Analysis and verification of the degree of bending isotropy of thin laminated composites manufactured using non-crimp fabric

Abstract

Laminated composites generally provide anisotropic stiffness properties depending on their stacking sequences. These anisotropic properties are not desired for certain applications that are subjected to multi-directional loadings requiring optimal stiffness properties in all directions. As a result, this paper aims to present a formulation for assessing the degree of isotropy of thin laminated composites. The starting laminate configuration chosen is a quasi-isotropic laminate in which extensional isotropy is inherently satisfied. As for the degree of bending isotropy, it can be quantified from a generalized eigenvalue problem of the bending stiffness tensor. A new lay-up strategy is proposed for thin-ply non-crimp fabric (NCF), and a parametric design study was conducted on quasi-isotropic NCF to maximize the degree of bending isotropy. Using mid-plane symmetric and non-symmetric laminate configurations, the maximum degree of bending isotropy is achieved for non-symmetric ones. A fully isotropic laminate (FIL) can be obtained when both in-plane and bending stiffness tensors are equivalent to the isotropic stiffness counterpart with null coupling stiffness. Starting with a quasi-isotropic 3-ply NCF as the building block, a non-symmetric fully isotropic laminate with 18 plies was obtained. Two laminate designs composed of NCFs with different degrees of bending isotropy were manufactured for experimental verification using three-point bending testing, and the results validate a direct relation between the degree of bending isotropy and the flexural stiffnesses at multiple directions.