In this work the constitutive behaviour and the influence of casting defects on the failure of Ni-base single crystal (SC) superalloy components is investigated. It is well known that the presence of casting-related porosities can lead to the nucleation of microcracks under both creep and fatigue conditions and thus, ultimately produce component failure. A rate dependent crystallographic formulation is introduced to describe the inelastic deformation behaviour of the latest generation of single crystal superalloys. The evolution of the current dislocation and obstacle network is described through appropriate slip resistance and internal or back stress variables for each slip system. Good correlations are obtained between experimental data and numerical predictions within the 750◦C .- 950◦C temperature range and for < 001 > and < 111 > crystallographic orientations. The formulation is then numerically implemented into the FE method and used to investigate the deformation of a representative material volume containing a spherical void of approximately 20 micrometers diameter. The functional dependence of the void growth rates in terms of material anisotropy, stress state, temperature and interaction with a free surface is determined. It is shown that the rate of growth of casting defects in an infinite single crystal medium is strongly dependent on the applied triaxiality and relative orientation between the crystallographic axes and the applied stresses. Furthermore, it has been found that, for the acceleration of the defect growth rate as the result of its proximity to a free-surface to be non-negligible, the void needs to be within two diameters of the free surface. Based on the above results, a framework is proposed to describe the growth of embedded casting defects within superalloy single crystals under a given applied multiaxial stress state. The framework provides an explicit link between the mesoscopic (at the level of the voids) and the macroscopic length scales. A number of blunt notch bar creep specimens and thermo-mechanical cyclic stress-strain specimens were tested to validate both the rate dependent crystallographic formulation and the micro-mechanics void growth model. In addition microstructural analysis of the blunt notch bars provided necessary data for the development of a micro-crack initiation criterion. A life assessment methodology combining these models is developed and applied in the analysis of an actual gas turbine blade. It is expected that the understanding of defect growth kinetics and the corresponding damage accumulation will be beneficial in the design and life prediction of superalloy components.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:392399 |
| Date | January 2000 |
| Creators | Dennis, Roger James |
| Publisher | Imperial College London |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/10044/1/11302 |
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