Flaw-sensitivity of a tough hydrogel under monotonic and cyclic loads

Abstract Rupture of emerging tough hydrogels has received much attention in recent years. It is a fundamental question what length of initial flaws can significantly affect the rupture of a tough hydrogel. Here we study the rupture of a tough hydrogel, using samples with and without initial cuts, under monotonic and cyclic loads. We prepare six samples under the same conditions, either without initial cut, or with initial cuts of the same length, and load them monotonically to inspect the statistical variation of measured rupture stress, rupture stretch and work of rupture. We cycle the six samples in parallel to the same amplitude of stretch and record the number of cycles to rupture of each sample. The average number of cycles to rupture decreases with the amplitude of stretch; and an endurance stretch exists, below which the samples can sustain indefinite number of cycles without rupture. We find that when the initial cut is long, the endurance stretch decreases with the initial cut length. When the initial cut is short, the endurance stretch is insensitive to the initial cut length. We interpret this finding by a material-specific length, the endurance fractocohesive length. We compare the endurance fractocohesive lengths of the hydrogel and other materials, including elastomers, plastics, metals, and ceramics. It is hoped that similar experiments will be soon conducted for other hydrogels to guide their development.