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1. Aeroelastic tailoring with tow-steered composites 2. Aeroelastic tailoring with functionally-graded metals 3. Wing box topology optimization - Curvilinear rib/spar/stiffener placement - Cellular-based stringer topologies - Optimal topology of lightening holes within ribs/spars - 3D level set methods
Composite materials and structures are enabling technology for modern aerospace vehicles. Numerical-control manufacturing, including automated fiber placement (AFP), is widely used to fabricate these components. However, weight and performance requirements for the next generation of aerospace vehicles will push the state-of-art even further, and require development of more advanced materials, manufacturing, and structures technologies that are both affordable and efficient. The Integrated Structural Assembly of Advanced Composites (ISAAC) system is intended to address much of NASA?s critical research needs in advanced composites. The baseline ISAAC system, combining a commercial robot with multiple redundant degrees of freedom, a tool changer interface, and a special-purpose AFP end effector, enables precise and accurate additive manufacturing of composite structures, as well as development of advanced tow-steered structures with tailored load paths. The highly capable baseline system can quickly change end effectors, thus enabling further research for composites manufacturing. This extended system is similar to a high-speed machining center, where interchangeable cutters are used for different operations during metal structures fabrication. This capability enables future development, integration and assessment of new advanced manufacturing technologies, such as in-situ curing and NDE, and through-thickness reinforcements to reduce delaminations. End effectors with these advanced capabilities may be purchased, developed internally or with industry or academia, and then integrated onto the existing robotic platform to perform advanced manufacturing operations and develop new techniques and processes. Technologies, techniques and processes developed using this research-oriented system could then be transitioned to the broader composites industry.
The structural performance of two advanced composite tow-steered shells is assessed using experiments and analyses. The fiber angles vary continuously around the shell circumference, and one shell has all 24 tows applied during fabrication, while the second shell uses the fiber placement systems tow drop/add capability to achieve a uniform wall thickness. The shells are tested in compression, and their axial stiffnesses and buckling loads are estimated using linear finite element analyses. Scaled passenger and cargo door cutouts are machined into one side of each shell. Axial stiffnesses and buckling loads of the shells with cutouts are computed using linear finite element analyses. The retested shells carry over 90 percent of the axial stiffness, and 85 percent of the buckling loads, of the shells without cutouts. These small reductions demonstrate the potential of tow steering to mitigate adverse effects of cutouts on the structural performance. The good correlation between experimental buckling loads and linear buckling analyses noted here may result from their circumferential stiffness variation, reducing geometric imperfection sensitivity. Analysis models of both shells were evaluated without and with measured geometric imperfections superposed in different longitudinal orientations. Small variations in prebuckling stiffness and buckling load of the shells are observed.
Reduced weight is an ever-increasing demand within the aerospace industry, and non-traditional means of achieving this demand are increasingly being considered. There is also a desire to reduce the manual labor involved in attaching thousands of parts together with rivets and other fasteners. Using advanced stiffener concepts can avoid the weight of fasteners and streamline fabrication by reducing the part count, while still providing the mandatory strength requirements for flight certification. Described herein is a set of manufacturing trials that were completed at NASA Langley Research Center to investigate the feasibility of various integrated stiffener concepts. Fully composite stiffeners that can be integrated or attached by other means, either through bonding or interleaving them within the skin, can remove the fasteners and still maintain a rapid manufacturing rate. This study contains these advanced stiffener concepts, and the lessons learned and design iterations that resulted from the series of manufacturing trials. Each set of trials was completed with an end goal of using tow-steered integral stiffeners for flight structure in composite aircraft wings.
Commercial aircraft structures are frequently manufactured from carbon-epoxy materials because of their weight and stiffness advantages compared to metallic materials. Wing cover panels are regularly manufactured using an automated fiber placement (AFP) process, but current design and manufacturing methodologies do not fully take advantage of the opportunities afforded by AFP. Design and manufacturing studies were undertaken at the NASA Langley Research Center at the Integrated Structural Assembly of Advanced Composites (ISAAC) facility to quantify manufacturing benefits and limitations associated with AFP to create structurally efficient integral stiffeners as an alternative to bonded or mechanically fastened stiffeners. This methodology could save weight and remove failure mechanisms by reducing the need for rivets and bonding materials since the stiffener plies are interleaved within the skin plies. The use of AFP with integral stiffeners can open the design space, but a fundamental, systematic evaluation of manufacturing limitations is necessary. Manufacturing trials are described herein, where considered manufacturing variables included stiffener location, stiffener course staggering, stiffener widths, stiffener intersections, and material thicknesses for both the skin and stiffener plies. The manufacturing process and lessons learned from each trial are described, including the most successful current design which contains staggered stiffeners, non-traditional laminate angles, and a combination of multiple material thicknesses within the same laminate.