Numerical analyses of bending fatigue of four-step three-dimensional rectangular-braided composite materials from unit cell approach

This paper reports the numerical analyses of three-point bending fatigue behavior of four-step three-dimensional braided composite with three types of representative unit cell (RUC) model. The three types of unit cell models are the unit cell model for the interior, surface, and corner regions of the three-dimensional braided composite. A user-defined material subroutine was developed to characterize the stiffness matrix and fatigue damage evolution of the unit cells and furthermore the braided composite. The fatigue bending behaviors were calculated from the unit cell model with finite element method. It is found that the stress concentration and large deflection located at the middle of the braided composite specimen and both the ends of the bottom areas. As the increase of the stress level, the braided composite is subjected to larger deformation and faster damage accumulation, resulting in a fast fatigue failure. The RUC model could be extended to predict and design the bending fatigue performance of other three-dimensional braided composite structures.