13th International Conference on Fracture June 16–21, 2013, Beijing, China Figure 10: a) schematic illustration of the test principle of a cylindrical specimen with constant cross section under sea water flow, b) evolution of the stress amplitude along the longitudinal axis of the cylindrical specimen for a stress amplitude of 120 MPa in the central cross section, c) and d) optical picture of the specimen surface after 7.37×107 cycles in area 8 and area 12 loaded at different stress amplitudes. Figure 11: corrosion pits at the surface of a specimen under sea water flow during 13 h 53 min 20 s without any cyclic loading. Further investigations should be carried out to understand the physical phenomenon involved in the VHCF damage process assisted by corrosion. One may imagine ultrasonic fatigue tests with potentiostatic measurements to monitor corrosion current and potential evolution during a fatigue test. However, not immerged experiments (to avoid cavitation) is a big experimental difficulty for carrying such tests. Investigation of the corrosion pitting kinetics under cyclic loading is probably a key factor for a better understanding of fatigue corrosion process. In situ micro-tomography under both ultrasonic cyclic loading and sea water corrosion may an interesting way for future researches. 6. Conclusion and prospects Very high cycle fatigue tests were carried out up to 109 cycles on smooth specimens in hot rolled martensitic-bainitic steel under three different conditions (i) virgin specimens, (ii) pre-corroded specimens and (iii) under artificial sea water flow during the fatigue test. The fatigue strength at 108 cycles is significantly reduced by a factor of 74% compared to the virgin specimens and of 71% compared to the pre-corroded ones. The poor corrosion – fatigue strength is related to the size of the pits. The assessment of the crack growth shows that crack initiation dominates the total fatigue life when N>107 cycles. A strong coupling between environment and cyclic loading has been proven. Under cyclic loading, even at ultrasonic frequency, fatigue damage and corrosion pitting are interacting for creating crack initiation at very low stress amplitude where no fatigue crack initiates
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