STP415: Application of a Double Linear Damage Rule to Cumulative Fatigue

    Manson, S. S.
    Chiefchiefresearch engineer, National Aeronautics and Space Administration, Cleveland, Ohio

    Freche, J. C.
    Chiefchiefresearch engineer, National Aeronautics and Space Administration, Cleveland, Ohio

    Ensign, C. R.
    Chiefchiefresearch engineer, National Aeronautics and Space Administration, Cleveland, Ohio

    Pages: 29    Published: Jan 1967


    Abstract

    The validity of a previously proposed method of predicting cumulative fatigue damage in smooth ƈ-in.-diameter specimens based upon the concept of a double linear damage rule is investigated. This method included simplified formulas for determining the crack initiation and propagation stages and indicated that each of these stages could be represented by a linear damage rule. The present study provides a critical evaluation of the earlier proposal, further illuminates the principles underlying cumulative fatigue damage, and suggests a modification of the original proposal. Data were obtained in two stress level tests with maraged 300 CVM and SAE 4130 steels in rotating bending. Two strain level tests were conducted in axial reversed strain cycling with maraged 300 CVM steel. The investigation showed that in most cases the double linear damage rule when used in conjunction with originally proposed equations for determining crack initiation and propagation predicted fatigue life with greater or equal accuracy than the conventional linear damage rule. An alternate viewpoint of the double linear damage rule is suggested. This requires that a limited number of simple two-stress level tests be run to establish effective fatigue curves for what may be defined as Phases I and II of the fatigue process. These fatigue curves may then be used in the analysis of any spectrum of loads involving as loading extremes the two stresses used for their determination. Only limited verification of the new method has been obtained to date, and it must presently be limited to the study of smooth, ƈ-in.-diameter specimens. However, it may be considered as a first step in the direction of eventually predicting the effect of a complex loading history on the life of more complex geometrical shapes.

    Keywords:

    fatigue (materials), crack propagation, cumulative fatigue damage, linear damage rule, maraging steels, steels


    Paper ID: STP47237S

    Committee/Subcommittee: E08.06

    DOI: 10.1520/STP47237S


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