Evaluation of fatigue strength of AISI 4340 steel under hydrostatic pressure

The presence of hydrostatic pressure in the range of 10 to 600 bar (typical of sea depths from 100 meters to 6 kilometers) and cyclic wave loading in off-shore structures and equipments operating in deep-water environment requires suitable criteria accounting for high hydrostatic load during design of such structural elements. The presence of corrosive species due to sea water and marine organisms also adds complexity to this problem. Plain fatigue data evaluated under ambient conditions or fatigue data evaluated in atmospheric conditions under soaked sea water environment may not be appropriate for the design of systems subjected to hydrostatic pressure. High hydrostatic pressure alters the yield criterion of materials. Thus, it becomes essential to generate the fatigue strength data of structural steel in pressurized simulated sea water (3.5% NaCl solution) conditions. This paper presents the comparison of fatigue life data evaluated for a structural steel material under ambient conditions and under hyperbaric pressure conditions. A special hyperbaric chamber was designed for this purpose and 3.5% NaCl solution was pressurized into the chamber by a manual pump. Mechanical fatigue loading was applied on the specimen in the presence of two pressures, viz., 30 bar and 50 bar hydrostatic pressure. The results suggest that hydrostatic pressure has a significant influence on fatigue life data of steel. The fracture surfaces were examined using a scanning electron microscope which suggests change in fracture mode from being pure ductile to mixed mode of fracture. Work is in progress to evaluate the strength degradation of welded joints subjected to hyperbaric fatigue.