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The high frequency fatigue technique originated independently by Mason and Neppiras was used to study the influence of corrosive environment on the fatigue of SAE 1020 steel. With this technique, uniaxial tensile and compressive stresses are produced at the node of a “tuned” test specimen vibrating at its resonant frequency. Tests were conducted at two frequencies, namely 14 000 and 20 000 Hz. At these frequencies the reduction in fatigue strength and fatigue life in corrosive environment was comparable to the value obtained in the more familiar low frequency fatigue tests. The fatigue strength is reduced by 50 percent in 5 h at high frequency, whereas the same reduction in strength requires about 300 h at low frequency. Furthermore, the corrosion fatigue life is reduced two orders of magnitude by the superposition of a hydrostatic pressure of 1000 psig. The high frequency fatigue data in noncorrosive environment support the two distribution interpretation of Swanson; however, the value of the Weibull shape parameter increases considerably in the corrosive environment and tends to become unique. This change in probability distribution of fatigue life is brought about in less than 5 min of exposure to the environment. The mean life is reduced from 2 × 108 cycles to 5 × 106 cycles in such a short exposure time. Internal friction is affected significantly in a relatively short exposure to corrosion fatigue. The frequency at which the maximum damping is produced also changes after exposure to corrosion fatigue, These results emphasize the importance of the microstructural aspects in the mechanism of corrosion fatigue.
corrosion, seawater corrosion, fatigue (materials), fatigue strength at , N, cycles, internal friction, relaxation (mechanics), stresses, hydrostatic pressure, grain boundaries, notch sensitivity, steels, fatigue tests, immersion tests (corrosion)
Professor of Mechanical Engineering, The Catholic University of America, Washington, D.C.,