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Mapping three-dimensional deformation field in transparent soft materials : applications to fracture

Rong Long Department of Mechanical Engineering, University of Colorado Boulder


Soft polymers and gels have been applied in various new technologies including soft robotics, stretchable electronics, biomedical implants, and actuators with large shape change. For many of these applications, the soft materials’ ability to sustain mechanical stress and to resist fracture is crucial. Typically macroscopic fracture results from the growth of small defects such as cracks and voids. The deformation and stress fields near the defects are amplified in comparison to the remote loading, which then leads to local material damage, defect growth and the eventual global fracture. However, the detailed deformation field near defects, e.g. at the tip of a crack, is not well understood for soft materials due to the geometrical nonlinearity of large deformation and the complex material constitutive behaviors. Asymptotic solutions for crack tip fields in some hyperelastic solids are available, but experimental data are still needed to verify the region of validity for such solutions. In this presentation, I will describe an experimental method to measure the local three-dimensional (3D) deformation fields in soft materials. This method is based on tracking the displacements of fluorescent beads embedded in transparent soft materials and then converting the data to strain fields through interpolation. It consists of three major steps : i) in situ 3D imaging of the fluorescent beads along loading history ; ii) image analysis and displacement tracking for individual beads ; iii) interpolation to form continuous displacement and strain fields. This method, different from the widely-used digital image (or volume) correlation (DIC or DVC) method, can provide important experimental data for understanding the fracture mechanics of soft materials.


Rong Long is currently an Assistant Professor in the Department of Mechanical Engineering at University of Colorado at Boulder, leading the Nonlinear Mechanics Laboratory ( 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. His research interests include : continuum mechanics of soft materials, fracture mechanics, contact mechanics, adhesion and biomechanics.