Composite Stitching with Vacuum Consolidation

This NASA invention is a method enabling the use of a robotic arm outfitted with a stitching head to join two or more dry composite preforms previously joined with fasteners. The method accommodates the presence of fasteners via stitching around individual fasteners at a desired radius (termed “encapsulation”). The stitched seam enabled by NASA's invention serves several purposes – it is a means of joining of preforms, providing damage arrestation for cracks that form near fastener holes, and strengthening joints via through-thickness stitching. NASA’s method entails stitching a partial sinusoid-containing pattern in dry fiber composite preform substrate using a robotic, single-sided or double-sided needle stitching head in the following manner: (1) moving the stitching head to the proper location, (2) inserting the stitching needle and thread and capturing a stitch without moving the needle relative to the substrate, (3) continuing the stitching motion until the needle has returned to its starting position, (4) traversing the next point of entry without rotating the stitching head relative to its forward motion, (5) repeat steps 1-5 along a sinusoidal path. In addition to the general method, NASA has generated a control schema to develop the G-code that is used to command the stitching head / robotic arm to carry out the required operations. The control schema enables a stitching pattern that turns the stitching path through a 180 degree arc without requiring the stitching head or composite preform to rotate. This reduces the complexity of required stitching tooling and operations (since turning the stitching apparatus through this arc requires sufficient clearance in the “throat” of the apparatus), removes the constraint on preform length in the direction of the stitching path, and enables stitching around fasteners at very small radii.

Next generation aircraft are anticipated to be largely made with composite components, requiring significant increases in manufacturing rates of composites to meet the demand for a new fleet of aircraft. The higher rate manufacturing will require multiple advances, including rapid curing and lower processing temperatures. These requirements can be enabled by new processing methods such as isothermal rapidly cured composite parts. NASA has developed materials and methods that meet those stringent requirements for high-rate manufacturing. The innovators have demonstrated at least two families of new resin formulations that meet the expected high-rate manufacturing needs. These new formulations have been engineered to be infused and cured at the same (i.e., isothermal) temperature, below that of commercially available materials. The materials can then be removed from the mold while still hot without distorting the shape, thereby reducing the processing times by eliminating the need for cooling to occur in the mold. After a post-cure process - which takes 4 hours or less and can be performed in batches - the mechanical properties of NASA's next-gen composites. The related patent is now available to license. Please note that NASA does not manufacturer products itself for commercial sale.

This technology is a joint connector for application between two or more tubular parts, or to connect one or more tubular parts to a fixed structure. This attachment technology is more structurally efficient and reduces failure characteristics due to the uniformity of composite material across the joint. In comparison to a typical joint, this technology reduces weight while minimizing stress variations that lead to structural failure. Moreover, typical joints must be bonded or screwed together, which further reduces efficiency. The invented technology, however, is designed so that it is both bonded and mechanically locked by design rather than relying on separate mechanical fasteners. The result is a design that mitigates failure of a structural joint.

The AERoBOND and AERoBOND+ technologies are composite resin materials design innovations that enable new methods for composites joining and manufacturing. The resins are formulated with carefully selected off-set stoichiometries to delay/control the cure such that initial curing of individual components can be followed separately by joining/curing of components together. The ability to delay and control the co-cure joining step provides ease of manufacturing of multi-part composite structures, without compromising joint integrity. There are significant cost savings associated with eliminating fasteners and joint surface preparation steps. To date, the focus of the NASA development effort has been on novel epoxy-based prepreg formulations though other types of thermosets could be considered as well. The AERoBOND+ innovation provides an added adhesive layer to the AERoBOND joint design to improve the ability to join composite surfaces when these surfaces are less tightly matched. Conventional adhesives, e.g., film, paste, etc., are employed. By including an adhesive between the offset stoichiometric prepreg plies, the adhesive can fill the gaps between the bonding surfaces while maintaining reflowable AERoBOND layer interfaces. Since all interfaces are reflowable, they are much more tolerant of surface contamination, thereby mitigating a primary challenge for conventional adhesive bonding.

This technology enables the fabrication of co-cured structures without the need for an autoclave or oven large enough to contain the full-scale structure. Instead, sub-components can be prepared in a less expensive, smaller autoclave or oven with co-cure plys applied to the faying surfaces. A continuous, assembled structure can be prepared using a subsequent curing process in a heated press. The co-cured structures can be designed to meet FAA certification criteria for composite structures because no adhesive bond or mechanical fasteners are needed. The structure can be treated as a single, joint-free component.