13th International Conference on Fracture June 16–21, 2013, Beijing, China For the specimens tested under sea water flow the crack initiated all around the specimen surface due to several large corrosion pits. The size of the pits depends on the time (i.e. the number of cycles); their diameter is smaller on the pre-corroded specimens (30 to 80 µm) than under sea water flow (50 to 300 µm) (Figures 6 and 7). Furthermore, a lot of small cracks, perpendicular to the loading direction (mode I), were observed at the surface of the specimen tested under sea water flow (Figure 6C); whereas such small cracks were not observed on both virgin and pre-corroded specimens. This is characteristic of a corrosion/cyclic loading interaction. Since a significant decrease of the fatigue strength has been observed due to corrosion pits (seen as geometrical defects), it is important to determine if the crack propagation dominates or not the total fatigue life compared with the crack initiation phase. Figure 6: Specimen with pre-corrosion, σa=370 MPa, Nf=4.37×108 cycles Figure 7: Specimen tested under sea water flow, σa=160 MPa, Nf=1.8×108 cycles. 4. Assessment of the crack initiation and propagation duration The experimental da/dN = f(ΔK) curve for the investigated steel is shown in Figure 9. This shows that in air the mode I (R= –1) stress intensity threshold for R5 steel is around 3.3 MPa√m and as usual the crack growth rate is faster under sea water flow than in air [15]. The fatigue crack propagation duration was assessed according to the work of Paris et al. [8, 9], based on the ParisHertzberg-Mc Clintock crack growth model da dN =b( ∆Keff E√b ) 3 and ( ∆Keff E√b )=1 at the corner, where E is the elastic modulus and b is the Burger’s vector. The good agreement of this equation with our experimental data and b=0.285 nm was shown in [12]. Since in our experiments at 20 kHz the measurement of Kop is not possible, in first approximation ΔKeff≈Kmax was assumed and no water interaction with the stress intensity factor was considered.
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