Presentation on theme: "STRUCTURAL WRINKLING PREDICTIONS FOR MEMBRANE SPACE STRUCTURES"— Presentation transcript:
1 STRUCTURAL WRINKLING PREDICTIONS FOR MEMBRANE SPACE STRUCTURES David W. Sleight, Alexander Tessler, and John T. Wang Analytical and Computational Methods BranchNASA Langley Research CenterHampton, VAFEMCI Workshop 2002Innovative FEM Solutions to Challenging ProblemsMay 22-23, 2002
2 Outline Motivation Objectives Tension Field Theory for Predicting WrinklingThin-Shell Theory for Predicting WrinklingWrinkling AnalysesSummary
3 Motivation Future space missions enabled by large Gossamer systems Understanding and predicting the behavior of membrane structures is essential for design and assessment of their performanceMembrane OpticsSunshieldSolar SailStructural behavior, particularly wrinkling behavior, is highlynonlinear and difficult to predict.
4 Types of Wrinkles Structural Wrinkles Material Wrinkles Permanent deformationsCreasesResult from manufacturing or packagingTemporary deformationsLocalized bucklingResult from loading or boundary conditionsChange load paths within a membrane structureMaterial wrinkles are permanent out-of-plane deformations resulting from manufacturing or packaging processes. Material wrinkles remain after loading is removed.Structural wrinkles are temporary out-of-plane deformations caused by localized buckling of the thin film due to compressive loads.They depend upon the combination of applied loads and boundary conditionsStructural wrinkles change load paths within a membrane structure, keeping compressive stresses from forming
5 Problems Due to Structural Wrinkling Degraded performanceAffect maneuverability and stabilityPoor surface accuracyNGST TestWrinkles Give a 3-D Aspect to a 2-D Structure, a Cause for Concern with Structures Like the NGST Sunshield.Wrinkles Can Change Structural Responses to Out-of-Plane Loads, Important when Designing Structures such as Solar Sails.Solar SailInflatable Antenna
6 10 m, 2 Quadrant Solar Sail in LaRC 16m Vacuum Chamber
7 ObjectivesDevelop effective and robust FEA capabilities to predict structural wrinkles in membrane space structuresPredict surface distribution of structural wrinkles using:Membrane analysisShell analysis (with out-of-plane deformations)
8 Tension Field Theory for Predicting Wrinkling Originated by Wagner (1929) and Reissner (1938)Membranes have negligible bending stiffness and cannot sustain compressive stressesWrinkles are treated as infinitesimally close to one anotherOut-of-plane deformations of wrinkles cannot be determinedTension field Theory (TFT) has been implemented byvarying linear elastic material propertiesintroducing a wrinkle strainformulating a ‘relaxed’ strain energy densityWrinkling occurs when a compressive stress develops.Troughs and Crests of Wrinkles run Parallel to Major Principal Stress AxisWrinkles only transmit load in direction of the troughs and crests.
9 Stein-Hedgepeth Theory (SHT) - 1961 Membrane cannot carry compressive stressTwo types of regions:TautWrinkledEffects of wrinkling are accounted for using a variable Poisson’s ratio that permits “over-contraction” in the direction of minor principal stressesWrinkles are aligned with the major principal stress axisA generalized modification of the previous membrane wrinkling theoryWrinkles form in straight lines along direction of major principal stresses - load transfer in the wrinkled region is along these linesClosed-form solutionss2=0Wrinkle lines10
10 Iterative Membrane Property (IMP) Method (Miller and Hedgepeth, 1982) FE implementation of the SHTGeometrically nonlinear analysisWrinkle criteria based on element stress/strain statesTaut: s2 > 0 (isotropic material)Wrinkled: 1 > 0 & s2 ≤ 0 (orthotropic material)Slack: 1 < 0 (zero stiffness material)Finite element implementation into ABAQUSAdler-Mikulas (2000)
11 Thin-Shell Theory for Predicting Wrinkling Membrane theory cannot predict the amplitude and shape of wrinklesShell theory includes both membrane and bending flexibilitiesenables post-buckling responsecan predict amplitude and shape of wrinklesGeometrically nonlinear analysis is necessary to predict the structural behavior
12 Thin-Shell Wrinkling Analyses Using ABAQUS Geometrically nonlinear FE analysisUsing reduced integration thin-shell element (S4R5)Imperfections are added to initial geometrymode shapes from buckling analysisrandom imperfectionsEmploy ABAQUS with STABILIZE parameter to automatically add damping for preventing unstable and singular solutionsfor accuracy, use lowest possible value for which convergence can be achieved
13 Wrinkling Analyses Square Membrane Loaded in Tension Rectangular Membrane Loaded in Shear
14 Square Membrane Loaded in Tension (Blandino, Johnston, et al, 2002) Applied LoadApplied LoadKapton membrane: 2.54 x 10-2 mm thickApplied Loads: 2.45 N (Isothermal)
15 Wrinkling Results Wrinkled Region Experimental Results (Blandino, Johnston, et al)ABAQUS IMP Results10,000 elementsM3D3/M3D4 Membrane Elements
16 Rectangular Membrane Loaded in Shear (Wong and Pellegrino, 2002) 380 mmd = 3 mm128 mmFixedKapton foil: 25 mm thick
17 Wrinkling Results 3960 elements 5% initial random imperfections Out-of-plane Deflection
18 Summary Future space missions enabled by Gossamer structures Effective and robust analysis tools requiredStructural wrinkles constitute a major concernAffect surface topology and behavior/performanceFEA analyses using ABAQUS to predict structural wrinklingMembrane analyses with IMP methodThin-shell geometrically nonlinear analyses
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