13th International Conference on Fracture June 16–21, 2013, Beijing, China Figure 3: S-N curves of the R5 steel under fully reversed tension at 20 kHz. The fatigue tests were carried out at 20 kHz under corrosive environment and may be seen first as non representative experiments because of the high loading frequency compared with the characteristic time of the corrosion process. However the results of our experimental data are not so far from the recommendations provided by the International Classification Societies of Off-shore Systems (for design purpose the fatigue strength at 108 cycles in sea water should be around 30 MPa). The competition between cyclic loading and corrosion is discussed later in this paper. 3.2. SEM observation of the fracture surfaces On virgin specimens, even if some unusual internal crack initiations were observed (Figure 5), most of the fatigue cracks initiated at the specimen surface over the cycle range (106 – 109 cycles). Surface defects were the origin of the cracks for non-corroded specimens and corrosion pits for both pre-corroded specimens and specimens tested under sea water flow (Figures 6 and 7). Figure 5: Fracture surface of R5 steel virgin specimens (a) Internal crack initiation σa= 380 MPa, N= 2.78×106 cycles, (b) surface crack initiation σa=395 MPa, Nf =5.61×108 cycles.
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