STP1298: A Process Model for the Initiation of Stress-Corrosion Crack Growth in BWR Plant Materials

    Akashi, M
    Senior research engineer and research engineer, Research Institute, Ishikawajima-Harima Heavy Industries Co, Ltd., Tokyo,

    Nakayama, G
    Senior research engineer and research engineer, Research Institute, Ishikawajima-Harima Heavy Industries Co, Ltd., Tokyo,

    Pages: 15    Published: Jan 1997


    Abstract

    The process of initiating the stress-corrosion crack growth comprises following six elementary processes; namely, (I) the incubation period, (II) the process of nucleating the corrosion pits (or corrosion crevices), (III) the process of the growth of pits (or crevices), (IV) the process of initiating microcracks, (V) the process of the propagation of microcracks, (VI) the process of coalescing the microcracks, leading into the process of steady propagation of the main crack. The processes I, III, and V fall into the category of the deterministic process, while the processes II, IV, and VI the stochastic process. This paper deals with the last three stochastic elementary processes by analyzing the stress-corrosion cracking behavior for each stage, and examines the models developed thereby to predict the initiation life of each. Following observations and conclu-sions have been made: (1) the critical pit depth to initiate the microcrack is approximately 20 μm for the carbon steel, and the micro-cracks are non-propagative semicircular cracks of approximately 50 μm in depth; (2) the processes II of the pit nucleation, IV of the microcrack initiation, and VI of the microcrack coalescence can all be represented to the Poissonian stochastic process; (3) inasmuch as the total crack-initiation life is determined by a stochastic process comprising these Poissonian processes concatenated in series, the probability distribution of the life leading to the onset of steady propagation of a main crack can be represented by the exponential distribution; (4) the applied stress exerts its influence mainly on the rate of proliferation of microcracks, but hardly on the propagation of microcrack, critical pit depth, and the probability of the pit's gener-ating microcracks; and (5) inasmuch as the proliferation rate of microcrack is represented by a linear function in applied stress, the distribution lower limit of the main-crack initiation life is in inverse proportion to the applied stress.

    Keywords:

    stress-corrosion cracking, boiling water reactor, initi-ation process, crack coalescence, Poissonian stochastic process, exponential distribution, life prediction


    Paper ID: STP19959S

    Committee/Subcommittee: G01.99

    DOI: 10.1520/STP19959S


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