Degradation and Fatigue Involving Dissipated Processes

Irreversible material degradation due to cyclic mechanical loading is investigated utilizing the concept of thermodynamic entropy, plastic strain energy, and temperature slope measurement. Uniaxial tension-compression and fully-reversed bending fatigue tests are performed over a wide range of loading conditions with metallic and composite materials subject to both constant- and variable-amplitude loading. A methodology is developed for the estimation of the fatigue fracture entropy (FFE) and fatigue toughness of metallic specimens in a rapid fashion. It is found that the FFE and the fatigue toughness of each material tested are within a small band. The value of FFE is found to be unique for a given type of a material, substantiating that FFE can be regarded as a material property. The concept of FFE is applied to study the effect of stress concentration on a metallic specimen. It is found that the FFE of a V-notched specimen with certain amount of stress concentration is fairly constant. A formula is derived for the prediction of the fatigue life of a V-notched specimen based on the fatigue test results of an un-notched specimen. The concept of FFE is utilized to study the high-cycle fatigue (HCF) of carbon steel 1018. As the stress levels in HCF are substantially smaller than the yield strength of the material, a considerable amount of anelastic energy is present in the hysteresis loop along with plastic strain energy. We propose a method to calculate anelastic energy so that entropy generation can be estimated. Finite element simulations are performed to validate the assumptions made in the development of the methodology. It is found that the FFE of this material remains within a specific band both for the low- and high-cycle fatigue. A methodology is developed for the prediction of the remaining fatigue life (RFL) of a specimen with prior history of loading in a non-destructive (NDT) fashion based on the slope of temperature rise obtained from the specimen under cyclic loading. This method which uses thermographic technique has been validated with API 5L X52, carbon steel 1018, and Glass/Epoxy composite with promising results. This approach is further extended to derive a correlation between the damage parameter and the temperature slope obtained from a fatigued specimen. This correlation and the so-called master curve of damage evolution are employed to develop a methodology for the prediction of the RFL of a metallic specimen

On the Degradation of Lubricating Grease

A comprehensive literature review on physical and chemical degradation monitoring and life estimation models for lubricating greases is presented in chapter one. Degradation mechanisms for lubricating grease are categorized and described, and an extensive survey of the available empirical and analytical grease life estimation models including degradation monitoring standards and methods are presented. In chapter two, irreversible thermodynamic theory is employed to study the mechanical degradation of lubricating grease. A correlation between the mechanical degradation and entropy generation is established and the results are verified experimentally using a rheometer, a journal bearing test rig, and a modified grease worker machine. It is shown that the degradation rate is linearly related to the entropy generation, and that it can be used for estimation of the mechanically degraded grease life. In chapter three, a model is presented that uses the principles of irreversible thermodynamics to predict the life of a lubricating grease undergoing mechanical shearing action. Here we restrict our attention to operating temperatures far below the initial activation energy needed to initiate chemical degradation or base oil evaporation. Thus, mechanical degradation is the dominant degradation process. The predictions of the model are validated using the experimental results obtained by testing three greases subjected to different shear rates and temperatures. In chapter four, mechanical life of grease in an elastohydrodynamic (EHL) line contact between two steel rollers is studied. Grease traction curves are measured and reported in different conditions. Three successive lubricating phases of “Fully grease covered rollers”, “Slippage and grease separation” and “Formation of liquid lubricant reservoir” are observed and their behaviors are examined. The traction of the grease is monitored during a long term mechanical degradation process. Our mechanical life prediction model is applied to the lubricating grease at the contact. In chapter five, chemical degradation is studied from an energy point of view. A theory is introduced based on acquired experimental results, and is verified using a roller tester rig. The theory is used to estimate the chemical life of a grease at different temperatures. Summary and conclusions are given in Chapter six along with recommendations for future studies

AN ENTROPIC THEORY OF DAMAGE WITH APPLICATIONS TO CORROSION-FATIGUE STRUCTURAL INTEGRITY ASSESSMENT

This dissertation demonstrates an explanation of damage and reliability of critical components and structures within the second law of thermodynamics. The approach relies on the fundamentals of irreversible thermodynamics, specifically the concept of entropy generation due to materials degradation as an index of damage. All failure mechanisms that cause degradation, damage accumulation and ultimate failure share a common feature, namely energy dissipation. Energy dissipation, as a fundamental measure for irreversibility in a thermodynamic treatment of non-equilibrium processes, leads to and can be expressed in terms of entropy generation. The dissertation proposes a theory of damage by relating entropy generation to energy dissipation via generalized thermodynamic forces and thermodynamic fluxes that formally describes the resulting damage. Following the proposed theory of entropic damage, an approach to reliability and integrity characterization based on thermodynamic entropy is discussed. It is shown that the variability in the amount of the thermodynamic-based damage and uncertainties about the parameters of a distribution model describing the variability, leads to a more consistent and broader definition of the well know time-to-failure distribution in reliability engineering. As such it has been shown that the reliability function can be derived from the thermodynamic laws rather than estimated from the observed failure histories. Furthermore, using the superior advantages of the use of entropy generation and accumulation as a damage index in comparison to common observable markers of damage such as crack size, a method is proposed to explain the prognostics and health management (PHM) in terms of the entropic damage. The proposed entropic-based damage theory to reliability and integrity is then demonstrated through experimental validation. Using this theorem, the corrosion-fatigue entropy generation function is derived, evaluated and employed for structural integrity, reliability assessment and remaining useful life (RUL) prediction of Aluminum 7075-T651 specimens tested

Entropy as a material response to fatigue in metals and related thermographic assessment

The dissertation comprehensively proposes multi non-destructive testing methods investigation in mechanical degradation of metallic materials. First, high cycle fatigue of an unalloyed medium carbon steel SAE1045 is quantified investigated through thermographic measurement; a short-term evaluation procedure is proposed based on the intrinsic thermal dissipation model to determine the S-N curves by performing two constant amplitude tests and one load increase test. Second, a unified approach is developed by evaluating the evolution of intrinsic dissipation and microplasticity. This plasticity is derived from temperature and is correlated to the fatigue process and related to fatigue life. Furthermore, a modified fracture fatigue entropy (FFE) method is modeled to evaluate the fatigue damage. It is shown that the FFE can be better used as an index to trace the fatigue damage as an irreversible degradation of a metallic material of its non-linearity. Finally, the mechanical and magnetic behavior of an ultrafine-grained medium manganese transformation-induced plasticity steel is investigated in its plastic instability. Lüders bands are characterized by digital image correlation and magnetic Barkhausen noise (MBN), and the final results show that MBN can be used as a potential means for the non-destructive evaluation for the strengthening of this steel.In der Dissertation werden mehrere zerstörungsfreie Prüfverfahren zur Untersuchung der mechanischen Degradation metallischer Werkstoffe vorgeschlagen. Erstens wird das Ermüdungsverhalten in dem Zeitfestigkeitsbereich (engl. high cycle fatigue) eines unlegierten Kohlenstoffstahls (SAE1045/C45E) durch thermografische Messungen quantifiziert ermittelt, wobei ein Kurzzeitverfahren zur Lebensdauerberechnung abgeleitet wird, das auf dem Modell der intrinsischen Wärmedissipation basiert, sodass die Bestimmung der Wöhlerkurven mit zwei Einstufenversuchen und einem Laststeigerungsversuch ermöglicht wird. Darüber hinaus wird ein einheitlicher Ansatz entwickelt, indem die Entwicklung der intrinsischen Dissipation und der Mikroplastizität bewertet wird. Diese Plastizität wird hierbei von der Temperatur abgeleitet, mit dem Ermüdungsprozess korreliert und auf die Ermüdungslebensdauer bezogen. Zudem wird eine modifizierte Fracture Fatigue Entropy-Methode (FFE) modelliert, um die Ermüdungsschädigung zu evaluieren. Schließlich wird das mechanische und magnetische Verhalten in der plastischen Instabilität eines ultrafeinkörnigen TRIP-Stahls mit mittlerem Mangangehalt untersucht. Lüders-Bänder werden durch digitale Bildkorrelation (DIC) und magnetisches Barkhausenrauschen (MBN) charakterisiert. Die Ergebnisse zeigen, dass MBN als potenzielles Mittel für die zerstörungsfreie Bewertung der Festigkeit dieses Stahls verwendet werden kann

Modelamiento de la dinámica termo-mecánica presente en un par deslizante con fines de predecir desgaste

Resumen: El daño progresivo que se produce en la superficie de un componente como resultado de su movimiento relativo a las partes adyacentes de trabajo, tiene profundas consecuencias económicas que implican no sólo los costes de sustitución, sino también los gastos relacionados con la inactividad de la máquina y pérdida de producción, haciéndose necesario estudiar, analizar y modelar el proceso de deslizamiento entre sólidos. Se tiene conocimiento del desarrollado de diferentes modelos estáticos que correlacionan algunas propiedades mecánicas con el fin predecir la tasa de desgaste en procesos de deslizamiento entre sólidos, o de modelos dinámicos que describen los cambios térmicos y su influencia directa en el desgaste progresivo de uno o de ambos sólidos de un par mecánico. También se ha aplicado el potencial de las leyes de la termodinámica para analizar la fricción en sistemas de ingeniería, consolidando estudios en el desarrollo de modelos que representan y explican el comportamiento energético de dichos sistemas, logrando obtener modelos capaces de relacionar variables termodinámicas con variables mecánicas y así predecir el comportamiento de pares deslizantes. Sin embargo no se encuentra en la literatura un consenso de cuáles son las variables que representan, describen y explican el proceso de desgaste en deslizamiento entre sólidos, donde se relacione tanto las variables clásicas del proceso como el coeficiente de fricción, la carga y la velocidad, sino también la temperatura, con el aumento en la tasa de desgaste de un proceso de deslizamiento. En este trabajo se planteó desarrollar un modelo que relacionara las dinámicas térmicas y mecánicas con el fin de predecir el aumento y/o disminución de la tasa de desgaste en un par deslizante. Para el desarrollo del modelo se debió: Primero modelar la generación y transferencia térmica, donde se halló la “flash temperature” y, Segundo, modelar los fenómenos disipativos de degradación, donde se cuantifica la tasa de desgaste. El Modelado de la “flash temperature” se logró con el desarrollo de una ecuación constitutiva que representa el calor generado por el contacto entre microasperezas. Esta ecuación sirvió como entrada de energía para el modelo térmico desarrollado, con el cual se obtuvo un modelo finito dimensional, capaz de representar la dinámica de la llamada “Flash Temperature”. Esta variable ha sido difícil de estudiar por lo intrincado de los conceptos fenomenológicos que la componen (dinámica instantánea, relación en el aumento en el flujo entrópico, posibles cambios en las propiedades de los materiales, entre otros). En este trabajo se logra determinar y cuantificar la influencia directa que tiene la “Flash Temperature” sobre el aumento y/o disminución de la tasa de desgaste en un proceso de deslizamiento entre sólidos. Paralelamente se desarrolló un segundo modelo acoplado al anterior, modelo de los fenómenos disipativos de degradación, tomando como base la segunda Ley de la Termodinámica en el cual se calculó el aumento de la entropía del sistema mediante un balance de diferentes mecanismos de disipación en intercaras tribológicas con el fin de predecir la tasa de desgaste presente en el proceso de deslizamiento. En la utilización de algunos de estos mecanismos de degradación se realizaron cambios a los postulados originales y, además, se propone una ecuación que relaciona el cambio entrópico con la pérdida de masa por unidad de tiempo en el mecanismo de abrasión, específicamente para el proceso de microcorte. La validación del modelo se realizó mediante la comparación de los resultados obtenidos en la simulación con: a) Los resultados obtenidos en el desarrollo de ensayos de desgaste realizados en el Laboratorio de Tribología y Superficies de la Universidad Nacional (R. Arrubla and C. Ochoa, 2011) y b) con los resultados publicados por el autor (H. A. Abdel-aal, 2003). Finalmente, es importante considerar que el modelo propuesto se rige por la fenomenología que representa el sistema, sin embargo está delimitado por los supuestos que se tienen en cuenta para el desarrollo del modelo. La gran cantidad de variables y parámetros que pueden ser incluidos en un posible modelo haría que este aumente su nivel de precisión. Pero teniendo en cuenta las variables incluidas y supuestos propuestos se tiene que el modelo desarrollado realiza estimaciones precisas y predicciones certerasAbstract: The progressive damage that occurs at the surface of a component as a result of its motion relative to a counterbody has significant consequences which involve not only the costs of replacement but also the expenses related to machine downtime and production losses. This makes necessary to study, analyze and model the entropy balance associated with sliding solids. Such study requires the development of stationary models that correlate some mechanical properties to predict the wear rate of a rubbing pair, as well as dynamic models that describe the thermal changes and their direct influence on the progressive wear of one or both solids involved. It is also needed to apply the thermodynamics laws to analyze the friction in engineering systems in order to develop models that represent the energy balance of these systems. A deep literature survey showed that no consensus has been reached regarding the variables needed to accurately describe and explain the wear process of a sliding pair, since a number of process variables such as the coefficient of friction, load and relative speed have to be combined with thermal factors such as temperature-dependence of mechanical properties, thermal conductivity, among others. In this work, a thermo-mechanical model was developed to predict the wear rate of a WC/Co--Ti6Al4V sliding pair. The model was composed of 2 main functional blocks: First, modeling of the process of generation and transfer of heat, which allowed finding the flash temperature variations. Secondly, modeling of the dissipative phenomena of degradation allowed obtaining the wear rate. The dynamics of the flash temperature was studied thanks to a finite dimensional model, whose data input was obtained through the development of a constitutive equation that represents the heat generated by the contact between micro-asperities. In parallel, a second model based on the second law of thermodynamics was developed and coupled to the previous model. This second model calculated the increase of entropy of the tribological system by proposing a balance among the different dissipation mechanisms present in order to predict the wear rate of the sliding pair. As a consequence, an equation that relates the entropy change with the mass loss per unit time was obtained, with the main assumption that abrasion is the dominant wear mechanism. The validation model was performed by comparing the results obtained in the simulation to: a) The experimental data from wear tests performed at the Laboratory of Tribology and Surfaces National University (R. Arrubla and C. Ochoa, 2011) and b) the results published by H. A. Abdel-aal, 2003. It is worth noticing that the large number of variables and parameters that can be included in a model like the one developed in this work makes its level of precision quite difficult to increase. In spite of this limitation, the results of the present work showed that the assumptions made led to accurate estimates and reasonably good predictionsMaestrí

Continuum and crystal strain gradient plasticity with energetic and dissipative length scales

This work, standing as an attempt to understand and mathematically model the small scale materials thermal and mechanical responses by the aid of Materials Science fundamentals, Continuum Solid Mechanics, Misro-scale experimental observations, and Numerical methods. Since conventional continuum plasticity and heat transfer theories, based on the local thermodynamic equilibrium, do not account for the microstructural characteristics of materials, they cannot be used to adequately address the observed mechanical and thermal response of the micro-scale metallic structures. Some of these cases, which are considered in this dissertation, include the dependency of thin films strength on the width of the sample and diffusive-ballistic response of temperature in the course of heat transfer. A thermodynamic-based higher order gradient framework is developed in order to characterize the mechanical and thermal behavior of metals in small volume and on the fast transient time. The concept of the thermal activation energy, the dislocations interaction mechanisms, nonlocal energy exchange between energy carriers and phonon-electrons interactions are taken into consideration in proposing the thermodynamic potentials such as Helmholtz free energy and rate of dissipation. The same approach is also adopted to incorporate the effect of the material microstructural interface between two materials (e.g. grain boundary in crystals) into the formulation. The developed grain boundary flow rule accounts for the energy storage at the grain boundary due to the dislocation pile up as well as energy dissipation caused by the dislocation transfer through the grain boundary. Some of the abovementioned responses of small scale metallic compounds are addressed by means of the numerical implementation of the developed framework within the finite element context. In this regard, both displacement and plastic strain fields are independently discretized and the numerical implementation is performed in the finite element program ABAQUS/standard via the user element subroutine UEL. Using this numerical capability, an extensive study is conducted on the major characteristics of the proposed theories for bulk and interface such as size effect on yield and kinematic hardening, features of boundary layer formation, thermal softening and grain boundary weakening, and the effect of soft and stiff interfaces

Structural Thermomechanical Models for Shape Memory Alloy Components

Thermally responsive shape memory alloys (SMA) demonstrate interesting properties like shape memory effect (SME) and superelasticity (SE). SMA components in the form of wires, springs and beams typically exhibit complex, nonlinear hysteretic responses and are subjected to tension, torsion or bending loading conditions. Traditionally, simple strength of materials based models/tools have driven engineering designs for centuries, even as more sophisticated models existed for design with conventional materials. In light of this, an effort to develop strength of materials type modeling approach that can capture complex hysteretic SMA responses under different loading conditions is undertaken. The key idea here is of separating the thermoelastic and the dissipative part of the hysteretic response by using a Gibbs potential and thermodynamic principles. The dissipative part of the response is later accounted for by a discrete Preisach model. The models are constructed using experimentally measurable quantities (like torque–twist, bending moment–curvature etc.), since the SMA components subjected to torsion and bending experience an in-homogeneous non-linear stress distribution across the specimen cross-section. Such an approach enables simulation of complex temperature dependent superelastic responses including those with multiple internal loops. The second aspect of this work deals with the durability of the material which is of critical importance with increasing use of SMA components in different engineering applications. Conventional S-N curves, Goodman diagrams etc. that capture only the mechanical loading aspects are not adequate to capture complex thermomechanical coupling seen in SMAs. Hence, a novel concept of driving force amplitude v/s number of cycles equivalent to thermodynamical driving force for onset of phase transformations is proposed which simultaneously captures both mechanical and thermal loading in a single framework. Recognizing the paucity of experimental data on functional degradation of SMAs (especially SMA springs), a custom designed thermomechanical fatigue test rig is used to perform user defined repeated thermomechanical tests on SMA springs. The data from these tests serve both to calibrate the model and establish thermodynamic driving force and extent of phase transformation relationships for SMA springs. A drop in driving force amplitude would suggest material losing its ability to undergo phase transformations which directly corresponds to a loss in the functionality/smartness of SMA component. This would allow designers to set appropriate driving force thresholds as a guideline for analyzing functional life of SMA components

On the Role of Entropy Generation in Processes Involving Fatigue

In this paper we describe the potential of employing the concept of thermodynamic entropy generation to assess degradation in processes involving metal fatigue. It is shown that empirical fatigue models such as Miner’s rule, Coffin-Manson equation, and Paris law can be deduced from thermodynamic consideration

On the Role of Entropy Generation in Processes Involving Fatigue

In this paper we describe the potential of employing the concept of thermodynamic entropy generation to assess degradation in processes involving metal fatigue. It is shown that empirical fatigue models such as Miner’s rule, Coffin-Manson equation, and Paris law can be deduced from thermodynamic consideration