Crack patterns over uneven substrates

TL;DR

This study investigates how the shape of an underlying substrate influences crack patterns in thin layers, revealing transitions from wavy to ladder-like to isotropic cracks, and provides a model to predict these patterns.

Contribution

It introduces a model based on Griffith fracture criteria and new metrics to characterize crack pattern transitions over uneven substrates.

Findings

Crack patterns change with layer thickness from wavy to isotropic.

Order parameters effectively measure crack alignment and transitions.

The model predicts crack pattern types based on substrate shape and layer properties.

Abstract

Cracks in thin layers are influenced by what lies beneath them. From buried craters to crocodile skin, crack patterns are found over an enormous range of length scales. Regardless of absolute size, their substrates can dramatically influence how cracks form, guiding them in some cases, or shielding regions from them in others. Here we investigate how a substrate's shape affects the appearance of cracks above it, by preparing mud cracks over sinusoidally varying surfaces. We find that as the thickness of the cracking layer increases, the observed crack patterns change from wavy to ladder-like to isotropic. Two order parameters are introduced to measure the relative alignment of these crack networks, and, along with Fourier methods, are used to characterise the transitions between crack pattern types. Finally, we explain these results with a model, based on the Griffith criteria of fracture, that identifies the conditions for which straight or wavy cracks will be seen, and predicts how well-ordered the cracks will be. Our metrics and results can be applied to any situation where connected networks of cracks are expected, or found.