Analytical and Numerical Approaches on the Stiffness of Magnetorheological Fluid Filled Spring

A solid mechanical spring generally exhibits uniform stiffness. This thesis studies a mechanical spring filled with magnetorheological (MR) fluid to achieve controllable stiffness. The hollow spring filled with MR fluid is subjected to a controlled magnetic field in order to change the viscosity of the MR fluid and thereby to change the overall stiffness of the spring. MR fluid is considered as a Bingham viscoplastic linear material in the mathematical model. The goal of this research is to study the feasibility of such spring system by analytically and numerically computing the effects of MR fluid on the overall spring stiffness. For this purpose, spring mechanics and MR fluid behavior are studied to increase the accuracy of the analytical analysis. Numerical simulations are also performed to generate some assumptions, which simplify calculations in the analytical part of the analysis. The accuracy of the present analytical approach is validated by comparing the results to previously known experimental results. Overall stiffness variations of the spring, calculated through the developed equations, are also discussed for different spring designs. Simulation of a helical hollow spring with an annular cross section filled with MR fluid is performed using ANSYS by means of two-way Fluid-Structural Interaction (FSI). The simulation shows that MR fluid effect is capable of controlling the stiffness of the spring in some ranges

Additively Manufactured Dielectric Elastomer Actuators: Development and Performance Enhancement

The recently emerging and actively growing areas of soft robotics and morphing structures promise endless opportunities in a wide range of engineering fields, including biomedical, industrial, and aerospace. Soft actuators and sensors are essential components of any soft robot or morphing structure. Among the utilized materials, dielectric elastomers (DEs) are intrinsically compliant, high energy density polymers with fast and reversible electromechanical response. Additionally, the electrically driven work principle allows DEs to be distributed in a desired fashion and function locally with minimum interference. Thus, a great effort is being made towards utilizing additive manufacturing (AM) technologies to fully realize the potential of DE soft actuators and sensors. While soft sensors have received more attention and development due to their simpler implementation, DE actuators (DEAs) set stricter AM and electrode material requirements. DEAs’ layered structure, compliant nature, and susceptibility to various defects make their manufacturability challenging, especially for non-trivial biomimetic soft robotics geometries. This dissertation comprehensively analyzes DE materials’ transition into a soft actuator using AM to facilitate effective DEA soft actuator fabrication. Closely interrelated fabrication techniques, material properties, and DEA geometries are analyzed to establish a fundamental understanding of how to implement high-quality DEA soft actuators. Furthermore, great attention is paid to enhancing the performance of printed DEAs through developing printable elastomer and electrode materials with improved properties. Lastly, performance enhancement is approached from the design point of view by developing a novel 3D printable DEA configuration that actuates out-of-plane without stiffening elements

Additively Manufactured Morphing Structures with Embedded Smart Actuators

Observing volant creatures has demonstrated that adapting the shape of the wing to the changing flight environment increases flight efficiency and performance. Current aerial vehicles have stiff aerodynamic surfaces that limit any adapting capability. The development of the concept of fully morphing structures is enabling the creation of bio-inspired, adaptable structures with outstanding performance. However, current morphing structures suffer from poor implementation that often brings more drawbacks than advantage to the final product. This research focuses on an effective implementation of morphing technology to fully realize it\u27s potential. This can be achieved by employing a novel additive manufacturing method that can fabricate morphing structures with integrated and distributed actuation systems. Dielectric elastomer actuators (DEAs) are one of the most intensively studied soft, smart actuators due to their promising electromechanical properties. As such, this project utilizes DEAs as the primary material for the morphing structure. Preliminary work has been completed in selecting and validating the additive manufacturing method as well as material selection and improvement. The main goal of this research is to implement additive manufacturing coupled with morphing structures to design, build and test a fully morphing wing structure suitable for small aerial vehicles

Additively manufactured unimorph dielectric elastomer actuators: Design, materials, and fabrication

Dielectric elastomer actuator (DEA) is a smart material that holds promise for soft robotics due to the material’s intrinsic softness, high energy density, fast response, and reversible electromechanical characteristics. Like for most soft robotics materials, additive manufacturing (AM) can significantly benefit DEAs and is mainly applied to the unimorph DEA (UDEA) configuration. While major aspects of UDEA modeling are known, 3D printed UDEAs are subject to specific material and geometrical limitations due to the AM process and require a more thorough analysis of their design and performance. Furthermore, a figure of merit (FOM) is an analytical tool that is frequently used for planar DEA design optimization and material selection but is not yet derived for UDEA. Thus, the objective of the paper is modeling of 3D printed UDEAs, analyzing the effects of their design features on the actuation performance, and deriving FOMs for UDEAs. As a result, the derived analytical model demonstrates dependence of actuation performance on various design parameters typical for 3D printed DEAs, provides a new optimum thickness to Young’s modulus ratio of UDEA layers when designing a 3D printed DEA with fixed dielectric elastomer layer thickness, and serves as a base for UDEAs’ FOMs. The FOMs have various degrees of complexity depending on considered UDEA design features. The model was numerically verified and experimentally validated through the actuation of a 3D printed UDEA. The fabricated and tested UDEA design was optimized geometrically by controlling the thickness of each layer and from the material perspective by mixing commercially available silicones in non-standard ratios for the passive and dielectric layers. Finally, the prepared non-standard mix ratios of the silicones were characterized for their viscosity dynamics during curing at various conditions to investigate the silicones’ manufacturability through AM

Effects of Ferroelectric Fillers on Composite Dielectric Elastomer Actuator

Integrating nano- to micro-sized dielectric fillers to elastomer matrices to form dielectric composites is one of the commonly utilized methods to improve the performance of dielectric elastomer actuators (DEAs). Barium titanate (BaTiO3) is among the widely used ferroelectric fillers for this purpose; however, calcium copper titanate CaCu3Ti4O12 (CCTO) has the potential to outperform such conventional fillers. Despite their promising performance, CCTO-based dielectric composites for DEA application are studied to a relatively lower degree. Particularly, the composites are characterized for a comparably small particle loading range, while critical DEA properties such as breakdown strength and nonlinear elasticity are barely addressed in the literature. Thus, in this study, CCTO was paired with polydimethylsiloxane (CH3)3SiO[Si(CH3)2O]nSi(CH3)3 (PDMS), Sylgard 184, to gain a comprehensive understanding of the effects of particle loading and size on the dielectric composite properties important for DEA applications. The dielectric composites’ performance was described through the figures of merit (FOMs) that consider materials’ Young’s modulus, dielectric permittivity, and breakdown strength. The optimum amounts of the ferroelectric filler were determined through the FOMs to maximize composite DEA performance. Lastly, electromechanical testing of the pre-stretched CCTO-composite DEA validated the improved performance over the plain elastomer DEA, with deviations from prediction attributed to the studied composites’ nonlinearity