Mark J. Silver

Predictive Elastothermodynamic Damping in Finite Element Models by Using a Perturbation Formulation

A method is presented by which elastothermodynamic damping can be included in finite element formulations for design analysis. In this method, elastothermodynamic damping theory is combined with a perturbation method previously developed for viscoelastic modeling. A key aspect of this approach is that it projects elastothermodynamic damping onto the undamped mode shapes of the structure. A finite element formulation is developed and presented for beams in both bending and extension. The finite element formulation creates nonsparse, nonsymmetric damping and stiffness matrices. Results with this method for various cases are discussed. After validation against the classic Zener model damping prediction, the method is applied to the analysis of damping in a three-dimensional truss. The results show that elastothermodynamic damping is higher for modes with a larger portion of their strain energy due to local member bending rather than extension. Through examples it is shown that to maximize elastothermodynamic damping in a truss, both the member cross section and the truss mode shapes must be considered.

Modeling of Snap-Back Bending Response of Doubly Slit Cylindrical Shells

This paper presents an investigation of the bending-induced buckling response of doubly slit cylindrical shells. The goal of this work is to determine a design-relevant parametric model of the load-displacement response during stowing and deployment of folding hinges built from opposing doubly slit cylindrical shells. This relationship will assist in the design and modeling of structures built from these folding hinges for use in elastically deployed structures. Results of an investigation into computational analysis procedures for modeling the snap-back problem are presented. A review of the literature indicates that previous work has not dealt with the snap-back phenomenon that needs to be considered for the folding hinge application. The few analytical models that can be used to verify these finite element model predictions are presented, and comparisons are made to the current finite element results. A test apparatus is described which imposes controlled displacement and/or rotation at the end of the doubly slit cylindrical shells up to and through snap-back, while end moment and planar forces are measured.

Experimental Characterization of Lightweight Strain Energy Deployment Hinges

Elastic, lenticular hinges have been used for many years to provide simple, reliable, repeatable deployment of spacecraft structural components. Recent advances in high performance elastically flexible composites have opened up the application field for this useful deployment component by providing lighter, more thermally stable hinges as well as designs that can meet a much wider range of requirements. This paper describes recent advances in composite hinge performance through a combination of analysis and testing.

Dynamic Modeling of the Folding of Multi-Shell Flexible Composites

Flexible composite structural members have been key features in many deployable space structure designs. However designers of deployment systems that require complex folds, tighter packaging, more deployment torque and higher stiffness when deployed have been slow in adopting flexible composites. One reason for this is the difficulty in analyzing the complex strain field in flexing composite shells. This difficulty is exacerbated when multiple, separate shells come in contact with each other. This paper describes Finite Element modeling techniques used to analyze the strain and deployment force in multi-shell flexing composites. In the paper two Finite Element codes are compared with respect to the affect of various solution parameters on solution accuracy and time.

Controlled Displacement Snap-Through of Tape Springs: Modeling and Experiment

This paper presents an analysis of the buckling of tape springs due to axial compressive loads and end moments. The motivation for this work is to derive design guidelines for axial loading of hinges made from tape springs. A previously derived non-dimensionalized, nonlinear analytical model of the buckling problem including load eccentricity and geometric imperfections is presented. The finite difference method is used to solve this analytical model. An experimental apparatus has been built to verify the results of the analytical model. Preliminary results from the finite difference model and experiments are presented.

The Strain Energy Deployed High Expansion Outer Barrel Assembly

Future large aperture telescopes will require several deployable subsystems, including Primary Mirrors (PM), Secondary Mirror Support Structures (SMSS), and Optical Barrel Assemblies (OBA). The three subsystems must be specifically designed to package together but then deploy separately and operate without disturbing each other. This work presents a point design for a combined SMSS and OBA deployable structure for a non-deployed PM telescope. The architecture uses elastic strain energy composites to provide high precision, lightweight, reliable, and repeatedly verifiable deployment. A full scale proof of concept was built using non-flight materials and successfully stowed and deployed many times. Component and full system tests and analyses were performed to estimate the performance for an equivalent system using flight materials.

Strain Energy Deployment of Telescope Outer Barrel Assemblies

While some work has been done on the packaging and deployment of large space telescopes, the design of the accompanying Optical Barrel Assemblies (OBAs) that act as thermal and light shrouds for the telescopes have been relatively neglected. These systems must be lightweight, stiff, strong, reliable, and function without disturbing the overall telescope system. To investigate this problem, a modular, deployable structural system based on lightweight flexible composites has been developed. This system is called the Strain Energy Deployable OBA (SEDOBA). This paper describes the design, fabrication and testing of a deployable OBA based on this modular system approach for a 2-3m diameter telescope.

Experimental Measurement of Picometer Scale Spontaneous Vibrations in a Precision Deployable Boom under Thermal Loading

This paper reports observations and analysis of picometer scale spontaneous vibrations in a precision deployable boom under thermal loading. The structural test article is a deployable boom previously flown in space. It exhibited spontaneous vibrations during the temperature rise following a night to day transition on orbit. In an attempt to reproduce the spontaneous vibrations on the ground, the test article was thermally loaded within a mechanically stabilized test environment. Spontaneous vibrations were induced in these ground experiments. The vibrations were at a scale of motion for which current theories would not expect such a release. The amplitudes of these vibrations were on the order of a few dozen picometers, and the frequency was near 1500 Hz. Evidence of wave dispersion was detected in the vibrations.

Structural Characterization of Carbon Nanotube Rope Films for Gossamer Structure Applications

Recent work has focused on making macroscopic materials from carbon nanotubes (CNTs) with the goal of maintaining the impressive structural characteristics of individual CNT. This paper presents a comparison of three-point bending and tensile extension methods for measuring the modulus of the new macroscopic epoxy infiltrated film samples made from aligned and unaligned CNT ropes. A method for measuring the film thickness to within 10 µm is also presented. Two processing variations are used in making the samples: incorporating micro-particles in the CNT network and densifying the aligned samples. The measured tensile moduli fall between 5 and 8 GPa for aligned and up to 10.5 GPa for unaligned samples with tensile moduli being between 40% and 100% larger than the bending