Soft materials have been utilized in a wide range of emerging applications such as soft robotics, stretchable electronics, and biomedical implants. Many of these applications require the underlying soft materials to be stretchable and yet fracture resistant. A key mechanism of toughness enhancement is to introduce energy dissipation and thus shield a crack from the external loading. However, fracture mechanics of such soft dissipative materials involves nonlinear deformation and complex material behaviors, and as a result quantitative prediction of the fracture toughness is very challenging.This seminar will start with a discussion about crack propagation in a pure shear specimen. Three ingredients are needed to predict the fracture toughness: 1) an accurate constitutive model to capture hysteresis and viscoelasticity; 2) the nonlinear stress/strain fields near the crack tip; and 3) a valid local fracture criterion at crack tip. I will describe our recent and ongoing works along these three directions. Specifically, I will first describe our efforts to develop physics-based constitutive models for the damage and viscoelasticity in soft polymers. After that I will talk about asymptotic solutions for crack tip fields in hypereleastic solids under large deformation. I will also present our current development of a new experimental method to measure crack tip strain fields during the initiation of a pre-existing crack and the subsequent crack propagation based on tracking randomly distributed tracer points on the surface of fracture test samples. Rong Long is currently an Assistant Professor in the Department of Mechanical Engineering at University of Colorado at Boulder.  Prior to that he was an Assistant Professor at University of Alberta in 2013-2014, a Research Associate at University of Colorado in 2012, and a Postdoctoral Associate at Cornell University in 2011. He received his Ph.D. degree in Theoretical and Applied Mechanics from Cornell University in 2011 and his B.S. degree in the same field from University of Science and Technology of China in 2006. He received the Young Adhesion Scientist Award from the Adhesion Society in 2014 and the Ralph E. Powe Junior Faculty Enhancement Award from Oak Ridge Associated Universities in 2015. His research interests include: continuum mechanics of soft materials, fracture mechanics, contact mechanics, adhesion and biomechanics.