The Effects of Atmosphere on Fatigue Crack Propagation

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Fatigue tests have been carried out on carbon steel specimens in air, in dry air and in argon as an inert atmosphere to observe the effects of atmospheres on the rate of crack propagation. Eddy current method was applied to measure the propagation of fatigue cracks continuously, and the followings have been made clear. (1) Moisture and oxygen in air appear to have small deleterious effects on the crack initiation. In the crack propagation period the effect of adsorption of oxygen is the most significant and the rate of crack growth remarkably decreases in an inert atmosphere. This may be due to the smaller reduction of surface energy in argon than in air because no chemisorption occurs on the freshly created surfaces at the crack tip in an inert atmosphere. (2) Under the high-low two step stresses fatigue tests, the crack remains dormant after the transition to the lower stress level owing to the residual stresses at the crack tip produced by the previous higher stresses. Under the two step atmosphere tests of argon and air, similar phenomena are observed to those under the two step tests, and the effects of atmospheres on crack rate are made clear.
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