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An experimental and analytical study was conducted on the influence of matrix cracking on delamination in (+ 15/90n/-15)s glass-epoxy laminates subjected to monotonically increasing tension loads. Four different layups are examined with the number of symmetric 90° layers varying from 0 to 3 (n = 0, 1, 2, 3). Experimental results show that local delaminations form at the intersection of matrix cracks in the + 15° plies and the free edge. A quasi-three-dimensional (Q3D) finite-element analysis is used to examine free-edge stresses in undamaged laminates. Q3D results show that: (1) in-plane transverse tensile stresses exist in the +15° plies near the free edges of all of the laminates and (2) only the interlaminar shear stress is high at the + 15/θ interface. Three-dimensional (3D) finite-element analyses are used to model a + 15° ply crack near the laminate free edge. The 3D analysis results indicate that large tensile interlaminar normal, as well as shear, stresses develop at the intersection of the matrix crack and the free edge. This suggests that the interlaminar normal stress plays a significant role in the failure of these laminates and emphasizes the importance of modeling matrix cracking in delamination analyses of composite structures.
composite materials, laminates, delamination, interlaminar fracture, matrix ply cracking, three-dimensional finite-element analysis, free-edge effect, glass-epoxy
Research engineer, McDonnell Douglas Helicopter Co., Mesa, AZ
Senior scientist, Aerostructures Directorate, U.S. Army Aviation Research and Technology Activity (AVSCOM), NASA Langley Research Center, Hampton, VA