A new failure mechanism in thin film by collaborative fracture and delamination: interacting duos of cracks

TL;DR

This paper investigates a novel failure mechanism in thin films involving collaborative fracture and delamination, revealing a characteristic detachment width and providing experimental, numerical, and theoretical insights into the pattern formation.

Contribution

It introduces a new failure mode involving coupled cracks and delamination in thin films, with a focus on the characteristic width and its dependence on material properties.

Findings

The system selects an equilibrium width about 25 times the film thickness.

The width is independent of fracture and adhesion energies.

A simplified energy-based model explains pattern formation mechanisms.

Abstract

When a thin film moderately adherent to a substrate is subjected to residual stress, the cooperation between fracture and delamination leads to unusual fracture patterns, such as spirals, alleys of crescents and various types of strips, all characterized by a robust characteristic length scale. We focus on the propagation of a duo of cracks: two fractures in the film connected by a delamination front and progressively detaching a strip. We show experimentally that the system selects an equilibrium width on the order of 25 times the thickness of the coating and independent of both fracture and adhesion energies. We investigate numerically the selection of the width and the condition for propagation by considering Griffith's criterion and the principle of local symmetry. In addition, we propose a simplified model based on the criterion of maximum of energy release rate, which provides insights of the physical mechanisms leading to these regular patterns, and predicts the effect of material properties on the selected width of the detaching strip.