Title: Toughness of Hydrogels

Abstract: Poroelastic hydrogels display dissipative behavior due to liquid diffusion, and defining fracture toughness is an open issue. Our primary interest is fibrous gels such as cartilage, blood clots, and carbon-nanotube-based sponges with absorbed oils which suffer a reduction in volume by the expulsion of liquid under uniaxial tension, which directly affects crack-tip fields and energy release rates. We have carried out experiments on fibrin gels coupled with a large-deformation continuum model to investigate the initiation of crack growth. The continuum model is formulated for isotropic fibrous gels that exhibit a range of behaviors volume increasing to volume decreasing in uniaxial tension by changing the ratio of two material parameters. The direction of liquid fluxes around cracks is shown to depend on whether the gel locally increases or decreases in volume. The energy release rate for cracks is computed using a surface-independent integral and it is shown to have two contributions -- one from the stresses in the solid network, and another from the flow of liquid. The contribution to the integral from liquid permeation tends to be negative when the gel exhibits volume decrease, which effectively is a crack shielding mechanism. Both from correlations with experiments and from simulations based on a critical stretch fracture criterion, we show that both contributions are required to characterize the fracture toughness of these materials. Some historical perspective on fracture toughness of dissipated solids will also be discussed.

Bio: John L. Bassani received a B.S. in Mechanical Engineering from Lehigh University in 1973 and worked as a Structural Engineer at Fairchild-Republic Aviation in New York before beginning graduate studies. He received a M.S. in Applied Mechanics from Lehigh University in1975 and Ph.D. in Engineering from Harvard University in 1978 on the mechanics of polycrystalline plasticity. During the next two years, he was a post doc and lecturer and then an Assistant Professor of Mechanical Engineering at the Massachusetts Institute of Technology focusing on the mechanics of high-temperature crack growth. Since 1980 he has been on the faculty of Mechanical Engineering and Applied Mechanics at the University of Pennsylvania and has held the Richard H. and S. L. Gabel Professorship since 1993. He was Chair of MEAM for 11 years, from 1997-2005 and 2008-2011. He also holds appointments in Materials Science and Engineering, the Laboratory for Research on the Structure of Matter, and the Institute of Medicine and Engineering at Penn. He has held visiting professor positions at the University of California at Santa Barbara, Harvard University, and Brown University. He is the recipient of the Presidential Young Investigator Award, Fellow of the American Society of Mechanical Engineers, Midwest Mechanics Lecturer, Board of Directors of the Society of Engineering Science and President of SES in 2008, and 2019 American Society of Mechanical Engineers Daniel C. Drucker Medal. He has served on the editorial boards of Modelling and Simulation in Materials Science and Engineering, Interface Science, Journal of Applied Mechanics, and Mechanics of Materials. Professor Bassani's research interests include: the relationship between properties of discrete and continuous media; interfacial mechanics; formation and properties of nanostructures; adhesion of shells, mechanics of living cells and tissue; plastic deformation of crystals and polycrystals; material stability and localized deformation; mechanics of fracture and fatigue; and material degradation under extreme environments.