Evaluation of Fatigue Strength of Alloy Steel Pipe Under Influence of Hydrostatic Pressure

Abstract

The influence of hydrostatic pressure on the performance of the material is becoming increasingly important in several applications such as ocean engineering, nuclear engineering and petroleum engineering. It is required to meet the increasing demand to analyze the influence of hydrostatic pressure under fatigue load. The magnitude of hydrostatic pressure increases with increase in depth of the water from surface level. For example at a depth of 2 km from water surface, 200 bar hydrostatic pressure will act on the submerged body. In addition to that when components are used in subsea environment, corrosion will occur in the component. So it is necessary to determine what will be the effect on fatigue life of the components which are subjected to hydrostatic pressure with fatigue load in a corrosion environment. This analysis will be taken into consideration while designing machine element which is used for underwater application. 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 alloy steel pipe under ambient conditions and under hyperbaric pressure conditions. To generate fatigue strength data under the influence of hydrostatic pressure, experiment has been done in MTS machine setup which consists of high-pressure chamber assembly and computer-controlled servo-hydraulic test system for applying cyclic load and differential hydrostatic pressure. Here both external and internal pressures are applied with the axial load to the specimen pipe. The S–N curve for alloy steel pipe (ASTM 106 grade B) is evaluated for two differential hydrostatic pressures at 10 bars internal with 30 bar external and 30 bar internal with 50 bar external pressure which are applied to the test specimen pipe.