High Strain Rate Behavior of Liquid Crystal Elastomers

TL;DR

This work investigates the high strain rate behavior of isotropic-genesis polydomain liquid crystal elastomers (I-PLCEs) across tension and compression, using a tensile drop-tower to reach $10^{2}$ s$^{-1}$ and a split Hopkinson bar to reach $1.95 imes10^{4}$ s$^{-1}$. It develops a thermodynamically consistent macroscopic model that extends the Bladon-Warner-Terentjev framework by incorporating viscoelasticity and compressibility with internal variables $(\Lambda,\Delta,Q)$ to capture nematic-domain kinetics and reorientation. The experiments show clear rate-dependent stiffening and a suppression of soft-elastic behavior at high rates, which are well captured by a single parameter set of the new model, highlighting the coupling between mesogen kinetics and polymer viscosity. The findings provide a predictive framework for designing I-PLCEs for energy absorption and soft actuation under dynamic loading, with implications for impact protection and soft robotics.

Abstract

Liquid crystal elastomers are rubbery solids that couple liquid crystalline order and deformation. This coupling leads to properties that are attractive for a number of applications in soft robotics and energy absorption. This paper is motivated by the latter application, and provides a systematic experimental study of a particular class of liquid crystal elastomers -- the isotropic genesis polydomain liquid crystal elastomers -- over a wide range of strain rates. An important aspect of this study is a novel tensile drop-tower that enables tensile strain rates of 100 s$^{-1}$ that are important to application but previously inaccessible. The paper also extends a recently proposed constitutive model to the high strain rate regime, and shows that it can be fit to describe the observed behavior across the spectrum of examined behavior.

Figures (13)

• Figure 1: Schematic drawing (left) and image (right) of the drop weight tensile testing system. Note that the striker is not shown in the right image.
• Figure 2: Schematic drawing of the split Hopkinson Pressure Bar (SHPB) setup consisting of impactor, input bar, and output bar
• Figure 3: Rheological model of LCE
• Figure 4: Behavior of I-PLCE in tension at quasistatic strain rate of $\dot{\varepsilon} = 10^{-2}$ s$^{-1}$.
• Figure 5: Typical results of a test at 3.4 m/s striker velocity or 100 s$^{-1}$ nominal strain rate. (a) Specimen before and after the test. (b) Snapshot of the specimen during the test superposed with the DIC strain rate in the stretching direction. s(c) Green-Lagrange strain as a function of time. (d) Piola-Kirchhoff or engineering stress as a function of time