A logical framework for the analysis of enhancement of fatigue crack propagation by the ambient atmosphere

Abstract

It is shown that low subthreshold fatigue crack growth rates essentially depend on the environment. This is explained by the presence of water vapor because the influence of the environment cannot be adequately described by the closure of cracks in the presence of oxides. The effect of the environment was analyzed on the basis of the experimental data with regard for the adsorption of molecules of water vapor by the newly-formed surface near the crack tip which accelerates the process of crack growth but does not affect its fundamental mechanisms and the subsequent process of hydrogen-assisted growth of fatigue cracks. This type of behavior in air can be calledpure corrosion fatigue because it is induced by the joint synergetic action of stresses in the vicinity of the crack tip and aggressive media on the new-ly-formed surfaces. It cannot be described as a simple superposition of the ordinary mechanism of fatigue depending on loading cycles and the processes depending on the static component of loading and time and requires the complete description of the indicated processes. This is why the law of growth of fatigue cracks in the media is not established up to now. We suggest new promising directions of investigations; first of all, this is true for the analysis of fatigue damage in the prefracture zone under the influence of hydrogen. The most difficult problem is to clarify the factors responsible for fatigue damage and the processes accompanying the process of fatigue damage or being its inherent part. Deep under-standing of the process of growth of fatigue cracks in aggressive media is necessary for the analysis of the mechanisms of hydrogen-assisted fracture.