Cooperating Cracks in Two-Dimensional Crystals

TL;DR

This study explores how cracks interact across layers in 2D crystals, revealing unique attraction and repulsion behaviors that influence material toughness and can be engineered for better fracture resistance.

Contribution

The paper introduces a combined experimental, simulation, and theoretical framework to understand and control crack interactions in multilayer 2D materials, highlighting the role of interlayer shear.

Findings

Parallel cracks attract, anti-parallel cracks repel in bilayer 2D crystals.

Interlayer shear modifies crack driving forces and stress intensity factors.

Material toughness increases with shear stiffness and decreases with crack spacing.

Abstract

The pattern development of multiple cracks in extremely anisotropic solids such as bilayer or multilayer two-dimensional (2D) crystals contains rich physics, which, however, remains largely unexplored. We studied crack interaction across neighboring 2D layers by transmission electron microscopy and molecular dynamics simulations. Parallel and anti-parallel ('En-Passant') cracks attract and repel each other in bilayer 2D crystals, respectively, in stark contrast to the behaviors of co-planar cracks. We show that the misfit between in-plane displacement fields around the crack tips results in non-uniform interlayer shear, which modifies the crack driving forces by creating an antisymmetric component of the stress intensity factor. The cross-layer interaction between cracks directly leads to material toughening, the strength of which increases with the shear stiffness and decreases with the crack spacings. Backed by the experimental findings and simulation results, a theory that marries the theory of linear elastic fracture mechanics and the shear-lag model is presented, which guides the unconventional approach to engineer fracture patterns and enhance material resistance to cracking.