R. Brett Williams

Nonlinear tensile and shear behavior of macro fiber composite actuators

The Macro Fiber Composite (MFC) actuator, developed at the NASA Langley Research Center, offers much higher flexibility and induced strain levels (~2000μσ, peak-to-peak) than its monolithic piezoceramic predecessors. The focus of this work is twofold; to measure the four independent linear elastic engineering constants of the orthotropic MFC actuator under short-circuit electrical boundary conditions using standard tensile testing procedures, and to use these experimental results to characterize the nonlinear tensile and shear stress–strain behavior and Poisson effects using various plastic deformation models. The results can then be readily incorporated into the piezoelectric constitutive equation and ultimately into structural actuation models that accurately consider nonlinear mechanical behavior.

Manufacturing and Mechanics-Based Characterization of Macro Fiber Composite Actuators

The use of piezoelectric ceramic materials for structural actuation is a fairly well developed practice that has found use in a wide variety of applications. However, just as advanced composites offer many benefits over traditional engineering materials for structural design, actuators that utilize the active properties of piezoelectric fibers can improve upon many of the limitations encountered with monolithic piezoceramic devices used to control structural dynamics. This paper discusses the Macro Fiber Composite (MFC) actuator, which utilizes piezoceramic fibers, for example, lead zirconate titanate (PZT), embedded in an epoxy matrix for structural actuation. An overview of the MFC assembly process is presented, followed by a cure kinetics model that describes the behavior of the thermosetting matrix. This empirical model is seen to agree closely with the experimental data. Lastly, a hybrid classical lamination theory is developed to predict the linear elastic properties of the MFC package as a function of the PZT fiber lamination angle.Copyright

TEMPERATURE-DEPENDENT THERMOELASTIC PROPERTIES FOR MACRO FIBER COMPOSITE ACTUATORS

This research effort models the thermoelastic properties of the macro fiber composite actuator as a function of temperature. The required temperature-dependent properties of each constituent material are obtained, and the orthotropic layer properties are calculated using a variety of micromechanics models, with the most accurate being selected based on a comparison with ANSYS finite element models. Equations for the four independent stiffness parameters and two coefficients of thermal expansion of the entire actuator are derived using a classical lamination approach. These results agree closely with an ANSYS finite element model of the unit cell of the macro fiber composite actuator.

Manufacturing and Cure Kinetics Modeling for Macro Fiber Composite Actuators

The use of piezoelectric ceramic materials for structural actuation is a fairly welldeveloped practice that has found use in a wide variety of applications. However, just as advanced composites offer many benefits over traditional engineering materials for structural design, actuators that utilize the active properties of piezoelectric fibers can improve upon many of the limitations encountered with monolithic piezoceramic devices used to control structural dynamics. This paper discusses the Macro Fiber Composite (MFC) actuator, which utilizes piezoceramic fibers, for example, lead zirconate titanate (PZT), embedded in an epoxy matrix for structural actuation. An overview of the MFC assembly process is presented, followed by a cure kinetics model that describes the behavior of the thermosetting epoxy matrix. This empirical model is seen to agree closely with the experimental data.

Limitations of Using Membrane Theory for Modeling PVDF Patches on Inflatable Structures

An underlying goal in structural modeling is to use the simplest mathematics possible that captures the physics of a problem accurately. Inflatable structures are normally fabricated from thin films, so they are often modeled as membranes, i.e., structural elements that cannot resist bending moments. Researchers have recently been looking at active control of inflated structures, so this raises the question of whether membrane theory can account for the effects of surface-mounted piezopolymer patches used as either sensors or actuators. This work discusses these effects on the dynamic behavior of a flat, rectangular coupon section and assesses the patch’s ability to sense and actuate transverse deflections of the thin film substrate using traditional membrane theory. The Rayleigh-Ritz method was employed to approximate the natural frequencies and mode shapes of this layered system. While including the additional mass of the patch, traditional membrane theory was unable to account for the added stiffness of the patch layer. When the piezoelectric behavior of the patch was considered, membrane theory failed to model the PVDF as a useful sensor. Also, excitation of transverse vibrations was not possible using membrane theory, which does not allow application of bending moments. However, PVDF actuation was modeled as an applied in-plane force, which allowed the patch the ability to suppress out-of-plane disturbances by altering the tension in the base layer as a function of applied voltage. This article discusses the limitations associated with using traditional membrane theory to analyze the dynamic behavior of thin-layered systems as well as model the interaction between an active PVDF patch and the torus substrate.

An analytical model of the mechanical properties of the single crystal Macro-Fiber Composite actuator

While exhibiting powerful characteristics, monolithic piezoelectric sensors and actuators suffer from many drawbacks due to inherent material properties and implementation issues. As a result of their stiff structure and primary operating principle, monolithic piezoelectric wafers perform poorly in a variety of important engineering applications. Piezoelectric Fiber Composites (PFCs) offer one possible solution to these limitations. Mechanically flexible and functioning on the basis of the d33 effect, these actuators enable and improve many piezoelectric applications. The NASA-Langley Research Center recently developed the Macro-Fiber Composite (MFC) actuator to address several shortcomings in the operational characteristics of competing PFC packages. While the construction of this actuator results in many advantages over comparable PFCs, potential exists for improvement in the design of the MFC. Thus, the single-crystal MFC is proposed. Single-crystal PMN, a specific piezoceramic compound, comprises the piezoceramic fibers of the proposed device, contributing larger piezoelectric coupling, higher bandwidth and higher stiffness to the MFC configuration. Development of this new actuator necessitates extensive characterization of its electromechanical properties. This paper describes the development and computational results of a short-circuit stiffness model that produces the four independent mechanical properties which describe the single-crystal MFC. Modeling results are compared to those of the standard MFC.

Lightweight Deployable Sunshade Concepts for Passive Cooling for Space-Based Telescopes

Deployable multi-layer sunshade concepts were developed to meet the thermal and mechanical requirements for two versions of the EPIC (Experimental Probe of Inflationary Cosmology) mission study for NASA’s Einstein Inflation Probe. The sunshield’s primary function is keeping sunlight off the progressively colder parts of the instrument. This goal is accomplished using 3 shades, each an extension of 3 stages of a rigid V-groove cooler. The layers of the sunshade are arranged so the V-grooves have a large view angle out to space for maximum radiative cooling. The thin, aluminized-Kapton layers of the sunshade have low thermal conductivity and thus do not contribute to the effective area for passive cooling. This article discusses two deployment designs. One concept is based on a simple hinged scheme using high-TRL components and is envisioned for smaller, less expensive sunshades ( ~10m). Some of the mechanical design considerations and modeling of the lenticular struts and the dynamics of the sunshade system are also included.

Structural Feasibility Analysis of a Robotically Assembled Very Large Aperture Optical Space Telescope

This paper presents a feasibility study of robotically constructing a very large aperture optical space telescope on-orbit. Since the largest engineering challenges are likely to reside in the design and assembly of the 150-m diameter primary reflector, this preliminary study focuses on this component. The same technology developed for construction of the primary would then be readily used for the smaller optical structures (secondary, tertiary, etc.). A reasonable set of ground and on-orbit loading scenarios are compiled from the literature and used to define the structural performance requirements and size the primary reflector. A surface precision analysis shows that active adjustment of the primary structure is required in order to meet stringent optical surface requirements. Two potential actuation strategies are discussed along with potential actuation devices at the current state of the art. The finding of this research effort indicate that successful technology development combined with further analysis will likely enable such a telescope to be built in the future.

Local Effects of Piezopolymer Patches on Inflatable Space-Based Structures

Inflatable structures are often characterized as membranes (that is, structural elements that cannot resist bending moments). This simplification raises the question of whether membrane theory can account for the effects of active, surface-mounted piezopolymer patches. This work discusses these effects on the dynamic behavior of a flat, rectangular coupon section and assesses the patchs ability to sense and actuate transverse deflections of the thin-film substrate using traditional membrane theory. The Rayleigh-Ritz method was employed to approximate the natural frequencies and mode shapes of this layered system. Although including the additional mass of the patch, traditional membrane theory was unable to account for the added stiffness of the patch layer. When the piezoelectric behavior of the patch was considered, membrane theory failed to model the piezopolymer as a useful sensor. Also, excitation of transverse vibrations was not possible using membrane theory, which does not allow application of bending moments. However, piezoelectric actuation was modeled as applied in-plane forces, which enabled the patch to suppress out-of-plane disturbances by altering the tension in the base layer as a function of applied voltage.