Deformation localisation in stretched liquid crystal elastomers

TL;DR

This paper models two key deformation instabilities in stretched liquid crystal elastomers—necking and shear striping—using finite elasticity simulations, providing new insights into their mechanisms and characteristics.

Contribution

It introduces a finite element model that successfully captures necking and shear striping phenomena, with novel numerical confirmation of stripe width dependence on nematic penetration depth.

Findings

Necking can be modeled by increasing the uniaxial order parameter with stretch.

Shear striping involves a fixed uniaxial order parameter and shows stripe width dependence on nematic penetration depth.

Finite element simulations accurately reproduce observed deformation patterns in liquid crystal elastomers.

Abstract

We model within the framework of finite elasticity two inherent instabilities observed in liquid crystal elastomers under uniaxial tension. First is necking which occurs when a material sample suddenly elongates more in a small region where it appears narrower than the rest of the sample. Second is shear striping, which forms when the in-plane director rotates gradually to realign and become parallel with the applied force. These phenomena are due to the liquid crystal molecules rotating freely under mechanical loading. To capture necking, we assume that the uniaxial order parameter increases with tensile stretch, as reported experimentally during polydomain-monodomain transition. To account for shear striping, we maintain the uniaxial order parameter fixed, as suggested by experiments. Our finite element simulations capture well these phenomena. As necking in liquid crystal elastomers has not been satisfactorily modelled before, our theoretical and numerical findings related to this effect can be of wide interest. Shear striping has been well studied, yet our computed examples also show how optimal stripe width increases with the nematic penetration depth measuring the competition between the Frank elasticity of liquid crystals and polymer elasticity. Although known theoretically, this result has not been confirmed numerically by previous nonlinear elastic models.