13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- only 2.5-27% of that of the uncorroded condition [28]. Sriraman [12] presents a simple integrated deterministic model for life prediction in a high-strength aluminum alloy subject to pitting corrosion under cyclic stresses. The overall corrosion-fatigue life is the sum of crack initiation and propagation. At higher stress levels, there is not enough time for pits to develop and hence failure is not associated with stress concentration at the base of a pit, whereas life prediction at low stress amplitude is possible using only pit growth times [13]. Dolley [18] interpreted the reduction in fatigue life depending upon the pre-corrosion time and in turn the initial pit size. Rokhlin [19] established an empirical relation to predict fatigue life N= Nth(d/h) -3/4. Yongming Liu [41] predicted the probabilistic fatigue life by using an equivalent initial flaw size (EIFS) distribution, which is independent of applied load level and only uses fatigue limit and fatigue crack threshold stress intensity factor. A method for estimation of the cumulative distribution function (CDF) for the lifetime is demonstrated to predict the lifetime, reliability, and durability beyond the range of typical data by integrating the CDFs of the individual RVs into a mechanistically based model [42]. 4. Corrosion fatigue testing To further illustrate the effect of corrosion pit on fatigue life, the test on pre-pitted in air and in corrosion solution of 3.5% NaCl were conducted. Fig.2 shows the S-N data for air and corrosion fatigue tests. It can be seen that the air fatigue P1200 samples have the longest fatigue lifetimes. The stress concentration factor of the pre-pitted samples, with a pit aspect ratio of 0.11, is around 1.5 [33]. This geometry of defect significantly reduces the fatigue life by over 60%. At 298 MPa, the air fatigue life of the pre-pitted sample is only 16% of that of the P1200 samples, while the corrosion fatigue lives are further reduced. The corrosion fatigue strength reduced from 279 MPa (in air) to 126 MPa (in 3.5 % NaCl) at 107 cycles. A previous study by Masaki et al [34] showed that the fatigue strength of pitted specimens for 316NG at 108 cycles is approximately half that of unpitted specimens, where the SCF of the pre-pit was assumed to be approximately 2, almost equivalent to the fatigue strength reduction factor. However, the present study shows that the fatigue strength reduction factor is much greater than the stress concentration factor. The corrosion pits have smaller stress concentration factor than the pre-pit due to their smaller depth [33], implying that pre-pitted samples having longer fatigue lives than initially-smooth samples that develop pits within a corrosive solution. Furthermore, the smooth samples in 3.5%NaCl have shorter fatigue lives than the pre-pitted samples in air, indicating that electrochemical effects, i.e., localized corrosion, has a greater effect on fatigue life than mechanical effects, especially as stress levels fall below the in-air fatigue limit. 5. Modeling of corrosion pit development As literatures stated above that the corrosion pits can be simplified as semi-elliptical pits. Cracks originates from pits where the SCF is the biggest. To calculate the SCF around pits, a 3-D model is developed on a round bar under uniaxial tension and bending loading by using FEM. The 3-D model has various pit diameter (2c) and depth (a) ranging from 80 to 1000 μm. A total of 82878 finite elements and 118406 nodes are employed.
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