13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- effect of sour crude oil on the fatigue life of round notched specimen is pronounced in high stress region and tends to decrease as decreasing the stress. At nominal stress of about 470MPa the fatigue life in the sour crude oil decreased to about 10 % of the fatigue life in air. Decreasing the stress the effect of sour crude oil on fatigue life decreased. At nominal stress of about 200MPa specimen did not fail after 2x106 cycles. Fatigue life testing results on boxing fillet welded specimens in air and in sour crude oil are also shown in Fig 5. At axial stress of 150MPa specimen in sour crude oil did not fail even after 10 times of the number of cycles to fail in air. Macroscopic fracture surface observation for round notched bar specimen revealed that fatigue crack initiated at notched root as shown in Fig.6 a). It is apparent from SEM observation that corrosion pit was not observed at crack initiation area[Fig. 6 b)]. Brittle striation was predominant at crack propagation area [Fig.6 c)]. It is well understood that crack propagation rate of ship structural steels and line pipe steels is accelerated in sour crude oil environment[37,38 and 39]. Hydrogen effect on crack propagation rate of these steels and welded joints was drastically observed at highΔ K region. Cleavage fracture surface and brittle striation were predominant at crack propagation area. At axial stress of 196MPa a couple of small fatigue cracks initiated at notched root as shown in Fig.7 a).Figure 7.b) is one of the forced fracture surface from these cracks. The depth of the corrosion fatigue crack was 0.2mm and notched area was brown in color. But corrosion pit was not observed at fatigue crack initiation area [Fig.7 b)]. Ductile striation was predominant at crack propagation area [Fig.7c)]. This means that crack propagation rate was not affected by hydrogen in low stress region. After corrosion fatigue life test for 2393hrs crude oil exhibited less H2O content (340 to 210ppm), lower pH value(7.3 to 4.3) and more total S content than those before test. All these facts suggest that hydrogen (H2) was produced through the reaction of H2O and H2S in sour crude oil with steel. Thus, in high stress region it can be considered that atomic hydrogen (H) accumulated in the plastic zone at fatigue crack tip accelerate crack propagation rate and reduce the fatigue life of the specimen. In the low stress region, plastic zone size is small and hydrogen does not accelerate fatigue crack propagation rate. It is assumed that wedge effect [40] of crude oil retarded corrosion fatigue crack propagation rate of notched bar specimen and boxing fillet welded specimen. Corrosion fatigue mechanism in sour crude oil is schematically illustrated in Fig, 8. Corrosion fatigue behavior is a time dependent phenomenon. If corrosion pit formed at initiation area in Figure 5. S-N curves of round notched HT50(TMCP) specimen in air and sour crude oil environment[Ebara et al.[37]]
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