UCLA

A liquid crystal elastomer (LCE) combines cross-linked elastomers and rod-like liquidcrystals (LCs), presenting the hyperelasticity characteristics of elastomers and the unique properties associated with LCs. A LCE is a highly promising material utilized in the realm of actuators, soft robotics, and related applications, primarily attributed to its unique capability of spontaneous strain achieved through changes in ordering and director. This dissertation aims to characterize the stress-director coupling behavior under different loading conditions and its effect on crack-tip fields and fracture propagation, providing a comprehensive understanding of LCEs and guidance in further design. First, LCEs exhibit extremely slow relaxation due to a combination of viscous network deformation and director reorientation. We study the rate-dependent stress-director coupling behavior under various loading rates. Our experimental setup successfully captures rate- dependent stress, strain, and director reorientation in real-time, and distinguishes the two relaxation time scales of the network deformation and mesogen reorientation. Based on the experimental findings, a viscoelastic constitutive model is developed to manifest the relation between rate-dependent macroscopic deformation and microscopic director rotation in LCEs. This work provides a comprehensive investigation into and mechanistic understanding of the rate-dependent behavior of LCEs. Secondly, we explore the exceptional stress-director coupling effect around the crack tip. We examine edge-cracked LCEs samples subjected to tensile loading at different angles relative to the initial director. Unlike traditional elastomers, LCEs exhibit an elliptical stress/strain distribution, attributed to the significant and inhomogenous director reorientation at the crack tips. Notably, we observe a domain wall formation along a certain polar angle at the crack tip, caused by opposite director rotation. Moreover, LCEs with a tilted initial director to the loading exhibit much smaller crack openings and energy release rates compared to the parallel loading. We attribute these findings to a combined effect of bulk softening at the remote region and the formation of domains of opposite director rotation near the crack tip. In the end, we report the intriguing crack propagation behavior in LCEs during post-cut experiments with varying initial directors. A post-cut method, which induces a crack after the sample is stretched and held for a long time, is utilized to minimize the effect of bulk viscosity, enabling a systematical record of crack propagation rates and directions. Besides, a growth delay is observed after post-cutting, followed by steady-state propagation. The results reveal that cracks typically propagate perpendicular to the director ahead of the crack tip. During the steady stage, crack velocity increases with higher pre-stretching levels. Notably, anomalous growth occurs when strip domains and monodomain coexist at the crack tip, attributed to the lower fracture energy in the strip domain compared to monodomain. This study offers valuable insights into fracture behavior and provides significant contributions to the development of fracture criteria.