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Quasi-static tension tests were conducted on AS4/3501-6 graphite epoxy (02/θ2/-θ2)s laminates, where θ was 15, 20, 25, or 30°. Dye penetrant-enhanced X-radiography was used to document the onset of matrix cracking in the central - θ° plies and the onset of local delaminations in the θ/-θ interface at the intersection of the matrix cracks and the free edge. Edge micrographs taken after the onset of damage were used to verify the location of the matrix cracks and local delaminations through the laminate thickness.
A quasi-3D finite element analysis was conducted to calculate the stresses responsible for matrix cracking in the off-axis plies. Laminate plate theory indicated that the transverse normal stresses were compressive. However, the finite element analysis yielded tensile transverse normal stresses near the free edge. Matrix cracks formed in the off-axis plies near the free edge where in-plane transverse stresses were tensile and had their greatest magnitude. The influence of the matrix crack on interlaminar stresses is also discussed.
composite material, graphite epoxy, delamination, matrix crack
Directorate, NASA Langley Research Center, Hampton, VA
Wichita State University, Wichita, KS