Solution Chemistry Modification within Corrosion-Fatigue Cracks

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Abstract
There exists in the literature several observations indicating that the electrolyte within corrosion-fatigue cracks can become modified relative to the bulk solution. Since the rate of fatigue-crack growth should depend upon electrolyte chemistry near the crack tip, it is important that the conditions under which such modification occurs and the role of influential variables be recognized. This paper represents an initial analysis of how fatigue variables might influence mixing between the crack and bulk solutions. The rationale considers that such mixing is governed primarily by net momentum of the periodically exhausted and ingested electrolyte. It is shown that this momentum should be directly proportional to the crack opening angle, cyclic frequency, and the cube of crack length. Decreasing mean stress also contributes to a momentum increase, with the latter becoming large for stress functions that result in crack closure during a portion of each cycle. Other factors which are considered include temperature, stress-wave form, specimen geometry, test method, and applied current density. The significance of the projected trends is discussed within the frame of commonly employed corrosion-fatigue test procedures.