The fundamentals of designing deployable structures with elastic memory composites

Elastic Memory Composite (EMC) materials exhibit many favorable qualities for deployable space structures and have piqued a broad interest within America's deployable space structures industry. EMC materials are similar to traditional fiber-reinforced composites except for the use of a thermoset shape memory resin that enables EMC materials to achieve much higher failure strains than traditional composites when exercised through a very specific thermomechanical load cycle. This unique behavior is exploited in the development of EMC structural components that exhibit very high packaging-strain capability while guaranteeing predictable postdeployment structural performance. This paper presents a review of the current state of development of EMC technologies and EMC deployable space structure components. The paper is intended primarily to help potential users of EMC materials develop a better understanding of the special capabilities of EMC materials and considerations that must be applied while engineering with EMC materials.

Furlable structures are a class of deployable structures that utilize distributed material strain as a means for compact packaging. Elastic Memory Composite (EMC) materials are ideally suited for furlable structures because they offer high strength, stiffness, and strain capacity for efficient packaging and deployed structural performance. EMC components are packaged in their soft-resin state, which enables higher packaging strains than traditional hard-resin composites. For example, packaging strains on the order of 2-5% are commonly achieved in carbon-fiber-reinforced EMC materials whereas non-EMC, carbon-fiber composites are limited to less than 1% strain. The present paper summarizes two recent advancements in the state-of-the-art of EMC materials: 1) improvments to the design of EMC longerons, key structural elements in furlable truss booms, and 2) advancements in analytical models for EMC material behavior through the use of a Multicontinuum Theory. Preliminary results are presented that illustrate the significant potential for these advancements to further improve the performance and capability of furlable EMC space structures.

Near-term and future spacecraft and satellites will require large ultra-lightweight structures and components that must be efficiently packaged for launch and reliably deployed on orbit. A new material technology called Elastic Memory Composite (EMC) materials, shows promise in meeting these needs. The EMC polymer matrix materials enable a fully cured composite structure or component to be deformed or folded for efficient packaging into a spacecraft or launch vehicle, then regain its original shape with no degradation or loss in mechanical or physical properties. A component using EMC materials is fabricated in its deployed, on orbit shape using conventional composite manufacturing processes. Then by heating the material and applying force this fully cued composite material can be folded or deformed for packaging. When cooled, it will retain the packaged shape indefinitely. When reheated the structure will regain its original shape with little or no external force. This packaging/deployment cycle is reversible. This paper reviews new developments in EMC materials technology including material properties, analytical and designs tools, testing and evaluation protocols, and new applications.

The growing application of TEMBO Elastic Memory Composite (EMC) materials for deployable space structures has highlighted the need for an improved understanding of the material’s mechanical behavior. Recent mechanical testing has revealed significant nonlinearity in the stress-strain response of highly-aligned, unidirectional EMC materials. It is hypothesized that the nonlinearity in the material’s longitudinal modulus is due to a combination of nonlinear behavior inherent in the carbon fiber reinforcement and processinduced wavy fiber imperfections in EMC laminates. A model is developed accounting for fiber nonlinearity and process-induced wavy fibers and using this model, a data-analysis protocol is developed that allows the relative contribution of these effects to be separated out of the observed global nonlinear stress-strain response. In essence, this protocol provides an indirect method of measuring the effective magnitude of the wavy fiber imperfections, given an accurate knowledge of the nonlinear elastic response of the fibers, or vice-versa. The results for the test laminates considered indicate that the nonlinearity in the fiber reinforcement is predominately responsible for the nonlinear behavior of the composite material. However, effects due to process-induced wavy fibers on the longitudinal modulus can become significant at relatively high compression strains.

Shape memory polymers (SMPs) are a new type of smart material, they perform large reversible deformation with a certain external stimulus (e.g., heat and electricity). The properties (e.g., stiffness, strength and other mechanically static or quasi-static load-bearing capacity) are primarily considered for conventional resin-based composite materials which are mainly used for structural materials. By contrast, the mechanical actuating performance with finite deformation is considered for the shape memory polymers and their composites which can be used for both structural materials and functional materials. For shape memory polymers and their composites, the performance of active deformation is expected to further promote the development in smart active deformation structures, such as deployable space structures and morphing wing aircraft. The shape memory polymer composites (SMPCs) are also one type of High Strain Composite (HSC). The space deployable structures based on carbon fiber reinforced shape memory polymer composites (SMPCs) show great prospects. Considering the problems that SMPCs are difficult to meet the practical applications in space deployable structures in the recent ten years, this paper aims to research the mechanics of deformation, and failure of SMPCs. In the overall view of the shape memory polymer material’s nonlinearity (nonlinearity and stress softening in the process of pre-deformation and recovery, relaxation in storage process, irreversible deformation), by the multiple verifications among theory, finite element and experiments, one obtains the deformation and mechanism for the process of “pre-deformation, energy storage and actuation and its non-fracture constraint domain. Then, the parameters of SMPCs will be optimized. Theoretical analysis is realized by the strain energy function, additionally considering the interaction strain energy between the fiber and the matrix. For the common resin-based or soft-material-based composites under pure bending deformation, we expect to uniformly explain the whole process of buckling occurrence, evolution and finally failure, especially for the early evolution characteristics of fiber microbuckling inside the microstructures. The research results are meaningful for the practical applications for SMPC deployable structures in space. Considering the deformation mechanisms of SMPCs, the local post-microbuckling is required for the unidirectional fiber reinforced composite materials, at the conditions of its large geometrical deflection. The cross section of SMPC is divided into three areas: non-buckling stretching area, non-buckling compressive area, and buckling compressive area. Three variables are considered: critical buckling position, and neutral plane, the fiber buckling half-wavelength. Considering the condition of the small strain and large displacement, the strain energy expression of the SMP/fiber system was derived, which contains two types, e.g., strain energy of SMP and fiber. According to the minimum energy principle, the expression for all key parameters were derived, including the critical buckling curvature, neutral plane position, the buckling half-wavelength, fiber buckling amplitude, and strain.

Elastic-memory composites (EMCs) is a class of polymeric composites that can be folded into a very compact shape and later deployed to the original shape upon reheating. They have significant potential in developing large deployable structures greater than 10m in size that can be stowed in a small launch vehicle and deployed in space to predetermined levels of precision. Generally, the composite is folded by heating it above the glass transition temperature of the resin. The primary deformation mode that allows EMCs to achieve significantly higher effective strains than traditional composites is microbuckling of the compressed fibers. The magnitude of this microbuckling dictates whether the laminate behaves elastically or if the laminate exhibits material failure such as fiber breakage, or fiber kinking. This paper explores the micromechanics of both types of fiber-deformation modes associated with the bending of such soft-resin composites. An analysis method is developed and correlated with experimental data to provide an estimate for the level of folding strain that a soft-resin laminate can achieve based on microbuckling-induced material failure. Additionally, insight is provided into the effects of fiber and matrix material properties on the formation and magnitude of the kink mode. Tests were also conducted to determine the deployment accuracy after a number of folding/deployment cycles to help design deployable sensor structures with these materials.

A new deployable solar array system named RAPDAR™ (Roll-out and Passively Deployed Array) is being developed using elastic memory composite (EMC) booms and Thin-Film Photovoltaic (TFPV) cells. For stowage, the 2.5” diameter EMC booms are heated, flattened, and rolled (along with the TFPV array) around a 4” tip spool. The booms maintain their stowed geometry until heated by radiant solar energy, then deploy with the TFPV array in a controlled fashion by unrolling away from the spacecraft. Passive deployment of EMC booms was demonstrated in a thermal vacuum test, using infrared lamps to simulate the sun with the chamber walls cooled with liquid nitrogen. The booms were covered with MLI to increase their temperature during deployment and decrease their temperature during operation of the solar array. The 8’-long booms were cantilevered horizontally in the chamber with no offloading or secondary support and self-deployed at a rate of 3 inches per minute. A full-scale RAPDAR™ flight experiment is also under development that will use 20’-long booms to deploy an array 5’ wide, which produces roughly 800 watts of power. A full-scale engineering unit of this experimental array will be deployed in a solar-thermal vacuum chamber in April of 2007. This paper presents all testing to validate the solar-heated self-deployment of the booms, including in-air testing, thermal-vacuum testing, repeatability testing, and qualification deployments. The test plan for the full-scale solar simulator deployment is also presented.

range of 1%-1.5%, yet EMC laminates reinforced with carbon fibers are capable of nominal bending strain values of greater than 5% without damage. The primary mechanism that allows these high packaging strains is microbuckling and nonlinear, post-microbuckling response of the carbon fibers. Although progress has been made in predicting EMC behavior through the development of practical, rule-of-thumb design equations, a thorough analytical treatment of the onset of microbuckling and post-microbuckled mechanics of EMC has not yet been achieved. The present paper is a survey of classical microbuckling solutions and an assessment of their applicability for predicting microbuckling response in EMC materials during packaging and deployment. A key conclusion is that these solutions are based on incorrect kinematic assumptions for EMC microbuckling, and hence result in significant errors in computed critical stress/strain and microbuckle wavelength. It is, however, likely that the approach used to derive the classical solutions may be used with modifications to derive more accurate solutions for EMC laminates.

In construction industries, deployable structure has the capability to transform the structures components into predetermined configurations, safe and designated final known direction to achieve their architectural function. This construction technique provides ease of transportation and erection, more space and rapid completion time. This review aims to provide an overview of current potential deployable structure and system towards prefabrication modular construction application to overcome space and transportation limitation. This paper reviews the principle, application, characteristics, types and classification and design consideration of the deployable structures. Based on the literature reviewed, the deployable structure with simple folding configuration principle in linear direction is practical in application. Beside, the strength and stiffness of deployable structure depends on the proper design, material quality, elements thickness, dimension, joint rigidity and deployment technology. However, shape and direction of movement for deployable structure elements need to be determined in the initial stage to minimize the design failure. The findings from this review will contribute to the transformation of prefab volumetric construction practice to solve the limited space and transportation constraints issues.

Controlled shape-morphing metallic components for deployable structures

Multistable structures have widespread applications in the design of deployable aerospace systems, mechanical metamaterials, flexible electronics, and multimodal soft robotics due to their capability of shape reconfiguration between multiple stable states. Recently, the snap-folding of rings, often in the form of circles or polygons, has shown the capability of inducing diverse stable configurations. The natural curvature of the rod segment (curvature in its stress-free state) plays an important role in the elastic stability of these rings, determining the number and form of their stable configurations during folding. Here, we develop a general theoretical framework for the elastic stability analysis of segmented rings (e.g., polygons) based on an energy variational approach. Combining this framework with finite element simulations, we map out all planar stable configurations of various segmented rings and determine the natural curvature ranges of their multistable states. The theoretical and numerical results are validated through experiments, which demonstrate that a segmented ring with a rectangular cross-section can show up to six distinct planar stable states. The results also reveal that, by rationally designing the segment number and natural curvature of the segmented ring, its one- or multiloop configuration can store more strain energy than a circular ring of the same total length. We envision that the proposed strategy for achieving multistability in the current work will aid in the design of multifunctional, reconfigurable, and deployable structures.

Bistable composite tape springs are often used as deployable structures in aerospace applications because of their ability to be tailored for compact size, deployed lengths and stiffness. Analytical studies of the coupling between stress-strain and moment-curvature of thin curved shells have been used to estimate the total stored strain energy in different tape spring configurations. The strain energy as function of the curvatures reveals the fundamental stability behavior of the tape springs in the different configurations. The present study investigates the minimum strain energy path between two stable equilibrium points and the passage through the unstable saddle point along this path. For a coiled fiber-reinforced composite tape spring, the strain energy level for the local minimum and the saddle points will change with respect to time due to viscoelastic effects. To explain the fundamental mechanical behavior of viscoelastic bistable tape springs, a two-dimensional (2D) model composed of rigid links, elastic springs and viscous dampers was proposed. It was shown how the 2D model can be made strain energy-equivalent to a shell model with respect to the minimum strain energy paths. The 2D model was able to capture the fundamental behavior of viscoelastic tape springs during both snap-through and snap-back paths.

Vibration isolation and precision pointing control will be essential to the success of future space science missions such as Next Generation Space Telescope (NGST). This research investigated the use of actively controlled piezoelectric stack actuators to reduce the transmission of mechanical disturbances to the optics and to coarsely point the telescope. Initial actuator requirements were based on approximate expected motions on the spacecraft side (due to unbalanced reaction wheel assemblies), the structural dynamic properties of the flexible, deployable truss structure that supports the active optics, and the allowable motions on the telescope side. Piezoelectric actuator design was based on flextensional motion amplification, with sizing details determined using results from finite element analysis. A compressive pre-load was maintained on the fragile piezoceramic stacks via a mechanical interference (center block) fit into the actuator frame. Device performance characteristics, including blocked force, passive stiffness, free displacement, motion amplification factor, natural vibration frequencies, and electromechanical coupling coefficient were determined numerically and experimentally, with good agreement. An initial controlled isolation test of the entire system reduced the transmissibility by more than 80% over the frequency range from 4-20 Hz. Finally, an active isolation system with dual sensor feedback was implemented and shown to successfully reduce telescope motion by a factor of 10 (from 4-40 Hz) and to provide lower-frequency pointing capability. Introduction As space systems continue to grow in complexity and require greater precision, piezoelectric materials will be investigated for an increasing number of actuator applications, including precision pointing and vibration isolation and control. There are several approaches to vibration isolation, which have been successful in the past. Vibration isolation can be achieved through passive or active techniques or a combination of the two. The chosen method of isolation depends primarily on the application and requirements of the system. Precision pointing, on the other hand, is achieved solely by active control systems, which combine the necessary structural and mechanical components with a control design. Pointing systems are primarily limited by the quality of the mechanical system as well as the drive system, which is actuating movement. The structure (holding the pay load) to be controlled can be a rigid body, quasi-static flexible body, or a dynamic flexible body. With the classical arrangements of sensors and actuators in these control systems, only the rigid body configuration is commonly controlled. NGST (Next Generation Space Telescope) is the successor to the Hubble Space Telescope (HST) mission. NGST is a deployable 8-meter diameter truss structure holding the telescope and instruments such as cameras and spectrometers that are required by the scientific goals of the mission. In order to meet the scientific goals or projects such as the Origins of Galaxies studies [Mather, et al] along with independent science projects dealing with current scientific interests, positioning the optics and minimizing vibratory loads are very important. Positioning control for the HST was done as a rigid structure pointed as a unit to a precision of a small fraction of an arcsecond, but NGST is a dynamic flexible structure with sub-micron positioning performance. For NGST, the isolation system is to be used to reduce the ultrafine pointing requirements on the fine pointing system (fast steering mirror and adjusters distributed across the mirror) by as much as 1 2 orders of magnitude. This isolation system is aimed to reduce the estimated 30-nm peak disturbances at the mast to 6-nm or less in a range of 4 4 0 Hz, with most concern on the 15 20 Hz region. By substantially reducing these axial disturbances, the dynamic, wavefront error correction requirements will also be reduced, and the fine pointing system will have less of a burden. In order to achieve these requirements, piezoelectric stack actuators are ideal because they can provide sufficiently high output forces with large bandwidths. Individually, piezoelectric stack displacements are extremely small, on the order of approximately 2000 jiie; therefore, multiple stacks can be used in series to produce the desired output. In addition to this, mechanical amplification devices can be used to increase actuation displacement by reducing the actuation force. Compliant mechanisms, flexurehinged mechanisms and flextensional mechanisms Copyright © 2001 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. (c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. are commonly used for motion amplification, depending on the application. The flextensional actuator used in this research is shown in Figure I and consists of four stacks of piezoceramics connected in parallel to a flextensional mechanism that amplifies and converts the output

This research deals with materials for deployable large space structures such as large space antennas and tube-type structure support members. Motors are often used as actuators for deployable structures, but the use of functional materials such as shape memory alloys and shape memory polymers used in the field of adaptive structures is also an effective ways from the viewpoint of weight reduction. In this report, we focus on shape memory polymer, and propose a composite material called CF-CNT-SMP by adding thermal conductivity by CNT to the conventional CF-SMP, which is a composite material with carbon fiber that is in charge of high tension. Then the efficiencies of this CF-CNT-SMP are reported by acquiring material properties and shape recovery performance.

During this seminar, I will talk about three of the projects I’ve been involved with at NASA in the last few years: 1. **The Human Exploration Research Analog (HERA)** is a ground-based astronaut analog mission run at NASA’s JSC in Houston to study and evaluate impacts on the crew due to isolation, remoteness, and confined habitation. NASA scientists use the collected data to develop and verify countermeasures to reduce or mitigate psychological and physiological effects for future Deep Space missions. This simulation was a 45-day trip to Mars’s moon Phobos and back with the goal of performing geological operations with complete communications delays in effect. 2. **Astrobee** is a new class of free-flying robots that operates in the interior of the International Space Station (ISS). In addition to being a research platform for microgravity free-flying robotics, Astrobee improves the efficiency of ISS operations by providing flight and payload controllers with a mobile camera and a sensor platform. 3. NASA is developing new deployable structures and material technologies for solar sail propulsion systems destined for future low-cost deep space missions. NASA’s Advanced Composite Solar Sail System (ACS3) uses composite materials in its novel, lightweight booms that deploy from a Cubesat. Data obtained from ACS3 will guide the design of future larger-scale composite solar sail systems that could be used for several deep space exploration missions.