Mechanics and Mechanisms of Slow Crack Propagation in Brittle Hydrogels

Abstract

Several experiments have reported rate dependent roughening of crack surfaces in brittle hydrogels following slow crack propagation, 0.1–1 mm/s. We conduct in-situ 2D imaging of an internal plane of propagating cracks and volumetric imaging of stationary cracks using confocal microscopy for very slow crack speeds, 0.01 mm/s, in thin brittle hydrogel specimens. We seek to better understand the mechanisms at play in these slow crack roughening events by observing both crack propagation and renucleation. Additionally, we study the crack tip region and crack shape to infer fracture toughness of the gel. Observations suggest that toughening mechanisms are at play in these surface roughening events and lead to crack arrest followed by renucleation at a “weak” point in the crack surface. Based on these observations, we hypothesize that there are two interconnected mechanisms at play: The crack is slow enough that the gel “sees” the stress concentrations at the crack tip and water is forced out of the gel while there is also time for viscoelastic and plastic processes to occur.