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A special-purpose, semi-analytical solution method for determining the stress and deformation fields in a thin laminated-composite cylindrical shell with an elliptical cutout is presented. The analysis includes the effects of cutout size, shape, and orientation; non-uniform wall thickness; oval-cross-section eccentricity; and loading conditions. The loading conditions include uniform tension, uniform torsion, and pure bending. The analysis approach is based on the principle of stationary potential energy and uses Lagrange multipliers to relax the kinematic admissibility requirements on the displacement representations through the use of idealized elastic edge restraints. Specifying appropriate stiffness values for the elastic extensional and rotational edge restraints (springs) allows the imposition of the kinematic boundary conditions in an indirect manner, which enables the use of a broader set of functions for representing the displacement fields. Selected results of parametric studies are presented for several geometric parameters that demonstrate that analysis approach is a powerful means for developing design criteria for laminated-composite shells.

An analysis-based approach for developing shell-buckling design criteria for laminated-composite cylindrical shells that accurately account for the effects of initial geometric imperfections is presented. With this approach, measured initial geometric imperfection data from six graphite-epoxy shells are used to determine a manufacturing-process-specific imperfection signature for these shells. This imperfection signature is then used as input into nonlinear finite element analyses. The imperfection signature represents a first-approximation mean imperfection shape that is suitable for developing preliminary-design data. Comparisons of test data and analytical results obtained by using several different imperfection shapes are presented for selected shells. These shapes include the actual measured imperfection shape of the test specimens, a first-approximation mean imperfection shape, with and without plus or minus one standard deviation, and the linear-bifurcation-mode imperfection shape. In addition, buckling interaction curves for composite shells subjected to combined axial compression and torsion loading are presented that were obtained by using the various imperfection shapes in the analyses. A discussion of the nonlinear finite element analyses is also presented. Overall, the results indicate that the analysis-based approach presented for developing reliable preliminary-design criteria has the potential to provide improved, less conservative buckling-load estimates and to reduce the weight and cost of developing buckling-resistant shell structures.

The new Space Shuttle superlightweight external fuel tank e ew for the e rst time on 2 June 1998 (Space Transportation System-mission 91 ). We present results of elastic linear-bifurcation buckling and nonlinear analyses of one of its major components; that is, the liquid-oxygen tank. The contents include an overview of the structure and a brief description of the e nite element code that was used to conduct the analyses. Results are presented that illustrate three distinctly different types of nonlinearresponsephenomena for thin-walled shells that aresubjected to combined mechanical and thermal loads that launch-vehicle shell designers may encounter. A procedure is demonstrated that can beused by structural analysts and designers to obtain reasonable, conservative estimates of linear-bifurcation, buckling-load knockdown factors for shells that are subjected to complex loading conditions or to characterize the effects of initial geometric imperfections on nonlinear shell response phenomena. Results are also presented that show that the superlightweight liquid-oxygen tank can carry loads in excess of twice the values of the operational prelaunch loads considered and that a e uid-e lled launch-vehicle shell can be highly sensitive to initial geometric imperfections. Presentedherearee vepapersonlarge-scaleanalysesofacomplexshellstructure.Thee rst,longerpaper,“ NonlinearBehaviorofSpaceShuttleSuperlightweight Liquid-OxygenTankUnderPrelaunchLoads,” coversthestructure,theanalysistechnique,someloading cases,andexperimentalverie cationonthemethod.Itis followed by a companion paper, “ Modeling and NonlinearStructural Analysis of a Large-ScaleLaunch Vehicle,” which goes into greaterdepth on themethods, and then by three shorter papers, “ Effects of Welding-Induced Imperfections on Behavior of Space Shuttle Superlightweight Tank,” “ Nonlinear Behavior of Space Shuttle Superlightweight Tank Under Booster Ascent Loads,” and “ Nonlinear Behavior of Space Shuttle Superlightweight Tank Under End-of-Flight Loads,” covering interesting behaviorunder differing load cases. Thesepapersare intended to stand ontheir own (and hencehavesomeredundant introductory material) but are complementary to one another, and so they are presented here together.

Results of linear-bifurcation and nonlinear analyses of the Space Shuttle superlightweight (SLWT) external liquid-oxygen (LO2) tank for an important early booster ascent loading condition are presented. These results for thin-walled linear elastic shells that are subjected to combined mechanical and thermal loads illustrate an important type of response mode that may be encountered in the design of other liquid-fuel launch vehicles. Linear-bifurcation analyses are presented that predict several nearly equal eigenvalues that correspond to local buckling modes in the forward ogive section of the LO 2 tank. In contrast, the nonlinear response phenomenon is shown to consist of short-wavelength bending deformations in the forward-ogive and barrel sections of the LO2 tank that grow in amplitude in a stable manner with increasing load. Imperfection sensitivity analyses are presented that show that the presence of several nearly equal eigenvalues does not lead to a premature general instability mode for the forward-ogive section. For the linear-bifurcation and nonlinear analyses, the results show that accurate predictions of the response of the shell generally require a large-scale, high-e delity, e nite element model, and that a design based on a linear-bifurcation buckling analysis and a buckling-load knockdown factor is overly conservative. Results are presented that show that the SLWT LO 2 tank can support loads in excess of approximately 2.6 timesthevaluesof theoperationalloads considered. In addition, results are presented that show that local bending deformations may cause failure of the thermal protection system (TPS) at load levels less than the load level corresponding to structural collapse. Results are presented that can be used to estimate the load level at which TPS failure is likely to occur.

Results of linear-bifurcation buckling and nonlinear analyses of the Space Shuttle superlightweight external liquid-oxygen tank are presented for an important prelaunch loading condition. These results show the effects of actual, measured welding-induced initial geometric imperfections on an important response mode for thin-walled shellsthataresubjectedtocombinedmechanicalandthermalloads.Thistypeofinitialgeometricimperfectionmay be encountered in the design of other liquid-fuel launch vehicles. Results are presented that show that the liquidoxygen tank will buckle in the barrel section, but at load levels nearly four times the magnitude of the operational load level, and will exhibit stable postbuckling behavior. The actual measured imperfections are located in this sectionofthetank. Resultsofimperfection sensitivity analysesarepresented thatshowthat thelargestdegradation in the apparent membrane stiffnesses of the liquid-oxygen tank barrel section is caused by an imperfection shape that is in the form of the linear-bifurcation buckling mode with a relatively small amplitude. These results also show that the effect of the relatively large-amplitude measured imperfection is benign.

Results of elastic, linear-bifurcation buckling and nonlinear analyses of the new Space Shuttle superlightweight external liquid-oxygen tank are presented for an important end-of-e ight loading condition. These results illustrate an important type of response mode for thin-walled shells subjected to combined mechanical and thermal loads that may be encountered in the design of other liquid-fuel launch vehicles. Linear-bifurcation buckling analyses are presented that predict several nearly equal eigenvalues that correspond to local buckling modes in the aft dome of the liquid-oxygen tank. In contrast, the nonlinear response phenomenon is shown to consist of a shortwavelength bending deformation in the aft elliptical dome of the liquid-oxygen tank that grows in amplitude in a stable manner with increasing load. Imperfection sensitivity analyses are presented that show that the presence of several nearly equal eigenvalues does not lead to a premature general instability mode for the aft dome. For the linear-bifurcation and nonlinear analyses, the results show that accurate predictions of the response of the shell generally require a large-scale, high-e delity, e nite element model, and that a design based on a linear-bifurcation buckling analysis and a buckling-load knockdown factor is overly conservative. Results are also presented that showthatthesuperlightweightliquid-oxygentankcansupportloadsin excessofapproximately1.9 timesthevalues of the operational loads considered.

The Space Shuttle Challenger (STS 51-L) accident led to an intense investigation of the structural behavior of the solid rocket booster (SRB) tang and clevis field joints. Results are presented of axisymmetric shell analyses that parametrically assess the structural behavior of SRB field joints subjected to quasi-steady-state internal pressure loading for both the original joint flown on mission STS 51-L and the redesigned joint flown for the first time after the STS 51-L accident on the Space Shuttle Discovery. Discussion of axisymmetric shell modeling issues and details is presented and a generic method for simulating contact between adjacent shells of revolution is described. Results are presented that identify the performance trends of the joints for a wide range of joint parameters. An important finding is that the redesigned joint exhibits significantly smaller O-ring gap changes and much less sensitivity to joint clearances than the original joint. For a wide range of joint parameters, the result presented indicate that the redesigned joint provides a much better pressure seal than the original joint.

The advantages of using a hierarchical analysis approach to calculate the buckling load of an axially compressed composite cylindrical shell is demonstrated using an example taken from a recent experimental program. The Delft Interactive Shell DEsign COde (DISDECO) shell design code is used for this hierarchical analysis approach to provide an accurate prediction of the critical buckling load of the actual shell structure. DISDECO includes the influence of the boundary conditions, initial geometric imperfections, the effects of stiffener and load eccentricities, and the effects of prebuckling deformations caused by edge constraints in the analysis. It is shown that the use of DISDECO makes it relatively simple to proceed step by step from simple to more complex models and solution procedures. As a final step in the hierarchical analysis approach, the critical buckling load and the estimated imperfection sensitivity of the shell are verified by conducting an analysis of a large finite element model with one of the current generation two-dimensional shell analysis codes with advanced capabilities needed to represent both geometric and material nonlinearities.

NONLINEAR ANALYSIS OF THE SPACE SHUTTLE SUPERLIGHTWEIGHTLO 2 TANK: PART I - BEHAVIOR UNDER BOOSTER ASCENT LOADSRichard D. Young, Michael P. Nemeth, Timothy J. Collins, and James. H. Starnes, Jr.NASA Langley Research CenterHampton, Virginia 23681-0001Presented at the 39th AIAA/ASME/ASCE/AHS/ASC Structures,Structural Dynamics, and Material ConferenceAIAA Paper No. 98-1838Long Beach, CahforniaApril 20-23, 1998

The buckling and postbuckling behaviour of thin plates with a cut-out and made of advanced composite materials are research topics of great practical importance. For example, composite plate-like subcomponents with cut-outs are being considered for use in many types of aerospace structures owing to their high stiffness-to-weight and strength-to-weight properties. These properties could ultimately yield substantial weight savings for aircraft structures.