Abstract
Phase one report: Crack fields (colonies) in dents are often observed in liquid pipe lines. Because of their colonial appearance, these cracks in dents are often thought to be associated with stress corrosion cracking (SCC). However, a recent full-scale dent fatigue testing under a PRCI mechanical damage program showed that crack colonies in dents can be produced by fatigue. This observation facilitated PRCI to launch a further study of the cracking mechanism in dents using samples extracted from a liquid pipeline. A total of 6 pipe samples containing dent with crack/metal loss were investigated. Evidences from this investigation showed that cracks in dents are aligned in an axial direction with appearance similar to crack colony. Fractographic analyses showed that the mechanism for cracking in these dents was fatigue. No evidence of stress corrosion cracking (SCC) was found. Fractographic analyses also showed that cracks in the colony were associated with a corrosion pit, suggesting corrosion pits are the initiation sites for fatigue crack. A combination of corrosion pitting and fatigue crack growth is the overall mechanism for the observed cracking, that is, corrosion may be the first degrading mechanism followed by the fatigue crack growth. Based on the understanding of the mechanism for cracking, a review is given to the currently available pit-to-crack transition and overall life prediction models. Applicability and limitations of these models to cracks in dent are discussed. Gaps and areas for further study are discussed. An example of rate competing between pit and crack growth and for overall life estimate is illustrated. In this report, sample selection and the approach used in this investigation are presented first. The findings from fractographic analysis are summarized. Currently available modelling efforts for pitto-fatigue are reviewed. Gaps and further research areas are discussed.
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