Various alternative composite materials like, textile composites, especially woven, are being developed and tried in place of conventional multidirectional laminates, because they have better properties in mutually orthogonal directions and out of plane properties than the multidirectional laminates. In structural applications, predictions of the elastic modulii, Poisson’s ratios from the weave architecture and the properties of the constituents are required. There are various parameters that characterize the weave architecture of woven laminate composites. In repeated loading, the matrix cracking occurs, degrading matrix properties. This in turn degrades the effective properties of the woven composites. Analytical models are necessary to study the effects of these parameters on the behavior of woven fabric composites and to design efficient woven structure for particular application. The objective of the current paper is to study the effect of matrix cracking on the effective properties of the twill woven composites by degrading the matrix properties. First the effective properties of the composite material, without any matrix cracks, are determined by using an analytical model, which predicts the stiffness of the twill woven composites. This model takes into account effects of the actual fabric structure with various tow cross sections by considering tow undulations and continuity along both the fill and warp directions.
In twill woven composites, there is a weaker matrix along with the stronger medium i.e. the glass fibers. The matrix first fails and then the load is transferred to the fibers. The matrix properties are degraded in the resin pockets and also in the transverse tows only. This is because normally matrix cracking first occurs in the transverse tows (cracks originate in the tows that run in the transverse direction to the loading). The degradation of the properties includes the Young’s Modulus and the Poisson’s ratio of the resin. With these modified properties of the resin, the homogeneous properties of the transverse tows are calculated. At each degradation step the effective properties of the composite material are evaluated. This progressive failure is continued till the matrix loses most of its strength.