Mechanical Behaviour and Simulation of Alloys Used in Jet Engines

Thermo-mechanical fatigue and creep behaviour are two of the most studied topics when characterising the performance of materials used for jet engines components. However, there are situations, such as blade-off events or foreign object damage, in which impact loads are the main thread to the structural integrity. An efficient design process of the engine components requires both an experimental campaign and numerical simulations. Numerical simulations are useful to analyse the influence of different parameters, such as impact conditions, thickness, materials, without performing expensive experimental tests. The prediction capabilities of the simulations rely on the accuracy of the material models. Nevertheless, the Johnson-Cook (JC) model, one of the most widely used for impulse loading, cannot describe the positive temperature dependence, also known as yield-strength anomaly (YSA), of some of the alloys that are present in jet engines (superalloys). Moreover, the heterogeneous grain structure of directionally solidified (DS) turbine blades results in a complex response that must be analysed through advance simulations techniques like the crystalplasticity finite-element method (CPFEM). This technique has been used by other researchers to investigate fatigue behaviour of DS structures; however, to the author’s knowledge, no work has been found which analyses the mechanical response of DS superalloys at high strain rates using CPFEM. In this thesis, a JC-type material model to describe the response of alloys with YSA under high strain rates or impact loading is developed. The model is suitable for computational purposes in the industry its implementation in commercial finite-element codes, such as Abaqus, is not tedious. The constitutive equation describes the flow-stress dependence on temperature through super-posing two terms with an empirical law. The first term accounts for the general response of the alloy (plastic-strain hardening and thermal softening) while the second term models the YSA region with a phenomenological law. The model is calibrated for three different alloys tested under high strain rates and different temperatures using the split-Hopkinson bar technique. The chosen alloys were a cobalt-base superalloy (Co-12Al-10W), a maraging steel (VascoMax C-250) and a directionally solidified nickel-based superalloy (MAR-M247). In all cases, the capability of the model to replicate the experimental dynamic flow behaviour at different temperatures is demonstrated. Finally, the anisotropic flow stress behaviour of the MAR-M247 DS alloy is studied in detail through the CPFEM technique under quasi-static and dynamic loading. Virtual samples of the whole gauge length of the specimens are generated considering the grain structure and orientations of the DS superalloy. The elastic-visco-plastic response of each crystal is modelled with phenomenologicalbase equations but taking into account the dislocation-dislocation interaction among the different slip systems. The constants of the model are fitted with the information from tests parallel to the grain-growth direction and the simulation strategy predicts with high accuracy the experimental response in the perpendicular direction. The mechanical constants of the quasi-polycrystal and polycrystal version of the alloy are also predicted with accuracy. The simulations reveal that in oligocrytalline structures, such as the one presented by the alloy, the yield-strength value is controlled by the grains with higher Schmid factor, while this influence decreases when plastic strain increases. Moreover, the analysis of stress micro-fields confirms that perpendicular grains to the loading axis are prone to nucleate cavities due to the pronounced local-stress increase factor