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The effect of laminate stacking sequence and lamination angle on the notched strength of 6-ply graphite/epoxy laminates is investigated. Data is presented for the three different stacking sequences of a 6-ply laminate comprised of two unidirectional plies and two pairs of angled plies: [±θ/0]s, [0/±θ]s, and [+θ/0/ −θ]s for θ equal to 15°, 30°, 45°, 60°, 75°, and 90°. Notch geometry is limited to circular holes of which four different diameters were used: 3.175 mm, 6.35 mm, 9.525 mm, and 12.7 mm. Five specimens of each type (hole diameter and laminate) were tested monotonically to failure in tension. In addition, five unnotched specimens of each laminate were also tested. The fracture stresses of the notched specimens are correlated using the method proposed by Mar and Lin and a modified version of the point stress criterion of Whitney and Nuismer. The data shows that there is a distinct stacking sequence effect on notched fracture stress. The fracture stresses of the notched [+θ/0/ −θ]s laminates differ significantly from those of the [±θ/0]s and [0/±θ]s laminates that are generally the same for a given laminate and hole diameter. It is suggested that these differences are caused by out-of-plane effects of interlaminar stresses (delamination) or the elastic constraint of neighboring plies or both. These factors change as the stacking sequence and lamination angle are varied.
composite materials, notch sensitivity, fracture strength, delamination, composite fracture parameter, stacking sequence
Draper Assistant Professor of Aeronautics and Astronautics, Technology Laboratory for Advanced Composites, Massachusetts Institute of Technology, Cambridge, MA