Fracture and Fatigue of Liquid Crystal Elastomers

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This project will investigate fracture of liquid crystal elastomers under a single or multiple cycles of loading. A liquid crystal elastomer is an elastomer with liquid crystal molecules on a polymer network aligning in a certain direction. Liquid crystal elastomers find broad applications in stimuli-responsive actuators and soft robotics. Therefore, understanding their fracture behavior and preventing their failure have practical significance. Studying nonlinear fracture mechanics of liquid crystal elastomers, which involves coupling between stretching of polymer networks and reorientation of liquid crystal molecules, is also fundamentally unique and important. This project has broader impacts in education and technology. The research work will be included into a new graduate class, integrated into research projects for graduate and undergraduate students, and developed into demonstrations for K-12 outreach activities. In addition, understanding fracture of liquid crystal elastomers will have long-term impacts in the fields of stimuli-responsive actuators and soft robotics by preventing failure of liquid crystal elastomers during operation and designing durable liquid crystal elastomers for reliable devices.The objective of this project is to establish a nonlinear fracture mechanics theory for liquid crystal elastomers, and experimentally characterize their fracture and fatigue processes and properties. This project will use both simulations and experiments to determine the crack-tip fields, energy release rates, and fracture energies of liquid crystal elastomers under a single or multiple cycles of loading. A viscoelastic field theory of liquid crystal elastomers coupling the fields of stress and liquid crystal orientation, along with the cohesive zone model will be established and used to computationally calculate the time-dependent crack-tip fields and energy release rates, to simulate crack propagation, and to determine the fracture energies of polydomain and monodomain liquid crystal elastomers. Experimentally, mode I and mode III fracture energies of liquid crystal elastomers will be measured by the pure shear and trousers tests, and the crack-tip fields will be observed using digital image correlation and transmission circular polariscopy.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.