Magic angles and cross-hatching instability in hydrogel fracture

TL;DR

This study investigates the fracture surface roughness in gelatin gels, revealing anisotropic roughness patterns and a velocity-dependent cross-hatching instability caused by macrosteps drifting at specific angles.

Contribution

It uncovers the existence of magic angles and a sub-critical cross-hatching instability in hydrogel fracture, linking macrostep behavior to strain-hardened zone properties.

Findings

Maximum roughness occurs at velocity-independent angles.

A sub-critical instability leads to cross-hatched fracture patterns.

Step heights relate to the strain-hardened zone width.

Abstract

The full 2D analysis of roughness profiles of fracture surfaces resulting from quasi-static crack propagation in gelatin gels reveals an original behavior characterized by (i) strong anisotropy with maximum roughness at $V$-independent symmetry-preserving angles, (ii) a sub-critical instability leading, below a critical velocity, to a cross-hatched regime due to straight macrosteps drifting at the same magic angles and nucleated on crack-pinning network inhomogeneities. Step height values are determined by the width of the strain-hardened zone, governed by the elastic crack blunting characteristic of soft solids with breaking stresses much larger that low strain moduli.