Experimental Measurement of Tearing and Cutting in Highly Deformable Solids Relating to the Mechanical Origin of Crack Blunting-Mediated Toughness

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This award aims to determine the relationship between cutting and tearing failure modes in order to re-examine and provide new insight into the mechanical origin of tearing in highly deformable materials. Failure in these rubber-like materials governs design rules in applications ranging from transportation to biomaterials. Unlike many rigid materials, cracks in rubber-like materials can blunt macroscopically as the material undergoes large deformations. This complicates the stress state at the tip of the crack, which varies depending on the material. Current understanding of the failure mechanism cannot explain the energy differences in two different loading geometries, cutting versus tearing. Potential benefits include design criteria to serve as targets to guide the fabrication of new materials. Results can be generalized to describe puncture and insertion of needles in soft tissue; these failure mechanisms play a key role in diagnostics and guided-needle therapeutics. This project will provide educational opportunities facilitated by the PI as many of the experiments in this project are accessible to undergraduate researchers. The PI will recruit female undergraduate students to participate in data-gathering throughout the project.Highly deformable, crack-blunting solids exhibit fracture energies many times larger than predicted by simple cohesive models. Researchers have postulated more complex models that indicate fracture is governed by the local mechanics of the tip. These local mechanics include the material's large strain response, tip radius, and local fracture energy. To fully characterize each of these important features, the PI will supplement experimental mechanical failure data with in situ and post-mortem imaging, documenting crack tip radius, near-tip strain, and fracture mechanism. The mechanism map resulting from this experimental characterization will guide examination and, if necessary, re-interpretation of existing models describing the mechanical origin of tearing in highly deformable materials. Once established for the standard plane stress geometry, the relationship quantified in this project, between geometrically controlled (cutting) and remote-load (tearing) failure energies, will be applied to relate needle-mediated failure to standard, mode I fracture energy values.