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Esse estudo aborda o cotidiano de gestantes usuárias de crack no período anterior à internação em hospital materno-infantil. O tema se mostra relevante devido ao crescente número de usuários de crack no Brasil, à gravidade cada vez maior desses casos, à necessidade de maior compreensão por parte dos profissionais da saúde a respeito do assunto e à escassa produção existente. O presente estudo tem como objetivo geral compreender o cotidiano de gestantes usuárias de crack. Os objetivos específicos são identificar as expectativas das gestantes usuárias de crack com a sua internação, conhecer como era o cotidiano da gestante usuária de crack antes da internação e descrever o tipo vivido de gestantes usuárias de crack. Trata-se de um estudo qualitativo, embasado no referencial da fenomenologia social de Alfred Schutz, por esse tipo de pesquisa permitir uma aproximação do sujeito de estudo, levando à busca pela compreensão de suas ações e percepções. O estudo foi realizado na Unidade de Internação Psiquiátrica do Hospital Materno-Infantil Presidente Vargas (HMIPV), por meio de entrevista fenomenológica com gestantes usuárias de crack internadas na Unidade de Psiquiatria do Hospital Materno Infantil Presidente Vargas. A coleta de dados foi realizada no período de junho a novembro de 2012. Da análise das entrevistas emergiram duas Categorias concretas: O cotidiano das gestantes usuárias de crack antes da internação e Expectativas das gestantes usuárias de crack com a internação. Os aspectos destacados dentro da primeira Categoria concreta foram: O uso da droga interferia no auto-cuidado; Meios para conseguir a droga; Preocupação com a gestação e com a saúde do bebê; Distanciamento da família. Dentro da segunda Categoria concreta foram discutidos os seguintes aspectos: Cessar o uso da droga; Manter um trabalho, uma ocupação; Voltar ao cuidado dos filhos; Fortalecer o apoio da família; Manter o tratamento e o medo da recaída. / This study approaches the daily life of a pregnant crack user before hospitalization at a maternal infant hospital. The subject shows itself relevant due to an increased number of crack users in Brazil, the increasing severity in these cases, the need for more comprehension from health professionals on this matter, and the scarce existing production. This study has as a general goal to comprehend the daily life of a pregnant crack user. The specific goals are to identify the intentions of pregnant crack users and their hospitalization; to know how the pregnant crack user daily life, before the hospitalization was and to describe the typical life of a pregnant crack user. This is a qualitative study, based on the Alfred Schutz´s social phenomenology framework, and this type of research allows an approximation of the subject of study, leading to the search for comprehension of their actions and perceptions. The study was conducted at the President Vargas Maternal Infant Hospital (HMIPV) at the Psychiatry Hospitalization Unit, through phenomenological interviews with pregnant crack users hospitalized at the Psychiatry Unit from the Presidente Vargas Maternal Infant Hospital. The data collection was conducted in the period from June to November of 2012. From the interview analyses’ were revealed two Tangible Units: The daily life of a pregnant crack user before the hospitalization and the pregnant crack user intentions about their hospitalization. The highlighted aspects inside the first Tangible Unit were: The drug use interfered in their self-care; ways to get drug, concern about their pregnancy, family distance. Inside the second Tangible Unit were discussed the following aspects: Ceasing drug use; keeping a job, an occupation; getting back to children care; strengthening the support of family; maintaining the treatment and the fear of relapse. / Este estudio aborda el cotidiano de usuarias de crack embarazadas en el periodo anterior a la internación en un hospital materno-infantil. El tema se muestra relevante debido al creciente número de usuarios de crack en Brasil, a la gravedad cada vez mayor de esos casos, a la necesidad de mayor comprensión por parte de los profesionales de la salud acerca de la temática y a la escasa producción existente. El presente estudio tiene como objetivo general comprender el cotidiano de usuarias de crack embarazadas. Los objetivos específicos son identificar las intenciones de las usuarias de crack embarazadas con su internación, conocer como era el cotidiano de la usuaria de crack embarazada antes de la internación y describir el tipo vivido de usuarias de crack embarazadas. Se trata de un estudio cualitativo, embasado en el referencial de la fenomenología social de Alfred Schutz, por ese tipo de investigación se permite una aproximación del sujeto de estudio, llevando a la búsqueda por la comprensión de sus acciones y percepciones. El estudio fue realizado en la Unidad de Internación Psiquiátrica del Hospital Materno-Infantil Presidente Vargas (HMIPV), por medio de entrevista fenomenológica con usuarias de crack embarazadas internas en la Unidad de Psiquiatría del Hospital Materno Infantil Presidente Vargas. La colección de datos fue realizada en el periodo de junio a noviembre de 2012. Del análisis de las entrevistas emergieron dos Categorias concretas: El cotidiano de las usuarias de crack embarazadas antes de la internación e Intenciones de las usuarias de crack embarazadas con la internación. Los aspectos destacados dentro de la primera Unidad Concreta fueron: El uso de la droga interfería en el autocuidado; Medios para conseguir la droga; Preocupación con el embarazo y con la salud del bebé; Distanciamiento de la familia. Dentro de la segunda Unidad Concreta fueron discutidos los siguientes aspectos: El término del uso de la droga; Mantener un trabajo, una ocupación; Volver al cuidado de los hijos; Fortalecer el apoyo a la familia; Mantener el tratamiento y el miedo a la recaída.

Esse estudo aborda o cotidiano de gestantes usuárias de crack no período anterior à internação em hospital materno-infantil. O tema se mostra relevante devido ao crescente número de usuários de crack no Brasil, à gravidade cada vez maior desses casos, à necessidade de maior compreensão por parte dos profissionais da saúde a respeito do assunto e à escassa produção existente. O presente estudo tem como objetivo geral compreender o cotidiano de gestantes usuárias de crack. Os objetivos específicos são identificar as expectativas das gestantes usuárias de crack com a sua internação, conhecer como era o cotidiano da gestante usuária de crack antes da internação e descrever o tipo vivido de gestantes usuárias de crack. Trata-se de um estudo qualitativo, embasado no referencial da fenomenologia social de Alfred Schutz, por esse tipo de pesquisa permitir uma aproximação do sujeito de estudo, levando à busca pela compreensão de suas ações e percepções. O estudo foi realizado na Unidade de Internação Psiquiátrica do Hospital Materno-Infantil Presidente Vargas (HMIPV), por meio de entrevista fenomenológica com gestantes usuárias de crack internadas na Unidade de Psiquiatria do Hospital Materno Infantil Presidente Vargas. A coleta de dados foi realizada no período de junho a novembro de 2012. Da análise das entrevistas emergiram duas Categorias concretas: O cotidiano das gestantes usuárias de crack antes da internação e Expectativas das gestantes usuárias de crack com a internação. Os aspectos destacados dentro da primeira Categoria concreta foram: O uso da droga interferia no auto-cuidado; Meios para conseguir a droga; Preocupação com a gestação e com a saúde do bebê; Distanciamento da família. Dentro da segunda Categoria concreta foram discutidos os seguintes aspectos: Cessar o uso da droga; Manter um trabalho, uma ocupação; Voltar ao cuidado dos filhos; Fortalecer o apoio da família; Manter o tratamento e o medo da recaída. / This study approaches the daily life of a pregnant crack user before hospitalization at a maternal infant hospital. The subject shows itself relevant due to an increased number of crack users in Brazil, the increasing severity in these cases, the need for more comprehension from health professionals on this matter, and the scarce existing production. This study has as a general goal to comprehend the daily life of a pregnant crack user. The specific goals are to identify the intentions of pregnant crack users and their hospitalization; to know how the pregnant crack user daily life, before the hospitalization was and to describe the typical life of a pregnant crack user. This is a qualitative study, based on the Alfred Schutz´s social phenomenology framework, and this type of research allows an approximation of the subject of study, leading to the search for comprehension of their actions and perceptions. The study was conducted at the President Vargas Maternal Infant Hospital (HMIPV) at the Psychiatry Hospitalization Unit, through phenomenological interviews with pregnant crack users hospitalized at the Psychiatry Unit from the Presidente Vargas Maternal Infant Hospital. The data collection was conducted in the period from June to November of 2012. From the interview analyses’ were revealed two Tangible Units: The daily life of a pregnant crack user before the hospitalization and the pregnant crack user intentions about their hospitalization. The highlighted aspects inside the first Tangible Unit were: The drug use interfered in their self-care; ways to get drug, concern about their pregnancy, family distance. Inside the second Tangible Unit were discussed the following aspects: Ceasing drug use; keeping a job, an occupation; getting back to children care; strengthening the support of family; maintaining the treatment and the fear of relapse. / Este estudio aborda el cotidiano de usuarias de crack embarazadas en el periodo anterior a la internación en un hospital materno-infantil. El tema se muestra relevante debido al creciente número de usuarios de crack en Brasil, a la gravedad cada vez mayor de esos casos, a la necesidad de mayor comprensión por parte de los profesionales de la salud acerca de la temática y a la escasa producción existente. El presente estudio tiene como objetivo general comprender el cotidiano de usuarias de crack embarazadas. Los objetivos específicos son identificar las intenciones de las usuarias de crack embarazadas con su internación, conocer como era el cotidiano de la usuaria de crack embarazada antes de la internación y describir el tipo vivido de usuarias de crack embarazadas. Se trata de un estudio cualitativo, embasado en el referencial de la fenomenología social de Alfred Schutz, por ese tipo de investigación se permite una aproximación del sujeto de estudio, llevando a la búsqueda por la comprensión de sus acciones y percepciones. El estudio fue realizado en la Unidad de Internación Psiquiátrica del Hospital Materno-Infantil Presidente Vargas (HMIPV), por medio de entrevista fenomenológica con usuarias de crack embarazadas internas en la Unidad de Psiquiatría del Hospital Materno Infantil Presidente Vargas. La colección de datos fue realizada en el periodo de junio a noviembre de 2012. Del análisis de las entrevistas emergieron dos Categorias concretas: El cotidiano de las usuarias de crack embarazadas antes de la internación e Intenciones de las usuarias de crack embarazadas con la internación. Los aspectos destacados dentro de la primera Unidad Concreta fueron: El uso de la droga interfería en el autocuidado; Medios para conseguir la droga; Preocupación con el embarazo y con la salud del bebé; Distanciamiento de la familia. Dentro de la segunda Unidad Concreta fueron discutidos los siguientes aspectos: El término del uso de la droga; Mantener un trabajo, una ocupación; Volver al cuidado de los hijos; Fortalecer el apoyo a la familia; Mantener el tratamiento y el miedo a la recaída.

Esse estudo aborda o cotidiano de gestantes usuárias de crack no período anterior à internação em hospital materno-infantil. O tema se mostra relevante devido ao crescente número de usuários de crack no Brasil, à gravidade cada vez maior desses casos, à necessidade de maior compreensão por parte dos profissionais da saúde a respeito do assunto e à escassa produção existente. O presente estudo tem como objetivo geral compreender o cotidiano de gestantes usuárias de crack. Os objetivos específicos são identificar as expectativas das gestantes usuárias de crack com a sua internação, conhecer como era o cotidiano da gestante usuária de crack antes da internação e descrever o tipo vivido de gestantes usuárias de crack. Trata-se de um estudo qualitativo, embasado no referencial da fenomenologia social de Alfred Schutz, por esse tipo de pesquisa permitir uma aproximação do sujeito de estudo, levando à busca pela compreensão de suas ações e percepções. O estudo foi realizado na Unidade de Internação Psiquiátrica do Hospital Materno-Infantil Presidente Vargas (HMIPV), por meio de entrevista fenomenológica com gestantes usuárias de crack internadas na Unidade de Psiquiatria do Hospital Materno Infantil Presidente Vargas. A coleta de dados foi realizada no período de junho a novembro de 2012. Da análise das entrevistas emergiram duas Categorias concretas: O cotidiano das gestantes usuárias de crack antes da internação e Expectativas das gestantes usuárias de crack com a internação. Os aspectos destacados dentro da primeira Categoria concreta foram: O uso da droga interferia no auto-cuidado; Meios para conseguir a droga; Preocupação com a gestação e com a saúde do bebê; Distanciamento da família. Dentro da segunda Categoria concreta foram discutidos os seguintes aspectos: Cessar o uso da droga; Manter um trabalho, uma ocupação; Voltar ao cuidado dos filhos; Fortalecer o apoio da família; Manter o tratamento e o medo da recaída. / This study approaches the daily life of a pregnant crack user before hospitalization at a maternal infant hospital. The subject shows itself relevant due to an increased number of crack users in Brazil, the increasing severity in these cases, the need for more comprehension from health professionals on this matter, and the scarce existing production. This study has as a general goal to comprehend the daily life of a pregnant crack user. The specific goals are to identify the intentions of pregnant crack users and their hospitalization; to know how the pregnant crack user daily life, before the hospitalization was and to describe the typical life of a pregnant crack user. This is a qualitative study, based on the Alfred Schutz´s social phenomenology framework, and this type of research allows an approximation of the subject of study, leading to the search for comprehension of their actions and perceptions. The study was conducted at the President Vargas Maternal Infant Hospital (HMIPV) at the Psychiatry Hospitalization Unit, through phenomenological interviews with pregnant crack users hospitalized at the Psychiatry Unit from the Presidente Vargas Maternal Infant Hospital. The data collection was conducted in the period from June to November of 2012. From the interview analyses’ were revealed two Tangible Units: The daily life of a pregnant crack user before the hospitalization and the pregnant crack user intentions about their hospitalization. The highlighted aspects inside the first Tangible Unit were: The drug use interfered in their self-care; ways to get drug, concern about their pregnancy, family distance. Inside the second Tangible Unit were discussed the following aspects: Ceasing drug use; keeping a job, an occupation; getting back to children care; strengthening the support of family; maintaining the treatment and the fear of relapse. / Este estudio aborda el cotidiano de usuarias de crack embarazadas en el periodo anterior a la internación en un hospital materno-infantil. El tema se muestra relevante debido al creciente número de usuarios de crack en Brasil, a la gravedad cada vez mayor de esos casos, a la necesidad de mayor comprensión por parte de los profesionales de la salud acerca de la temática y a la escasa producción existente. El presente estudio tiene como objetivo general comprender el cotidiano de usuarias de crack embarazadas. Los objetivos específicos son identificar las intenciones de las usuarias de crack embarazadas con su internación, conocer como era el cotidiano de la usuaria de crack embarazada antes de la internación y describir el tipo vivido de usuarias de crack embarazadas. Se trata de un estudio cualitativo, embasado en el referencial de la fenomenología social de Alfred Schutz, por ese tipo de investigación se permite una aproximación del sujeto de estudio, llevando a la búsqueda por la comprensión de sus acciones y percepciones. El estudio fue realizado en la Unidad de Internación Psiquiátrica del Hospital Materno-Infantil Presidente Vargas (HMIPV), por medio de entrevista fenomenológica con usuarias de crack embarazadas internas en la Unidad de Psiquiatría del Hospital Materno Infantil Presidente Vargas. La colección de datos fue realizada en el periodo de junio a noviembre de 2012. Del análisis de las entrevistas emergieron dos Categorias concretas: El cotidiano de las usuarias de crack embarazadas antes de la internación e Intenciones de las usuarias de crack embarazadas con la internación. Los aspectos destacados dentro de la primera Unidad Concreta fueron: El uso de la droga interfería en el autocuidado; Medios para conseguir la droga; Preocupación con el embarazo y con la salud del bebé; Distanciamiento de la familia. Dentro de la segunda Unidad Concreta fueron discutidos los siguientes aspectos: El término del uso de la droga; Mantener un trabajo, una ocupación; Volver al cuidado de los hijos; Fortalecer el apoyo a la familia; Mantener el tratamiento y el miedo a la recaída.

The first part of this dissertation employs a three-dimensional elastic-plastic finite element model of straight-through crack growth to correlate four well-known methodologies characterizing fatigue crack closure. The compliance offset and the adjusted compliance ratio (ACR) are experimental methods, whereas the node displacement and the contact stress methods are numerical approaches. Evolutions of crack closure from all four methodologies are compared for a numerical model of a single edge-cracked tension specimen subjected to different levels of constant amplitude cyclic loading. In the second part, a detailed two dimensional stress analysis is conducted for a single pin-joint under plane stress conditions. This study investigates the influence of material nonlinearity, friction, and pre-existing residual stresses from cold-working process on the local radial and hoop stress levels around the pin-loaded hole. Next, the beneficial influence of cold working process is quantified by computing the Mode I stress intensity factors style='mso-bidiont-style:normal'>KI for a single radial crack eanating from a side of a loaded hole. Two different loading configurations are considered: (a) an open hole in tension, (b) a pin-loaded hole. The stress intensity factors are computed using the J integral solutions and the weight functions specific to the crack configuration. The reductions in KI values due to different levels of cold-working process are presented for a range of crack lengths. The final part of the research involves a numerical investigation of an on-line crack compliance technique that is used for experimental measurements of residual stress fields along the crack growth path. A finite rectangular sheet is considered with a single crack emanating from a side of a central hole. The residual stress field is introduced around the hole by cold-working simulation. As part of validation, the normalized residual stress intensity factors computed using the on-line crack compliance technique are compared with those from the style='mso-bidiont-style:normal'>J-integral approach for the case of elastic crack growth. The influence of crack tip plasticity on the performance of the on-line crack compliance technique is studied by comparing the solutions of the elastic and elastic-plastic crack growth models.

From its discovery, crack closure was recognized as a key aspect in understanding the fatigue crack growth process. Considering the condition of plane stress, a vast amount of research has been conducted experimentally, analytically, and numerically to understand the complex process of fatigue crack growth and crack closure. Nonzero crack opening stress values are routinely observed, and it seems that there is a general agreement regarding the incidence of the phenomenon under plane stress. However, investigations regarding crack closure under plane strain conditions are less abundant. Moreover, the existence of crack closure under the plane strain state of the stress has been questioned. The importance of accurate measurements of closure to predict adequately fatigue crack growth rates should not be underestimated. Models employed to predict fatigue crack growth rates rely on plasticity-induced crack closure concepts, and the validity of plasticity-induced crack closure depends on crack closure measurements. Crack closure measurements can be performed with Elber’s Method, the ASTM standard(Compliance offset method), or it may be done alternatively by the compliance ratio (CR) or the adjusted compliance ratio method (ACR). In this research, a small scale yielding two-parameter modified boundary layer analysis is performed to study the occurrence of plasticity-induced fatigue crack closure under constant amplitude loading and plane strain conditions. A wide range of T-stresses and KI levels are considered in the finite element analysis with the purpose of exploring the behavior of the crack opening stress. Crack closure was observed for some values of T-stress. Other values of T-stress resulted in an absence of closure under steady state conditions. In addition, an elastic-plastic finite element model was used to simulate a growing fatigue crack with WARP3D software. The computed displacements were used to determine the effective stress intensity factor range ΔKeff with the ASTM standard compliance offset approach, the (CR) method, and the (ACR) method. Finally, measurement location effects on ACR and the ability of ACR method to remove residuals stresses were investigated.

Design based on damage tolerance concepts has become mandatory in high technology structures. These concepts are also essential for evaluating life extension of aged structures which are in service beyond originally stipulated life. Fracture analysis of such structures in the presence of single or multiple three-dimensional flaws is essential for this approach. Surface cracks are the most commonly occurring flaws and development of accurate methods of analysis for such cracks is essential for structural integrity evaluation of newly designed or aged structures. The crack fronts of these surface flaws are usually approximated mathematically to be of either part-elliptical or part-circular in geometry. In this thesis, some of the issues related to fatigue crack growth of single and multiple surface cracks are studied in detail. Here emphasis is given to the development of simple and accurate post-processing techniques to estimate stress intensity factors for surface cracks, development and/or implementation of simple numerical methods to simulate three-dimensional single and multiple cracks in fatigue and their experimental verification. Modified virtual crack closure integral (MVCCI) technique for estimation of strain energy release rates has been improved (chapter II) to deal with curved crack front and unequal elements across the crack front. The accuracy of this method is evaluated and presented in this chapter for certain benchmark surface flaw problems. The improved MVCCI is used in the investigation of interaction between multiple surface cracks in three-dimensional solids. The interaction effects are studied for both interacting and coalescing phases as observed to occur in the growth of multiple surface cracks. Extensive numerical work is performed to study the effects of various parameters such as aspect ratio, thickness ratio, interspacing on the interaction factors. These solutions are used in formulating empirical equations to estimate interaction factors. This facilitated the development of a simple semi-analytical method to study fatigue crack growth of multiple cracks. The growth of surface cracks under fatigue loading in the finite width specimens of an aero-engine superalloy has been studied experimentally (presented in chapter III). Four configurations for single semi-elliptical cracks are considered. Fatigue crack growth is simulated by two models viz. two degrees of freedom and "multi degrees of freedom with ellipse fit'. These models are sometimes referred to as semi-analytical models as the crack growth is predicted by numerical integration combining Paris equation with an empirical form of stress intensity factor solution. In order to use two degrees of freedom model for fatigue crack growth prediction of semi-elliptical cracks, empirical solution for the Ml range of geometric parameters for stress intensity factor is required for the considered configurations. The available Newman-Raju solution is useful for this purpose within a limited range of surface crack length to width (c/W) of the specimen. Based on the present finite element results, the empirical equations are developed for extended values of c/W. It is well understood that the fatigue prediction for two-dimensional crack can be improved by inclusion of crack closure effects. Usually, in semi-analytical models for growth of surface cracks under fatigue loading, the crack closure is included as a ratio of crack closure factor at surface and depth locations of semi-elliptical crack. In the present work, this ratio for the considered material of specimens is obtained by an experimental study. The difference in characteristics of preferred propagation path between semi-elliptical crack in a finite width plate and a wide plate is clearly brought out. Current crack growth predictions for most of the structures are based on the presence of only a single crack. However, in structures several cracks may initiate simultaneously within a stress critical zone and may interact depending upon their geometry, spatial location, structure geometry and mode of loading. In this work various configurations of twin semi-elliptical cracks have been studied by experiments. The beachmarks created on the specimens during experiments are used in the investigation of crack shape progression during fatigue. A three degrees of freedom crack growth model for interacting and coalescing cracks has been proposed. The experimentally determined crack shape and lives have been compared with the corresponding values from numerical simulation. The correlation of experimental results with numerical predictions was carried out through improved MVCCI for eight-noded brick elements. This has worked well in the configurations analysed. However, it is known in literature that there are benefits of using 20-noded singular elements. There could be special situations where the regular elements could fail, and singular elements could be essential. For this purpose, further development of MVCCI were carried out using 20-noded quarter-point elements (presented in chapter IV). Also a novel technique of decomposed crack closure integral (DCCI) was developed (presented in chapter V) for both regular and singular elements to represent the variation of MVCCI more accurately along the crack front. It is well known that quarter-point elements at crack front produce the required singularity at the crack tip and give accurate stress distribution with fewer degrees of freedom than conventional elements. Thus to develop more efficient post-processing tools, the MVCCI expressions are formulated for 20-noded singular quarter-point element for various assumptions regarding stress and displacement distributions in the elements across the crack front. A comprehensive study is presented (chapter IV) on MVCCI for 20-noded singular brick element including various simplified expressions for three-dimensional part-through cracks in pure and mixed-mode state of deformation of fracture. The developed MVCCI expressions are also valid for 15-noded quarter-point Penta elements. The reduction in model size can further be obtained if 12-noded three-dimensional singular element is employed at the crack front and eight-noded elements are used away from the crack front. The MVCCI expressions are also developed for 12-noded singular element and their accuracy is evaluated by numerical solutions. Presently, MVCCI, estimates the average stress intensity factor at the center of each element along the crack front. In this thesis, a Decomposed Crack Closure Integral (DCCI) is formulated to represent an assumed variation of stress intensity factor along the crack front in each element. The DCCI is formulated for 8-noded brick, 20-noded conventional brick and 20-noded singular brick elements. The numerical examples presented here deal with three-dimensional problems of patch repair technology and part-through cracks. The technique showed a major advantage for the patch repair problems where SIF variations along the crack front are of significance and large mesh sizes are computationally expensive. This along with MVCCI for 12-noded and 20-noded singular elements formed a part of the work on development of accurate and effective post-processing tools. It is expected that the present work will be helpful in damage tolerance design and assessment of aerospace structures and the experimental work performed as a part of this thesis will enhance confidence in the damage tolerance analysis. The thesis is concluded in chapter VI presenting the contributions of this thesis and projecting future lines of work possible in this area.

Crack propagation in structures when subjected to fatigue loading, follows three different phases namely - short crack growth, stable crack growth and unstable crack growth. Accurate fatigue life prediction demands the consideration of every crack propagation phase rather than only the stable crack growth stage. Further, the use of existing crack growth laws in structures with small cracks under-predicts the growth rate compared to experimentally observed ones, thereby leading to an unsafe design and keeping the structure in a potentially dangerous state. In the present work, an attempt is made to establish fatigue crack propagation laws for plain concrete, reinforced concrete and concrete-concrete jointed interfaces from first principles using the concepts of dimensional analysis and self-similarity. Different crack growth laws are proposed to understand the behavior in each of the three regimes of the fatigue crack growth curve. Important crack growth characterizing material and geometrical parameters for each zone are included in the proposed analytical models. In real life applications to structures, the amplitude of cyclic loading rarely remains constant and is subjected to a wide spectrum of load amplitudes. Furthermore, the crack growth behavior changes in the presence of high amplitude load spikes within a constant amplitude history and this is incorporated in the model formulation. Using scaling laws, an improved understanding of the scaling behavior on different parameters is achieved. The models describing different regimes of crack propagation are finally unified to obtain the entire crack growth curve and compute the total fatigue life. In addition, crack growth analysis is performed for a reinforced concrete member by modifying the model derived for plain concrete in the Paris regime. Energy dissipation occurring due to shake-down phenomenon in steel reinforcement is addressed. The bond-slip mechanism which is of serious concern in reinforced concrete members is included in the study and a method is proposed for the prediction of residual moment carrying capacity as a function of relative crack depth. The application of the proposed analytical model in the computation of fatigue crack growth is demonstrated on three practical problems – beam in flexure, concrete arch bridge and a patch repaired beam. Through a sensitivity study, the influence of different parameters on the crack growth behavior is highlighted.

The integrity of nearly all engineering structures are threatened by the presence of cracks. Structural failure occurs if a crack larger than a critical size exists. Although most well designed structures initially contain no critical cracks, subcritical cracks can grow to failure under fatigue loading, called fatigue crack growth (FCG). Because it is impossible or impractical to prevent subcritical crack growth in most applications, a damage tolerant design philosophy was developed for crack sensitive structures. Design engineers have taken advantage of the FCG threshold concept to design for long fatigue lives. FCG threshold (DKth) is a value of DK (crack-tip loading), below which no significant FCG occurs. Cracks are tolerated if DK is less than DKth. However, FCG threshold is not constant. Many variables influence DKth including microstructure, environment, and load ratio. The current research focuses on load ratio effects on DKth and threshold FCG. Two categories of load ratio effects are studied here: extrinsic and intrinsic. Extrinsic load ratio effects operate in the crack wake and include fatigue crack closure mechanisms. Intrinsic load ratio effects operate in the crack-tip process zone and include microcracking and void production. To gain a better understanding of threshold FCG load ratio effects (1) a fatigue crack closure model is developed to consider the most likely closure mechanisms at threshold, simultaneously, and (2) intrinsic load ratio mechanisms are identified and modeled. An analytical fatigue crack closure model is developed that includes the three closure mechanisms considered most important at threshold (PICC, RICC, and OICC). Crack meandering and a limited amount of mixed-mode loading are also considered. The rough crack geometry, approximated as a two-dimensional sawtooth wave, results in a mixed-mode crack-tip stress state. Dislocation and continuum mechanics concepts are used to determine mixed-mode crack face displacements. Plasticity induced crack closure is included by modifying an existing analytical model, and an oxide layer in the crack mouth is modeled as a uniform layer. Finite element results were used to verify the analytical solutions for crack-tip stress intensity factor and crack face displacements. These results indicate that closure for rough cracks can occur at two locations: (1) at the crack-tip, and (2) at the asperity nearest the crack-tip. Both tip contact and asperity contact must be considered for rough cracks. Tip contact is more likely for high Kmax levels, thick oxide layers, and shallow asperity angles, a. Model results indicate that closure mechanisms combine in a synergistic manner. That is, when multiple closure mechanisms are active, the total closure level is greater than the sum of individual mechanisms acting alone. To better understand fatigue crack closure where multiple closure mechanisms are active (i.e. FCG threshold), these interactions must be considered. Model results are well supported by experimental data over a wide range of DK, including FCG threshold. Closure-free load ratio effects were studied for aluminum alloys 2024, 7050, and 8009. Alloys 7050 and 8009 were selected because load ratio effects at FCG threshold are not entirely explained by fatigue crack closure. It is believed that closure-free load ratio mechanisms occur in these alloys. Aluminum alloy 2024 was selected for study because it is relatively well behaved, meandering most load ratio effects are explained by fatigue crack closure. A series of constant Kmax threshold tests on aluminum alloys were conducted to eliminate fatigue crack closure at threshold. Even in the absence of fatigue crack closure load ratio (Kmax) effects persist, and are correlated with increased crack-tip damage (i.e. voids) seen on the fatigue crack surfaces. Accelerated FCG was observed during constant Kmax threshold testing of 8009 aluminum. A distinct transition is seen the FCG data and is correlated with a dramatic increase in void production seen along the crack faces. Void production in 8009 aluminum is limited to the specimen interior (plane-strain conditions), promoting crack tunneling. At higher values of Kmax (+_ 22.0 MPaà m), where plane-stress conditions dominate, a transition to slant cracking occurs at threshold. The transition to slant cracking produces an apparent increase in FCG rate with decreasing DK. This unstable threshold behavior is related to constraint conditions. Finally, a model is developed to predict the accelerated FCG rates, at higher Kmax levels, in terms of crack-tip damage. The effect of humidity (in laboratory air) on threshold FCG was studied to ensure that environmental effects at threshold were separated from load ratio effects. Although changes in humidity were shown to strongly affect threshold FCG rates, this influence was small for ambient humidity levels (relative humidity between 30% and 70%). Transient FCG behavior, following an abrupt change in humidity level, indicated environmental damage accumulated in the crack-tip monotonic plastic zone. Previous research implies that hydrogen (a component of water vapor) is the likely cause of this environmental damage. Analysis suggests that bulk diffusion is not a likely hydrogen transport mechanism in the crack-tip monotonic plastic zone. Rather, dislocation-assisted diffusion is presented as the likely hydrogen transport mechanism. Finally, the (extrinsic) fatigue crack closure model and the (intrinsic) crack-tip damage model are put in the context of a comprehensive threshold model. The ultimate goal of the comprehensive threshold model is to predict fatigue lives of cyclically loaded engineering components from (small) crack nucleation, through FCG, and including failure. The models developed in this dissertation provide a basis for a more complete evaluation of threshold FCG and fatigue life prediction. The research described in this dissertation was performed at NASA-Langley Research Center in Hampton, Virginia. Funding was provided through the NASA GSRP program (Graduate Student Researcher Program, grant number NGT-1-52174). / Ph. D.

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