A simple analysis using two-dimensional lamination theory combined with the ap propriate three-dimensional anisotropic constitutive equation is presented to show some rather surprising results for the range of values of the through-the-thickness ef fective poisson's ratio vxz for angle-ply laminates. Results for graphite-epoxy show that the through-the-thickness effective poisson's ratio can range from a high of 0.49 for a [90] laminate to a low of -0.21 for a [±25]S laminate. It is shown that negative values of vxz are also possible for other laminates.

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This paper presents the results of a zeroth-order solution for edge effects in angle-ply composite laminates obtained using perturbation tech niques and a limiting free body approach. The general solution for [±θ] laminates is applied to the special case of a [±45]s graphite/epoxy lamin ate. Interlaminar stress distributions are obtained as a function of the laminate thickness-to-width ratio h/b and compared to finite difference results. The solution predicts stable, continuous stress distributions, determines finite maximum tensile interlaminar normal stress σz, and provides mathe matical evidence for singular interlaminar shear stresses τ xz and τyz in [±45] graphite/epoxy laminates.

The dispersion of harmonic waves propagating in an arbitrary direction in a layered orthotropic elastic composite is studied. The full three-dimensional field equations of elasticity are considered, and the corresponding twelfth order characteristic determinant is examined. Numerical results are obtained and compared with the corresponding estimates of the new quotient method of Nemat-Nasser.

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Thermally induced transverse cracking in T300/5208 graphite-epoxy cross-ply laminates was investigated experimentally and theoretically. The six laminate configura tions studied were: [0/903] s, [02/902]s, [03/90] s, [90/03]s, [902/02] s, and [903/0]s. The thermal load required to initiate transverse cracking was determined experimentally and compared to theoretical prediction. Experimental results for the accumulation of transverse cracks, under cyclic thermal loading between —250°F and 250°F for up to 500 thermal cycles, are presented. The calculated in situ transverse lamina strength was deter mined to be at least 1.9 times the unidirectional lamina transverse tensile strength. All laminate configurations exhibited an increase in crack density with increasing thermal cycles.

An elasticity solution is utilized to analyze an orthotropic fiber in an isotropic matrix under uniform thermal load. The analysis reveals that stress distributions in the fiber are singular in the radial coordinate when the radial fiber stiffness (Crr) is greater than the hoop stiffness (Cθθ). Conversely, if Crr < Cθθ the maximum stress in the composite is finite and occurs at the fiber-matrix interface. In both cases the stress distributions are radically different than those predicted assuming the fiber to be transversely isotropic (Crr=Cθθ). It is also shown that fiber volume fraction greatly influences the stress distribution for transversely isotropic fibers, but has little effect on the distribution if the fibers are transversely orthotropic.

A criterion for predicting the direction of crack extension in orthotropic composite materials is presented. The criterion is based upon the normal stress and the anisotropic tensile strength on arbitrary planes about the tip of a crack. Results are ob tained, via finite element solutions, for: (a.) isotropic mixed mode fracture, (b.) cracks in unidirectional off-axis slotted composite tensile coupons and (c.) cracks in cross plied laminates. Comparisons are made with other theories and experimental results.