This study addresses the fatigue life of the head–to–skirt welds of tall distillation columns. Fatigue tests were done on two types of weld geometries which approximate the head–toskirt configurations. From the fatigue tests it was determined that the fatigue life of the experimental samples can be substantially improved by applying weld build–up between the head and the skirt. The expected fatigue life of the test samples was determined by way of calculation employing the so called Nominal–Stress–Approach, the Effective–Notch–Stress–Approach and the Stress–Life–Approach. For both the Nominal–Stress–Approach and the Effective–Notch–Stress–Approach the predicted fatigue life was found to be overly conservative compared to the experimental results. The Stress–Life–Approach predicted the fatigue life to within a factor of 1.3 for both the geometries under investigation when displacements due to welding are taken into account. If displacements due to welding is omitted this factor is increased, for the geometry without weld build–up, to 2. For the geometry with weld build–up the factor remains 1.3. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2011.

Rieck & Nedelman (1991) propuseram um modelo de regressão log-linear tendo como base a distribuição Birnbaum-Saunders (Birnbaum & Saunders, 1969a). O modelo proposto pelos autores vem sendo bastante explorado e tem se mostrado uma ótima alternativa a outros modelos propostos na literatura, como por exemplo, os modelos de regressão Weibull, gama e lognormal. No entanto, até o presente momento, não existe nenhum estudo tratando de refinamentos para as estatísticas da razão de verossimilhanças e escore nesta classe de modelos de regressão. Assim, um dos objetivos desta tese é obter um fator de correção de Bartlett para a estatística da razão de verossimilhanças e um fator de correção tipo-Bartlett para a estatística escore nesse modelo. Estes ajustes melhoram a aproximação da distribuição nula destas estatísticas pela distribuição qui-quadrado de referência. Adicionalmente, objetiva-se obter ajustes para a estatística da razão de verossimilhanças sinalizada. Tais ajustes melhoram a aproximação desta estatística pela distribuição normal padrão. Recentemente, uma nova estatística de teste foi proposta por Terrell (2002), a qual o autor denomina estatística gradiente. Esta estatística foi derivada a partir da estatística escore e da estatística de Wald modificada (Hayakawa & Puri, 1985). A combinação daquelas duas estatísticas resulta em uma estatística muito simples de ser calculada, não envolvendo, por exemplo, nenhum cálculo matricial como produto e inversa de matrizes. Esta estatística foi recentemente citada por Rao (2005): \"The suggestion by Terrell is attractive as it is simple to compute. It would be of interest to investigate the performance of the [gradient] statistic.\" Caminhando na direção da sugestão de Rao, outro objetivo da tese é obter uma expansão assintótica para a distribuição da estatística gradiente sob uma sequência de alternativas de Pitman convergindo para a hipótese nula a uma taxa de convergência de n^{-1/2} utilizando a metodologia desenvolvida por Peers (1971) e Hayakawa (1975). Em particular, mostramos que, até ordem n^{-1/2}, a estatística gradiente segue distribuição qui-quadrado central sob a hipótese nula e distribuição qui-quadrado não central sob a hipótese alternativa. Também temos como objetivo comparar o poder local deste teste com o poder local dos testes da razão de verossimilhanças, de Wald e escore. Finalmente, aplicaremos a expansão assintótica derivada na tese em algumas classes particulares de modelos. / The Birnbaum-Saunders regression model is commonly used in reliability studies.We address the issue of performing inference in this class of models when the number of observations is small. Our simulation results suggest that the likelihood ratio and score tests tend to be liberal when the sample size is small. We derive Bartlett and Bartlett-type correction factors which reduce the size distortion of the tests. Additionally, we also consider modified signed log-likelihood ratio statistics in this class of models. Finally, the asymptotic expansion of the distribution of the gradient test statistic is derived for a composite hypothesis under a sequence of Pitman alternative hypotheses converging to the null hypothesis at rate n^{-1/2}, n being the sample size. Comparisons of the local powers of the gradient, likelihood ratio, Wald and score tests reveal no uniform superiority property.

This thesis focuses on the design of new high pressure injection system application on diesel engine. The design came from company Zetor’s demands. For this application was chosen system Common Rail supplied by company Bosch. Assignment was separated into several tasks. Research of suitable components for low pressure section, design of waste system and high pressure injection tubes design including injector clamping. Due to injectors used in this system was necessary to redesign cover for valves and even new system to lead wires from outside of the engine to injectors. Injector is attached by screw connection, which is composed from screw and clamping jaw. These parts are subjected to an analytic calculations. Clamping jaw of injector is even subjected to a FEM analysis, which focuses on fatigue life. Finally, the drawings were documented.

Dvě pokročilé vysoce legované austenitické oceli s Fe-Ni-Cr matricí byly studovány za podmínek nízkocyklové únavy jak za pokojové tak vysoké teploty. Široká škála experimentálních a charakterizačních nástrojů byla použita ke studiu vzájemně souvisejících aspektů zahrnujících chemické složení slitin, mikrostrukturu, deformační mechanismy a celkovou odezvu materiálů na externě působící zatížení. Klíčové mechanismy a faktory definující mechanické vlastnosti a výkonnost v reálném provozu byly analyzovány a diskutovány v souvislosti s materiálovým designem. • Standardní únavové experimenty byly provedeny za pokojové teploty a teploty 700°C. Byly získány křivky cyklického zpevnění/změkčení, cyklické deformační křivky, Coffin-Manson a Wöhlerovy křivky. • Ke studiu změn mikrostrukturního stavu slitin v důsledku cyklického zatěžování za pokojové a zvýšené teploty byla použita široká škála technik charakterizace pomocí elektronové mikroskopie. • Únavové chování, pevnost a cyklická plastická odezva studovaných materiálů byla vysvětlena v souvislosti s mikrostrukturními změnami a mikrostrukturními aspekty deformačních mechanismů jak za pokojové tak za zvýšených teplot. • Bylo zjištěno, že Sanicro 25 vykazuje nejvyšší pevnostní charakteristiky ze všech materiálů stejné třídy. Výjimečné vlastnosti této slitiny jsou spojeny s populacemi dvou typů nanočástic, koherentními precipitáty bohatými na měď a nanočásticemi typu MX s charakteristikou disperzoidu. Tyto nanočástice mají klíčový vliv na pevnost a celkovou cyklickou odezvu. V důsledku interakcí s precipitáty způsobujících zachytávání je pohyb dislokací v Sanicro 25 významně zpomalen, což vede k potlačení normálních procesů zotavení obvykle vedoucích ke změně uspořádání dislokační struktury tak, aby byla celková vnitřní energie systému co nejnižší. Takové uspořádání je tvořeno například dislokačními buňkami. Jelikož jsou procesy zotavení potlačeny, dislokační struktura za vysokých teplot je charakteristická homogenní distribucí dislokací o vysoké hustotě s velkou mírou vzájemných interakcí. V kombinaci s dalšími mechanismy zpevnění jako jsou precipitáty a substituční prvky v tuhém roztoku, tyto deformační mechanismy vedou k významnému zvýšení cyklické pevnosti za vysokých teplot.

Fatique life extension of nuclear powerplants lies in the search for project reserves. This work deals with the evaluation of low-cycle fatigue of nuclear installations of the VVER type and the assessment of the influence of the computational model level. Fatigue tests of austenitic steel using optical method of digital image correlation for which the evaluation procedure is designed and used is performed. Selected model of plasticity with kimenatic (Chaboche) and combinated hardening (Chaboche, Voce) are calibrated from the obtained data. Subsequently, the durability of the test specimen is determined by computational modeling for different material models. From the comparison of the results of fatigue tests with the calculation, the material models suitable for the description of fatigue life and their validity are determined.

<p>  </p> <p>There has been a growing interest within the aerospace industry for shifting towards a digital twin approach, for reliable assessment of individual components during the product lifecycle - across design, manufacturing, and in-service maintenance, repair & overhaul (MRO) stages. The transition towards digital twins relies on continuous updating of the product lifecycle datasets and interoperable exchange of data applicable to components, thereby permitting engineers to utilize current state information to make more-informed downstream decisions. In this thesis, we primarily develop a framework to store, track, update, and retrieve product lifecycle data applicable to a serialized component, its features, and individual locations. </p> <p>From a structural integrity standpoint, the fatigue performance of a component is inherently tied to the component geometry, its material state, and applied loading conditions. The manufacturing process controls the underlying material microstructure, which in turn governs the mechanical properties and ultimately the performance. The processing also controls the residual stress distributions within the component volume, which influences the durability and damage tolerance of the component. Hence, we have demonstrated multiple use cases for fatigue life assessment of critical aerospace components, by using the developed framework for efficiently tracking and retrieving (i) the current geometric state, (ii) the material microstructure state, and (iii) residual stress distributions.</p> <p>Model-based definitions (MBDs) present opportunities to capture both geometric and non-geometric data using 3D computer-aided design (CAD) models, with the overarching aim to disseminate product information across different stages of the lifecycle. MBDs can potentially eliminate error-prone information exchange associated with traditional paper-based drawings and improve the fidelity of component details, captured using 3D CAD models. However, current CAD capabilities limit associating the material information with the component’s shape definition. Furthermore, the material attributes of interest, viz., material microstructures and residual stress distributions, can vary across the component volume. To this end, in the first part of the thesis, we implement a CAD-based tool to store and retrieve metadata using point objects within a CAD model, thereby creating associations to spatial locations within the component. The tool is illustrated for storage and retrieval of bulk residual stresses developed during the manufacturing of a turbine disk component, acquired from process modeling and characterization. Further, variations in residual stress distribution owing to process model uncertainties have been captured as separate instances of the disk’s CAD models to represent part-to-part variability as an analogy to track individual serialized components for digital twins. The propagation of varying residual stresses from these CAD models within the damage tolerance analysis performed at critical locations in the disk has been demonstrated. The combination of geometric and non-geometric data inside the MBD, via storage of spatial and feature varying information, presents opportunities to create digital replica or digital twin(s) of actual component(s) with location-specific material state information.</p> <p>To fully realize a digital twin description of components, it is crucial to dynamically update information tied to a component as it evolves across the lifecycle, and subsequently track and retrieve current state information. Hence, in the second part of the thesis, we propose a dynamic data linking approach to include material information within the MBDs. As opposed to storing material datasets directly within the CAD model in the previous approach, we externally store and update the material datasets and create data linkages between material datasets and features within the CAD models. To this end, we develop a model-based feature information network (MFIN), a software agnostic framework for linking, updating, searching, and retrieving of relevant information across a product’s lifecycle. The use case of a damage tolerance analysis for a compressor bladed-disk (blisk) is demonstrated, wherein Ti-6Al-4V blade(s) are linear friction welded to the Ti-6Al-4V disk, comprising well-defined regions exhibiting grain refinement and high residuals stresses. By capturing the location-specific microstructural information and residual stress fields at the weld regions, this information was accessed within the MFIN and used for downstream damage tolerant analysis. The introduction of the MFIN framework facilitates access to dynamically evolving as well as location-specific data for use within physics-based models.</p> <p>In the third part of thesis, we extend the MFIN framework to enable a physics-based, microstructure sensitive and location-specific fatigue life analysis of a component. Traditionally, aerospace components are treated as monolithic structures during lifing, wherein microstructural information at individual locations are not necessarily considered. The resulting fatigue life estimates are conservative and associated with large uncertainty bounds, especially in components with gradient microstructures or distinct location-specific microstructures, thereby leading to under usage of the component’s capabilities. To improve precision in the fatigue estimates, a location-specific lifing framework is enabled via MFIN, for tracking and retrieval of microstructural information at distinct locations for subsequent use within a crystal plasticity-based fatigue life prediction model. A use case for lifing dual-microstructure heat treated LSHR turbine disk component is demonstrated at two locations, near the bore (fine grains) and near the rim (coarse grains) regions. We employ the framework to access (a) the grain size statistics and (b) the macroscopic strain fields to inform precise boundary conditions for the crystal plasticity finite-element analysis. The illustrated approach to conduct a location-specific predictive analysis of components presents opportunities for tailoring the manufacturing process and resulting microstructures to meet the component’s targeted requirements.</p> <p>For reliably conducting structural integrity analysis of a component, it is crucial to utilize their precise geometric description. The component geometries encounter variations from nominal design geometries, post manufacturing or after service. However, traditionally, stress analyses are based on nominal part geometries during assessment of these components. In the last part of the thesis, we expand the MFIN framework to dynamically capture deviations in the part geometry via physical measurements, to create a new instance of the CAD model and the associated structural analysis. This automated workflow enables engineers for improved decision-making by assessing (i) as-manufactured part geometries that fall outside of specification requirements during the materials review board or (ii) in-service damages in parts during the MRO stages of the lifecycle. We demonstrate a use case to assess the structural integrity of a turbofan blade that had experienced foreign object damage (FOD) during service. The as-designed geometry was updated based on coordinate measurements of the damaged blade surfaces, by applying a NURBS surface fit, and subsequently utilized for downstream finite-element stress analysis. The ramifications of the FOD on the local stresses within the part are illustrated, providing critical information to the engineers for their MRO decisions. The automated flow of information from geometric inspection within structural analysis, enabled by MFIN, presents opportunities for effectively assessing products by utilizing their current geometries and improving decision-making during the product lifecycle.</p>

熱可塑性エポキシ樹脂を母材とするCFRTPの母材を高分子量化することにより静的および疲労特性の改善を行えることを示し，重量平均分子量の違いによりそれらの特性が大きく変化する変曲点が存在することを明らかにした．さらに，CFRTPの実用化のためにミシンステッチを利用したCFRTPテープ積層法を提案し，その試作機を作成することで，CFRTPテープ自動積層装置の実用性を明らかにした． / 博士(工学) / Doctor of Philosophy in Engineering / 同志社大学 / Doshisha University

Pokročilá vysoce legovaná austenitická nerezová ocel Sanicro 25 s Fe-Ni-Cr matricí byla studována za podmínek nízkocyklové únavy za pokojové a vysoké teploty 700 °C. Široká škála moderních experimentálních technik byla použita ke studiu vzájemně souvisejících efektů chemického složení slitiny, mikrostrukturních změn a deformačních mechanismů, které určují odolnost materiálu vůči poškození. Hlavní úsilí bylo zaměřeno na studium iniciace únavových trhlin a růstu krátkých trhlin, tedy dvě stádia, která hrají zásadní roli ve výsledné celkové délce únavového života materiálu v provozu. • Vnitřní deformační mechanismy byly korelovány s vývojem povrchového reliéfu, který je pozorován ve formě persistentních skluzových stop na povrchu. Bylo zjištěno, že vysoce planární charakter dislokačního skluzu způsobuje vysokou lokalizaci cyklické plastické deformace do persistentních skluzových pásů, což v důsledku vede k nukleaci “Stage I” trhlin, která je spojena s přítomností persistentních skluzových stop na povrchu ve všech studovaných vzorcích. Bylo zjištěno, že praskání dvojčatových hraníc je taktéž spojeno s přítomností persistentních skluzových stop podél povrchové stopy dvojčatové roviny. • Interkrystalická iniciace únavové trhliny byla pozorována pouze zřídka, a to za podmínek zatěžování amplitudami vysoké deformace. Bylo zjištěno že interkrystalická iniciace je spojena s přítomností persistentních skluzových stop na hranicích zrn. Hranice zrn praskají za podmínek externího tahového zatížení zejména z důvodu vysokého počtu nekompatibilit na hranicích zrn, které jsou způsobené tvarem persistentních skluzových stop. • Mechanismy růstu přirozených krátkých trhlin byly studovány na vzorcích vystavených nízkocyklove únava s nízkou i vysokou deformací. Role mikrostruktury byla analyzována pomocí experimentálních technik a diskutována. • Rychlosti šíření nejdelších trhlin byly měřeny na vzorcích s mělkým vrubem. Výsledky byly analyzovány použitím přístupů lomové mechaniky založených na amplitudě KI a J-integrálu stejně jako na amplitudě plastické deformace. Všechny přístupy byly diskutovány v souvislosti s Mansonovými-Coffinovými křivkami únavové životnosti. Jednoduchý mocninový zákon růstu krátkých trhlin založený na amplitudě plastické deformace ukazuje velice dobrou korelaci se zákonem únavové životnosti. • Byla studována role oxidace v podmínkách cyklického zatěžování za vysokých teplot. Bylo zjištěno, že křehké praskání zoxidovaných hranic zrn hraje hlavní roli v počátečních stádiích nukleace trhlin. Později po iniciaci se dráha růstu trhliny mění preferenčně na transkrystalickou. Dráha šíření trhlin je velmi podobná dráze zjištěné při cyklování za pokojové teploty.

Superaustenitická korozivzdorná ocel typu 22Cr25NiWCoCu určená pro vysokoteplotní aplikace v energetickém průmyslu byla studována za podmínek nízkocyklové únavy při pokojové a zvýšené teplotě. Jednotlivé vzorky byly podrobeny různým zátěžným procedurám, což umožnilo studium materiálové odezvy spolu s mechanismem poškození. Křivky cyklického zpevnění/změkčení, cyklického napětí a Coffin-Mansonovy křivky byly vyhodnoceny. Únavová životnost materiálu byla diskutována s ohledem na uplatňované mechanismy poškození, které se vyvinuly za specifických zátěžných podmínek. Standardní izotermální únavové experimenty byly provedeny při pokojové a zvýšené teplotě. Hysterezní smyčky zaznamenané během cyklického zatěžování byly analyzovány pomocí zobecněné statistické teorie hysterezní smyčky. Pro různé amplitudy napětí byla určena jak distribuce hustoty pravděpodobnosti interních kritických napětí (dále PDF), tak rovněž zjištěn její vývoj během cyklického namáhání. Zjištěné průběhy PDF byly korelovány s vývojem povrchového reliéfu a vnitřního dislokačního uspořádání zdokumentované pro obě teploty pomocí rastrovací elektronové mikroskopie (SEM) vybavené technikou fokusovaného iontového svazku (FIB), která umožnila rovněž efektivní studium nukleace povrchových únavových trhlin. Při cyklickém zatížení při pokojové teplotě byla pozorována lokalizace cyklické plastické deformace do perzistentních skluzových pásů (PSP). V místech, kde tyto PSP vystupují na povrch materiálu byly pozorovány perzistentní skluzové stopy (PSS) tvořené extruzemi a intruzemi. Postupné prohlubování intruzí, zejména na čele nejhlubší intruze, vede k iniciaci únavové trhliny. Odlišný mechanismus tvorby trhlin byl zjištěn při únavové zkoušce při zvýšené teplotě, kde zásadní roli hrál vliv prostředí. Rychlá oxidace hranic zrn a jejich následné popraskání představuje dominantní mechanismus v I. stádiu nukleace trhlin. Aplikace desetiminutové prodlevy v tahové části zátěžného cyklu vedlo k vývoji vnitřního (kavitačního) poškozování. Mechanismy vnitřního poškozování byly studovány na podélných řezech rovnoběžných s napěťovou osou zkušebních vzorků. Trhliny a jejich vztah k hranicím zrn a dvojčat byly studovány pomocí difrakce zpětně odražených elektronů (EBSD). Vliv prodlevy na únavovou životnost byl korelován s vývojem povrchového reliéfu a vnitřního poškození. Vzorky z uvedené oceli byly rovněž podrobeny zkouškám termomechanické únavy (TMF), při nichž se v čase mění jak zátěžná síla tak i teplota. Termomechanické únavové zkoušky v režimu soufázném (in-phase) a protifázném (out-of-phase) byly provedeny jak s prodlevou, tak i bez ní. Ve všech případech bylo pozorováno rychlé cyklické zpevnění bez ohledu na použitou amplitudu deformace, u vzorků testovaných v out-of-phase režimu byla zjištěna tendence k saturaci. Zkoumáním povrchového reliéfu za pomocí technik SEM a FIB byla odhalena přednostní oxidace hranic zrn a následné praskání těchto hranic kolmo k ose zatížení. Prodlevy v cyklech při maximálním napětí vedly ke zvýšení amplitudy plastické deformace a následně ke creepovému poškození ve formě vnitřních kavit a trhlin. Interkrystalické šíření trhlin bylo pozorováno na vzorcích testovaných v režimu in-phase. Vývoj poškození v režimu out-of-phase nebyl principiálně ovlivněn zařazením prodlevy do zátěžného cyklu. Charakteristickým znakem namáhání v režimu out-of-phase je nukleace několika trhlin v homogenní oxidické vrstvě jdoucích napříč zrny kolmo k ose zatěžování.

<div>In this thesis, the role of location-specific microstructural features in the fatigue performance of the safety-critical aerospace components made of Nickel (Ni)-base superalloys and linear friction welded (LFW) Titanium (Ti) alloys has been studied using crystal plasticity finite element (CPFE) simulations, energy dispersive X-ray diffraction (EDD), backscatter electron (BSE) images and digital image correlation (DIC).</div><div><br></div><div>In order to develop a microstructure-sensitive fatigue life prediction framework, first, it is essential to build trust in the quantitative prediction from CPFE analysis by quantifying uncertainties in the mechanical response from CPFE simulations. Second, it is necessary to construct a unified fatigue life prediction metric, applicable to multiple material systems; and a calibration strategy of the unified fatigue life model parameter accounting for uncertainties originating from CPFE simulations and inherent in the experimental calibration dataset. To achieve the first task, a genetic algorithm framework is used to obtain the statistical distributions of the crystal plasticity (CP) parameters. Subsequently, these distributions are used in a first-order, second-moment method to compute the mean and the standard deviation for the stress along the loading direction (σ_load), plastic strain accumulation (PSA), and stored plastic strain energy density (SPSED). The results suggest that an ~10% variability in σ_load and 20%-25% variability in the PSA and SPSED values may exist due to the uncertainty in the CP parameter estimation. Further, the contribution of a specific CP parameter to the overall uncertainty is path-dependent and varies based on the load step under consideration. To accomplish the second goal, in this thesis, it is postulated that a critical value of the SPSED is associated with fatigue failure in metals and independent of the applied load. Unlike the classical approach of estimating the (homogenized) SPSED as the cumulative area enclosed within the macroscopic stress-strain hysteresis loops, CPFE simulations are used to compute the (local) SPSED at each material point within polycrystalline aggregates of 718Plus, an additively manufactured Ni-base superalloy. A Bayesian inference method is utilized to calibrate the critical SPSED, which is subsequently used to predict fatigue lives at nine different strain ranges, including strain ratios of 0.05 and -1, using nine statistically equivalent microstructures. For each strain range, the predicted lives from all simulated microstructures follow a log-normal distribution; for a given strain ratio, the predicted scatter is seen to be increasing with decreasing strain amplitude and are indicative of the scatter observed in the fatigue experiments. Further, the log-normal mean lives at each strain range are in good agreement with the experimental evidence. Since the critical SPSED captures the experimental data with reasonable accuracy across various loading regimes, it is hypothesized to be a material property and sufficient to predict the fatigue life.</div><div><br></div><div>Inclusions are unavoidable in Ni-base superalloys, which lead to two competing failure modes, namely inclusion- and matrix-driven failures. Each factor related to the inclusion, which may contribute to crack initiation, is isolated and systematically investigated within RR1000, a powder metallurgy produced Ni-base superalloy, using CPFE simulations. Specifically, the role of the inclusion stiffness, loading regime, loading direction, a debonded region in the inclusion-matrix interface, microstructural variability around the inclusion, inclusion size, dissimilar coefficient of thermal expansion (CTE), temperature, residual stress, and distance of the inclusion from the free surface are studied in the emergence of two failure modes. The CPFE analysis indicates that the emergence of a failure mode is an outcome of the complex interaction between the aforementioned factors. However, the possibility of a higher probability of failure due to inclusions is observed with increasing temperature, if the CTE of the inclusion is higher than the matrix, and vice versa. Any overall correlation between the inclusion size and its propensity for damage is not found, based on inclusion that is of the order of the mean grain size. Further, the CPFE simulations indicate that the surface inclusions are more damaging than the interior inclusions for similar surrounding microstructures. These observations are utilized to instantiate twenty realistic statistically equivalent microstructures of RR1000 – ten containing inclusions and remaining ten without inclusions. Using CPFE simulations with these microstructures at four different temperatures and three strain ranges for each temperature, the critical SPSED is calibrated as a function of temperature for RR1000. The results suggest that critical SPSED decreases almost linearly with increasing temperature and is appropriate to predict the realistic emergence of the competing failure modes as a function of applied strain range and temperature.</div><div><br></div><div>LFW process leads to the development of significant residual stress in the components, and the role of residual stress in the fatigue performance of materials cannot be overstated. Hence, to ensure fatigue performance of the LFW Ti alloys, residual strains in LFW of similar (Ti-6Al-4V welded to Ti-6Al-4V or Ti64-Ti64) and dissimilar (Ti-6Al-4V welded to Ti-5Al-5V-5Mo-3Cr or Ti64-Ti5553) Ti alloys have been characterized using EDD. For each type of LFW, one sample is chosen in the as-welded (AW) condition and another sample is selected after a post-weld heat treatment (HT). Residual strains have been separately studied in the alpha and beta phases of the material, and five components (three axial and two shear) have been reported in each case. In-plane axial components of the residual strains show a smooth and symmetric behavior about the weld center for the Ti64-Ti64 LFW samples in the AW condition, whereas these components in the Ti64-Ti5553 LFW sample show a symmetric trend with jump discontinuities. Such jump discontinuities, observed in both the AW and HT conditions of the Ti64-Ti5553 samples, suggest different strain-free lattice parameters in the weld region and the parent material. In contrast, the results from the Ti64-Ti64 LFW samples in both AW and HT conditions suggest nearly uniform strain-free lattice parameters throughout the weld region. The observed trends in the in-plane axial residual strain components have been rationalized by the corresponding microstructural changes and variations across the weld region via BSE images. </div><div><br></div><div>In the literature, fatigue crack initiation in the LFW Ti-6Al-4V specimens does not usually take place in the seemingly weakest location, i.e., the weld region. From the BSE images, Ti-6Al-4V microstructure, at a distance from the weld-center, which is typically associated with crack initiation in the literature, are identified in both AW and HT samples and found to be identical, specifically, equiaxed alpha grains with beta phases present at the alpha grain boundaries and triple points. Hence, subsequent fatigue performance in LFW Ti-6Al-4V is analyzed considering the equiaxed alpha microstructure.</div><div><br></div><div>The LFW components made of Ti-6Al-4V are often designed for high cycle fatigue performance under high mean stress or high R ratios. In engineering practice, mean stress corrections are employed to assess the fatigue performance of a material or structure; albeit this is problematic for Ti-6Al-4V, which experiences anomalous behavior at high R ratios. To address this problem, high cycle fatigue analyses are performed on two Ti-6Al-4V specimens with equiaxed alpha microstructures at a high R ratio. In one specimen, two micro-textured regions (MTRs) having their c-axes near-parallel and perpendicular to the loading direction are identified. High-resolution DIC is performed in the MTRs to study grain-level strain localization. In the other specimen, DIC is performed on a larger area, and crack initiation is observed in a random-textured region. To accompany the experiments, CPFE simulations are performed to investigate the mechanistic aspects of crack initiation, and the relative activity of different families of slip systems as a function of R ratio. A critical soft-hard-soft grain combination is associated with crack initiation indicating possible dwell effect at high R ratios, which could be attributed to the high-applied mean stress and high creep sensitivity of Ti-6Al-4V at room temperature. Further, simulations indicated more heterogeneous deformation, specifically the activation of multiple families of slip systems with fewer grains being plasticized, at higher R ratios. Such behavior is exacerbated within MTRs, especially the MTR composed of grains with their c-axes near parallel to the loading direction. These features of micro-plasticity make the high R ratio regime more vulnerable to fatigue damage accumulation and justify the anomalous mean stress behavior experienced by Ti-6Al-4V at high R ratios.</div><div><br></div>