STP1186

    Proposed Framework for Thermomechanical Fatigue (TMF) Life Prediction of Metal Matrix Composites (MMCs)

    Published: Jan 1993


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    Abstract

    The framework of a mechanics of materials model is proposed for thermomechanical fatigue (TMF) life prediction of unidirectional, continuous-fiber metal matrix composites (MMCs). Axially loaded MMC test samples are analyzed as structural components whose fatigue lives are governed by local stress-strain conditions resulting from combined interactions of the matrix, interfacial layer, and fiber constituents. The metallic matrix is identified as the vehicle for tracking fatigue crack initiation and propagation. The proposed framework has three major elements. First, TMF flow and failure characteristics of in situ matrix material are approximated from tests of unreinforced matrix material, and matrix TMF life prediction equations are numerically calibrated. The macrocrack initiation fatigue life of the matrix material is divided into microcrack initiation and microcrack propagation phases. Second, the influencing factors created by the presence of fibers and interfaces are analyzed, characterized, and documented in equation form. Some of the influences act on the microcrack initiation portion of the matrix fatigue life, others on the microcrack propagation life, while some affect both. Influencing factors include coefficient of thermal expansion mismatch strains, residual (mean) stresses, multiaxial stress states, off-axis fibers, internal stress concentrations, multiple initiation sites, nonuniform fiber spacing, fiber debonding, interfacial layers and cracking, fractured fibers, fiber deflections of crack fronts, fiber bridging of matrix cracks, and internal oxidation along internal interfaces. Equations exist for some, but not all, of the currently identified influencing factors. The third element is the inclusion of overriding influences such as maximum tensile strain limits of brittle fibers that could cause local fractures and ensuing catastrophic failure of surrounding matrix material. Some experimental data exist for assessing the veracity of the proposed framework.

    Keywords:

    metal matrix composites, fatigue (metal), thermal fatigue, thermomechanical fatigue, bithermal fatigue, low cycle fatigue, high temperature fatigue, creep fatigue, life prediction, strainrange partitioning, crack initiation, crack propagation, thermal expansion


    Author Information:

    Halford, GR
    Senior scientific technologist and research engineers, NASA Lewis Research Center, Cleveland, Ohio

    Lerch, BA
    Senior scientific technologist and research engineers, NASA Lewis Research Center, Cleveland, Ohio

    Saltsman, JF
    Senior scientific technologist and research engineers, NASA Lewis Research Center, Cleveland, Ohio

    Arya, VK
    Resident research associate, University of Toledo, NASA Lewis Research Center, Cleveland, OH


    Paper ID: STP24256S

    Committee/Subcommittee: E08.05

    DOI: 10.1520/STP24256S


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