13th International Conference on Fracture June 16–21, 2013, Beijing, China flow rate of 1.52 ml/h. After the pre-corrosion process the specimens were first chemically cleaned and then cleaned with emery paper to remove the oxide layer. Many corrosion pits were created by the salt fog (Figure 2) their diameter is about 30 to 80 µm. To carry out VHCF tests in sea water environment a special corrosion cell was designed (see [12] for details). To avoid any cavitation it was decided to test the specimens under sea water flow (without immersion). This is representative of the splash zone of the mooring chains. To do that a peristaltic pump creates a flow of sea water (100 ml/min) on two opposite sides of the specimen surface in the tested area (diameter 3 mm). The sea water used was the A3 standard synthetic sea water; its chemical composition in % weight is: 24.53% NaCl, 5.2% MgCl, 4.09% Na2SO4, 1.16% Ca2Cl, 0.695% CaCl and 0.201% NaHCO3. The pH of this solution is 6.6 and no electrical potential was applied between the corrosion solution and the specimens. The temperature of the sea water was 20 – 25°C (room temperature). Figure 2: a) Surface of a specimen after pre-corrosion in the salt fog chamber, b) zoom on a corrosion pit. 2.1.3. Fatigue crack growth tests Fatigue crack growth tests were carried out under fully reversed tension (R=-1) in mode I following a similar methodology than prescribed in ASTM E647 standard. Since with the ultrasonic fatigue testing device the specimen was loaded under displacement control, the range of the stress intensity factor ΔK was computed according to references [13, 14]: ∆K=Uo( E 1−ν2)√ π a Y( a/ w) (1) with Y(a/w)=0.635( a/w)+1.731( a/ w)2−3.979( a/ w)3+1.963( a/w)4 . U0 is the displacement amplitude imposed at the top of the specimen by the horn of the ultrasonic fatigue testing machine, E is the dynamic modulus and ν is the Poisson ratio of the material, Y(a/w) is a function depending on the specimen geometry, a is the crack length and w is the width of the specimen: 3. Experimental results and discussion 3.1. Fatigue crack initiation tests Figure 4 shows the S-N curves of the crack initiation tests. A decreasing of around 50 MPa of the fatigue strength for the specimens with pre-corrosion is observed compared to the specimens without any corrosion. The scatter of the fatigue strength of pre-corroded specimens is larger than for virgin ones. There is a drastic effect of sea water flow on the fatigue strength of the R5 steel in the VHCF fatigue regime. Indeed, for the specimens tested under sea water flow, the fatigue strength at 107 cycles is around 300 MPa, not far from the value for pre-corroded specimens (360 MPa), but for 3×108 cycles the fatigue strength is around 100 MPa only. (a) (b)
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