Abstract
The influence of dehumidified silicone and paraffin oils with kinematic viscosities varying from 5 to 60,000 cS has been examined on fatigue crack propagation in a lower strength bainitic steel. Crack growth data at both low and high load ratios are compared with previous results for environments of moist air, dry gaseous hydrogen and dry gaseous helium. It is found that at low load ratios, growth rates in oil exceed those in moist air below 10 −6 mm/cycle, yet are lower than in moist air above 10 −6 mm/cycle. Furthermore there is a small but definite trend of higher growth rates in the higher viscosity oils. Such observations are discussed and interpreted in terms of three mutually competitive mechanisms specific to dry viscous environments, namely suppression of moisture-induced hydrogen embrittlement and/or metal dissolution, minimization of oxide-induced crack closure, and the hydrodynamic wedging action of the oil inside the crack. A quantitative analysis for the effect of crack closure induced by a viscous medium is developed in Part II of this paper.
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