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(25/-25/90n)s, n = 1/2, 1, 2, 3, 4, 6, 8, and (252/-252/902)s T300/934 graphite/epoxy laminate coupons were tested in tension and examined periodically by di-iodobutane (DIB) enhanced X-radiography following progressively larger increments of applied load. The tensile strain required to initiate transverse cracking in the 90-deg ply was found to depend on the thickness of the 90-deg layer. For n ≤ 3 delamination was initiated at the edge of the coupon. The onset of delamination occurred before or after transverse cracking, depending on the laminate construction. For n ≥ 4 delamination at the 25/90 interface was initiated across the entire width of the coupon by the formation of a transverse crack in the 90-deg ply. In some laminate constructions the location of specimen separation during final fracture was associated with the growth and coalescence of edge delaminations under either incrementally increasing or constant strain cyclic loading. Comparisons of experiments to the fracture sequence predictions by stress and energy analysis are made to assess our ability to account for the ply thickness dependence of fracture in these laminates.
composite laminates, graphite-epoxy, fracture, delamination, transverse cracking, X-radiography, tension testing, composite materials
Staff scientist, Lockheed Palo Alto Research Laboratory, Palo Alto, Calif.
Professor, Drexel University, Philadelphia, Pa.