'Spear and shield' inspire high toughness microstructure

A team led by Prof. NI Yong and Prof. HE Linghui from University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS) designed a discontinuous fibrous Bouligand (DFB) architecture, a combination of Bouligand and nacreous staggered structures. Systematic bending experiments for 3D-printed single-edge notched specimens with such architecture indicated that total energy dissipations are insensitive to initial crack orientations and show optimized values at critical pitch angles. The study was published in PNAS on June 22.

The survival war between the "spear" predators and "shield" preys in the biological world has inspired people that changing the microstructure of materials is an important way for structural materials to obtain extraordinary mechanical properties. The "spear" of mantis shrimps with Bouligand-type microstructures and the "shield" of abalone with nacreous staggered architectures are in research interest.

Inspired by the structural arrangements in the exoskeleton of aggressive crustaceans, where the chitin-protein nanofibrils with finite characteristic length are arranged in an overlapped array to form lamellae and the chitin-protein fibrils lamellae are arranged in the twisted plywood pattern, although not only the structure but also the mechanical properties of the materials, size, and geometry play important roles in the competition between mantis shrimp and abalone shells in nature, researchers hypothesized that the structure that exhibits the combination of the toughening mechanisms of crack twisting and crack bridging endows the mantis shrimps with remarkable fracture resistance as well as crack orientation insensitivity.

To prove the hypothesis, they designed a DFB architecture with the combination of Bouligand and nacreous staggered structures by 3D printing to examine how the fracture resistance depends on the controlled architectural parameters. Furthermore, they developed a fracture mechanics model to elucidate the mechanism of crack orientation insensitivity and maximum energy dissipations in DFB architecture.

The results showed that the sophisticated hybrid fracture mode due to the competition of energy dissipations between crack twisting and crack bridging arising in DFB architecture is identified as the origin of maximum fracture energy at a critical pitch angle.

This finding sheds light on how nature evolves materials to exceptional fracture toughness and crack orientation insensitivity. The provided design strategies with parameters selection principle enable the fabrication of formidable fracture-resistant fibrous composite systems that adapt to loads in various orientations.

This study not only reveals the origin of a microstructure for excellent fracture toughness of biomaterials, but also provides new biomimetic structural design ideas and performance optimization parameter selection principles for the preparation of high-performance advanced composite materials.

University of Science and Technology of China