STP1230

    Delamination of Carbon/Epoxy Laminates Under Cyclic Mode II Loading

    Published: Jan 1995


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    Abstract

    The influence of fiber orientation and the location of a delamination within the laminate on the delamination behavior under fatigue loading has not yet been considered sufficiently. In this study, the growth of delaminations between plies of different fiber orientation and on different locations over the thickness of the laminate under cyclic Mode II loading was characterized using the end-notched flexure (ENF) test. Laminates made of HTA/6376, a toughened carbon/epoxy, with unidirectional and quasi-isotropic layups were tested. Experiments were performed in stroke control at a frequency of 5 Hz and a displacement ratio of 0.1. Delamination growth per fatigue loading cycle, da/dN, was related to the strain energy release rate (SERR) GII associated with the Mode II (forward shear) loading. G was determined by an experimental compliance calibration procedure, correlating observed crack length with measured specimen compliance. A complete characterization of the delamination growth from the threshold of detectability to unstable delamination growth is given. A Paris law was fitted to the data which describes the experimental results rather well. It was observed that delamination growth always occurred along plies of 0° orientation. Depending on the specimens' lay up and the location of the crack initiation insert, intralaminar crack growth occurred until the crack reached the next 0° ply. Delaminations then propagated from this point. Due to the coupling of deformation modes and the rather complex damage pattern, a mixed mode loading may occur. The resulting delamination growth curves were found to lie within or to the right (i.e., delamination growth is slower for a given SERR) of the scatter band found for the specimens with midplane delamination and 0° bounding plies, with only few exceptions. Specimens with delaminations close to the laminate's surface did not fail by delamination from the insert but due to bending or interlaminar shear close to the laminate midplane. An analytical expression for the distribution of the strain energy release rate, II, over the thickness of the specimen is derived which, within the rather large scatter that was observed, characterizes the SERR determined experimentally fairly well. The experimental results suggest that it may be sufficient to characterize delamination propagation behavior under cyclic Mode II loading in carbon/epoxy utilizing specimens with unidirectional layup and midplane delaminations. However, further investigations are needed to confirm this conclusion.

    Keywords:

    composite materials, carbon/epoxy, fracture mechanics, delamination, Mode II, end-notched flexure, fatigue, strain energy release rate, unidirectional layup, quasi-isotropic layup


    Author Information:

    Kussmaul, K
    Professor and director, research engineer, and senior research engineer, Staatliche Materialpruefungsanstalt (MPA), University of Stuttgart, Stuttgart,

    Alberti, Mv
    Professor and director, research engineer, and senior research engineer, Staatliche Materialpruefungsanstalt (MPA), University of Stuttgart, Stuttgart,

    Eisele, U
    Professor and director, research engineer, and senior research engineer, Staatliche Materialpruefungsanstalt (MPA), University of Stuttgart, Stuttgart,

    Schneider, T
    Research engineer, Dornier Luftfahrt GmbH, Friedrichshafen,


    Paper ID: STP14008S

    Committee/Subcommittee: D30.06

    DOI: 10.1520/STP14008S


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