Shell Buckling Knockdown Factors

Metadata Updated: February 28, 2019

The Shell Buckling Knockdown Factor (SBKF) Project, NASA Engineering and Safety Center[HTML_REMOVED](NESC) Assessment #: 07-010-E, was established in March of 2007 by the NESC in[HTML_REMOVED]collaboration with the former NASA Constellation Program (CxP) and now the Space Launch System (SLS) Program. The SBKF Project has the goal of[HTML_REMOVED]developing and experimentally validating improved (i.e., less-conservative, more robust)[HTML_REMOVED]analysis-based shell buckling design factors (a.k.a., knockdown factors (KDFs)) and developing[HTML_REMOVED]design recommendations for launch vehicle structures. Shell buckling knockdown factors have been historically based on test data from laboratory-scale[HTML_REMOVED]test articles obtained from the 1930s through the 1960s. The knockdown factors are used to account[HTML_REMOVED]for the differences observed between the theoretical buckling load and the buckling load obtained[HTML_REMOVED]from test. However, these test-based KDFs may not be relevant for modern launch- vehicle designs,[HTML_REMOVED]and are likely overly conservative for many designs. Significant advances in structural stability[HTML_REMOVED]theory, high-fidelity analysis methods, manufacturing, and testing are[HTML_REMOVED]enabling the development of new, less conservative, robust analysis-based knockdown factors for[HTML_REMOVED]modern structural concepts. Preliminary design studies indicate that implementation of new[HTML_REMOVED]knockdown factors can enable significant weight savings in these vehicles and will help mitigate[HTML_REMOVED]some of NASA[HTML_REMOVED]s launch-vehicle development and performance risks, by reducing reliance on[HTML_REMOVED]large-scale testing, and providing high-fidelity estimates of as-built structural performance,[HTML_REMOVED]increased payload capability, and improved structural reliability.To achieve its KDF development and implementation goals, the SBKF Project is engaged in several[HTML_REMOVED]work areas including launch-vehicle design trade studies, subcomponent and component level design,[HTML_REMOVED]analysis and structural testing, and shell buckling design technology development including[HTML_REMOVED]analysis-method development, analysis benchmarking and standardization, and analysis-based KDF[HTML_REMOVED]development. Finite-element analysis is used extensively in all these work areas. In particular,[HTML_REMOVED]there are four main categories analyses conducted by SBKF and include:1) high-fidelity structural simulations, 2) imperfection sensitivity studies, 3) test article[HTML_REMOVED]design and analysis and 4) exploratory studies. Each of these types of analysis may have different[HTML_REMOVED]analysis objectives and utilize different modeling approaches that depend on the results required[HTML_REMOVED]to meet the Project needs. A description of the four main categories follows.[HTML_REMOVED]High-fidelity structural simulationsHigh-fidelity structural simulations are defined as simulations that can predict accurately the[HTML_REMOVED]complex behavior of a structural component or an assembly of components (e.g., virtual structural[HTML_REMOVED]test) and often require a significant level of modeling detail and knowledge of the structural[HTML_REMOVED]system (e.g., its physical behavior and expected variability). Models are considered high-fidelity[HTML_REMOVED]if results predicted with these models correlate with[HTML_REMOVED]test data to within a small range of variance and represent accurately the true physical behavior[HTML_REMOVED]of the structure. The permissible amount of variance is determined based on the analysis[HTML_REMOVED]requirements defined by the Project in accordance with the intended end use of the predicted data.[HTML_REMOVED]High-fidelity shell buckling analysis objectives considered by the SBKF Project often require the[HTML_REMOVED]accurate prediction of stiffnesses, local and global deformations, strains, load paths and[HTML_REMOVED]buckling-induced load redistribution, and buckling and failure loads and modes. To achieve these[HTML_REMOVED]analysis goals, the models typically must accurately represent loading and boundary conditions, and[HTML_REMOVED]expected or measured geometric and material variations (imperfections). It is expected that[HTML_REMOVED]high-fidelity models developed by SBKFwill predict effective axial stiffness (slope of the load versus end-shortening curve) within[HTML_REMOVED][HTML_REMOVED]2%, buckling loads and point displacements (displacement measured at a point) within[HTML_REMOVED][HTML_REMOVED]5%, and point strains within [HTML_REMOVED]10%. However, if the displacements or strains of interest are in a[HTML_REMOVED]high-gradient location, then the overall trend will be assessed for correlation.Imperfection sensitivity studiesImperfection sensitivity studies are used to assess the sensitivity of a structure[HTML_REMOVED]s nonlinear[HTML_REMOVED]response and buckling load to initial imperfections, such as geometric imperfections (imperfections[HTML_REMOVED]in the shell wall geometry including out-of-roundness or local dimples), and loading and material[HTML_REMOVED]non-uniformities. Geometric imperfections included in an analysis model can be based upon the[HTML_REMOVED]measured geometry of test articles or flight hardware, or they can be defined analytically using[HTML_REMOVED]eigenmode shapes or other perturbations.[HTML_REMOVED]The SBKF Project is developing analysis-based SBKFs (KDFs) that are derived from imperfection[HTML_REMOVED]sensitivity studies and several imperfection types are being investigated. First, a single[HTML_REMOVED]dimple-shaped imperfection is being used as a [HTML_REMOVED]worst-expected[HTML_REMOVED] imperfection shape and is similar to[HTML_REMOVED]the initial dimple that is observed in the shell wall at the onset of buckling. The dimple is[HTML_REMOVED]created in the shell by applying a radially inward lateral load at the mid-length of the cylinder.[HTML_REMOVED]The magnitude of the lateral load is held fixed and the active destabilizing load (e.g., axial[HTML_REMOVED]compression) is then applied until buckling occurs in the shell. The magnitude of the lateral load[HTML_REMOVED]is increased incrementally in subsequent buckling analyses until a minimum or lower-bound buckling[HTML_REMOVED]load is achieved. A second imperfection type used includes actual measured geometry data from[HTML_REMOVED]as-built launch-vehicle-like test articles and flight hardware. These measured geometric[HTML_REMOVED]imperfections are included in the model by adjusting the original geometrically perfect[HTML_REMOVED]finite-element mesh nodal coordinates to the perturbed imperfect geometry. Finally, the effects of[HTML_REMOVED]loading imperfections are investigated by applying localized concentrated loads on the ends of the[HTML_REMOVED]shell in combination with the geometric imperfections or separately. Loading imperfections can[HTML_REMOVED]occur due to manufacturing/machining variabilities and/or fit- up mismatch at component interfaces.Test article design and analysisTest article design and analysis encompass unique requirements that differ significantly from those[HTML_REMOVED]associated with the design of aircraft, spacecraft, or launch-vehicle structures. Aerospace[HTML_REMOVED]structures are designed and evaluated to ensure that they are able to sustain the required loads,[HTML_REMOVED]but they are not typically required to exhibit a specific controlling or critical failure mode[HTML_REMOVED](i.e., they are not typically designed such that a specific failure mode[HTML_REMOVED]has the minimum design margin). In contrast, test articles used in the SBKF Project are[HTML_REMOVED]designed and evaluated to ensure that a particular failure mechanism is exhibited during a[HTML_REMOVED]test so that the resulting test data may be used to validate modeling and analysis methodsfor predicting specific behaviors. In addition, the test articles are typically designed such[HTML_REMOVED]that they lie within the same design space as the full-scale structure they represent and[HTML_REMOVED]exhibit similar response characteristics.Exploratory studiesExploratory studies are typically quick assessments used to guide future detailed analysis[HTML_REMOVED]tasks. Data from these exploratory studies are not intended for future use or as decisional[HTML_REMOVED]data and are often only used by the analyst to make informed decisions on the direction of[HTML_REMOVED]future work. Thus, rigorous quality control and reporting of these analysis studies is[HTML_REMOVED]typically not required.The specific class of analysis and corresponding analysis and data requirements shall be determined[HTML_REMOVED]by the SBKF team leads and the analyst. The analysis approach shall be based on standard best[HTML_REMOVED]practices, when possible, and shall be uniform across all related analysis activities to ensure[HTML_REMOVED]consistency. However, deviations from standard practice may be required and/or new approaches may[HTML_REMOVED]be necessary to meet the analysis objectives. In such circumstances, the analyst and team lead will[HTML_REMOVED]work together to develop and validate any new approach required.

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Public: This dataset is intended for public access and use. License: U.S. Government Work

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Metadata Created Date August 1, 2018
Metadata Updated Date February 28, 2019

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Harvested from NASA Data.json

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Resource Type Dataset
Metadata Created Date August 1, 2018
Metadata Updated Date February 28, 2019
Publisher Space Technology Mission Directorate
Unique Identifier TECHPORT_33077
Maintainer Email
Public Access Level public
Bureau Code 026:00
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Harvest Object Id aa4b05ce-ada0-4710-a6bf-e832d5aa84f7
Harvest Source Id 39e4ad2a-47ca-4507-8258-852babd0fd99
Harvest Source Title NASA Data.json
Data First Published 2018-06-26
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Data Last Modified 2018-07-19
Program Code 026:027
Source Datajson Identifier True
Source Hash 73087857af48168e16f4cd8ce37238f3a579515b
Source Schema Version 1.1

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