A PREDICTIVE APPROACH TO THE IN-PLANE MECHANICAL PROPERTIES OF STITCHED COMPOSITE LAMINATES

This contribution attempts to model the alteration of the in-plane elastic properties in laminates caused by stitching, and to predict the in-plane effective tensile strength of the stitched composite laminates. The distortion of in-plane fibers is considered to be the main cause that affects the in-plane mechanical properties. A fiber distortion model is proposed to characterize the fiber misalignment and the fiber content concentration due to stitching. The undistorted region, the fiber distortion region, the resin-rich pocket and the through-thickness reinforcement section are taken into account. The fiber misalignment and inhomogeneous fiber content due to stitching have been formulated by introducing two parameters, the distortion width and maximum misalignment. It has been found that the ply stress concentration in stitched laminates is influenced by the two concurrent factors, the stitch hole and inhomogeneous fiber content. The stitch hole brings about the stress concentration whereas the higher fiber content at the local region induced by stitching restrains the local deformation of the composite. The model is used to predict the tensile strength of the [0/45/0/-45/90/45/0/-45]58 T300/QY9512 composite laminate stitched by Kevlar 29 yarn with different stitching configurations, showing an acceptable agreement with experimental data.

In-Plane Elastic Properties of Stitched Plain Weave Composite Laminate

A representative volume element method and a novel mesomechanical-based polyline model are proposed to describe the misalignment of in-plane fibers induced by the insertion of stitch thread.A multi-scale mathematical model of in-plane elastic parameters for stitched composite laminate is established with ply-angle and stitch parameters as well as material parameters taken into account.Based on the fabrication of specimens and the verification of experimental platform,the superposition influences of stitch on structural anisotropy are revealed by the developed theoretical model.Results indicate that the stitch orientation can increase the structural anisotropy.The decreases of stitch pitch and spacing as well as the increase of thread diameter obviously reduce the elastic and shear moduli of laminates.Furthermore,the elastic and shear moduli as well as Poisson’s ratios show sinusoidal changes with a period of 90°as the ply-angle increases.The theoretical model not only analyzes the in-plane mechanical properties of stitched laminate with ply-angle,but also lays a foundation for the dynamic studies of stitched sandwich structures with ribs in the future.

Experimental,analytical and numerical investigation on tensile behavior of twisted fiber yarns

Stitched composite materials are emerging as a promising material due to their high interlaminar strength,combined performance and light weight.The mechanical properties of stitch yarns are very essential for stitched composite structures.In this study,the tensile behaviors of the twisted fiber yarn in stitched composites were investigated experimentally,analytically and numerically.Two kinds of cross-sectional area of twisted yarn are proposed and discussed.The paper presents an intersecting circle model to describe the cross-section of twisted fiber yarns,and a physics-based theoretical model to predict the effective tensile moduli.The numerical models take into account the cross-sectional characteristic and the twist architecture.The investigation shows that:the sum of each fiber area should be used for experimental analysis;and the crosssectional area surrounded by the yarn profile should be used for theoretical predictions and finite element(FE)simulations.The relative errors of the prediction method and the FE simulation are less than 2%and 1%,respectively.The friction between the fibers is derived,and the effect of friction on mechanical properties is discussed.The investigation method will serve as a fundamental component of twisted fiber bundle/yarn analysis.