David W. Sleight

Effective modeling and nonlinear shell analysis of thin membranes exhibiting structural wrinkling

Thin solar sail membranes of very large span are being envisioned for near-term space missions. One major design issue that is inherent to these very flexible structures is the formation of wrinkling patterns. Structural wrinkles may deteriorate a solar sails performance and, in certain cases, structural integrity. A geometrically nonlinear, updated Lagrangian shell formulation is employed using the ABAQUS finite element code to simulate the formation of wrinkled deformations in thin-film membranes. The restrictive assumptions of true membranes as defined by tension field theory are not invoked. Two effective modeling strategies are introduced to facilitate convergent solutions of wrinkled equilibrium states. They include 1) the application of small, pseudorandom, out-of-plane geometric imperfections that ensure initiation of the requisite membrane-to-bending coupling in a geometrically nonlinear analysis and 2) the truncation of corner regions, where concentrated loads are prescribed, to improve load transfer, mesh quality, and kinematics and to reduce severe concentration of membrane stresses. The corner truncation necessitates replacing the concentrated force with a statically equivalent distributed traction. Several numerical studies are carried out, and the results are compared with recent experimental data. Good agreement is observed between the numerical simulations and experimental data.

Nonlinear Shell Modeling of Thin Membranes with Emphasis on Structural Wrinkling

Thin solar sail membranes of very large span are being envisioned for near-term space missions. One major design issue that is inherent to these very flexible structures is the formation of wrinkling patterns. Structural wrinkles may deteriorate a solar sails performance and, in certain cases, structural integrity. In this paper, a geometrically nonlinear, updated Lagrangian shell formulation is employed using the ABAQUS finite element code to simulate the formation of wrinkled deformations in thin-film membranes. The restrictive assumptions of true membranes, i.e. Tension Field theory (TF), are not invoked. Two effective modeling strategies are introduced to facilitate convergent solutions of wrinkled equilibrium states. Several numerical studies are carried out, and the results are compared with recent experimental data. Good agreement is observed between the numerical simulations and experimental data.

VACUUM DEPLOYMENT AN D TESTING OF A 20M SOLAR SAIL SYSTEM

Solar sails reflect photons streaming from the sun and transfer momentum to the sail. The thrust, though small, is continuous and acts for the life of the mission without the need for propellant. Recent advances in materials and ultra -low mass gossamer structures have enabled a host of useful missions utilizing solar sail propulsio n. This paper discusses the Phase 3 accomplishments of a 3 -phase 33 -month collaboration between L’Garde, Ball Aerospace, JPL, and NASA LaRC under the direction of the NASA In -Space Propulsion office to develop solar sails and to raise the technology readi ness level ( TRL ) to as close to 6 as possible though ground testing of large scale solar sail test articles . A comprehensive review of the program accomplishments to date finds the overall TRL level now in the 5 to 6 range which has increased from around 3 at the beginning of the program. This comprehensive ground test program included material, component, subsystem, system, and finally launch environment and thermal vacuum deployment tests . In conjunction FEA models were developed and validated with gr ound test data resulting in highly credible analysis techniques able to bridge the gap between ground test results and future large scale flight articles. This focused program will pave the way for a flight experiment of this highly efficient space propuls ion technology.

Parametric Studies of Square Solar Sails Using Finite Element Analysis

Parametric studies are performed on two generic square solar sail designs to identify parameters of interest. The studies are performed on systems-level models of full-scale solar sails, and include geometric nonlinearity and inertia relief, and use a Newton-Raphson scheme to apply sail pre-tensioning and solar pressure. Computational strategies and difficulties encountered during the analyses are also addressed. The purpose of this paper is not to compare the benefits of one sail design over the other. Instead, the results of the parametric studies may be used to identify general response trends, and areas of potential nonlinear structural interactions for future studies. The effects of sail size, sail membrane pre-stress, sail membrane thickness, and boom stiffness on the sail membrane and boom deformations, boom loads, and vibration frequencies are studied. Over the range of parameters studied, the maximum sail deflection and boom deformations are a nonlinear function of the sail properties. In general, the vibration frequencies and modes are closely spaced. For some vibration mode shapes, local deformation patterns that dominate the response are identified. These localized patterns are attributed to the presence of negative stresses in the sail membrane that are artifacts of the assumption of ignoring the effects of wrinkling in the modeling process, and are not believed to be physically meaningful. Over the range of parameters studied, several regions of potential nonlinear modal interaction are identified.

Simulating Nonlinear Deformations of Solar Sail Membranes Using Explicit Time Integration

In this study, the explicit time integration method is employed to predict deformation of highly flexible solar sail structural components. The nonlinear static analysis of a highly flexible ribbon structure is presented to demonstrate the need for having the explicit time integration method in the analysis toolbox for solar sail. Static analyses of the ribbon structure produce ambiquous results whereas the explicit time integration method determines the correct results. Extensive benchmarking examples are also presented to build confidence in the use of the explicit method. Previously determined nonlinear wrinkling deformations of solar sail membranes are found by the explicit method. As the explicit method is known to often require more computational time than nonlinear static methods, a study on mass scaling was also conducted. The computational times are reported for the nonlinear static and explicit time integration solutions to calibrate the advantage of using mass scaling for these problems.

Composite skin-stiffener debond analyses using fracture mechanics approach with shell elements

Abstract Fracture mechanics analyses of composite skin-stiffener debond configurations using shell elements are presented. Two types of debond configurations are studied: a flange skin strip debond configuration and a skin-stiffener debond configuration. The flange-skin strip configuration examines debond growth perpendicular to the stiffener while the skin-stiffener configuration examines debond growth parallel to the stiffener. Four-node and 9-node shell elements are used to model both debond configurations. The stiffener flange and skin are modeled as two different layers of elements whose translational degrees-of-freedom, in the bonded portion, of the corresponding flange and skin nodes are constrained to be identical. Strain energy release rate formulae are presented for both 4-node and 9-node element models based on the virtual crack closure technique (VCCT). In addition, average values of the strain energy release rates are calculated using a gradient method. The VCCT formulae and the gradient method are used to compute the strain energy release rates ( G -values) for both debond configurations. The G -values predicted by these methods are compared with those predicted by plane-strain and 3D finite element analyses. Excellent correlation is obtained among all the analysis results, thus helping to validate the VCCT formulae derived for the 4- and 9-node shell elements.

Debonding Failure of Sandwich-Composite Cryogenic Fuel Tank with Internal Core Pressure

A summary of the failure analyses and testing that were conducted to determine the cause of the X-33 liquidhydrogen tank failure is presented. Ply-level stress analyses were conducted to explain the formation of microcracks in the plies of the inner and outer facesheet laminates of the honeycomb sandwich walls of the tank under known thermal and mechanical loads. The microcracks allowed the ingression of liquid- and gaseous-hydrogen and gaseous-nitrogen purge gas that produced higher than expected sandwich core pressures in the tank. Single cantilever beam tests were used to determine the toughness of the interface between the facesheets and honeycomb core. Fracture mechanics analyses were developed to determine strain-energy release rates for known foreign object debris shapes and sizes and known and statistically possible core internal pressures. The fracture mechanics analyses were validated by comparing with results of blowoff tests that were fabricated from undamaged tank sandwich material. Strain-energy release rates from the validated analyses were then compared with known and statistically possible values of toughness determined from the single cantilever beam tests. These analyses and tests were then used to substantiate a scenario for failure of the X-33 liquid-hydrogen tank that includes microcracking of the inner facesheets and ensuing ingression of hydrogen and nitrogen, a low bondline strength and toughness, and the presence of foreign object debris. Nomenclature ¯ A −1 jk =i nverse of the laminate stiffness matrix transformed to the local coordinates a, b = dimensions of debond Eii = elastic modulus in the ith principal material

Geometrically Nonlinear Shell Analysis of Wrinkled Thin-Film Membranes with Stress Concentrations

Geometrically nonlinear shell finite element analysis has recently been applied to solar-sail membrane problems in order to model the out-of-plane deformations due to structural wrinkling. Whereas certain problems lend themselves to achieving converged nonlinear solutions that compare favorably with experimental observations, solutions to tensioned membranes exhibiting high stress concentrations have been difficult to obtain even with the best nonlinear finite element codes and advanced shell element technology. In this paper, two numerical studies are presented that pave the way to improving the modeling of this class of nonlinear problems. The studies address the issues of mesh refinement and stress-concentration alleviation, and the effects of these modeling strategies on the ability to attain converged nonlinear deformations due to wrinkling. The numerical studies demonstrate that excessive mesh refinement in the regions of stress concentration may be disadvantageous to achieving wrinkled equilibrium states, causing the nonlinear solution to lock in the membrane response mode, while totally discarding the very low-energy bending response that is necessary to cause wrinkling deformation patterns.

Finite Element Analysis and Test Correlation of a 10-Meter Inflation-Deployed Solar Sail

Under the direction of the NASA In-Space Propulsion Technology Office, the team of L Garde, NASA Jet Propulsion Laboratory, Ball Aerospace, and NASA Langley Research Center has been developing a scalable solar sail configuration to address NASAs future space propulsion needs. Prior to a flight experiment of a full-scale solar sail, a comprehensive phased test plan is currently being implemented to advance the technology readiness level of the solar sail design. These tests consist of solar sail component, subsystem, and sub-scale system ground tests that simulate the vacuum and thermal conditions of the space environment. Recently, two solar sail test articles, a 7.4-m beam assembly subsystem test article and a 10-m four-quadrant solar sail system test article, were tested in vacuum conditions with a gravity-offload system to mitigate the effects of gravity. This paper presents the structural analyses simulating the ground tests and the correlation of the analyses with the test results. For programmatic risk reduction, a two-prong analysis approach was undertaken in which two separate teams independently developed computational models of the solar sail test articles using the finite element analysis software packages: NEiNastran and ABAQUS. This paper compares the pre-test and post-test analysis predictions from both software packages with the test data including load-deflection curves from static load tests, and vibration frequencies and mode shapes from vibration tests. The analysis predictions were in reasonable agreement with the test data. Factors that precluded better correlation of the analyses and the tests were uncertainties in the material properties, test conditions, and modeling assumptions used in the analyses.

Ground Testing a 20-Meter Inflation Deployed Solar Sail

Solar sails have been proposed for a variety of future space exploration missions and provide a cost effective source of propellantless propulsion. Solar sails span very large areas to capture and reflect photons from the Sun and are propelled through space by the transfer of momentum from the photons to the solar sail. The thrust of a solar sail, though small, is continuous and acts for the life of the mission without the need for propellant. Recent advances in materials and ultra-low mass gossamer structures have enabled a host of useful space exploration missions utilizing solar sail propulsion. The team of L Garde, NASA Jet Propulsion Laboratory (JPL), Ball Aerospace, and NASA Langley Research Center, under the direction of the NASA In-Space Propulsion Office (ISP), has been developing a scalable solar sail configuration to address NASA s future space propulsion needs. The 100-m baseline solar sail concept was optimized around the one astronomical unit (AU) Geostorm mission, and features a Mylar sail membrane with a striped-net sail suspension architecture with inflation-deployed sail support beams consisting of inflatable sub-Tg (glass transition temperature) rigidizable semi-monocoque booms and a spreader system. The solar sail has vanes integrated onto the tips of the support beams to provide full 3-axis control of the solar sail. This same structural concept can be scaled to meet the requirements of a number of other NASA missions. Static and dynamic testing of a 20m scaled version of this solar sail concept have been completed in the Space Power Facility (SPF) at the NASA Glenn Plum Brook facility under vacuum and thermal conditions simulating the operation of a solar sail in space. This paper details the lessons learned from these and other similar ground based tests of gossamer structures during the three year solar sail project.