Effect of Hydrostatic Pressure on Corrosion-Fatigue at High Frequency

Abstract

An experimental investigation into the effect of hydrostatic pressure on corrosion-fatigue at a frequency of 20,000 cycles per sec is reported for a number of materials currently considered for use in the ocean environment. The high-frequency fatigue technique used for this study is based on the principle of producing longitudinal vibrations in a properly designed fatigue specimen at its resonant frequency. This, in turn, produces maximum uniaxial alternating strains at the specimen nodal point. The fatigue specimens are tested in a uniquely designed pressure chamber containing synthetic seawater under constant temperature. The investigation shows that the corrosion-fatigue life of some of the materials tested is significantly reduced when they are subjected to a hydrostatic pressure of 2000 psi as compared to that experienced at atmospheric pressure. It appears that the mechanism of the influence of hydrostatic pressure is primarily chemical in nature. This is evidenced by scanning electron microphotographs of specimens fatigued under hydrostatic pressure which reveal that the fracture surface is rougher and contains multiple crack zones, greater substructure secondary cracking, deeper rivering arrays, and innumerable crack branching. These features are not prevalent in specimens tested at atmospheric pressure.