Stability controlled crack initiation in nacre-like composite materials

Recent research has focused on the toughening mechanism of nacre in view of the cracking process. Crack initiation, often characterized as the sudden concentration of strain, has a strong relation with the emergence of the system's multi-solution and the change in convexity of the strain energy function. In this study, we use a cohesive spring system representing the micro-structure of nacre, and the Monte-Carlo (MC) algorithm to trigger the transition of the solution mode when the system loses its global stability. The stability evolution of the material can be divided into three stages. In stage I, the global and local stabilities are maintained and the strain energy function remains strictly convex. In stage II, the system loses local stability but global stability is maintained. In stage III, the system loses global stability and the strain concentration occurs. Stage II originates from the combination of shear and debond interface mechanisms and enhances the system's stability four-fold. Under certain conditions, stage II disappears with a simultaneous loss of both local and global stabilities. This phenomenon can be reproduced by both theoretical prediction and MC simulation. Several factors have shown relations with stability evolution, such as irreversible dissipation, micro-structure size, shear/debond cohesive strength, and the local cohesive bifurcation point. This study offers a view different from conventional fracture mechanics for exploring crack-toughening mechanisms of nacre. (C) 2019 Elsevier Ltd. All rights reserved.