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Our research is focused on fundamental field-coupled phenomena of adaptive materials and their integration into structures and devices. New methodologies that couple microstructure evolution with large deformation continuum mechanics are under investigation to unify a broad range of "smart" materials. Combinations of theory and numerical methods, including both determinstic and stochastic based models are being used to understand materials ranging from ferroelectric compositions, magnetostrictive compounds, shape memory alloys, and a number of functional elastomers and nanocomposites. These new methodologies are critical to advance the understanding of adaptive materials and facilitate the design of adaptive structures. Several specific examples on on-going projects are listed below.


Multi-phyiscs and Multiscale Modeling of Liquid Crystal Elastomers

We are exploring the field-coupled properties of liquid crystal elastomers (LCEs) for integration into a number of adaptive structure applications. These materials exhibit a variety of fascinating properties including photomechanical coupling, shape memory, electrostriction, and chemically induced deformation. Our current work is focused on understanding how liquid crystal domain structures create fast response, large deformation in the presence of polarized light and mechanical loads. Soft elasticity of a nematic phase liquid crystal elastomer is shown in the first image on the right. This monodomain to polydomain transitions that explain soft elastic behavior. The second image illustrates photoelastomer deformation from polarized light. A cubic B-spline plate model was used to obtain high speed computations for nonlinear control applications.

Unifying Model Development of Smart Materials

Material modeling techniques for active materials include a broad number of approaches that are typically focused on predicting a specific field-coupled constitutive relation. Limited work has been conducted on developing a unified theory. Such theories are useful for quantifying underlying field-coupled mechanics concepts to facilitate future materials development and design of adaptive structures. A theoretical approach and computational framework is under development that couples nonlinear continuum mechanics coupled to a set of order parameters that govern field-coupled microstructure evolution and phase changes. Unifying concepts are obtained which illustrate how material constants such as piezoelectric or magnetostrictive coefficients can be predicted without introducing explicit phenomenological parameters as well as nonlinear ferroelectric behavior. An example is shown to the left that illustrates complex soft elasticity in polydomain liquid crystal elastomers under uniaxial loading. In this case, the material model was cross-linked in the isotropic state similar to recent experimental results.

Structural Evolution of Protein Fibrils

Protein fibrils such as the RADA16-I protein have unique self-healling characteristics. These biological 1D nanostructures break into seed particles when exposed to ultrasonic radiation. Self-assembly and self-repair occurs when the ultrasonic radiation is turn-off. Many proteins require additional monomer to create self-repair; however, the RADA is unique. It will spontaneously self-heal. We are currently probing these materials through modeling and experimental comparisons to understand how to design self-healing structures. The top images are TEM results of 1D protein fibrils synthesized by the PI's collaborator, Dr. Anant Paravastu. The bottom images are simulations describing the structural-chemical behavior coupled with nonlinear continuum mechanics and mass flux that predicts protein evolution. Time increases from left to right in the experiments and simulation.

Active Flow Control

Steady flow microjets have shown significant enhancements in mitigating separation flow and reducing noise on a number of aircraft structures including STOVL (short take-off and vertical landing) aircraft, cavity bays, and rotor blades. However, variations in the mass flux is required to achieve a desire performance criteria. Pulsed mass flow injection is believed to enhance flow control; however, broadband actuators that can achieve the necessary momentum for superior control authority are limited. We are currently integrating piezoelectric materials into active microjet systems to understand high speed pulsed microflow. This collaboration, with the Advanced Aero Propulsion Laboratory (Dr. Farrukh Alvi), is focused on synergies between experimental fluid dynamics and next generation smart structures for aerospace applications. Two example is shown here. The first actuator, a piezohydraulic system, is used for fundamental pulsed flow characterization from quasi-static up to ~800 Hz. The second actuator eliminates hydraulic fluid by using an activel deforming converging-diverging nozzle that is control by a piezoelectric actuation.

Reduced order homogenization and nonlinear control

Adaptive materials such as piezoelectric and magnetostrictive materials provide broadband actuation for many micron and nanopositioning systems. One of the challenges with utilizing these materials requires a strong understanding of the constitutive behavior. Second, high speed computational material models are desired that can be integrated into control designs. We are currently expanding stochastic based 1D models to multi-axial loading for control of higher dimensional structures such as plates or 3D structures. Comparison with data on a PZT ceramic (Huber and Fleck, JMPS) is illustrated. Control experiments shown on the right have been conducted using these models within a nonlinear control design to control a magnetostrictive actuator at 1 kHz.

Multi-axial modeling and comparison with data. High speed tracking control of a magnetostrictive actuator.

Ferroelectric and Flexoelectric Thin Film Electromechanics

Ferroelectric materials are one of the most important technological materials for many electronic and electro-mechanical microsystems. Once the size of devices converges to the micron or nanoscale, new and unexpected behavior can occur. This was recently identified in micronscale ferroelectric thin film capacitor islands. Phase field modeling was used to illustrate abnormal polarization "back switching" for tetragonal phase lead zirconate titanate films grown in the <111> direction (see image). Defect structures, residual stress, charge compatible domain structures, and flexoelectricity contributed to the abnormal behavior. The nucleation of abnormal domain switching also strongly depended on the top electrode (TEL) thickness and lateral dimensions based on scaling laws based on theoretical predictions and finite element calculations. One example is shown by the plot of free energy vs. "defect" domain size which illustrates a change in the critical defect size for reverse switching.