Crack propagation and renucleation in soft brittle hydrogels

Crack tip opening displacement (CTOD) and fracture energy are determined from crack geometry and material properties for very slowly propagating cracks, less than 50 $$\upmu \mathrm {m/s}$$, in thin brittle hydrogels on the sub-millimeter scale. 2D fluorescent speckle images are captured using confocal microscopy during propagation, and 3D volumetric images are captured both before propagation begins and after the crack arrests. Fracture energy builds up until a critical value is reached and then remains constant as the crack propagates and eventually arrests when the energy is no longer sufficient for propagation. Once a crack arrests, more energy is needed for renucleation, suggesting that local toughening effects are at play. Based on observations of renucleation events and analysis of 3D crack shapes, this local toughening points to a mechanism for fracture surface roughening observed in the literature for slowly propagating cracks. Additionally, through-thickness variation in fracture energy, while expected from linear elastic fracture mechanics (LEFM) theory, suggests local toughening in the process zone which contributes to this roughening of crack surfaces.