STP811: The Role of Microplastic Deformation in Fatigue Crack Initiation

    James, MR
    Member Technical Staff, Rockwell International,

    Morris, WL
    Member Technical Staff, Rockwell International,

    Pages: 25    Published: Jan 1983


    Abstract

    A reference gage technique is used to measure the localized plastic strains which develop within individual surface grains during cyclic loading of A1 2219-T851. These are found to increase more rapidly in grains having large distances of slip between grain boundaries in the plane of the surface. Further fatigue causes cyclic hardening of the surface in the large grains as manifest by a reduction in the microplastic deformation. Maximum values of the local plastic strain and the cycles needed to achieve them are observed to be a function of environmental humidity and of internal hydrogen content.

    Humid air is found to decrease the development of microplasticity when compared with fatigue in dry air. Increased quantities of internal hydrogen (although in contents less than 1 ppm) substantially increase the rate of development of microplasticity early during fatigue and later accelerates cyclic hardening of the surface. A model based on a critical fracture strain criterion is formulated and used to calculate the numbers of constituent particles in the alloy fractured during fatigue. The model satisfactorily predicts the enhancement of particle fracture in alloys that contain hydrogen or that are fatigued in dry air. We conclude that the principal effect of these environmental factors on particle fracture arises because they alter the propensity of the surface to undergo microplastic deformation.

    Keywords:

    fatigue, crack initiation, crack nucleation, particle fracture, microplasticity, microstructure, aluminum alloys, environmental effects


    Paper ID: STP45339S

    Committee/Subcommittee: E08.05

    DOI: 10.1520/STP45339S


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