STP601

    Fracture Toughness Testing of Glassy Plastics

    Published: Jan 1976


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    Abstract

    Despite widespread use of linear elastic fracture mechanics to describe brittle fracture in glassy plastics, prior work has not established a valid fracture toughness criterion based on the unique aspects of polymer mechanical behavior. In this study, fracture toughness tests on poly(methyl methacrylate) (PMMA) were correlated with crack-tip crazing and fracture surface morphology in order to assess the influence of dissipative processes and crack growth modes on standard toughness test variables. Various Kc criteria were examined for their relevance to PMMA, a material with a velocity dependence of fracture typical of glassy plastics. Since crack-tip crazing is restricted in this material, crack extension is essentially elastic for all normal specimen dimensions and precrack conditions. The load instability at the maximum of the load-displacement record is distinguished from the point of intrinsic instability, which is indicated by the K versus ˙a relationship. The load instability leads to a geometry-dependent Kc criterion which can be described analytically, and procedures for measuring a Kc based on intrinsic instability are defined for a standard fracture test. Measured values are in reasonable agreement with estimates derived from K versus ˙a in the literature. The implications of the geometric effects on instability are discussed for other materials and specimen geometries.

    Keywords:

    crack propagation, fracture (materials), fracture tests, plastics


    Author Information:

    Margolis, RD
    Manager of engineering and technical service, GRTL Company, Birmingham, Mich.

    Dunlap, RW
    Professor of engineering and public affairs, Carnegie Institute of Technology, and professor of mechanics and polymer science, Mellon Institute of Science, Carnegie-Mellon University, Pittsburgh, Pa.

    Markovitz, H
    Professor of engineering and public affairs, Carnegie Institute of Technology, and professor of mechanics and polymer science, Mellon Institute of Science, Carnegie-Mellon University, Pittsburgh, Pa.


    Paper ID: STP28656S

    Committee/Subcommittee: E08.08

    DOI: 10.1520/STP28656S


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