Ratna Kumar Annabattula

Light-induced oscillatory behavior of liquid crystal polymer network (LCN) films has been demonstrated by several researchers in the past decade. Similarly, oscillations in LCN films under constant thermal stimulus have been reported recently, although the mechanism and the factors that govern the oscillatory behavior are not well understood. In this work, we study the dynamics of self-sustained oscillations exhibited by LCN films under a constant thermal stimulus through experiments and simulations. Geometrically asymmetric films such as a right triangle and an equilateral triangle are obtained from a twisted nematic square film. A multiphysics computational framework using the finite element method is developed to simulate the oscillatory behavior of the LCN films kept on a hot plate. The framework accounts for a coupling between heat transfer and mechanical deformations during the oscillations. Small temperature fluctuations (≈ 1 °C) coupled with gravity induced torque are shown to drive the oscillatory behavior at a specific plate temperature. We show for the first time that self-sustained oscillations can also be achieved in symmetric shapes, such as square films, by creating a thickness tapering between two opposite edges. The frequency of the oscillations is found to be in the range of 0.5 to 2.5 Hz for different geometries studied. The oscillation temperature depends on the mean thickness, size, and thickness profile of the films. As a possible application, we demonstrate a thermally actuated optical chopper using the oscillatory response of the films.

Nanocomposite coating films have been increasingly used in industrial applications because of their unique mechanical and physical properties. Residual stresses generated during the growth of sputter-deposited thin films due to a strain mismatch between the film and the substrate may lead to significant failure problems. Large residual stresses may generate buckling, delamination and film fracture. Although buckles with cracks in thin films have been experimentally observed, their origins are still not well understood. In this work, finite element simulations in Abaqus/Explicit are employed to study buckling and cracking in Ti-Si-N films on a silicon substrate. Residual stresses in the film are generated using two loading methods: 1) Eigenstrain is applied via a temperature field; 2) An initial stress field is applied. Cracking is modelled using an elastic material model with a brittle fracture criterion that takes into account the tensile strength of the material to initiate damage. It is found that while both loading methods lead to similar buckling patterns and stresses, an initial stress field generates premature film failure and thus the thermal field loading should be used. The numerical model fairly predicted the cracking patterns observed in the experiments. © (2014) Trans Tech Publications, Switzerland.

Lithium Metatitanate (Li2TiO3 or LMT) and Lithium Orthiosilicate (Li4SiO4 or LOS) are the prominent among the suitable candidate materials for breeders. Robust design of the breeder blankets requires a thorough understanding of the thermo-mechanical response of the breeder materials at different loading conditions. In this context, the material characterization plays a vital role. Solid breeder concept of various member nations comprises of pebbles in steel container. Determination of effective properties of the pebble beds helps in understanding and formulation of constitutive relations for such granular systems. In order to develop simulation tools to understand the complex thermo-mechanical response of the breeder units, the properties of the material should be known. Hence, the objective of this work is to characterize crush load of Li2TiO3 pebbles and investigate the thermo-mechanical response of the pebble beds. The study involves determining the crush load of individual pebbles and pebble-bed elastic modulus. Uniaxial oedometric tests are performed to investigate the influence of temperature on the effective elastic modulus of the pebble beds.

In process industries, the study of granular mixing can substantially optimize energy costs and the quality of final products. In this work, we study the mixing of binary granular systems assisted by fluid flow through the assembly. The granular system is exposed to continuous fluid flow from the bottom to the top. Particle mixing is simulated by the coupling of Computational Fluid Dynamics (CFD) and the Discrete Element Method (DEM). Parameters like particle stacking order, density ratio, and size ratio show a significant influence on mixing. The presence of tilted plates as a geometric intervention with different configurations plays a critical role in mixing efficiency. A new mixing index which captures the nature of contact creation, and the degree of mixing is proposed. The present work helps develop efficient strategies for particle mixing in fluidized beds.

The packing structure of the pebbles influences the thermo-mechanical behavior of the pebble beds in fusion breeder blankets. The packing structures at the wall of the container plays an important role in the extraction of heat energy out of the pebble bed to the surroundings. The packing fraction varies from the wall to the bulk region (away from wall). Hence, it is essential to understand the variation of packing fraction across the pebble bed. Packing structures were previously studied through X-ray tomography experiments and computer simulations. Computer simulations of packing were done using random closed packing algorithm, but without considering the gravity effect. Such simulations were able to predict only the wall-effect. In this paper, the packing structures are generated using discrete element method (DEM) by pouring the pebbles into prismatic containers under gravity. DEM simulations reveal that the extent of wall effect on the packing structure is not the same in all the directions. Due to the presence of gravity, the bottom wall (i.e. in the direction of gravity) induces regular packing up to 6 layers while the lateral walls influence is seen only up to 4 layers. Further, it is shown that the packing structure is also influenced by various filling strategies. The present work studies the evolution of packing structure through various filling strategies with an emphasis on the extent of wall effect on the packing structure.

The breeder materials proposed for the solid tritium breeding blanket concepts are ceramic lithium-based compounds in the form of pebble beds. Different fabrication processes have been developed to produce pebbles with a high sphericity. However, a small deviation from a perfectly spherical shape exists. In this paper the influence of non-sphericity on the mechanical behaviour of a pebble bed is assessed representing the currently produced pebbles by means of ellipsoidal particles. To this end, the in-house Discrete Element Method code KIT-DEM was further extended. The multi-sphere approach was implemented to generate the ellipsoidal particles while the existing random close packing algorithm was modified to create the assemblies. Uniaxial compression of the assemblies, under periodic boundary conditions, was simulated to investigate the bulk stress-strain behaviour of the bed. Sensitivity studies were carried out with different packing factors of the assembly and several aspect ratios of the particles. In agreement with previous studies carried on assemblies of spherical pebbles, the initial packing factor was found to noticeably affect the mechanical response of the investigated assemblies. Moreover, a remarkable influence of the shape on the mechanical behavior of the simulated assemblies was observed. Therefore it is concluded that for production techniques that result in poor sphericity, DEM simulations with non-spherical particles are necessary to reproduce realistic stress-strain behavior of pebble beds.

High performance thermoresponsive soft, controllable and reversible actuators are highly desirable for diverse applications. The practical implementation of the existing poly(N-isopropylacrylamide) (pNipam) based soft thermoresponsive actuators faces serious limitations due to their functional requirement of proximal bulk solvent medium. In this work, addressing this issue, we report the development of a bilayer based actuator composed of a solvent responsive biodegradable polymer and temperature responsive pNipam. The designed bilayer is capable of achieving reversible and irreversible actuation as needed when exposed to a physiological range of body temperature, without any solvent bath around. The solvent or water supplied by the pNipam layer at its lower critical solution temperature (LCST) builds a concentration gradient across the thickness of the polymer layer. The concentration gradient results in a strain gradient, causing an out-of-plane folding of the bilayer. The underlying coupled diffusion-deformation interaction during folding and unfolding is incorporated in the reported finite element model, capable of predicting actuation characteristics under different initial conditions. The combined experimental and modelling effort in this work highlights the possibility of engineering 2-dimensional films into complex 3-dimensional shapes, which could have potential applications in soft machines and robotics.

In the recent years, the phase field method for simulating fracture problems has received considerable attention. This is due to the salient features of the method: 1) it can be incorporated into any conventional finite element software; 2) has a scalar damage variable is used to represent the discontinuous surface implicitly and 3) the crack initiation and subsequent propagation and branching are treated with less complexity. Within this framework, the linear momentum equations are coupled with the diffusion type equation, which describes the evolution of the damage variable. The coupled nonlinear system of partial differential equations are solved in a ‘staggered’ approach. The present work discusses the implementation of the phase field method for brittle fracture within the open-source finite element software, FEniCS. The FEniCS provides a framework for the automated solutions of the partial differential equations. The details of the implementation which forms the core of the analysis are presented. The implementation is validated by solving a few benchmark problems and comparing the results with the open literature.

Recently researchers have developed soft actuators capable of morphing into complex shapes taking inspiration from nature. In this paper, we have developed a splay-nematic liquid crystal polymer network tapered actuator that can morph from a flat film to a cone, mimicking the blooming of single petal Calla Lily flower. We have demonstrated the formation of conical tubes through finite element simulations and experiments. The influence of tapering and alignment orientations with respect to the edge of the film on the cones is analyzed through simulations. The design with tapering and splayed alignments oriented at 45° to the edge is found to be the optimal choice for forming conical tubes.

In this paper we present a computational model to simulate surface waves on photo-responsive liquid crystal thin films under dynamic illumination. The influence of light attenuation is included to model the dynamic photo-mechanical response as a result of the forward (trans to cis) and the backward (cis to trans) isomerization of the embedded azobenzene molecules. The viscoelastic nature of the liquid crystal films is also estimated and incorporated in this study. The temporal response of the material under static spot illumination is analyzed first to understand the underlying mechanisms behind the observed response. The relation between the fraction of transformed molecules and the macroscopic light induced strains and stresses through the thickness is investigated to establish the significance of light attenuation. Subsequently, the model liquid crystal film is exposed to a rectangular illumination waveform to simulate a moving light source and thereby generate surface waves. Results are presented that relate the wave attributes, (i.e., the amplitude, frequency and phase lag) to the time scales associated with the moving light source, the molecular trans-cis isomerization reaction and the viscoelastic response of the material. A design guideline for producing surface waves with specific features (amplitude, phase) is presented.