions (SLE) and able to exhibit a remarkable dissipative capacity when a rare seismic event occurs. The codes currently in force provide that for seismic events characterized by a period of return comparable with the useful life of the structure (frequent or occasional events) the structures remain in the elastic field ensuring that the seismic energy is completely dissipated through viscous damping. Vice versa, the seismic energy must be dissipated through plastic engagement of parts of the structure, with wide and stable hysteresis cycles, for rare and very rare seismic events with a return period of about 500 years. The development of the hysteresis involves structural damage that have to be such as not to lead to the collapse of the structure in order to guarantee the protection of the life of those who occupy the building. The prediction of the behaviour of the structure in non-linear field for rare seismic events represents an aspect that only experimental research can describe in depth by developing new analytical models and innovative design philosophies. The execution of quasi-static tests can provide useful information in order to investigate the nonlinear behaviour of the members and the assemblages even if the forces or the displacement histories applied during the tests do not correspond exactly to the actions that occur during a real seismic event. The information obtained through these test procedures is however useful for calibrating analytical models and comparing the behaviour of structural components. The execution of tests on real scale structures is the best way to investigate the global behaviour of a structural system. For a more complete knowledge about the response in the dynamic field, the pseudo-dynamic tests represent a test protocol able to provide information of the structural response of a component or of a structure in a dynamic field through a static test. The main purpose of this work, developed within the FREEDAM research project financed by the European Community, is to develop an innovative beam-column connection. These innovative connections are equipped with an additional damper able to dissipating the energy deriving from destructive seismic events. The FREEDAM beam-column connection, through an appropriate design of the various components, is able to withstand frequent earthquakes and rare events without causing damage to the structural elements. The thesis is divided into six chapters. The Chapter 1 reports a brief introduction to the traditional beam-column connections, specifying the characteristics of the different types of connection and their influence on the behaviour of the Moment Resisting Frames. In the last part of the chapter the FREEDAM dissipative connection is presented, specifying its peculiarities and the benefits that its introduction into the structural system brings. The Chapter 2 is devoted to the description of the results obtained from an extensive experimental campaign developed at the STRENGTH laboratory of the University of Salerno, for the choice of material for the friction dampers used in the FREEDAM connections by carrying out a statistical characterization of the static and dynamic friction coefficients. The Chapter 3 collects the results of a further experimental campaign carried out at the University of Salerno laboratory and aimed at studying the tightening losses for pre-loading bolt systems equipped with different washers. In Chapter 4 a design procedure has been define for the FREEDAM beam-column connections, then this procedure has been applied in order to design two different types of connections that have been experimentally tested at the University of Coimbra Laboratory (PT). In the same chapter, the test layouts and the results obtained from the cyclic tests carried out on the nodes equipped with FREEDAM friction dampers have been described, finally developing models to the finite elements and comparing the experimental results with the computerized models. Finally, the Chapter 5 shows the results of the pseudo-dynamic tests carried out on a full-scale steel Moment Resistant Frame equipped in a first case with traditional full strength beam-column connections (dogbone) and in a second case equipped with the innovative connections proposed. These results have been compared to each other and with the results obtained from finite element models. [edited by Author]
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