Tensile Stiffness and Strength of Regular Braid Composites: Correlation of Theory with Experiments

This paper investigates the tensile behavior of plain regular braided fabric reinforced composites subjected to uniaxial load. An experimental program is performed to characterize the stiffnesses and strengths of a number of braid composites. Two different material systems, i.e., carbon/epoxy and glass/epoxy, were investigated in this study, each with three different braiding angles. A theoretical approach, based on a bridging micromechanics model, is employed to predict the tensile properties of the braid composites only using monolithic fiber and matrix properties and the fabric geometric information as input parameters. These parameters are easily obtainable before or after composite fabrication, and determination of them is described in the paper. Unit cell geometry of the braided fabric in the composite was represented by either elliptic or sinusoidal cross section combined with the same undulation function, and a comparative study has been performed. After the unit cell of the braid composite has been divided into slices and the bridging model has been applied, an assemblage based on iso-stress or iso-strain assumption was adopted to obtain the overall properties of the composite. Although both the assumptions give reasonable predictions for the stiffness of glass/epoxy braid composites, significant differences exist between the predictions from the iso-stress approach and those from the iso-strain approach for the strength of the glass/epoxy composites and for the stiffness and strength of the carbon/epoxy composites. The iso-strain approach combined with the elliptic geometric description exhibits the best accuracy, and the predicted stiffnesses and strengths for the two material systems thus obtained are all within 13% discrepancy with the experimental data.