Mode I fracture mechanisms and crack evolution behavior of three-dimensional braided composites

Abstract

Three-dimensional braided composites possess higher inter-laminar fracture toughness than two-dimensional laminates, while the mode I fracture toughness behavior of three-dimensional braided composites is still unknown. Here we report mode I fracture behavior of three-dimensional braided carbon fiber/epoxy composites tested with the double-cantilever beam method. The samples with a pre-crack were tested using the MTS device, and the fracture process was recorded to calculate the fracture toughness. The effects of different loading rates on fracture behavior were compared. The strain field at the crack tip was obtained by the digital image correlation technique. The damage morphologies were scanned by X-ray microcomputer tomography. It was found that the bridging effect of braiding yarns prevents cracks from initiation and propagation, while the intertwining points of the braiding structure cause crack bifurcation. The braided architecture provides an effective way to hinder crack propagation. The stiffness of composites tends to decrease as the loading rate rises. The higher loading rate results in a lower critical load level for crack propagation and reduces fracture toughness. The results show that the three-dimensional braided structure is an effective method for increasing the fracture toughness. We hope the results can be used for the design of higher fracture toughness composite structures.