Photosensitive Liquid Crystal High Projectile Liang Guangli Coupling 2-d Finite Element Model And Light To Bend Characteristics Analysis

Liquid crystal elastomers are cross-linked polmers with liquid crystalline mesogens. They combine the elasticity of polymers with the liquid crystalline properties so their mechanical behaviors have the characteristic of thermo-mechancial-order coupling, anisotropy and soft elasticity. Photochromic liquid crystal elastomers can deform largely under light, and even bend. Photochromic LCEs have potential applications as smart materials. In this paper, finite element method is carried out to study and analyze the light induced bending behavior of LCEs.Firstly, the light-induced constitutive relations are obtained by introducing the photoisomerization and its effect on the nematic-isotropic transitions into the nonlinear constitutive equations of LCEs which are based on the neo-classical elastic free energy. We linearize the stress-strain relation based on the assumption of infinitesimal strains and ignore the effect of the deformation on the light propagation and the effect of the stress on the order parameters. The constitutive equations for plane stress problem are derived. The stress-strain relations for the normal stress components look very similar to the linearly themoelastic Hooke’s law with the thermal strain replaced by the anisotropic opto strains. The shear stress in the plane of the director is not zero but proportional to the rotation of the director. Since the obtained stress-strain relation is very unusual, there is no ready-made material model in commercial finite element software. We make secondary development of the user material subroutine UMAT provided by the general software ABAQUS to achieve finite element analysis and modeling of light-induced bending in LCEs.Next, we discuss the effects of some parameters, such as light intensity, decay distance, temperature, geometry ration on the material porpertis and light induced bending pattern. The numerical results show that the light induced contraction and bending of two dimensional specimens can be simulated by our model. It is found that if the decay distance of the light due to the absorption is much large than the height of the specimen, approximately homogeneous contraction in length and expansion in height can be induced by uniform light illuminations. If the ratio of decay distance vs. height, d/h is not too large, the specimen will bend toward the incoming light in addition to a contraction in length and expansion in height. The bending seems quite identical for long and short specimens but depend strongly on the incoming light intensity. The dependence of the bending magnitude on the light intensity is generally non-monotonic, increasing firstly to a maximum and then decreasing to zero for very large light.Moreover, the distributions of the stresses and strains are analyzed in some details for specimens under uniform light illuminations. The results indicate that the driving force for the light induced bending is the normal stress component in the direction of the LC director. Although the shear stress is very small comparing to the main stress component, it is generally not zero under light illuminations in contrast to pure mechanical loading of soft LCEs. The Cauchy shear stain is generally not small but in the same order as the other normal strain components. Thus, the deformed transversal cross line does not remain normal any more, although it can remain straight after the light induced bending if the light intensity or the decay distance is not too small and the LCE specimen is in the deep nematic phase. Therefore, the classical Euler-Bernoulli Beam Theory cannot be directly applied to model the light induced bending of soft LCEs. The above unusual quasi-soft opto-mechanical behavior is certainly a consequence of the constitutive relations based on the neo-classical elastic energy of LCEs. We develop a modified Euler-Bernoulli Theory by considered shear effect.