Surprising Fracture Path Deflection in Liquid Crystal Elastomers Revealed
Liquid crystal elastomers (LCEs) are heavily employed in the field of soft robotics because they exhibit the such programmable reversible shape changing and anisotropic mechanical response. But new findings from the UCLA Department of Mechanical and Aerospace Engineering provide that there’s a cracking anomaly in LCEs that cracks the barrier: Under stress, these materials’ cracks are capable of spontaneously changing direction while shifting out- of plane path plan as governed by locally stressed induced molecular re- arrangement. This observation, experimentally verified and computational modelled, serves as a caution to the potential of designing durable with high-performance soft actuators and biomaterials.
1. Microstructure-Coupled Fracture Behavior
They consist of a crosslinked network polymer mixed with rod-forming liquid crystal mesogens prepared in an orientational field. These mesogensare orientational degrees of freedom which can rotate when loaded, and as a result the mechanical properties of the materialcan change. “Interestingly, the liquid crystals in LCEs can reorient their microstructure alignment when they’re pulled or stressed,” says UCLA principal investigator Lihua Jin. This reorientation of the director field leads to fracture paths that are not predicted by initial geometry, but rather generally run parallel to the evolution of the new orientation of the transformed directors, as previously found for crack paths in such biological materials as blood clots.
2. Experimental Evidence of Path Deflection
By systematical tensile measurements on monodomain LCE samples with edge-cracks, it was found that the initiation director orientation and the stretching rate of experiment had a great influence on the fracture angle. Cracks initiated at particular lengths and propagated parallel to the strain axis in the strained condition, but inclined in the relaxed state at low strainrates. On the other hand, higher rates corresponded to lower shear deformation and smaller fracture angles. Such rate dependence is unheard of in usual fiber‑oriented elastomers8, and represents unique relationships between deformation and molecular orientation inLCEs.
3. Predictive Phase-Field Fracture Modeling
To model these complex paths, the UCLAResearchers developed a rate-dependent Phase Field Fracture model using viscoelastic constitutive models for LCEs. The model includes a phase field variable approach to diffuse damage description avoiding the use of crack co-ordinates. Simulations of stress–stretch curves and fracture geometries for a range of rates, initial directors and sample shapes were reproduced by introducing the assumption that cracks prefer to grow normal to the director.
4. Influence of Geometry and Precracks
Results of the fracture-test-ing program on samples with deformed aspect ratios or two prec rucks indicated that pathcurvature could be exaggcrated or restrained by die shape. With longer precracks the non-dimensional slope of the initial upwards crack trajectory was lower due to its more shear deformation. In two-precrack geometries transmission cracks will even grow in opposite direc- tions depending on local director tilt and some of them would stop before meeting, which the model could realize, however not accurately regarding crack interactions.
5. Implications for Soft Robotics Durability
One such unexpected crack path variation may also account for the degradation of performance of LCE actuators in soft robotics including (i) multifunctional deformation using woven fiber, and (ii) untethered robots with on-board JO heating. Design of actuators must account for director rotation under load, especially for actuators loaded cyclically or multiaxially. Preliminary incorporation of fracture models at early stages may save premature failure in applications such as programmable robotic surfaces and the cardiac mimics pumps.
6. Lessons from Biomaterial Fracture Mechanics
Lessons from thefield of biomaterials, and in particular those for transient polymernetworks embedded in hydrogels, suggests that herein one cananticipate similar microstructure-mediated cracking. In these networks, reversible association anddynamic inhomogeneity govern the viscoelastic relaxation mechanism and crack behavior. We therefore conclude that the LCE fracture-toughness is linearly related to microstructural reorganization, a dependence consistent with both intrinsic toughness and bulk viscoelastic dissipation.
7. Path to Enhanced Material Design
This numerical tool, thus, offers a route to the engineering of LCEs with controllable director distributions in such a way as to impart greater resistance against fracture without significant loss in stiffness. By control of the alignment patterns, for example using mechanical, field based or surface templating methods, the crack path may be selectively re-oriented away from sensitive areas, enhancing actuator life. This is in line with the general direction of soft robotic design where we try to engineer material architectures for optimal performance while infusing them with redundancyfor robustness.
Rate- and orientation-dependent deflecting fracture paths have been observed in LCEs. Revisiting the problem of designing tough deformable media. It also serves a warning to mechanical engineers, materials scientists and roboticists ofthe need for considering dynamic microstructure explicitly in the analysis of fracture ifthey hope that next-generation soft robotics systems will be robust under the complex loadconditionsthat are encountered in practice.
