STP675

    Microstructural Aspects of the Threshold Condition for Nonpropagating Fatigue Cracks in Martensitic-Ferritic Structures

    Published: Jan 1979


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    Abstract

    A study has been made of the critical condition for the growth of microscopic fatigue cracks at the endurance limit from metallurgical considerations. The specimens employed have a microstructure with a matrix of ferrite and the second phase of martensite, the volume fraction of which is about 50 percent. All the non-propagating cracks nucleated in the ferrite matrix are stopped by the second-phase martensite at the endurance limit stress. The threshold condition for microscopic nonpropagating cracks in the ferrite matrix was investigated by restressing to slightly above the endurance limit. Subsequently it was found that the critical condition for the fatal growth of microscopic cracks into the second-phase martensite from the ferrite matrix can be evaluated by the linear theory of fracture mechanics. It is also recognized from the experiments that all the microscopic cracks in the ferrite matrix remain as nonpropagating unless the stress intensity factor at the crack tip exceeds the magnitude of the crack growth resistance of the second phase martensite, Kthm. Furthermore, experimental results show that the Kthm of the second-phase martensite is about 7.7 MPam. In addition, quantitative metallographic observations show that the critical length of the nonpropagating cracks at the endurance limits does not depend on the mean spacing of the second phase.

    Keywords:

    crack growth resistance, endurance limit, fatigue mechanism, linear fracture mechanics, martensite-ferrite microstructure, microscopic fatigue crack, nonpropagating crack, second phase spacing, threshold condition


    Author Information:

    Kunio, T
    Professor and assistant professor, Keio University, Yokohama, Japan

    Yamada, K
    Professor and assistant professor, Keio University, Yokohama, Japan


    Paper ID: STP35898S

    Committee/Subcommittee: E08.05

    DOI: 10.1520/STP35898S


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