STP836: Characterization and Analysis of Damage Mechanisms in Tension-Tension Fatigue of Graphite/Epoxy Laminates

    Jamison, RD
    Assistant professor, Mechanical Engineering Department, U.S. Naval Academy, Annapolis, Md.

    Schulte, K
    Research scientist, Institut für Werkstoff-Forschung, DFVLR,

    Reifsnider, KL
    Professor, Department of Engineering Science and Mechanics Virginia Polytechnic Institute and State University, Blacksburg, Va.

    Stinchcomb, WW
    Professor, Department of Engineering Science and Mechanics Virginia Polytechnic Institute and State University, Blacksburg, Va.

    Pages: 35    Published: Jan 1984


    Abstract

    The mechanisms by which subcritical and critical damage develops in several lamination geometries of T300/5208 and T300/914C graphite/epoxy material during tension-tension fatigue were closely examined. A damage analogue in the form of stiffness reduction was used to provide a framework by which the sequence of damage development could be correlated with mechanical response. Stiffness reduction, measured continuously during the course of cyclic loading, was shown to provide a reproducible characteristic correlation with percent of life expended. The relationship was observed to differ markedly among lamination geometries, but for a given geometry was found to clearly indicate the partition of the mechanical response into distinct regions in these characteristic curves. These regions, moreover, were shown to be predominated by particular damage mechanisms—some already discussed in the literature, others less well-recognized.

    Results of the observed damage development sequence for cross-ply and quasi-isotropic laminates are presented along with a preliminary association between this damage and the characteristic stiffness reduction curves for these geometries. The geometries used were characterized by distinct, predominant, early subcritical damage conditions. This secondary and subsequent damage development was examined in relation to known, predictable beginning state. Of particular emphasis in each case was the role of this developing damage state in the fracture of fibers in the 0-deg plies.

    Damage detection and characterization was accomplished using both nondestructive and microscopic techniques. Two techniques proved to be of considerable utility: penetrant-enhanced stereo X-ray radiography and scanning electron microscopy of coupons taken from penetrant-enhanced deplied, damaged specimens.

    A number of significant damage conditions, not heretofore reported, were observed: the production of interior delaminations at the 0/90-deg interfaces of [0,902]s laminates by the gradual growth of longitudinal cracks in the 0-deg plies; the existence of a dense distributed microcrack condition at all distinct interfaces of [0,90,±45]s laminates; the segregation of 0-deg fiber breaks in all laminates into zones coincidental with cracks in the adjacent plies; and, the appearance of shear fracture in 0-deg fibers associated with the passage of longitudinal splits.

    Mechanisms for each of these damage conditions are proposed in terms of the micromechanics of the predominant damage condition with which they are associated and the global stress state.

    Keywords:

    composite materials, fatigue (materials), damage mechanisms, graphite/epoxy nondestructive evaluation, fibers, X-ray radiography, stiffness changes, delamination, microcracking, matrix cracks, fiber fracture, fracture mechanics


    Paper ID: STP30196S

    Committee/Subcommittee: E08.01

    DOI: 10.1520/STP30196S


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