Zhang, WenleiLiu, XiaoWang, JikunTang, JingdaHu, JianLu, TongqingSuo, Zhigang2018-03-072018Zhang, Wenlei, Xiao Liu, Jikun Wang, Jingda Tang, Jian Hu, Tongqing Lu, and Zhigang Suo. 2018. “Fatigue of Double-Network Hydrogels.” Engineering Fracture Mechanics 187 (January): 74–93. doi:10.1016/j.engfracmech.2017.10.018.0013-7944http://nrs.harvard.edu/urn-3:HUL.InstRepos:34902467The discovery of tough hydrogels of many chemical compositions, and their emerging applications in medicine, clothing, and engineering, has raised a fundamental question: How do hydrogels behave under many cycles of stretch? This paper initiates the study of the fatigue behavior of the classic PAMPS/PAAM double network hydrogels discovered by Gong and her co-workers (Advanced Materials 15, 1155, 2003). We reproduce the hydrogels, and prepare samples of two types, with or without a crack cut before the test. When an uncut sample is subject to cyclic stretches, internal damage accumulates over thousands of cycles until a steady state is reached. When a cut sample is subject to cyclic stretches, the crack extends cycle by cycle if the amplitude of stretch is above a certain value. A threshold of energy release rate exists, below which the crack remains stationary as the sample is cycled. We find a threshold around 400 J/m2 for hydrogels containing PAAM networks of a low density of crosslinkers, and around 200 J/m2 for hydrogels containing PAAM networks of a high density of crosslinkers. The experimental findings are compared to the Lake-Thomas model adapted to the double-network hydrogels.en-USdouble-network hydrogelFatigue fractureShakedownThresholdJournal Article2017-10-2220172018-03-0710.1016/j.engfracmech.2017.10.018