Intrinsic-to-extrinsic transition in fracture toughness through structural design: A lesson from nature

TL;DR

This paper introduces a biomimetic design strategy inspired by biological materials to transition fracture toughness from an intrinsic to an extrinsic property, enabling unbounded toughness enhancement through structural design.

Contribution

It proposes a novel intrinsic-to-extrinsic transition mechanism for fracture toughness, demonstrated via simulations, modeling, and experiments, allowing size-dependent toughness enhancement.

Findings

Toughness can be increased without bound by structural design.

Biomimetic stress-strain behavior enables extrinsic toughness.

Experimental validation confirms the effectiveness of the strategy.

Abstract

Catastrophic failure of materials and structures due to unstable crack growth could be prevented if facture toughness could be enhanced at will through structural design, but how can this be possible if fracture toughness is a material constant related to energy dissipation in the vicinity of a propagating crack tip. Here we draw inspiration from the deformation behavior of biomolecules in load bearing biological materials, which have been evolved with a large extensibility and a high breaking strength beyond their elastic limit, and introduce an effective biomimetic strategy to enhance fracture toughness of a structure through an intrinsic to extrinsic (ITE) transition. In the ITE transition, toughness starts as an intrinsic parameter at the basic material level, but by designing a protein-like effective stress-strain behavior the toughness at the system level becomes an extrinsic parameter that increases with the system size without bound. This phenomenon is demonstrated through a combination of numerical simulations, analytic modeling and experiments, and leads to a biomimetic strategy which can be broadly adopted to enhance fracture toughness in engineering systems.