96 K. A. Mac Donald and G. Ravichandran corresponding to a mean SIF (K) andERR(G) of 850Pa√mand6.9 J/m2 respectively. The estimated size of the nonlinear region near the crack tip (rNL) is 28μm, which corresponds to about 22 pixels in the captured images, roughly the size of an appropriate digital image correlation subset for this speckle pattern. Errors in these calculated values are comparable to the errors reported for more traditional fracture experiments in similar soft gel materials [9]. Some features of crack renucleation in this networked polymer hydrogel were observed at the submillimeter scale such as intact ligaments and indicators of local toughening. However, to better understand nonlinear processes in this gel, higher resolution imaging of the crack tip nonlinear region is needed. In this study, we were able to use crack geometry and the polymer’s material properties to determine traditional measures of fracture toughness without needing to define and measure complex boundary conditions. We were also able to determine the scale at which we would expect nonlinear processes to affect the crack. Acknowledgments This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1144469 and the Designing Materials to Revolutionize and Engineer our Future (DMREF) program under Award No. DMS-1535083. Imaging was performed in the Biological Imaging Facility, with the support of the Caltech Beckman Institute and the Arnold and Mabel Beckman Foundation. 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