Mechanical performance of carbon-fibre braided composite tubes in tension and compression

This article examines the effect of braid angle on the mechanical performance of carbon-epoxy braided tubes in tension and compression. Vacuum-assisted resin transfer moulding is used to produce a variety of tubes with several combinations of 15◦and 20◦ braid angles. As uniaxial tensile testing of cylindrical tubes is not trivial, two tensile testing fixture designs are explored. It is found that a combination of mechanical and adhesive gripping produces repeatable fractures between the grips, with no slipping. Tubes with lower braid angles exhibit higher strengths both in tension and compression, as well as absorbing greater amounts of energy in compression.

The effect of loading rate on the in-plane shear strength of tri-axial braided composites

The in-plane shear strength of tri-axial braided composite materials was measured for three different braid angles (30°, 45°, and 60°) and two strain rates (0.001 s−1; 3 s−1) using the three-rail shear test. The in-plane shear strength was found to be sensitive to both—the braid angle and the strain rate. An increase of braid angle resulted in a reduction of shear strength, whilst an increase of loading rate resulted in an increase of shear strength of 8%–17%, depending on the braid angle.

A Modified Compact Tension Test for Characterization of the Intralaminar Fracture Toughness of Tri-Axial Braided Composites

The application of braided composite materials in the automotive industry requires an in-depth understanding of the mechanical properties. To date, the intralaminar fracture toughness of braided composite materials, typically used for describing post-failure behavior, has not been well-characterized experimentally. In this paper, a modified compact tension test, utilizing a relatively large specimen and a metallic loading frame, is used to measure the transverse intralaminar fracture toughness of a tri-axial braided composite. During testing, a relatively long fracture process zone ahead of the crack tip was observed. Crack propagation could be correlated to the failure of individual unit cells, which required failure of bias-yarns. The transverse interlaminar fracture toughness was found to be two orders of magnitude higher than the reference interlaminar fracture toughness of the same material. This is due to the fact, that intralaminar crack propagation requires breaking of fibers, which is not the case for interlaminar testing.

A novel synergistic multi-scale modeling framework to predict micro- and meso-scale damage behaviors of 2D triaxially braided composite

A novel synergistic multi-scale modeling framework with a coupling of micro- and meso-scale is proposed to predict damage behaviors of 2D-triaxially braided composite (2DTBC). Based on the Bridge model, the internal stress and micro damage of constituent materials are respectively coupled with the stress and damage of tow. The initial effective elastic properties of tow (IEEP) used as the predefined data are estimated by micro-mechanics models. Due to in-situ effects, stress concentration factor (SCF) is considered in the micro matrix, exhibiting progressive damage accumulation. Comparisons of IEEP and strengths between the Bridge and Chamis’ theory are conducted to validate the values of IEEP and SCF. Based on the representative volume element (RVE), the macro properties and damage modes of 2DTBC are predicted to be consistent with available experiments and meso-scale simulation. Both axial and transverse damage mechanisms of 2DTBC under tensile or compressive load are revealed. Micro fiber and matrix damage accumulations have significant effects on the meso-scale axial and transverse damage of tows due to multi-scale coupling effects. Different from existing meso-/multi-scale models, the proposed multi-scale model can capture a crucial phenomenon that the transverse damage of tow is vulnerable to micro fiber fracture. The proposed multi-scale framework provides a robust tool for future systematic studies on constituent materials level to larger-scale aeronautical materials.