A Multiscale Fracture Model to Reveal the Toughening Mechanism in the Bioinspired Bouligand Structure

The Bouligand structure has been observed in a variety of biological materials, such as lamellar bone and arthropod cuticles. It is a hierarchical architecture that exhibits excellent damage-resistant performance, which arouse many interests for the mechanists and structure engineers. However, there still lacks a deep understanding of the toughening mechanisms in the Bouligand structure. For the purpose of revealing the toughening effect of twisting cracks, this paper developed a multiscale fracture mechanics model with considering the non-homogeneity and anisotropic properties. Firstly, the macro and micro constitutive properties of the Bouligand structure are analyzed. Then, a multiscale fracture model is established to characterize the energy release rates and the local stress intensity factors at the crack front of twisting cracks which are formed within the Bouligand structure. Based on the model, serious of digital calculations are carried out. The digital results demonstrate that the decrease of the local energy release rate can be attributed to two mechanisms. One is that the multiscale structure causes the stress release of the crack tip nearby. The other is that the twisting crack leads to the loading mode transformation from the single-mode to the mixed-mode, which is the main reason of the fracture toughness increasing. The research results shown in this paper can provide structure engineers some suggestive guidelines for the design of high-performance composites.