A research team led by Prof. NI Yong and Prof. HE Linghui from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences uncovered the propagation and toughening mechanism of tortuous crack front in bioinspired anisotropic heterogeneities, and developed an optimization design for toughness amplification by manipulating microstructural orientation. The study was published in Nature Communications.

Crack-fronts in heterogeneous materials interact with the non-uniform structure, resulting in 3D tortuous crack-tip configuration that significantly increases fracture resistance. However, due to the complex nonlinear coupling between the 3D crack-front geometry and the heterogeneous structures, quantitative relationships of heterogeneity, crack-tip geometry, and toughness remain unclear.

In recent years, bioinspired heterogeneity has gained attention for improving material toughness. But most studies on it assumed straight crack-front propagation, overlooking the 3D tortuous crack-front geometry and the additional toughening effects it induces.

In this study, researchers used 3D-printed bioinspired heterostructures as a model system. They conducted fracture tests on the 3D-printed samples, and found that the straight crack front was distorted to a 3D helical crack-front configuration when interacted with heterostructures, deviating from the original pathway. Compared to linear toughening mechanisms, the 3D twisted crack geometry provides additional toughening effects.

Simulations and theoretical analysis revealed that heterogeneous structures influence the driving forces along the crack front, inducing an orientation-dependent mixed fracture mode of crack twisting and crack bridging. This interplay led to the formation of helical crack-tip geometries, with nonlinear relationships between crack-tip geometry, fracture toughness, and fiber orientation. 

Based on the findings, researchers designed a heterogeneous plywood system with enhanced toughness by adjusting the structural parameters.

This study reveals the physical mechanism behind the toughening effect of crack front in heterogeneous material, offering new ideas for future design of bioinspired heterogeneous materials.