Abstract

The development of multiaxial laminates consisting of layers of collimated, high performance fibers in a polymer matrix provided the opportunity to design material properties for each application. The earliest analytical methods for determining stresses in the individual laminae assumed linear strain variation through the laminate thickness (Kirchhoff assumption). For much of the design work with composite laminates this simplification was both appropriate and convenient. Yet in 1969 it was postulated by the leading engineers and scientists (such as Dr. N.J. Pagano and Dr. J. E. Ashton) that a planar state of stress within each laminae could not exist near a laminate free-edge. The argument rested on the fact that inplane shearing stresses, induced in the angle-ply laminae of a laminate under normal external loads, could not exist at the free-edge.

Numerous investigators began analyses of the three-dimensional stress state in finite-width laminates including Pagano, Pipes, Puppo and Evenson. Several important developments came for this early work. First, the interlaminar load transfer mechanism between laminae in finite-width laminates was fully articulated as a boundary-layer phenomenon and the influence of stacking sequence on the sign of the interlaminar normal stress for bidirectional laminates was discovered. The roles of laminate material anisotropy and geometric variables such as laminae-to-laminate thickness and laminae-to-laminate width were clarified. A singularity in the interlaminar normal stress at the free-edge was postulated in this early work.

The development of three-dimensional and anisotropic finite-element methods and increases in computation power lead to the further investigation of the afore mentioned phenomenon and to the ability to analyse the three-dimensional state of stress occurring at numerous geometric discountinuities typically found in composite structure. These analyses have confirmed many of the findings of the earlier work and provide powerful tools for investigations required in the future.

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