13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- ( ) ( ) ( ) 0 0 0 21 2 sin 2 2sin 2 sin2 0, 0 ν θ β θ β θ β − − − − − = ≠ ⎡ ⎤ ⎣ ⎦ (4) ( ) I 0 II 0 sin 3cos 1 0 K K θ θ + − = (5) 2 II II 0 I II I II 155.5 83.4 K K K K K K θ ⎡ ⎤ ⎡ ⎤ = ° − ° ⎢ ⎥ ⎢ ⎥ + + ⎣ ⎦ ⎣ ⎦ m (6) Table 2. Prediction of crack growth deflection angles based on different models α Minimum strain energy density (SED) criterion [2] Criterion of Richard [4] Maximum tangential stress (MTS) criterion [1] Experiment 0 0 0 0 21° 30° 35° 28.6° 26.2° 25° 60° 55.26° 52.1° 48.98° 50° 5. Conclusions In this study, the growth behavior of fatigue cracks in the HAZ of a welded joint under mixed mode loading conditions were studied by utilizing CTS specimens. The FCG morphology was characterized by in situ SEM observations in vacuum. The main conclusions are listed as follows. (1) Mode I fatigue behavior in the HAZ was influenced by local microstructure anisotropy and the gradient distribution of material strength. The crack growth path tended to deflect into the lower strength weld metal, showing a “zigzag” mode. (2) With the increasing of mixity in mode I/II fatigue loadings, FCG rate in the HAZ was decreased whereas the initial angles for crack branching were increased. Shear and branch cracks were competitive along the crack path, and fatigue crack was often initiated from grain boundary and the interface of lath martensites. Eventually the cracks tended to orient themselves into a pure mode I direction. (3) The initial branch crack direction was predicted based on existed models, and calculation from the MTS criterion showed comparable agreement with experimental data. Acknowledgements The authors are grateful for the supports provided by National Natural Science Foundations of China (51205131) and the Natural Science Foundation of Shanghai (12ZR1442900). Ming-Liang Zhu would also appreciate the support from Chen Guang project (12CG33) by Shanghai Municipal Education Commission and Shanghai Education Development Foundation. References [1] F. Erdogan, G.C. Sih, On the Crack Extension in Plates Under Plane Loading and Transverse Shear, J Basic Eng, 85 (1963) 519-525. [2] G.C. Sih, Mechanics of Fracture Initiation and Propagation, in, Kluwer, The Netherlands, 1991. [3] R.J. Nuismer, An energy release rate criterion for mixed mode fracture, Int J Fract, 11 (1975) 245-250. [4] H.A. Richard, M. Fulland, M. Sander, Theoretical crack path prediction, Fatigue Fract Eng Mater Struct, 28 (2005) 3-12. [5] J. Qian, A. Fatemi, Mixed mode fatigue crack growth: A literature survey, Eng Fract Mech, 55 (1996) 969-990.
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