Soft materials are often subject to repeated deformation in many applications, e.g., soft electronics, actuators, and wearable devices, leading to premature failures. Fracture and fatigue are two common failure modes. Therefore, strong, tough, self-healing, and anti-fatigue hydrogels and elastomers are highly desirable. Towards this goal, we combine the chemical and mechanical principles to design flaw-insensitive hydrogels and investigate how self-healing and energy dissipation affect fatigue fracture; we study the geometry and rheology of hydrogels on fatigue fracture; we also study the tribology of hydrogel surfaces and device an approach to reduce friction. The basic understanding of fracture and fatigue can help design and develop fatigue-resistant and self-heal materials.

Soft materials implanted in the body are vulnerable, as they are continuously subject to repeated and prolonged deformation in a corrosive environment (e.g., water, pH, salt, enzymes, chemicals, temperature). These multiple attacks are typically concurrent and the resulting material behaviors are often complicated. We study the mechanics and chemistry coupling of soft materials, in particular, the fracture process of mechanochemistry. We have studied how the presence of water accelerates crack propagation. For example, a widely-used biocompatible PDMS elastomer suffers stress-assisted hydrolysis and the crack growth in water is orders of magnitude faster than that in ambient air; We have also tested the mechanochemistry in a biodegradable PLA plastics and showed that the crack velocity is insensitive to load but is sensitive to humidity and pH. These findings will aid the development of degradable or non-degradable soft materials for implantation.

Mechanical instabilities are ubiquitous on the surfaces of soft materials. Harnessing various modes of surface instabilities can create intricate and versatile morphologies that can be used in many practical applications such as stretchable electronics, smart surfaces, and adaptive lenses. To better understand surface instabilities in various material systems, we employ theory, simulation, and experiments to study various surface modes, including wrinkles, Euler buckling, and rigid rotation in an elastic bilayer system, and harness the wrinkle-to-ridge transition to achieve high aspect ratio wrinkles. Moreover, we also investigate the crease instability in plastic materials and hydrogels and ratcheting instability in plastic liquids.

Relevant papers on this topic.

• Bai, R., Yang, J., Morelle, X.P., Yang, C. and Suo, Z., 2018. Fatigue Fracture of Self-Recovery Hydrogels. ACS Macro Letters, 7(3), pp.312-317
• Bai, R., Yang, J. and Suo, Z., 2019. Fatigue of hydrogels. European Journal of Mechanics-A/Solids, 74, pp.337-370
• Bai, R., Chen, B., Yang, J. and Suo, Z., 2019. Tearing a hydrogel of complex rheology. Journal of the Mechanics and Physics of Solids, 125, pp.749-761.
• Bai, R., Yang, J., Morelle, X.P. and Suo, Z., 2019. Flaw-Insensitive Hydrogels under Static and Cyclic Loads. Macromolecular Rapid Communications, 40(8), p.1800883.
• Mu, R., Yang, J., Wang, Y., Wang, Z., Chen, P., Sheng, H. and Suo, Z., 2020. Polymer-filled macroporous hydrogel for low friction. Extreme Mechanics Letters, p.100742.
• Chu, C.K., Joseph, A.J., Limjoco, M.D., Yang, J., Bose, S., Thapa, L.S., Langer, R., and Anderson, D.G., 2020. Chemical Tuning of Fibers Drawn from Extensible Hyaluronic Acid Networks. Journal of the American Chemical Society, 142(46), pp.19715-19721.
• Yang, X.#, Yang, J.#, Chen, L. and Suo, Z., 2019. Hydrolytic crack in a rubbery network. Extreme Mechanics Letters, p.100531.
• Yang, X., Steck, J., Yang, J., Wang, Y. and Suo, Z., 2021. Degradable plastics are vulnerable to cracks. Engineering, 7(5), pp.624-629.
• Yang, J., Illeperuma, W. and Suo, Z., 2020. Inelasticity increases the critical strain for the onset of creases on hydrogels. Extreme Mechanics Letters, p.100966.
• Yang, J., Jin, L., Hutchinson, J.W. and Suo, Z., 2019. Plasticity retards the formation of creases. Journal of the Mechanics and
  Physics of Solids, 123, pp.305-314.
• Ouchi, T.#, Yang, J.#, Suo, Z. and Hayward, R.C., 2018. Effects of Stiff Film Pattern Geometry on Surface Buckling Instabilities of Elastic Bilayers. ACS Applied Materials & Interfaces, 10(27), pp.23406-23413
• Auguste, A., Yang, J., Jin, L., Chen, D., Suo, Z. and Hayward, R.C., 2018. Formation of high aspect ratio wrinkles and ridges on elastic bilayers with small thickness contrast. Soft Matter, 14, 8545-8551
• Huang, J.#, Yang, J.#, Jin, L., Clarke, D.R. and Suo, Z., 2016. Pattern Formation in Plastic Liquid Films on Elastomers by Ratcheting. Soft Matter, 12(16), pp.3820-3827. 
• Yang, J. and Nie, G., 2014. Analysis of Sinusoidal Interfacial Wrinkling of an Anisotropic Film Sandwiched Between Two Compliant Layers. Journal of Applied Mechanics, 81(9), p.091013.