Multi-scale characterisation and damage analysis of 3D braided composites under off-axis tensile loading

This study investigates the influence of off-axis tensile loading (0°, 45°, 60° and 90°) on the mechanical behavior of 3D braided composites. Here, digital image correlation is utilized to characterize the full-field deformation and strain distribution in the composites. Scanning electron microscope and micro-computed tomography techniques are also employed to study the deformation and damage mechanisms in the fractured specimens. An off-axis stiffness prediction model for 3D braided composites is proposed, and a finite element model of braided composites containing porosity is established. A user-defined material subroutine is also developed to implement the damage model as a function of off-axis loading angle. The experimental and simulation results demonstrate that the application of off-axis loading causes significant variations in the spatial orientation angle (γ, φ) of the yarn. This phenomenon gives rise to two distinct stages in which the modulus of the composite initially increases, followed by a subsequent decrease. Furthermore, the dominant failure mechanisms in these on-axis samples are found to be yarn breakage and matrix cracking. Also, matrix cracks and filaments pull-out are observed in the off-axis samples.