STP1298: Strain Energy Density - Distance Criterion for the Initiation of Stress Corrosion Cracking of Alloy X-750

    Hall, MM
    Manager and Senior Engineer, Bettis Atomic Power Laboratory, Westinghouse Electric Corporation, West Mifflin, PA

    Symons, DM
    Manager and Senior Engineer, Bettis Atomic Power Laboratory, Westinghouse Electric Corporation, West Mifflin, PA

    Pages: 15    Published: Jan 1997


    Abstract

    A strain energy density-distance criterion was previously developed and used to correlate rising-load KC initiation data for notched and fatigue precracked specimens of hydrogen precharged Alloy X-750. This criterion, which was developed for hydrogen embrittlement (HE) cracking at 93°C, is used here to correlate static-load stress corrosion cracking (SCC) initiation times obtained at 360°C in deaerated water using smooth geometry, notched and fatigue precracked specimens. According to this initiation criterion, the onset of crack growth occurs when a critical strain energy density is attained at a microstructurally characteristic distance from notch and crack tips. In the previous HE study the characteristic distance for crack initiation was found to be one grain diameter, independent of notch radius, applied stress intensity factor and hydrogen level. For the current SCC study, initiation is shown to occur at a variable distance from notch and crack tips. The initiation site varies from very near the crack tip, for highly loaded sharp cracks, to a site that is one grain diameter from the notch, for lower loaded, blunt notches. The existence of hydrogen gradients, which are due to strain-induced hydrogen trapping in the strain fields of the notch and crack tips of the SCC test specimens, is argued to be responsible for variation in the crack initiation site. A variable crack initiation site for the SCC specimens is shown to be consistent with a fixed initiation site for the HE specimens by considering the source of the hydrogen, which is external to the specimen in the SCC study and internal to the specimen in the HE study.

    Keywords:

    SCC initiation, strain energy density criterion, hydrogen


    Paper ID: STP19960S

    Committee/Subcommittee: G01.99

    DOI: 10.1520/STP19960S


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