As part of the Space Technology Game Changing Development Program (GCDP) Composite Cryotank Technology Development (CCTD) contract, Boeing fabricated a 2.4 m diameter test article as a precursor to a 5.5 meter cryotank design, fabrication, and test. This component encompasses several challenging design features: (a) one-piece co-cured/co-bonded spherical geometry with integral skirts, (b) out-of-autoclave curing materials, (c) permeation resistant thin/hybrid ply laminate skins, and (d) thin and thick off-angle slit tape (tow) construction. The component was built on a 24 piece collapsible composite tool using robotic fiber placement. This paper details the tooling and manufacturing flow with an emphasis on process development building block activities. Lessons learned are compiled that will be used to help guide the build of a 5.5 m diameter tank during the next phase of the CCTD contract.
The development of durable bonded joint technology for assembling composite structures is an essential component of future space technologies. While NASA is working toward providing an entirely new capability for human space exploration beyond low Earth orbit, the objective of this project is to design, fabricate, analyze, and test a NASA patented durable redundant joint (DRJ) and a NASA/Boeing co-designed fluted-core joint (FCJ). The potential applications include a wide range of sandwich structures for NASA's future launch vehicles. Three types of joints were studied -- splice joint (SJ, as baseline), DRJ, and FCJ. Tests included tension, after-impact tension, and compression. Teflon strips were used at the joint area to increase failure strength by shifting stress concentration to a less sensitive area. Test results were compared to those of pristine coupons fabricated utilizing the same methods. Tensile test results indicated that the DRJ design was stiffer, stronger, and more impact resistant than other designs. The drawbacks of the DRJ design were extra mass and complex fabrication processes. The FCJ was lighter than the DRJ but less impact resistant. With barely visible but detectable impact damages, all three joints showed no sign of tensile strength reduction. No compression test was conducted on any impact-damaged sample due to limited scope and resource. Failure modes and damage propagation were also studied to support progressive damage modeling of the SJ and the DRJ.
Foams made from engineered thermoplastic resin systems possess high glass transition temperatures. When compared to most thermoset resins, it was anticipated that lightly cross-linked foam candidates might survive high temperature/ pressure cures. This is especially true if the viscosity decline (i.e., loosening of weak secondary bonds) during sandwich bond cycles were found to be gradual enough to be controllable. To test this supposition, a low-cost five-step screening down-select methodology was utilized: (a) survey and evaluate vendor data, (b) pre-screen candidate foams using a vertical flame test, (c) characterize basic material properties using room temperature quasi-static compression testing, (d) evaluate failure mechanisms via elevated temperature compression testing, and (e) validate material functionality by conducting flat and contoured autoclave sandwich panel bond trials. None of the three candidate materials evaluated individually met all research requirements. Still, the population taken as a whole exhibited the potential for fulfilling the study's objectives. It is hypothesized that foam chemistry modifications, cell structure manipulation, and/or additional cure cycle process parameter management could lead to extended usage of foam in structurally demanding applications.
We present in this paper an extension to the ML type system by which it is possible to statically estimate all untrapped exceptions that can be raised by executing a program. This type system can handle polymorphic information on exceptions. A prinicipal extended type exists and can be computed for any well-typed expression.
