Comparisons on impact fracture behavior between three-dimensional four directional and five directional braided composite materials

Fracture behaviors of three-dimensional braided composites could be designed from braided preform structures. Here, we compare the fracture behaviors of three-dimensional four direction (3D4d) and five directional (3D5d) braided composites under impact loading. A drop weight impact tester combined with a high-speed photography system was employed to characterize the progressive damages and fracture behaviors, which include crack growth and crack mouth opening displacement. The two-dimensional digital image correlation method was utilized for evaluating deformation contours and strain field. A finite element analyses model was established to reveal impact damage evolution, crack growth, stress distribution, and energy absorption at microstructure level. We found that axial yarns had great effect on the crack propagation paths. The stress at the notch bottom caused yarn breakage and then resulted in a sharply growing crack. The axial yarns in the 3D5d braided composites impede the progress of cracks under transverse impact. The 3D5d braided composites have smaller crack mouth opening displacement than that of the 3D4d composites. The braided preform is the main energy absorption component. With the same volume fraction, axial yarns exhibit the highest energy absorption. Due to the existence of axial yarns, the resistance to the fracture and crack growth of the 3D5d braided composites is better than the 3D4d braided composites.