In situ measurement of three-dimensional deformation and stress fields with applications to soft material fracture

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Soft polymers and gels have emerged as an exciting class of materials due to their high deformability, bio-compatibility and responsiveness to various external stimuli. They are widely applied in areas including stretchable electronics, pressure sensitive adhesives, biomedical implants, and actuators with large shape change. A bottleneck for these applications is the ability of soft materials to resist fracture caused by the growth of small defects. There is a limited understanding on how soft materials break, largely due to the complex nonlinear mechanics and the lack of experimental tools to examine mechanical stresses near defects. The proposed research program aims to advance fundamental understandings on soft material fracture, and to identify guidelines for the engineering of tough polymers and gels with enhanced ability to resist fracture. This will greatly expand the scope of their applications, examples including tough gels for artificial cartilage and durable elastomers for soft robotics. To achieve this goal, we will develop a novel experimental method that can measure three-dimensional strain and stress fields in soft materials. This method will be combined with theoretical and computational analyses to map the detailed distribution of stresses near defects such as cracks or cavities and understand how the stresses cause defect growth and eventually lead to macroscopic fracture. Emphasis will also be placed on understanding the effect of nonlinear elasticity and material microstructure on defect growth. In addition to understanding soft material fracture, the experimental method to be developed in this proposal can potentially become a transformative tool for characterizing mechanics of soft materials. Two PhD students will be trained through the proposed research activities.