13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- 5. Conclusion The main objective of this study was to investigate the effect of residual stresses on the fatigue crack growth behavior of a stainless steel alloy CA6NM welded joint. From the results, the following conclusions can be drawn. In as-welded specimens, many clues point to the fact that tensile residual stresses are present at the crack tip and are having an effect on the fatigue crack growth behavior. This hypothesis is supported by: (1) A higher effective load ratio indicated by the agreement between the mean R = 0.1 experimental base metal fatigue crack growth rate and the R = 0.7 base metal fatigue crack growth curve. (2) An effective stress intensity factor range ratio equal to unity for all crack lengths meaning that a crack opening mechanism is present. (3) A negative closure load on an as-welded specimen at a crack length of 20 mm. It is also believed that some tensile residual stresses remain after post-weld heat treatment, as evidenced by the effective stress intensity factor range ratio being superior to Newman’s prediction for plasticity-induced crack closure. In addition, an increase in crack closure when the crack is in the base metal points to the fact that the tensile residual stresses were relaxed with crack growth and that closure mechanisms become fully active. This study showed that tensile residual stresses, which are strongly believed to be present in the as-welded specimens, caused the crack to remain fully open, this having a detrimental effect on the fatigue crack growth resistance of CA6NM welds. Moreover, the fatigue crack growth resistance was found to be improved in the heat treated specimens compared to the as-welded specimens. This is attributed to the residual stresses relaxation after tempering and with crack growth, allowing closure mechanisms to become active and reducing the fatigue crack growth rates. This shows the importance of a good residual stress level control through post-weld heat treatment. Acknowledgements This study was made possible by the support of Alstom Hydro, Hydro-Québec and the National Science and Engineering Research Council of Canada (NSERC). The authors wish to thank Prof. Yves Verreman for his invaluable technical insights as well as technologists Carlo Baillargeon and Benedict Besner for their help with the fatigue testing procedure. References [1] M. Sabourin, D. Thibault, D. A. Bouffard, and M. Levesque, "New parameters influencing hydraulic runner lifetime," presented at the 25th IAHR Symposium on Hydraulic Machinery and Systems, Timisoara, Romania, 2010. [2] D. Thibault, P. Bocher, M. Thomas, M. Gharghouri, and M. Côté, "Residual stress characterization in low transformation temperature 13%Cr–4%Ni stainless steel weld by neutron diffraction and the contour method," Mat Sci Eng A-Struct, vol. 527, pp. 6205-6210, 2010. [3] É. Moisan, M. Sabourin, M. Bernard, and T. Bui-Quoc, "Residual stress measurements in hydraulic turbine welded joints," presented at the IAHR 23rd Symposium on Hydraulic Machinery and Systems, Yokohama, Japan, 2006. [4] Y. B. Lee, C. S. Chung, Y. K. Park, and H. K. Kim, "Effects of redistributing residual stress on the fatigue behavior of SS330 weldment," Int J Fatigue, vol. 20, pp. 565-573, 1998.
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