This work concerns the fatigue crack growth behaviour of nickel base single crystal superalloys. The main industrial application of this class of materials is in gas turbine blades, where the ability to withstand severe mechanical loading in combination with high temperatures is required. In order to ensure the structural integrity of gas turbine blades, knowledge of the fatigue crack growth behaviour under service-like conditions is of utmost importance. The aim of the present work is both to improve the understanding of the crack growth behaviour of single crystal superalloys and also to improve the testing and evaluation methodology for crack propagation under thermomechanical fatigue loading conditions. Single crystal superalloys have anisotropic mechanical properties and are prone to localization of inelastic deformation along the close-packed planes of the crystal lattice. Under some conditions, crystallographic crack growth occurs along these planes and this is a complicating factor throughout the whole chain of crack propagation life simulation; from material data generation to component calculation. Fatigue crack growth testing has been performed, both using conventional isothermal testing methods and also using thermomechanical fatigue crack growth testing. Experimental observations regarding crystallographic crack growth have been made and its dependence on crystal orientation and testing temperature has been investigated. Quantitative crack growth data are however only presented for the case of Mode I crack growth under isothermal as well as thermomechanical fatigue conditions. Microstructural investigations have been undertaken to investigate the deformation mechanisms governing the crack growth behaviour. A compliance based method for the evaluation of crack opening force under thermomechanical fatigue conditions was developed, in order to enable a detailed analysis of the test data. The crack opening force evaluation proved to be of key importance in the understanding of the crack driving force under different testing conditions.

This work concerns the fatigue crack growth behaviour of nickel base single crystal superalloys. The main industrial application of this class of materials is in gas turbine blades, where the ability to withstand severe mechanical loading in combination with high temperatures is required. In order to ensure the structural integrity of gas turbine blades, knowledge of the fatigue crack growth behaviour under service-like conditions is of utmost importance. The aim of the present work is both to improve the understanding of the crack growth behaviour of single crystal superalloys and to improve the testing and evaluation methodology for crack propagation under thermomechanical fatigue loading conditions. Single crystal superalloys have anisotropic mechanical properties and are prone to localization of inelastic deformation along the close packed planes of the crystal lattice. Under some conditions, crystallographic crack growth occurs along these planes, and this is a complicating factor throughout the whole chain of crack propagation life simulation; from material data generation to component calculation. Crack growth testing has been performed, both using conventional isothermal testing methods and using thermomechanical fatigue crack growth testing. Experimental observations regarding crystallographic crack growth have been made and its dependence on crystal orientation and testing temperature has been investigated. Quantitative crack growth data are presented for the case of Mode I crack growth under isothermal as well as thermomechanical fatigue conditions. Microstructural investigations have been undertaken to investigate the deformation mechanisms governing the crack growth behaviour. A compliance-based method for the evaluation of crack opening force under thermomechanical fatigue conditions was developed, to enable a detailed analysis of the test data. The crack opening force evaluation proved to be of key importance for the understanding of the crack driving force under different testing conditions. The influence of hold time on crack growth behaviour was analysed, both in terms of creep crack growth and in terms of creep effects on the crack opening force. The transition between non-crystallographic and crystallographic crack growth was studied in detail and a criterion was developed to enable accurate predictions of this transition under a wide range of loading conditions representative for gas turbine blades.

Detta arbete behandlar sprickpropageringsbeteendet hos monokristallina nickelbaserade superlegeringar. Den huvudsakliga industriella tillämpningen för denna materialgrupp är som skovelmaterial i gasturbiner, vilket kräver förmågan att motstå hög mekanisk belastning i kombination med höga temperaturer. För att säkerställa gasturbinskovlarnas hållfasthet är kunskap om sprickpropageringsbeteendet, under driftsliknande förhållanden, av yttersta vikt. Målet med detta arbete är både att förbättra förståelsen för monokristallina nickelbaslegeringars sprickpropageringsbeteende och att förbättra metodiken för sprickpropageringsprovning med termomekanisk utmattningsbelastning. Monokristallina superlegeringar har anisotropa mekaniska egenskaper och en benägenhet till lokalisering av inelastisk deformation längs kristallgittrets tätpackade plan. Under vissa förhållanden sker kristallografisk spricktillväxt längs dessa plan och detta är en komplicerande faktor genom hela kedjan av sprickpropageringssimulering; från materialdatagenerering till komponentberäkningar. Sprickpropageringsprovning har utförts både med konventionella isoterma provningsmetoder och med termomekanisk sprickpropageringsprovning. Kristallografisk sprickpropagering har studerats experimentellt för att klargöra dess beroende av kristallorientering och temperatur. Kvantitativa sprickpropageringsdata presenteras för sprickväxt i Modus I, under isoterm såväl som termomekanisk utmattningsbelastning. Mikrostrukturundersökningar har gjorts för att studera deformationsmekanismerna som styr sprickpropageringsbeteendet. En kompliansbaserad metod för att utvärdera spricköppningskraften vid termomekanisk utmattning har utvecklats, för att möjliggöra en djupgående analys av provdatat. Spricköppningsutvärderingen visade sig ha en nyckelroll för förståelsen av drivkraften för sprickpropagering vid olika provningsförhållanden. Inverkan av hålltid på sprickpropageringsbeteendet analyserades både med avseende på krypspricktillväxt och med avseende på inverkan av krypdeformation på spricköppningskraften. Omslaget från icke-kristallografisk till kristallografisk sprickpropagering studerades i detalj och ett kriterium utvecklades för att möjliggöra tillförlitliga prediktioner av detta omslag under ett brett spann av förhållanden, representativa för gasturbinskovlar.