Study of the fatigue strength in the gigacycle regime of metallic alloys used in aeronautics and off-shore industries

This work concerns the study of the gigacycle fatigue strength of two materials, the first one used in gearboxes of turbines for helicopters and the second one used in offshore chain lines (petroleum platforms). A cast aluminium alloy AS7G06-T6 is studied. This alloy is used in the frame of the gearboxes of helicopters turbines. It is submitted to very high number of cycles due to the working duration of the engine (3,000 hours) and the rotation speed of the turbine in operating conditions: around 30,000 rpm. Specimens from real components were tested at both room temperature and 150 °C in air, and at three different stress ratio s, to investigate both the temperature and the mean stress effect on the gigacycle fatigue strength. The fatigue strength at high number of cycles (109) and the crack initiation cause have been investigated. A hot rolled low alloy steel, designed R5 according to the international denomination of the International Classification Societies of offshore systems is also studied. This steel is used for manufacturing mooring chains for offshore petroleum platforms in the European North Sea. It is requested to have high strength, as well as good corrosion resistance. Indeed, chains are designed for 30 years, and they are loaded at low frequency (~0.5 Hz) due to the waves in sea water environment during long time (25 to 30 years) all the day and all the year long. This represents more than 108 cycles. Specimens from R5 steel chains were tested in fatigue at different R ratios and at three different conditions; pre-corroded specimens in air, and virgin specimens in air and under sea water flow simultaneously. Fatigue strength in the very high cycle regime and the causes of the crack initiation were investigated. In particular, it is looked for understand the effect of the defects on gigacycle fatigue strength. The fatigue failure in AS7G06-T6 cast aluminium alloy is due to porosities of manufacturing. For R5 steel, corrosion pits are the cause of fatigue failure. An assessment of the crack growth duration is proposed and shows that beyond to 10^7 cycles the fatigue life is dominated by the crack initiation stage and not by propagation.