Shell Buckling Knockdown Factors

The Shell Buckling Knockdown Factor (SBKF) Project, NASA Engineering and Safety Center (NESC) Assessment #: 07-010-E, was established in March of 2007 by the NESC in collaboration with the former NASA Constellation Program (CxP) and now the Space Launch System (SLS) Program. The SBKF Project has the goal of developing and experimentally validating improved (i.e., less-conservative, more robust) analysis-based shell buckling design factors (a.k.a., knockdown factors (KDFs)) and developing design recommendations for launch vehicle structures.

Shell buckling knockdown factors have been historically based on test data from laboratory-scale test articles obtained from the 1930s through the 1960s. The knockdown factors are used to account for the differences observed between the theoretical buckling load and the buckling load obtained from test. However, these test-based KDFs may not be relevant for modern launch- vehicle designs, and are likely overly conservative for many designs. Significant advances in structural stability theory, high-fidelity analysis methods, manufacturing, and testing are enabling the development of new, less conservative, robust analysis-based knockdown factors for modern structural concepts. Preliminary design studies indicate that implementation of new knockdown factors can enable significant weight savings in these vehicles and will help mitigate some of NASA’s launch-vehicle development and performance risks, by reducing reliance on large-scale testing, and providing high-fidelity estimates of as-built structural performance, increased payload capability, and improved structural reliability.

To achieve its KDF development and implementation goals, the SBKF Project is engaged in several work areas including launch-vehicle design trade studies, subcomponent and component level design, analysis and structural testing, and shell buckling design technology development including analysis-method development, analysis benchmarking and standardization, and analysis-based KDF development. Finite-element analysis is used extensively in all these work areas. In particular, there are four main categories analyses conducted by SBKF and include:
1) high-fidelity structural simulations, 2) imperfection sensitivity studies, 3) test article design and analysis and 4) exploratory studies. Each of these types of analysis may have different analysis objectives and utilize different modeling approaches that depend on the results required 
to meet the Project needs. A description of the four main categories follows. 

High-fidelity structural simulations

High-fidelity structural simulations are defined as simulations that can predict accurately the complex behavior of a structural component or an assembly of components (e.g., virtual structural test) and often require a significant level of modeling detail and knowledge of the structural system (e.g., its physical behavior and expected variability). Models are considered high-fidelity if results predicted with these models correlate with test data to within a small range of variance and represent accurately the true physical behavior of the structure. The permissible amount of variance is determined based on the analysis requirements defined by the Project in accordance with the intended end use of the predicted data. High-fidelity shell buckling analysis objectives considered by the SBKF Project often require the accurate prediction of stiffnesses, local and global deformations, strains, load paths and buckling-induced load redistribution, and buckling and failure loads and modes. To achieve these analysis goals, the models typically must accurately represent loading and boundary conditions, and expected or measured geometric and material varia