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.

Im Rahmen des Forschungsprojekts ”TMF-Rissverlaufsberechnung für ATL-Heißteile“ des FVV e.V. wurden Untersuchungen zur Entwicklung eines Rissfortschrittsgesetzes unter thermomechanischer Wechselbeanspruchung (thermo-mechanical fatigue, TMF) für das austenitische Gusseisen Ni-Resist D-5S durchgeführt. Ziel der experimentellen Arbeiten war es, mit einseitig gekerbten Proben (SENT) eine Datenbasis für Risswachstum unter TMF-Belastung zu schaffen. Das Werkstoffverhalten des betrachteten austenitischen Gusseisens Ni-Resist D-5S wurde mithilfe eines validierten viskoplastischen, temperaturabhängigen Materialmodells modelliert, das zur Berücksichtigung großer Verzerrungen und Rotationen am Riss auf große Deformationen erweitert wurde. Zur Beurteilung des Rissfortschritts unter TMF wurde die zyklische Rissspitzenöffnungsverschiebung (ΔCTOD) als geeigneter Beanspruchungsparameter verwendet. Für die Simulation der Rissausbreitung wurde ein automatischer FEM-Algorithmus mit inkrementeller, adaptiver Neuvernetzung entwickelt. Dabei wurden Verformungen und inelastische Zustandsvariablen auf das neue Netz übertragen. Der Einfluss verschiedener Parameter innerhalb der Simulationsstruktur wurde analysiert. Ein verbesserter Mapping-Algorithmus zur Übertragung der Zustandsvariablen wurde entwickelt. Mithilfe begleitender 2D FEM-Simulationen wurden die Rissfortschrittskurven des Werkstoffs basierend auf den experimentellen Daten ermittelt und unter Anwendung des ΔCTOD Konzepts in parametrisierter Form dargestellt. Zusätzlich wurden Rissfortschrittsgesetze durch den Einsatz von Machine-Learning-Konzepten bestimmt. Dies ermöglicht erstmals eine quantitative Vorhersage der Rissentwicklung unter Beanspruchungsbedingungen mit Großbereichsfließen unter Berücksichtigung von TMF. / As part of the research project ”TMF crack propagation calculation for ATL hot parts“ of the FVV e.V., an investigation was carried out to develop a crack propagation law under thermomechanical fatigue (TMF) for the austenitic cast iron Ni-Resist D-5S. The aim of the experimental work was to create a database for crack growth under TMF loading with single edge notch tension specimens (SENT). The material behavior of the austenitic cast iron under consideration, Ni-Resist D-5S, was modeled using a validated viscoplastic temperature dependent material model, which was extended to large deformations to account for large distortions and rotations in the crack. The cyclic crack opening displacement (ΔCTOD) was used as a suitable loading parameter to assess crack propagation under TMF. An automatic FEM algorithm with incremental adaptive remeshing was developed for the simulation of crack propagation. In the process, the deformations and inelastic state variables were transferred to the new mesh. The influence of different parameters within the simulation structure was analyzed. An ideal mapping algorithm for the transfer of state variables was developed. With the help of accompanying 2D FEM simulations, the crack propagation curves of the material were determined on the basis of the experimental data and presented in parameterized form using the concept ΔCTOD. Furthermore, crack propagation laws were determined using machine learning concepts. This allows for the first time a quantitative prediction of crack development under loading conditions with large scale yielding taking TMF into account.

Thermomechanical fatigue is a fatigue failure caused by combined thermal and mechanical loading cycles where both stresses and temperature can vary with time. A cohesive zone model simulates material failure within the finite element method by describing material behaviour and damage evolution through a traction-separation relation. In this work, a viscoplastic cohesive zone model is developed in order to describe the fatigue and fracture behaviour of high performance superalloys undergoing thermomechanical fatigue loading. A micromechanically motivated cohesive potential, incorporating fatigue and creep damage variables, is proposed and its thermodynamic consistency is established. The characteristic behaviour of the cohesive law during cyclic loading is described in terms of both traction and energy. The developed cohesive zone model is applied to simulate the lifetime behaviour in a smooth specimen. The effect of loading parameters such as temperature, dwell period, and strain rate on isothermal, in-phase and out-of-phase thermomechanical fatigue is successfully demonstrated. Using crack growth simulations in a boundary layer model, the relation between the local behaviour of the cohesive zone and fracture parameters like stress intensity factor, J-integral and crack tip opening displacement is established. The three stages of fatigue crack growth rate curves that are experimentally observed were obtained through simulations. Crack growth in a corner crack specimen is simulated under cyclic stress-controlled loading to obtain the experimentally observed crack profiles. Material parameters for the Nickel based superalloy MAR-M247 are identified using experimental data from the literature.

Main extrinsic and intrinsic modifiers factors of crack growth rate in AA7050-T7451 were assessed in order to provide tools for aeronautical structures designers. These tools cover most necessary information to project aircraft\'s structures using the studied alloy, under damage tolerance philosophy. The experimental methodology consisted of use CT specimens, on TL and LT rolling direction to test its behavior under different conditions of stress ratio, force waveform, and the environment. The stress ratio values were 0.1 and 0.5, the force waveform used were sine and trapezoidal or Dwell under normal air laboratory conditions and salt fog 3.5%NaCl weight in order to simulate the marine environment. In Dwell tests, results were checked with the electrical potential drop technique (DCPD) in addition to the crack opening displacement (COD) method. Using the Walker coefficients, calculated on the present research, could be projected accurately the crack propagation behavior on Paris region and do fatigue life predictions using da/dN and S-N diagrams for different stress ratio values. The corrosion environment increases both crack growth rate and ΔKth due to oxides formation on the crack path that generates a crack closure effect. Dwell carrying makes decrease the crack growth rate by decreasing the slope of the Paris line on log (da/dN) versus log (ΔK) curve, instead of shifting down the line as occurs on titanium alloys. Rolling direction change from LT to TL increase the FCG rate in both threshold and Paris region, where the rate change use to be small. / Os principais fatores modificadores extrínsecos e intrínsecos da taxa de propagação de trincas na liga AA7050-T7451 foram avaliados para fornecer subsídios para projetistas de estruturas aeronáuticas, com base na filosofía de tolerância ao dano. A metodologia experimental consistiu em ensaiar corpos de prova do tipo compact tension (CT) da liga nas direções de laminação TL e LT, para verificar seu comportamento sob diferentes razões de tensões, forma de onda e condição ambiente. Os valores de razão de tensão estudados foram 0,1 e 0,5, as formas de onda foram senoidal e trapezoidal ou de Dwell, em condições normais de laboratório, ao ar, e névoa salina 3,5% NaCl, em massa, para simular um ambiente marinho. No caso dos ensaios Dwell, os resultados foram conferidos pelo método de queda de potencial eléctrico (QPE), além do método de flexibilidade elástica. Usando os coeficientes de Walker calculados a partir dos resultados obtidos, pôde-se projetar com precisão o comportamento da propagação de trinca na região de Paris e prever a vida em fadiga usando os diagramas da/dN e S-N para diferentes valores da razão de tensões. O ambiente corrosivo aumenta tanto a taxa de propagação de trinca, quanto o valor de ΔKth por causa da formação de óxidos na trajetória da trinca, que geram um efeito de fechamento sobre a mesma. Quanto à forma de onda, verificou-se que o carregamento Dwell diminui a taxa de propagação de trinca, diminuindo a inclinação das curvas log (da/dN) versus log (ΔK) na região de Paris, ao invés de deslocá-la paralelamente como ocorre com ligas de titânio. A mudança da direção de laminação de LT para TL aumenta a taxa de propagação de trinca por fadiga (PTF) tanto na região de threshold, quanto na região de Paris, onde a mudança de taxa é pequena.

Due to the increasing use of cyclically loaded cast aluminum components in automotive and aerospace applications, fatigue and fatigue crack growth characteristics of aluminum castings are of great interest. Despite the extensive research efforts dedicated to this topic, a fundamental, mechanistic understanding of these alloys' behavior when subjected to dynamic loading is still lacking. This fundamental research investigated the mechanisms active at the microstructure level during dynamic loading and failure of conventionally cast and SSM Al-Si-Mg alloys. Five model alloys were cast to isolate the individual contribution of constituent phases on fatigue resistance. The major constituent phases, alpha-Al dendrites, Al/Si eutectic phase, and Mg-Si strengthening precipitates were mechanistically investigated to relate microstructure to near-threshold crack growth (Delta Kth) and crack propagation regimes (Regions II and III) for alloys of different Si composition/morphology, grain size, secondary dendrite arm spacing, heat treatment. A procedure to evaluate the actual fracture toughness from fatigue crack growth data was successfully developed based on a complex Elastic-Plastic-Fracture-Mechanics (EPFM/J-integral) approach. Residual stress-microstructure interactions, commonly overlooked by researches in the field, were also comprehensively defined and accounted for both experimentally and mathematically, and future revisions of ASTM E647 are expected.

Made available in DSpace on 2017-07-21T20:43:45Z (GMT). No. of bitstreams: 1 Luis Otavio Ribas Lima.pdf: 8124964 bytes, checksum: d16bc8c64f9e15d57f770b1b271d6b3b (MD5) Previous issue date: 2014-06-18 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Aluminum alloys have been the primary material of choice for the aircraft due to their properties such as low density, high mechanical and corrosion resistance. Commercial aircraft apply aluminum alloys for the fuselage, wings and supporting structure due to the extensive knowledge in design and production of aluminum components, and most importantly, aluminum alloys continue to be developed, keeping it highly competitive. A great development happen with the heat-treatable alloys, which allow improvement of the mechanical properties. Among this alloys stand out the Al-Zn-Mg-Cu series, known for high strength, toughness and corrosion resistance. The improvement of those alloys occurs by the precipitation of nanometric particles MgZn2, called η phase. This study’s aim was to promote an interrupted heat treatment T6I4-65 in an AA7050 aluminum alloy, with fatigue crack growth resistance as priority. Interrupted heat treatments’ goal is optimizing the consumption of solute atoms during process of nucleation and growth of precipitates as a finely dispersion. The T6I4-65 condition obtained was analyzed by differential scanning calorimetry, DSC, optical and electronic microscopy, mechanical tests as hardness, tensile and fatigue crack growth. The T6I4-65 treatment results in a microstructure with a fine dispersion of precipitated phase η’, about 75% smaller than those resulting from in current use, T7451. This microstructure resulted in a reduction of up to 24% in fatigue crack growth rate compared to that resulting from T7451 treatment, keeping the ductility of 17% of area reduction and yield strength higher than 400MPa. / Ligas de alumínio são o principal material de uso na indústria aeronáutica devido a suas propriedades como baixa densidade, alta resistência mecânica e a corrosão. Aeronaves comerciais utilizam ligas de alumínio em sua fuselagem, asas e na estrutura de suporte devido ao extenso conhecimento no projeto e produção de componentes em alumínio, e mais importante, as ligas de alumínio continuam a serem desenvolvidas, mantendo-se altamente competitivas. Ocorreu um grande avanço com o desenvolvimento das ligas tratáveis termicamente, que permitiram a otimização das propriedades mecânicas. Entre estas ligas tratáveis, destaca se a família Al-Zn-Mg-Cu, conhecidas pela alta resistência mecânica, tenacidade e resistência a corrosão. O aperfeiçoamento destas ligas ocorre pela precipitação de partículas nanométricas de MgZn2, conhecida como fase η. O objetivo deste trabalho foi obter um tratamento térmico interrompido T6I4-65 em uma liga de alumínio AA7050 com prioridade ao aumento de resistência à propagação de trinca por fadiga. Tratamentos interrompidos tem por objetivo otimizar o consumo de átomos de soluto durante os processos de nucleação e crescimento dos precipitados endurecedores na liga na forma de dispersão finamente dispersa. A condição T6I4-65 obtida foi analisada por meio de calorimetria diferencial de varredura, DSC, microscopia ótica e eletrônica de varredura e transmissão, ensaios mecânicos dureza, tração e propagação de trinca por fadiga. Este tratamento resultou em uma microestrutura com uma dispersão de finos precipitados de fase η’, cerca de 75% menores que os resultantes de tratamentos de uso corrente, T7451. Esta microestrutura promoveu a redução de até 24% na taxa de propagação de trinca por fadiga em comparação à resultante do tratamento T7451, mantendo grande ductilidade, até 17% de redução de área e limite de escoamento superior a 400MPa.

Ultrafeinkörnige metallische Werkstoffe haben verstärkt wissenschaftliche Bedeutung erlangt. Um dieser neuartigen Werkstoffklasse über die grundlagenorientierte Forschung hinaus einen Einsatz in technischen Anwendungen zu ermöglichen, ist es notwendig, deren Verhalten unter verschiedenen einsatzrelevanten Belastungsbedingungen vorhersagen zu können. In der vorliegenden Arbeit wird das Schädigungsverhalten einer ultrafeinkörnigen Aluminiumlegierung in den Bereichen der hochzyklischen (HCF) und niedrigzyklischen (LCF) Ermüdung sowie des Rissfortschritts untersucht. Im Mittelpunkt steht dabei die Identifikation der mikrostrukturell wirksamen Mechanismen bei der Entstehung und Ausbreitung von Ermüdungsrissen. Es werden ein homogen ultrafeinkörniger und ein bimodaler Zustand sowie verschiedene duktilitätsoptimierte Zustände betrachtet und systematisch der Einfluss der Korngröße, der Korngrößenverteilung, der Ausscheidungscharakteristik sowie der Festigkeit und Duktilität auf das Ermüdungs- und Rissfortschrittsverhalten ermittelt. Die Untersuchungen zeigen, dass das Schädigungsverhalten der ultrafeinkörnigen Aluminiumlegierung insbesondere durch die Korngröße und Korngrößenverteilung sowie den Kohärenzgrad der festigkeitssteigernden Ausscheidungen beeinflusst wird.

This thesis is on fatigue crack growth experiments and assessments of fatigue crack growth rates. Both constant and variable amplitude loads in two different materials are considered; a nickel based super-alloy Inconel 718 and a stainless steel 316L. The considered load levels extend from small scale yielding (SSY) to large scale yielding (LSY) for both materials. The effect of different load schemes on the fatigue crack growth rates is investigated on Inconel 718 and compact tension specimens in Paper A. It is concluded that load decreasing schemes give a to high Paris law exponent compared to constant or increasing load amplitude schemes. Inconel 718 is further analyzed in Paper B where growth rates at variable amplitude loading in notched tensile specimens are assessed. The predictions are based on the fatigue crack growth parameters obtained in Paper A. The crack closure levels are taken into consideration and it is concluded that linear elastic fracture mechanics is incapable of predicting the growth rates in notches that experience large plastic cyclic strains. Even if crack closure free fatigue parameters are used and residual stresses due to plasticity are included. It is also concluded that crack closure free and nominal fatigue crack growth data predict the growth rates equally well. However, if the crack closure free parameters are used, then it is possible to make a statement in advance on the prediction in relation to the experimental outcome. This is not possible with nominal fatigue crack growth parameters. The last three papers consider fatigue crack growth in stainless steel 316L. Here the load is defined as the crack tip opening displacement parameter. Paper C constitutes an investigation on the effect of plastic deformation on the potential drop and consequently the measured crack length. It is concluded that the nominal calibration equation obtained in the undeformed geometry can be used at large plastic deformations. However, two conditions must be met: the reference potential must be taken in the deformed geometry and the reference potential needs to be adjusted at every major change of plastic deformation. The potential drop technique is further used in Paper D and Paper E for crack length measurements at monotonic LSY. Constant amplitude loads are considered in Paper D and two different variable amplitude block loads are investigated in Paper E. The crack tip opening displacement is concluded in Paper D to be an objective parameter able to characterize the load state in two different geometries and at the present load levels. Furthermore, if the crack tip opening displacement is controlled in an experiment and the local load ratio set to zero, then only monotonic LSY will appear due to extensive isotropic hardening, i.e. elastic shake-down. This is also the reason why the linear elastic stress-intensity factor successfully could merge all growth rates, extending from SSY to monotonic LSY along a single line in a Paris law type of diagram, even though the generally accepted criteria for SSY is never fulfilled. For the variable amplitude loads investigated in Paper E, the effect of plastic deformation on measured potential drop is more pronounced. However, also here both the crack tip opening displacement parameter and the linear elastic stress-intensity factor successfully characterized the load state. / <p>QC 20130108</p>

The ability to predict fatigue crack growth rates is essential in safety critical systems. The discovery of fatigue crack closure in 1970 caused a flourish of research in attempts to simulate this behaviour, which crucially affects crack growth rates. Historically, crack tip plasticity models have been based on one-dimensional rays of plasticity emanating from the crack tip, either co-linear with the crack (for the case of plane stress), or at a chosen angle in the plane of analysis (for plane strain). In this thesis, one such model for plane stress, developed to predict fatigue crack closure, has been refined. It is applied to a study of the relationship between the apparent stress intensity range (easily calculated using linear elastic fracture mechanics), and the true stress intensity range, which includes the effects of plasticity induced fatigue crack closure. Results are presented for all load cases for a finite crack in an infinite plane, and a method is demonstrated which allows the calculation of the true stress intensity range for a growing crack, based only on the apparent stress intensity range for a static crack. Although the yield criterion is satisfied along the plastic ray, these one-dimensional plasticity models violate the yield criterion in the area immediately surrounding the plasticity ray. An area plasticity model is therefore required in order to model the plasticity more accurately. This thesis develops such a model by distributing dislocations over an area. Use of the model reveals that current methods for incremental plasticity algorithms using distributed dislocations produce an over-constrained system, due to misleading assumptions concerning the normality condition. A method is presented which allows the system an extra degree of freedom; this requires the introduction of a parameter, derived using the Prandtl-Reuss flow rule, which relates the magnitude of slip on complementary shear planes. The method is applied to two problems, confirming its validity.

Main extrinsic and intrinsic modifiers factors of crack growth rate in AA7050-T7451 were assessed in order to provide tools for aeronautical structures designers. These tools cover most necessary information to project aircraft\'s structures using the studied alloy, under damage tolerance philosophy. The experimental methodology consisted of use CT specimens, on TL and LT rolling direction to test its behavior under different conditions of stress ratio, force waveform, and the environment. The stress ratio values were 0.1 and 0.5, the force waveform used were sine and trapezoidal or Dwell under normal air laboratory conditions and salt fog 3.5%NaCl weight in order to simulate the marine environment. In Dwell tests, results were checked with the electrical potential drop technique (DCPD) in addition to the crack opening displacement (COD) method. Using the Walker coefficients, calculated on the present research, could be projected accurately the crack propagation behavior on Paris region and do fatigue life predictions using da/dN and S-N diagrams for different stress ratio values. The corrosion environment increases both crack growth rate and &Delta;Kth due to oxides formation on the crack path that generates a crack closure effect. Dwell carrying makes decrease the crack growth rate by decreasing the slope of the Paris line on log (da/dN) versus log (&Delta;K) curve, instead of shifting down the line as occurs on titanium alloys. Rolling direction change from LT to TL increase the FCG rate in both threshold and Paris region, where the rate change use to be small. / Os principais fatores modificadores extrínsecos e intrínsecos da taxa de propagação de trincas na liga AA7050-T7451 foram avaliados para fornecer subsídios para projetistas de estruturas aeronáuticas, com base na filosofía de tolerância ao dano. A metodologia experimental consistiu em ensaiar corpos de prova do tipo compact tension (CT) da liga nas direções de laminação TL e LT, para verificar seu comportamento sob diferentes razões de tensões, forma de onda e condição ambiente. Os valores de razão de tensão estudados foram 0,1 e 0,5, as formas de onda foram senoidal e trapezoidal ou de Dwell, em condições normais de laboratório, ao ar, e névoa salina 3,5% NaCl, em massa, para simular um ambiente marinho. No caso dos ensaios Dwell, os resultados foram conferidos pelo método de queda de potencial eléctrico (QPE), além do método de flexibilidade elástica. Usando os coeficientes de Walker calculados a partir dos resultados obtidos, pôde-se projetar com precisão o comportamento da propagação de trinca na região de Paris e prever a vida em fadiga usando os diagramas da/dN e S-N para diferentes valores da razão de tensões. O ambiente corrosivo aumenta tanto a taxa de propagação de trinca, quanto o valor de &Delta;Kth por causa da formação de óxidos na trajetória da trinca, que geram um efeito de fechamento sobre a mesma. Quanto à forma de onda, verificou-se que o carregamento Dwell diminui a taxa de propagação de trinca, diminuindo a inclinação das curvas log (da/dN) versus log (&Delta;K) na região de Paris, ao invés de deslocá-la paralelamente como ocorre com ligas de titânio. A mudança da direção de laminação de LT para TL aumenta a taxa de propagação de trinca por fadiga (PTF) tanto na região de threshold, quanto na região de Paris, onde a mudança de taxa é pequena.