Fatigue fracture of tough hydrogels

TLDR: In this article, the authors studied the fatigue fracture of a polyacrylamide-alginate hydrogel and found that the stress-stretch curve changes cycle by cycle, and reaches a steady state after thousands of cycles.

About: This article is published in Extreme Mechanics Letters.The article was published on 2017-09-01 and is currently open access. It has received 177 citations till now. The article focuses on the topics: Fracture mechanics & Self-healing hydrogels.

Figures

Fig. 1 Shakedown after prolonged cycling. (a) A sample of tough hydrogel is cyclically stretched in the pure shear test. (b) The loading profile of the test. Shakedown of stress-stretch curves was observed in tests with different loading frequencies, and shows no significant rate dependence: The loading frequency is (c) 0.25 Hz, (d) 0.125 Hz.

Fig. 4 Crack growth as a function of the number of cycles N, under different maximum stretches. The crack growth reaches the steady state after thousands of cycles, and becomes approximately a linear function of N.

Fig. 7 Comparison between a polyacrylamide hydrogel and a tough hydrogel in fatigue fracture. (a) The crack growth of the polyacrylamide hydrogel is faster (steeper slope) than that of the tough hydrogel, even under a less critical condition (smaller and G per cycle). (b) The fatigue threshold of the tough hydrogel is higher than that of the polyacrylamide hydrogel. The slope of dc/dN vs. G in the tough hydrogel is much lower than that in the polyacrylamide hydrogel. Both factors show that tough hydrogels have better fatigue fracture resistance than polyacrylamide hydrogels.

Fig. 6 (a) A polyacrylamide hydrogel maintains stable stress-stretch loops over thousands of cycles. (b) The maximum stress in each cycle remains nearly constant over cycles.

Fig. 2 (a) The maximum stress in each cycle decreases and approaches a steady state. (b) The stress-stretch curves after 2000 cycles for different maximum stretches .

Fig. 5 (a) The energy release rate is calculated by integrating the stress-stretch curve in the steady state of the material far ahead of the crack tip. (b) In the steady state, the extension of crack per cycle, dc/dN, is plotted as a function of energy release rate G. A threshold is found, below which no crack propagation in the steady state occurs. Close above the threshold, dc/dN is found as a linear function of G. dc/dN then increases rapidly with G.