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The effect of laminate thickness on the interlaminar stresses in rectangular quasi-isotropic laminates under uniform axial strain was studied. Laminates from 8-ply to infinitely thick were analyzed. Thick laminates were synthesized by stacking (45/0/−45/90) ply groups, rather than grouping like plies. Laminates with and without delaminations were studied.
In laminates without delaminations, the free-edge interlaminar normal stress distribution in the outer ply groups was insensitive to total laminate thickness. The interlaminar normal stress distribution for the interior ply groups was nearly the same as for an infinitely thick laminate. In contrast, the free-edge interlaminar shear stress distribution was nearly the same for inner and outer ply groups and was insensitive to laminate thickness. In laminates with delaminations, those delaminations near the top and bottom surfaces of a thick laminate have much larger total strain-energy-release rates (GT) and Mode I-to-total (GI/GT) ratios than delaminations deep in the interior. Therefore delaminations can be expected to grow more easily near the surfaces of a laminate than in the interior. This is consistent with experimental results reported in the literature. Also, near surface delaminations in thin laminates tend to have larger strain-energy release rates than corresponding near surface delaminations in thick laminates.
composite laminates, interlaminar stresses, thickness effects, stress analysis, delamination, strain-energy-release rates
Research Engineer, NASA Langley Research Center, Hampton, VA
Senior Scientist, Analytical Services and Materials, Inc., Tabb, VA