A meso-scale finite element-based model has been developed to evaluate the uniaxial and biaxial strength characterizations of three-dimensional five-directional braided composites. The representative volume cells are established based on the realistic geometry cross sections. A continuum damage mechanics model, which is implemented as a user-defined material subroutine in the ABAQUS commercial finite element code, is introduced to predict the mechanical responses and the failure locus under uniaxial and biaxial loadings. For the verification of numerical results, a series of tensile and compressive tests are conducted along the longitudinal and transverse directions. The results show that under uniaxial loading, the calculated max stress and the final damage morphology agree well with the experimental data. Moreover, it is found that the interaction effect between the axial stress and the shear stress is not apparent, whereas that between two axial stresses becomes significant. It is expected that such a numerical investigation will provide useful information for building a strength criterion of three-dimensional five-directional braided composites.

Microstructure of 3D five-directional braided composites determines its mechanical properties. Microscopic evidence of the microstructure reveals that the cross-sections of the axial yarn are variable along its central axial due to the lateral compression from the adjacent braiding yarns. On the basis of microscopic observations, a dense but non-intersecting geometry internal unit cell model of 3D five-directional braided composites is established. By using the unti-cell model, a 3D mesomechanical FEA is proposed to predict the effective elastic properties of 3D five-directional braided composites. Compared with existing model, present model is more effective to calculate the mechanical properties of 3D five-directional braided composites with more than 50% fiber volume fraction.

The effect of normal load on the frictional and wear behaviour of carbon fiber is investigated by simulating the tow-on-tool friction relevant to the beating-up motion of three-dimensional (3 D) weaving process. The true number of contact filaments over a range of normal loads is calculated by characterizing the cross-section parameters of carbon tow. The real contact area is calculated on the basis of the filaments amount by Hertzian contact model. The friction force values obtained from multiplying the real contact area with shear strength are closely with the measured results. The coefficient of friction increases with the increase of normal loads. When the normal load is 250, 400 and 600 mN, the tensile loss rate of the carbon tow after friction test is 6.3%, 23.2% and 42.4% respectively. The filaments reveal multiple fracture damage patterns which are caused by stretching, shearing and compression during the weaving process.

A study is conducted with the aim of evaluating the transverse compressive behaviors of three-dimensional five-directional braided composites based on the meso-scale representative volume cells. Finite element models with three kinds of interior braiding angles, 20°, 30°, and 45°, are established on the basis of the realistic cross-section shape of each yarn. A damage mechanics model, which is implemented by user-defined material subroutines in ABAQUS/Standard, is used to investigate the mechanical responses and the damage initiation/evolution throughout the transverse compression. For the verification of numerical results, a series of transverse compression tests are conducted. Numerical results indicate that the transverse compression strength of three-dimensional five-directional braiding composite is strongly dependent on the matrix properties, and is not very sensitive to the braiding angle. The predicted results show good agreement with the relevant experimental data, demonstrating the applicability of meso-scale finite element model.