V = 0.68 f V = 0.58 f 010 [MPa] θ o = 850 C V = 0.68 f V = 0.58 f θ o = 850 C (a) ( b ) Distance from the crack tip [mm] Distance from the crack tip [mm] [MPa] Figure 6: Distribution of (a) the stress component normal to the crack faces and (b) the mean hydrostatic stress (= (σ100 +σ010 +σ001)/3), as a function of the distance from the tip of the notch and the γ’ volume fraction In on-going work, parametric studies of the type shown here coupled with oxygen diffusion from the environment into the crack faces are being conducted and the resulting information used to formulate appropriate fracture mechanics-based formulation for crack growth predictions under creep-fatigue interaction. CONCLUDING REMARKS This work has shown that, in single crystal alloys reinforced with a high precipitate volume fraction, such as Nibase superalloys, the decrease in the precipitate content due to environmental effects can have a strong effect on the local stresses responsible for both cleavage and stable crack growth. Such effects must be incorporated into fracture-mechanics based crack growth models to assure the accuracy of hot section component life assessments. ACKNOWLEDGEMENTS Financial support for this work by the EPSRC (UK) through grant GR/N12312 and by Alstom Power (UK) are gratefully acknowledged. The authors are grateful to Dr. F. Andrieux for her assistance in the calibration of the crystallographic model. References [1] Andrieu, E. and Pineau, A. (1999), Study of the Coupled Phenomena Involved in the Oxidation Assisted Intergranular Cracking in Ni-Base Superalloys, J. de Physique IV, V. 9 , pp. 3-12. [2] Martinez-Esnaola, J.M., Martin-Meizoso, A., Affeldt, E.E., Bennett, A. and Fuentes, M. (1997), High Temperature Fatugue in Single Crystal Superalloys, Fatigue Fract. Engng. Mater. Struct., V. 20 , pp. 771-788. [3] Busso, E.P., Meissonnier, F., and O’Dowd, N.P. (2000), Gradient-Dependent Visco-Plastic Deformation of Two-Phase Single Crystals. J. Mechanics Physics Sol., V. 48 , pp. 2333-2361. [4] Meissonnier, F., Busso, E.P., and O’Dowd, N.P. (2001), Finite Element Implementation of a Non-Local Visco-Plastic Crystallographic Formulations. Int. Journal of Plasticity, V. 17, pp. 601-640. [5] Busso, E.P., A Crystallographic Formulation for Superalloy Single Crystals with Explicit Microstructural Length Scales. Part I: Model Formulation. Submitted for publication. [6] ABAQUS V. 5.8 (1999), Hibbitt, Karlsson and Sorensen Inc., Providence, RI. 6
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