Toughening mechanism of crack bridging in bioinspired Bouligand structures

The bioinspired Bouligand structure is a hierarchical and non-homogeneous architecture, which has been observed in lamellar bone and the exoskeleton of lobsters. It exhibits excellent damage-resistant performance. However, toughening mechanisms in this structure are still not clear. This paper presents a multiscale fracture model to reveal the toughening mechanisms of crack bridging. Firstly, the anisotropic property is derived based on the micro-structural parameters. Then, the crack-bridging model is established, which systematically considers the toughening effect of the in-plane normal stress and the out-of-plane shear stress in the bridging zone. Finally, the toughening mechanism is investigated. The results demonstrate that the initiation fracture toughness predominantly arises from the enhancement of the intrinsic matrix fracture toughness due to the material anisotropy and heterogeneity. The ascending crack resistance curve is principally associated with the in-plane closing normal stress within the bridging zone. The periodic micro-fluctuations observed in the crack propagation resistance curve are primarily attributed to the out-of-plane shear forces present in the bridging zone. Increasing the fiber slenderness ratio and toughening the interfacial matrix can significantly improve the toughness. These results can not only reveal the toughening mechanism of the Bouligand structure but also provide guidelines for the design of high-performance composites.

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