13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- Figure 5. The COD measurements during the creep-fatigue hold period for cycles 9 and 62. The COD measurements were clearly higher for cycle 62 than for cycle 9. This is what would be expected, since at a later point in the test the crack would have grown causing a larger COD measurement. Also, for both cycles the slope is positive, indicating that the crack opening increased during the hold period. 4. Conclusions DIC was used to measure both the crack tip deformation and crack opening displacement during a 120 second hold period of a creep-fatigue crack growth cycle. The crack tip strain measurements can be used to determine crack tip creep rates during a creep-fatigue cycle. The results for both the strain measurements and the COD measurements showed that a positive slope existed over this 120 second period. The results shown herein are only for a very limited study, and it is expected that a greater understanding of the crack tip behavior could be gained by conducting these tests over many more cycles and different test conditions. Nevertheless these results suggest that creep and crack opening take place during the hold period studied for this material under these conditions and that they can be measured using the DIC system. Acknowledgements The author would like to thank graduate students Jason Ray, Milica Damjanovic, and Alex McCool for assistance in this work. Also, the in-kind support from AAR Precision, specifically Alan Ballenger and Willie Michaely, for machining the samples, is very much appreciated. This research was supported by NSF Contract #: DMR-1151109. That support is gratefully acknowledged. References [1] K.O. Findley, J.L. Evans, A. Saxena, A critical assessment of fatigue crack nucleation and growth models for Ni- and Ni,Fe-base superalloys, Int Mater Reviews, 25 (2011) 49-71. [2] Evans, J.L. and Saxena, A., Elevated temperature crack growth rate model for Ni-base superalloys, in: Proceedings of the 12th International Conference on Fracture, Ottawa, Canada, July 12-17, 2009. [3] J.L. Evans, Method for comparing the crack tip kinetics during creep-fatigue loading of nickel-base superalloys, Mater Sci Eng, A, 528 (2011) 5306-08. [4] E. Andrieu, R. Molins, H. Ghonem, A. Pineau, Intergranular crack tip oxidation mechanism in a nickel-based superalloy. Mater Sci Eng A Struct Mater Prop Microstruct Process, A154 (1) (1992) 21-28.
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