Stabilizing Nonstationary Electrochemical Time Series

Electrochemical systems are ideal working-horses for studying oscillatory dynamics. Experimentally obtained time series, however, are usually associated with a spontaneous drift in some uncontrollable parameter that triggers transitions among different oscillatory patterns, despite the fact that all controllable parameters are kept constant. Herein we present an empirical method to stabilize experimental potential time series. The method consists of applying a negative galvanodynamic sweep to compensate the spontaneous drift and was tested for the oscillatory electro-oxidation of methanol on platinum. For a wide range of applied currents, the base system presents spontaneous transitions from quasi-harmonic to mixed mode oscillations. Temporal patterns were stabilized by galvanodynamic sweeps at different rates. The procedure resulted in a considerable increase in the number of oscillatory cycles from 5 to 20 times, depending on the specific temporal pattern. The spontaneous drift has been associated with uncompensated oscillations, in which the coverage of some adsorbed species are not reestablished after one cycle; i.e., there is a net accumulation and/or depletion of adsorbed species during oscillations. We interpreted the rate of the galvanodynamic sweep in terms of the time scales of the poisoning processes that underlies the uncompensated oscillations and thus the spontaneous slow drift.

Evaluation of the fretting fatigue behaviour of commercially pure titanium

Fretting fatigue occurs when the contact surfaces of two components undergo small oscillatory movement while they are subjected to a clamping force. A cyclic external load gives rise to the early initiation of fatigue cracks, thus reducing their service life. In this paper, the fretting fatigue behaviour of commercially pure titanium flat samples (1.5 mm thick) is evaluated. A fretting device composed of a frame, load cell, and two screw-mounted cylindrical fretting pads with convex extremities was built and set to a servo-hydraulic testing machine. The fatigue tests were conducted under load control at a frequency of 10 Hz and stress ratio R = 0.1, with various contact load values applied to the fretting pads. Additional tests under inert environment allowed assessing the role of oxidation on the wear debris formation. The fracture surfaces and fretting scars were analysed via scanning electron microscopy in order to evaluate the surface damage evolution and its effect on the fatigue crack features. The effect of the fretting condition on the S-N curve of the material in the range of 10(4)-10(6) cycles is described. Fatigue crack growth calculations allowed estimating the crack initiation and propagation lives under fretting conditions. The effect of the fretting condition in fatigue life is stronger for the lower values of cyclic stress and does not seem to depend on the contact loading value.

Dinâmica populacional de moscas-das-frutas e de Triozoida limbata (Hemiptera: Psyllidae) e danos de Costalimaita ferruginea (Coleoptera: Chrysomelidae) e de T. limbata em pomar de goiaba submetido a sistema de racionalização de inseticidas

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Caracterización multivariada de los temporales para su aplicación en el dimensionamiento del manto principal

En esta tesis se presenta una metodología para la caracterización del oleaje, dentro del marco de las nuevas Recomendaciones para Obras Marítimas (ROM 0.0.-00 y ROM 1.0-09), por ser una de las principales acciones que afectan a la estabilidad de las estructuras marítimas. Debido al carácter aleatorio intrínsecamente multivariado de la acción considerada, las tormentas, su caracterización paramétrica se realiza en términos de funciones cópula uniparamétricas. Las variables consideradas son altura de ola significante del pico de la tormenta, el periodo medio asociado y la magnitud, o número de olas, de todo el ciclo de solicitación. Para establecer un patrón teórico de evolución de la tormenta que permita extrapolar las muestras fuera de la región con datos se analizan los modelos teóricos existentes, comprobándose que no reproducen adecuadamente las tormentas constituidas por estados de mar con un peso importante de oleaje swell. Para evitar esta limitación se proponen cuatro modelos teóricos de evolución de tormentas con distintas formas geométricas. El análisis de los modelos existentes y los propuestos pone de relieve que el Modelo Magnitud Equivalente de Tormenta (EMS= Equivalent Magnitude Storm) con la forma triangular es el que mejor adapta las tormentas constituidas por estados de mar típicos del viento. Para tormentas con un mayor grado de desarrollo, el modelo teórico de tormenta EMS con la forma trapezoidal es el adecuado. De las aproximaciones propuestas para establecer el periodo medio de los sucesivos estados de mar del ciclo de solicitación. la propuesta por Martín Soldevilla et al., (2009) es la más versátil y , en general , mejor reproduce la evolución de todo tipo de tormentas. La caracterización de las tormentas se complementa con la altura de ola máxima. Debido a la mayor disponibilidad y longitud temporal de los datos sintéticos frente a las registros, la práctica totalidad de los análisis de extremos se realizan con tormentas sintéticas en las que la distribución de olas individuales es desconocida. Para evitar esta limitación se utilizan modelos teóricos de distribución de olas acordes a las características de cada uno de los estados de mar que conforman la tormenta sintética. Para establecer dichas características se utiliza la curtosis y en función de su valor la altura de ola máxima se determina asumiendo una determinada distribución de olas. Para estados de mar lineales la distribución de olas individuales de Rayleigh es la considerada. Para condiciones no lineales de gran ancho de banda el modelo de distribución de olas propuesto por Dawson, (2004) es el utilizado y si es de banda estrecha las predicciones de (Boccotti, (1989), Boccotti et al., (2013)) se compara con las resultantes del modelo de Dawson. La caracterización de la evolución de las tormentas en términos multivariados es aplicada al estudio de la progresión del daño del manto principal de diques en talud, y al rebase de las olas. Ambos aspectos cubren el segundo objetivo de la tesis en el que se propone una nueva formulación para el dimensionamiento de mantos constituidos por bloques cúbicos de hormigón. Para el desarrollo de esta nueva formulación se han utilizado los resultados recogidos en los estudios de estabilidad del manto principal de diques talud realizados en modelo físico a escala reducida en el Centro de Estudios de Puertos y Costas (CEDEX) desde la década de los 80 empleando, en su mayoría, bloques paralelepípedos cúbicos de hormigón. Por este motivo y porque los últimos diques construidos en la costa Española utilizan este tipo de pieza, es por lo que la formulación planteada se centra en este tipo de pieza. Después de un primer análisis de las fórmulas de cálculo y de evolución existentes, se llega a la conclusión de que es necesario realizar un esfuerzo de investigación en este campo, así como ensayos en laboratorio y recogida de datos in-situ con base a desarrollar fórmulas de evolución de daño para mantos constituidos por piezas diferentes a la escollera, que tenga en cuenta las principales variables que condiciona su estabilidad. En esta parte de la tesis se propone un método de análisis de evolución de daño, que incluye el criterio de inicio de avería, adecuada para diques en talud constituidos por bloque cúbicos de hormigón y que considera la incidencia oblicua, el daño acumulado y el rebase. This thesis proposes a methodology to estimate sea waves, one of the main actions affecting the maritime structures stability, complying with (ROM 0.0.-00 & ROM 1.0-09.Due to the multivariate behavior of sea storms, the characterization of the structures of sea storms is done using copula function. The analyzed variables are the significant height wave, mean period and magnitude or number of waves during the storm history. The storm evolution in terms of the significant height wave and the mean period is also studied in other to analyze the progressive failure modes. The existing models of evolution are studied, verifying that these approximations do not adjust accurately for developed waves. To overcome this disadvantage, four evolution models are proposed, with some geometrical shapes associated to fit any development degree. The proposed Equivalent Magnitude Storm model, EMS, generally obtains the best results for any kind of storm (predominant sea, swell or both). The triangle is recommended for typical sea storms whereas the trapezoid shape is much more appropriate for more developed storm conditions.The Martín Soldevilla et al., (2009) approach to estimate the mean period is better than others approaches used.The storm characterization is completed with the maximum wave height of the whole storm history. Due to synthetic historical waves databases are more accessible and longer than recorded database, the extreme analyses are done with synthetic data. For this reason the individual waves’ distribution is not known. For that limitation to be avoided, and depending on the characteristics of every sea states, one theoretical model of waves is choose and used. The kurtosis parameter is used to distinguish between linear and nonlinear sea states. The Rayleigh model is used for the linear sea states. For the nonlinear sea states, Dawson, (2004) approach is used for non-narrow bandwidth storms, comparing the results with the Boccotti, (1989), Boccotti et al., (2013) approach, with is used for narrow bandwidth storms. The multivariate and storm evolution characterization is used to analyze of stone armour damage progression and wave overtopping discharge. Both aspects are included in the second part of the thesis, with a new formula is proposed to design cubes armour layer. The results the stability studies of armour layer, done in the Centre for Harbours and Coastal Studies (CEDEX) laboratory are used for defining a new stability formula. For this reason and because the last biggest breakwater built in Spain using the cube, the damage progression is analyze for this kind of concrete block. Before to analyze the existing formulae, it is concluded that it is necessary more investigation, more tests in laboratory and data gathering in situ to define damage evolution formulae to armour of other kind of pieces and that takes to account the principal variables. This thesis proposed a method to calculate the damage progression including oblique waves, accumulated damage, and overtopping effect. The method also takes account the beginning of the movement of the blocks.

Estudo experimental e numérico de juntas adesivas em T com aderentes de alumínio

Nos dias de hoje, a ligação adesiva de estruturas complexas que não poderiam ou não seriam tão fáceis de ser fabricadas numa só peça é cada vez mais usual. As juntas adesivas têm vindo a substituir muitos outros métodos de ligação, como por exemplo ligações aparafusadas, rebitas ou soldadas, devido às vantagens de facilidade na sua fabricação, resistência superior e capacidade de unir materiais diferentes. Por esta razão as juntas adesivas têm vindo a ser aplicadas cada vez mais em várias industrias como aeroespacial, aeronáutica, automóvel, naval e calçado. O tipo de adesivo a usar em determinada aplicação é principalmente escolhido consoante as suas características mecânicas e o tipo de resposta pretendida às solicitações impostas. Como exemplo de adesivo resistente e frágil existe o Araldite® AV138. Por outro lado, o adesivo Araldite® 2015 é menos resistente, mas apresenta maior ductilidade e flexibilidade. Além dos adesivos Araldite® comerciais, existem adesivos de poliuretano que combinam características de elevada resistência com características de grande ductilidade e flexibilidade, como por exemplo o Sikaforce® 7752. Esta dissertação tem como objetivo estudar experimentalmente e numericamente, através de modelos de dano coesivo (MDC), o comportamento de diferentes configurações de junta em T quando sujeitas a solicitações de arrancamento. Consideram-se os adesivos anteriormente mencionados para testar as juntas sob diferentes tipos de adesivos. A junta em T é constituída por 2 aderentes em L de alumínio e um aderente base também em alumínio, unidos por uma camada de adesivo. Experimentalmente é feito um estudo da resistência da junta com a variação da espessura dos aderentes em L (tP2). Com a análise numérica são estudadas as distribuições de tensões, evolução do dano, modos de rotura e resistência. Além disso, realizou-se um estudo numérico da existência ou não de adesivo de preenchimento na zona da curvatura dos aderentes em L nas tensões e na resistência da junta. Mostrouse que a variação da geometria nos aderentes em L, a presença de adesivo de preenchimento e o tipo de adesivo têm uma influência direta na resistência de junta. Os ensaios experimentais validaram os resultados numéricos e permitiram concluir que os MDC são uma técnica precisa para o estudo das geometrias das juntas em T.

Statistical description of pattern evolution in damage-fracture

A model of dynamical process and stochastic jump has been put forward to study the pattern evolution in damage-fracture. According to the final states of evolution processes, the evolution modes can be classified as globally stable modes (GS modes) and evolution induced catastrophic modes (ElC modes); the latter are responsible for fracture. A statistical description is introduced to clarify the pattern evolution in this paper. It is indicated that the appearance of fracture in disordered materials should be depicted by probability distribution function.

Evolution of Localized Damage Zone in Heterogeneous Media

Evolution of localized damage zone is a key to catastrophic rupture in heterogeneous materials. In the present article, the evolutions of strain fields of rock specimens are investigated experimentally. The observed evolution of fluctuations and autocorrelations of strain fields under uniaxial compression demonstrates that the localization of deformation always appears ahead of catastrophic rupture. In particular, the localization evolves pronouncedly with increasing deformation in the rock experiments. By means of the definition of the zone with high strain rate and likely damage localization, it is found that the size of the localized zone decreases from the sample size at peak load to an eventual value. Actually, the deformation field beyond peak load is bound to suffer bifurcation, namely an elastic unloading part and a continuing but localized damage part will co-exist in series in a specimen. To describe this continuous bifurcation and localization process observed in experiments, a model on continuum mechanics is developed. The model can explain why the decreasing width of localized zone can lead stable deformation to unstable, but it still has not provided the complete equations governing the evolution of the localized zone.

Evolution of radiographic joint damage in rituximab-treated versus TNF-treated rheumatoid arthritis cases with inadequate response to TNF antagonists

Observational studies have suggested that patients with rheumatoid arthritis (RA) who experience inadequate response to anti-tumour necrosis factor (anti-TNF) agents respond more favourably to rituximab (RTX) than to an alternative anti-TNF agent. However, the relative effectiveness of these agents on long-term outcomes, particularly in radiographic damage, remains unclear.

Storm evolution characterization for analysing stone armour damage progression

Storm evolution is fundamental for analysing the damage progression of the different failure modes and establishing suitable protocols for maintaining and optimally sizing structures. However, this aspect has hardly been studied and practically the whole of the studies dealing with the subject adopt the Equivalent triangle storm. As against this approach, two new ones are proposed. The first is the Equivalent Triangle Magnitude Storm model (ETMS), whose base, the triangular storm duration, D, is established such that its magnitude (area describing the storm history above the reference threshold level which sets the storm condition),HT, equals the real storm magnitude. The other is the Equivalent Triangle Number of Waves Storm (ETNWS), where the base is referred in terms of the real storm's number of waves,Nz. Three approaches are used for estimating the mean period, Tm, associated to each of the sea states defining the storm evolution, which is necessary to determine the full energy flux withstood by the structure in the course of the extreme event. Two are based on the Jonswap spectrum representativity and the other uses the bivariate Gumbel copula (Hs, Tm), resulting from adjusting the storm peaks. The representativity of the approaches proposed and those defined in specialised literature are analysed by comparing the main armour layer's progressive loss of hydraulic stability caused by real storms and that relating to theoretical ones. An empirical maximum energy flux model is used for this purpose. The agreement between the empirical and theoretical results demonstrates that the representativity of the different approaches depends on the storm characteristics and point towards a need to investigate other geometrical shapes to characterise the storm evolution associated with sea states heavily influenced by swell wave components.

Study of surface evolution in the wheel-rail interface

This paper aims to trace surface evolution in the wheel-rail interface using data obtained from a twin-disc testing machine and the surface replication technique. Changes in the surface profile of the rail testing disc are explicitly analysed according to the wear mechanism, which helps elaborate a better understanding of the attrition of asperities during the wearing-in process of surface modification. The surface profile amplitude was seen to decrease during the initial running-in phase of the experiment cycle, and after reaching a saturation value, the profile amplitude then increased. Ultimately the results show that grinding will roughen the rail surface and the wheel-rail contact conditions will then remove this surface damage to some saturation value of the profile height. The variation in the rail surface profile beyond this point is then only dependant on the contact conditions which exist between the wheel and rail during normal operation.

Wear particle analysis and the evolution of the plastically deformed layer on Australian rail steel

Particle analysis methodology is presented, together with the morphology of the wear debris formed during rolling contact fatigue. Wear particles are characterised by their surface topography and in terms of wear mechanism. Rail-wheel materials are subjected to severe plastic deformation as the contact loading progresses, which contributes to a mechanism of major damage in head-hardened rail steel. Most of the current methodologies involve sectioning of the rail-wheel discs to trace material damage phenomena such as crack propagation and plastic strain accumulation. This paper proposes methodology to analyse the development of the plastically deformed layer by sectioning wear particles using the focussed ion beam (FIB) milling method. Moreover, it highlights the processes of oxidation and rail surface delamination during unlubricated rolling contact fatigue.

Genetic Algorithm for Damage Assessment

Genetic Algorithms (GAs) are recognized as an alternative class of computational model, which mimic natural evolution to solve problems in a wide domain including machine learning, music generation, genetic synthesis etc. In the present study Genetic Algorithm has been employed to obtain damage assessment of composite structural elements. It is considered that a state of damage can be modeled as reduction in stiffness. The task is to determine the magnitude and location of damage. In a composite plate that is discretized into a set of finite elements, if a jth element is damaged, the GA based technique will predict the reduction in Ex and Ey and the location j. The fact that the natural frequency decreases with decrease in stiffness is made use of in the method. The natural frequency of any two modes of the damaged plates for the assumed damage parameters is facilitated by the use of Eigen sensitivity analysis. The Eigen value sensitivities are the derivatives of the Eigen values with respect to certain design parameters. If ωiu is the natural frequency of the ith mode of the undamaged plate and ωid is that of the damaged plate, with δωi as the difference between the two, while δωk is a similar difference in the kth mode, R is defined as the ratio of the two. For a random selection of Ex,Ey and j, a ratio Ri is obtained. A proper combination of Ex,Ey and j which makes Ri−R=0 is obtained by Genetic Algorithm.

An experimental study on cracking evolution in concrete and cement mortar by the b-value analysis of acoustic emission technique

Notched three point bend (TPB) specimens made with plain concrete and cement mortar were tested under crack mouth opening displacement (CMOD) control at a rate of 0.0004 mm/s and simultaneously acoustic emissions (AE) released were recorded during the experiments. Amplitude distribution analysis of AE released during concrete was carried out to study the development of fracture process in concrete and mortar specimens. The slope of the log-linear frequency-amplitude distribution of AE is known as the AE based b-value. The AE based b-value was computed in terms of physical process of time varying applied load using cumulative frequency distribution (Gutenberg-Richter relationship) and discrete frequency distribution (Aki's method) of AE released during concrete fracture. AE characteristics of plain concrete and cement mortar were studied and discussed and it was observed that the AE based b-value analysis serves as a tool to identify the damage in concrete structural members. (C) 2012 Elsevier Ltd. All rights reserved.

An elastoplastic constitutive relation of whisker-reinforced composite for meso damage: Part I - formulation

An elastoplastic constitutive relation is developed for meso damage of whisker-reinforced composites. A model is constructed that includes orientation distribution of whiskers and slip systems as well as interface and crystal sliding. Evolution of damage will be addressed. Given in Part I is the formulation while examples will be illustrated in Part II.

A micromechanical damage theory for brittle materials with small cracks

For brittle solids containing numerous small cracks, a micromechanical damage theory is presented which accounts for the interactions between different small cracks and the effect of the boundary of a finite solid, and includes growth of the pre-existing small cracks. The analysis is based on a superposition scheme and series expansions of the complex potentials. The small crack evolution process is simulated through the use of fracture mechanics incorporating appropriate failure criteria. The stress-strain relations are obtained from the micromechanics analysis. Typical examples are given to illustrate the potential capability of the proposed theory. These results show that the present method provides a direct and efficient approach to deal with brittle finite solids containing multiple small cracks. The stress-strain relation curves are evaluated for a rectangular plate containing small cracks.