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No description available.

GKN Aerospace AB, Sweden (GAS) is one of the leading companies taking up the charge in manufacturing components using Additive Manufacturing(ed) (AM) techniques in the aerospace sector. They are a hub of engineering and they are a supplier of engine and engine components to the world’s leading aero-engine manufacturers, and airframes to civil and military aircraft manufacturers. A phenomenon that is of interest to designers at GAS is the effects of dwell times on high temperature fatigue, especially how this phenomenon affects the fatigue properties of Laser Powder Bed Fusion (LPBF) Inconel 718 (IN718). IN718 is a versatile alloy that can be used at relatively high temperatures and has excellent weldability and is one of the newer materials replacing expensive materials such as Titanium (and its alloys) in the aerospace industry. The aerospace industry has been pushing for an increase in parts manufactured using AM processes because of the advantage the AM process grants to the production process, however a new manufacturing process for an industry needs to be studied and researched from a failure perspective, i.e. the prominent mode of failure for components manufactured using AM and the underlying factors influencing the failure mechanism must be studied. This thesis explores a solution to predict the life of components based on experimental crack propagation tests wherein the test specimens were subjected to the phenomenon mentioned above. A literature survey was conducted researching ways to model this phenomenon and the factors affecting it. The methods found in the literature survey were far too complex to model for the purposes of this thesis, additionally the methods described in the literature were empirical methods describing the phenomenon, rather than a fundamental study of factors causing the phenomenon and ways to model their influence on the life of the component. Hence, a simple method based on the Palmgren-Miner linear damage summation rule which was coded in the form of a FORTRAN code was utilized to compute the life of the components. Software runs predicting life of physical experiments were conducted and inferences about the predictive method were drawn. The limitations of this method were understood and possible solutions were explored, based on which conclusions were drawn regarding the method’s efficacy in predicting the life of the specimens that underwent dwell loading during fatigue cycling. Finally, the method was applied to a case study to understand the effectiveness of the method. / GKN Aerospace AB, Sverige (GAS) är ett av de ledande företagen som tar upp kampen vid tillverkning av komponenter med hjälp av additiv tillverkning (AM) inom flyg- och rymdsektorn. De är ett nav för ingenjörskonst och de är en leverantör av motorer och motorkomponenter till världens ledande tillverkare av flygmotorer, och civila och militära flygplanstillverkare. Ett fenomen som är av intresse för designers på GAS är effekterna av uppehållstider på högtemperaturutmattning, särskilt hur detta fenomen påverkar utmattningen egenskaper hos Laser Powder Bed Fusion (LPBF) Inconel 718 (IN718). IN718 är en mångsidig legering som kan användas vid relativt höga temperaturer och har utmärkt svetsbarhet och är ett av de nyare materialen som ersätter dyra material såsom titan (och dess legeringar) inom flygindustrin. Flygindustrin har drivit på för en ökning av delar som tillverkas med additiva tillverkningsprocesser på grund av den fördel som tillverkningsprocessen ger en ny tillverkningsprocess för en industri behöver dock studeras och forskat ur ett misslyckandeperspektiv, dvs. det framträdande sättet att misslyckas för komponenter som tillverkats med hjälp av additiv tillverkning och de bakomliggande faktorer som mekanismen måste studeras. Denna avhandling utforskar en lösning för att förutsäga livslängden för komponenter baserat på experimentella sprickutbredningstester där provexemplaren utsattes för fenomenet som nämns ovan. En litteraturstudie genomfördes för att undersöka olika sätt att modellera detta fenomenet och de faktorer som påverkar det. Metoderna som framkom i litteraturstudien var alldeles för komplexa för att modellera för denna avhandling, dessutom är metoderna som beskrivits i litteraturen var empiriska metoder som beskriver fenomenet, snarare än en grundläggande studie av de faktorer som orsakar fenomenet och sätt att modellera deras inverkan på komponentens livslängd. Därav en enkel metod baserad på Palmgren-Miners linjära skadesummeringsregel som kodades i form av en FORTRAN-kod användes för att beräkna livslängden för komponenterna. Programvarukörningar som förutspådde livslängden för fysiska experiment genomfördes och slutsatser om den prediktiva metoden drogs. Begränsningarna med denna metod förstods och möjliga lösningar utforskades. Som låg till grund för de slutsatser som drogs om metodens effektivitet när det gäller att förutsäga livslängden för de prover som genomgick uppehållsbelastning underutmattningscykling. Slutligen tillämpades metoden på en fallstudie för att förstå effektiviteten avmetod.

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Additive manufacturing (AM) introduces high variability in the microstructure and defect distributions, compared with conventional processing techniques, which introduces greater uncertainty in the resulting fatigue performance of manufactured parts. As a result, qualification of AM parts poses as a problem in continued adoption of these materials in safety-critical components for the aerospace industry. Hence, there is a need to develop precise and accurate, physics-based predictive models to quantify the fatigue performance, as a means to accelerate the qualification of AM parts. The fatigue performance is a critical requirement in the safe-life design philosophy used in the aerospace industry. Fatigue failure is governed by the loading conditions and the attributes of the material microstructure, namely, grain size distribution, texture, and defects. In this work, the crystal plasticity finite element (CPFE) method is employed to model the microstructure-based material response of an additively manufactured Ni-based superalloy, Inconel 718 (IN718). Using CPFE and associated experiments, methodologies were developed to assess multiple aspects of the fatigue behavior of IN718 using four studies. In the first study, a CPFE framework is developed to estimate the critical characteristics of porosity, namely the pore size and proximity that would cause a significant debit in the fatigue life. The second study is performed to evaluate multiple metrics based on plastic strain and local stress in their ability to predict both the modes of failure as seen in fractography experiments and estimate the scatter in fatigue life due to microstructural variability as obtained from fatigue testing. In the third study, a systematic analysis was performed to investigate the role of the simulation volume and the microstructural constraints on the fatigue life predictions to provide informed guidelines for simulation volume selection that is both computationally tractable and results in consistent scatter predictions. In the fourth study, validation of the CPFE results with the experiments were performed to build confidence in the model predictions. To this end, 3D realistic microstructures representative of the test specimen were created based on the multi-modal experimental data obtained from high-energy diffraction experiments and electron backscatter diffraction microscopy. Following this, the location of failure is predicted using the model, which resulted in an unambiguous one to one correlation with the experiment. In summary, the development of microstructure-sensitive predictive methods for fatigue assessment presents a tangible step towards the adoption of model-based approaches that can be used to compliment and reduce the overall number of physical tests necessary to qualify a material for use in application.