Sustained macroscopic deflected cracking in Nickel based superalloys : mechanism and design criteria

Abstract

In this EngD thesis the phenomenon of sustained macroscopic deflected crack growth (SMDCG) during fatigue in Nickel based Superalloys is discussed, also referred to as ‘teardrop’ cracking in previous work. In a corner notched bend (CNB) specimen a fatigue crack usually grows in a quarter-circular plane perpendicular to the stress axis, the SMDCG however exhibits considerable deflection from this plane at the free surfaces, so that a central planar region is enclosed by large shear terraces (thus forming the ‘teardrop’). This is difficult to characterise in terms of component lifing (both in terms of crack path and crack growth rates), and this thesis aims to further understanding of the underlying mechanisms and to develop assessment methodologies for lifing of such cracks in aeroengine components. The SMDCG effect has been observed under externally applied mode I loading in Udimet 720Li with relatively fine grain sizes of 6-18?m, which are candidate materials for aero engine turbine discs. CNB and single edge bend notch bend (SENB) fatigue tests have been carried out on cast and wrought and powder metallurgy variants of Udimet 720Li and SMDCG was observed at 300°C in air and at 300, 600 and 650°C in vacuum. A consistent measure of the onset of deflection has been defined; this is the crack tip stress intensity factor range ?K at which the sustained deflection from the free surface exceeds the expected shear lip area, which is estimated as the distance of the monotonic plane stress plastic zone size from the free surface. This has been used to characterise samples from other research programmes and aeroengine components from rigtests, that have shown some form of deflected crack growth, to assess whether they exhibit SMDCG or whether the apparent deflection was caused by the expected plane stress region, or other factors such as tunnelling due to creep effects or complex overall loading conditions. Fracture surfaces exhibiting SMDCG have been studied via SEM and the fractographic analysis shows a competition between local shear crack growth and mode I crack growth occurs in both the macroscopically deflected and planar regions. This indicates that the macroscopic deflection is triggered by the stress state at the free surface. Further detailed analysis of the deflected crack tip with focussed ion beam (FIB) serial sectioning combined with EBSD analysis has allowed a detailed 3D reconstruction of the crack tip interaction with local microstructure. This, together with TEM foils extracted via FIB in the same region, have confirmed that the SMDCG in these systems is not linked to any local texture effects or surface microstructural differences. Lifetime predictions for the CNB samples have been carried out based on Paris law coefficients from the SENB tests, and showed shorter lifetimes than the actual samples with deflected crack growth. Whilst this could be partially due to unaccounted for initiation effects, it could also indicate that significant extrinsic shielding caused by the deflected crack growth may actually increase fatigue lifetimes. To assess the driving force evolving during SMDCG, an Alicona Infinite Focus profilometre has been used to map the complex 3D crack shapes, from where tilt angles could be measured to define a mixed mode plane stress SENB test, with the aim of replicating the stress state that had given rise to the locally deflected shear growth and to explicitly measure crack growth behaviour in the deflected regions. However this was not achieved with this test set-up, as the achievable ?Keff was not high enough to trigger the deflection throughout the sample. As a result the 3D map was next used to define Finite Element model to assess the local crack tip stress state of the complex deflected crack path. Crack driving forces that have been identified at the point the deflected crack path becomes self-sustaining, could be used in a lifing model to predict crack paths in aeroengine components, together with the measured onset of deflection.

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Identifiers

ORCID for P.A.S. Reed: orcid.org/0000-0002-2258-0347

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