NASA's Shell Buckling Knockdown Factor (SBKF) project has the goal of developing new analysis-based shell buckling design factors (knockdown factors) and design and analysis technologies for launch vehicle structures. Preliminary design studies indicate that implementation of these new knockdown factors can enable significant reductions in mass and mass-growth in these vehicles. However, in order to validate any new analysis-based design data or methods, a series of carefully designed and executed structural tests are required at both the subscale and full-scale levels. This paper describes the design and analysis of three different orthogrid-stiffened metallic cylindrical-shell test articles. Two of the test articles are 8-ft-diameter, 6-ft-long test articles, and one test article is a 27.5-ftdiameter, 20-ft-long Space Shuttle External Tank-derived test article. Introduction uckling is an important and often critical consideration in the design of lightweight launch-vehicle structures; therefore, robust, validated design criteria for thin-walled shells are needed to achieve optimal designs of these structures. Unfortunately, the current design guidelines 1-4 have not been updated since they were first published in the late 1960's and early 1970's, and may not be able to take full advantage of the modern materials, precision manufacturing processes, and new structural concepts needed to produce the next generation of affordable and efficient launch vehicles. To this end, a design technology development program at NASA, the Shell Buckling Knockdown Factor (SBKF) project, is currently working to revise the existing design factors and recommendations for buckling-critical shell structures. 5 To support the development and validation of these new design factors, the SBKF project is conducting a series of shell buckling tests on large-scale, integrally-stiffened aluminum cylinders. Currently, SBKF is targeting specific structural configurations including isogrid-stiffened and orthogrid-stiffened cylinders for large-diameter heavy-lift launch vehicles. These large metallic cylinders are typically constructed by welding several curved panel sections together along longitudinal weld lands to form a complete circular cylinder (Fig. 1). The validation testing requires that the test article designs and data obtained from the tests are representative of these types of large-scale launch vehicle cylinder structures, and that certain behavioral characteristics and failure modes shall be isolated and studied. For example, these types of integrally-stiffened, welded structures can exhibit several different failure modes including global buckling, local skin-pocket buckling (i.e., buckling of the thin skin between stiffeners), weld land buckling (buckling of the unstiffened welded joint regions), and stiffener buckling and crippling. NomenclatureOne of the challenges in the test program is to design each test article to exhibit a specific failure mode or sequence of failure modes. However, imperfection sensitivity in these buckl...

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Fiber-reinforced polymer composites are widely used in the aerospace industry due to their high stiffness and strength-to-weight ratios. However, their applicability can be limited by their relatively low interlaminar properties when compared to metallic alternatives. Through-thickness reinforcement approaches, such as stitching, z-pinning, needling, tufting, and three-dimensional weaving, have been developed in recent decades to enhance the interlaminar properties of composites. Stitching is considered to be an efficient and cost-effective method to reinforce composites in the through-thickness direction. Additionally, stitch parameters (stitch density, linear thread density, thread material, pretension, etc.) highly influence the in-plane and out-of-plane properties. This paper summarizes results from over one hundred papers on the influence of stitch parameters on fracture energy, interlaminar strength, and impact characteristics of stitched composite laminates, sandwich composites, and high-temperature composites. Much of the research on the influence of stitch parameters has focused on thermoset polymer matrix composites (PMCs), while fewer studies have investigated the impact of stitch parameters on high temperature or sandwich composites. Modification of existing and new test methods have been developed to adequately measure the effectiveness of stitching on the out-of-plane behavior of PMC panels. Results demonstrate that out-of-plane properties of PMCs are highly dependent on stitch parameters and can be enhanced by through-thickness stitching.

New analysis-based shell buckling design factors (aka knockdown factors), along with associated design and analysis technologies, are being developed by NASA for the design of launch vehicle structures. Preliminary design studies indicate that implementation of these new knockdown factors can enable significant reductions in mass and mass-growth in these vehicles and can help mitigate some of NASA's launch vehicle development and performance risks by reducing the reliance on testing, providing high-fidelity estimates of structural performance, reliability, robustness, and enable increased payload capability. However, in order to validate any new analysis-based design data or methods, a series of carefully designed and executed structural tests are required at both the subscale and fullscale level. This paper describes recent buckling test efforts at NASA on two different orthogrid-stiffened metallic cylindrical shell test articles. One of the test articles was an 8-ftdiameter orthogrid-stiffened cylinder and was subjected to an axial compression load. The second test article was a 27.5-ft-diameter Space Shuttle External Tank-derived cylinder and was subjected to combined internal pressure and axial compression.

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