13th International Conference on Fracture June 16–21, 2013, Beijing, China Very High Cycle Fatigue Strength of a High Strength Steel under Sea Water Corrosion Thierry PALIN-LUC1)*, Claude BATHIAS2) 1) Arts et Metiers ParisTech, I2M, UMR CNRS, Universite Bordeaux 1, Esplanade des Arts et Metiers, 33405 Talence Cedex, France 2) Universite Paris X, LEME, 50 rue de Sevres, 92410 Ville d'Avray, France *Corresponding author: thierry.palin-luc@ensam.eu Abstract The effect of sea water corrosion on the gigacycle fatigue strength of a martensitic-bainitic hot rolled steel used for producing off-shore mooring chains for petroleum platforms was studied. Three different environments and conditions: (i) air; (ii) air after pre-corrosion, (iii) air under real time artificial sea water flow (in situ) were considered for testing smooth specimens under fully reversed tension between 106 and 1010 cycles. A drastic effect of corrosion was observed. For the fatigue life greater than 108 cycles, the fatigue strength is reduced by a factor more than 5 compared with non corroded specimens (virgin). Corrosion pits due to pre-corrosion, if any, or pits resulting from corrosion in real time during cyclic loading are the crack initiation sites. The calculation of the mode I stress intensity factor at hemispherical surface defects (pits) combined with the Paris-Hertzberg-Mc Clintock crack growth rate model showed that the fatigue crack initiation regime represents most of the fatigue life in the VHCF regime. Original additional experiments shew physical evidences that the fatigue strength in the gigacycle domain under sea water flow is mainly governed by the corrosion process with a strong coupling between cyclic loading and corrosion. Keywords Gigacycle fatigue, corrosion, steel, crack initiation, crack growth 1. Introduction Mooring chains for off-shore petroleum platforms are loaded in VHCF regime and in sea water environment. They are designed for 30 years that is around 109 cycles because of the ocean waves 24 hours a day. The aim of this study is to study the gigacycle fatigue strength of the low-alloy steel used for manufacturing such chains and the effects on this strength of both pre-corrosion and corrosion in sea water environment. Many studies carried out on steel and aluminum alloys in the gigacycle regime have demonstrated that there is not a fatigue limit for such metals beyond 107 cycles as was believed in the past [1, 2]. It has been shown that fatigue cracks initiate mainly at surface defects in the short fatigue life regime, but may shift to subsurface in the long life range [3]. Other studies have shown that defects like non-metallic inclusions, pores [4] or pits [5 - 7] are the key factors, which control the fatigue properties of metals in very high cycle fatigue (VHCF). Furthermore, in some works it has been proven that crack initiation dominates the total fatigue life of specimens in gigacycle fatigue [8, 9]. On the other hand, the influence of static deformation has been studied confirming that both passive and transpassive current densities on stainless steel reach a maximum in the course of growing plastic strain [10]. Other works have investigated the dissolution of chemical products in stainless steel with active, passive and intermediate potentials where it is presumed that dissolution is enhanced during cyclic loading [11]. But the effect of aqueous corrosion on the fatigue strength with regard to crack initiation in the VHCF regime is not very much studied in literature. In this regime the material is loaded in its elastic domain at the macroscopic scale. In this work the effect of sea water corrosion (under free potential) on the VHCF strength will be studied. After presenting the investigated material and the experimental conditions both for crack initiation tests and crack propagation tests, experimental results are presented and discussed with regard to the SEM observation of the fracture surface. It is shown that a drastic effect of sea water corrosion decreases the fatigue strength of the steel under sea water flow. The assessment of the crack growth helps us to show that crack initiation dominates the total fatigue life in the gigacycle regime.
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