Fatigue of double-network hydrogels

• Published in: Engineering fracture mechanics 2018-01, Vol.187, p.74-93
• Main Authors: Zhang, Wenlei, Liu, Xiao, Wang, Jikun, Tang, Jingda, Hu, Jian, Lu, Tongqing, Suo, Zhigang
• Format: Article
• Language: English
• Subjects: Chemical composition; Crosslinking; Damage accumulation; Density; Double-network hydrogel; Energy release rate; Fatigue fracture; Fractures; Hydrogels; Materials fatigue; Organic chemistry; Shakedown; Threshold
• ISSN: 0013-7944; 1873-7315
• DOI: 10.1016/j.engfracmech.2017.10.018

•Initiate the study of the fatigue behavior of the classic double network hydrogels.•Damage accumulates over thousands of cycles for an uncut sample.•Crack extends over thousands of cycles for a cut sample.•Determine a fatigue threshold and compare with the Lake-Thomas model. The 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 [25]. 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.