STP893: Fractographic and Microstructural Examination of Compression Failures in Wet Compression Graphite/Epoxy Coupons

    Kar, RJ
    manager, Materials and Processes Laboratories, Advanced Systems Division; and manager, Microstructural and Chemical Analyses Laboratories, Aircraft Division, Northrop Corporation, Hawthorne, CA

    Herfert, RE
    manager, Materials and Processes Laboratories, Advanced Systems Division; and manager, Microstructural and Chemical Analyses Laboratories, Aircraft Division, Northrop Corporation, Hawthorne, CA

    Kessler, RT
    manager, Materials and Processes Laboratories, Advanced Systems Division; and manager, Microstructural and Chemical Analyses Laboratories, Aircraft Division, Northrop Corporation, Hawthorne, CA

    Pages: 18    Published: Jan 1986


    Abstract

    AS1-graphite/3501-6 epoxy composites exhibit degradation in mechanical properties during testing at elevated temperatures, with laminate moisture contents of 1% or greater. To investigate the effects of environmental variables on the compressive properties, a fractographic and microstructural study was carried out on multi-ply graphite/epoxy coupons overloaded in compression at different temperatures and with varying moisture contents. The analytical techniques used consisted of macroscopic and microscopic fracture examination, optical metallography, and quantitative image analysis (stereology). It was determined that coupons tested at room temperature and low moisture had macroscopic and microscopic fracture characteristics that differed from the characteristics of similar coupons tested at elevated temperatures and higher moisture contents. Microstructural analysis of the fractured coupons helped establish the failure progressions through the laminates, and in conjunction with image analyses, indicated that there was excess resin in 0° orientation plies of the coupons examined. It was concluded that (1) at room temperature, the matrix did not appear to play a role, that is, catastrophic fracture initiated in the outer plies of the composite due to gradual loss of stiffness during overload; and (2) softening of the resin (matrix), due to testing close to the glass transition temperature, was instrumental in controlling the fracture behavior at elevated temperatures, and the resin-rich areas provided increased susceptibility to moisture absorption that led to general weakening of the composite.

    Keywords:

    composite materials, graphite/epoxy, AS1/3501-6 composites, fracture (materials), compression, elevated temperature, microstructure, image analysis


    Paper ID: STP35346S

    Committee/Subcommittee: D30.04

    DOI: 10.1520/STP35346S


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