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The strength distribution of 90-deg plies in the laminates (±θ/90n)s with θ = 0 deg and 45 deg and n = 2, 3, and 6 is described in the form of a two-parameter Weibull distribution. A common feature for both orientations of surface layers is that at decreasing 90-deg ply thickness the characteristic strength and shape parameter increase. The reason is that the surface layers suppress a growth of the defects in the 90-deg ply which are lying close to the interface with the surface layers. This causes the apparent shape parameter to increase from a value a = 6.4 for a thick 90-deg ply (which is comparable to the shape parameter determined on a unidirectional laminate) to a shape parameter of a = 10.47 for a thin 90-deg ply (n = 2) in case of the (02/90n)s laminate. Further, it is shown that crack formation in the 90-deg plies of the (±45/90n)s laminates is strongly influenced by the severe edge stresses. This causes the applied one-dimensional fracture strain criterion to be inaccurate for the (±45/90n)s laminates.
composite materials, graphite composites, cross-ply cracking, Weibull strength distribution, edge effect, carbon fiber reinforced plastic (CFRP)
Scientist, Institute for Materials Research, Cologne,