CompositesWorld
Issue link: https://cw.epubxp.com/i/632301
FEBRUARY 2016 62 CompositesWorld FOCUS ON DESIGN Boeing Research and Technology based its AFP system on a stock Kuka KR500 robot and controller (Kuka Robotics Corp., Shelby Township., MI, US). Its simplifed fber delivery path, enabled by the on-head spools, reduced issues with passing fber through a highly articulated transport path. Its compact package permitted the necessary range of motion and demonstrated the layup accuracy and speed required to meet engineering and material out-time requirements. A key to tank fabrication was a collapsible, segmented mandrel that could be extracted after cure from an elliptical opening in the tank (see "Learn More," p. 63). Material-wise, the cryotank was made of Cytec's CYCOM 5320-1 out-of-autoclave (OOA) prepreg, using a combination of thick plies (145 g/m²), which can be placed relatively quickly, and thin plies (70 g/m²), which create a micro-crack-resistant laminate that helps prevent hydrogen permeation, reports Dr. Jim Sutter, a composites expert at NASA Glenn Research Center (Cleveland, OH, US). For the skirt design, says McCarville, "We needed something stif and tailorable with low-temperature capability." Te Boeing team ruled out a honeycomb-cored sandwich structure because bondlines experience structural knockdowns at cryo-tempera- tures, McCarville explains. Two alternative skirt designs also were considered and dropped. Te frst, skin/stringer construction, is used prevalently in commercial aircraft and involves laminate skins stifened with longitudinal I-beam, T-beam or hat stringers. Te second, isogrid structure, has upstanding legs that cross over each other in a gridwork pattern. "Te fber-reinforced laminate legs are typi- cally 1 inch to 2 inches [25.4 mm to 50.8 mm] tall and look like a beehive with a rigid exterior," McCarville says. Unlike honey- comb-cored and futed-core panels, which have inner and outer facesheets, isogrids feature only an outer skin. McCarville says that on an 8.4m-diameter real-world tank, these options would be heavier than sandwich constructions. In the end, the team adopted the futed core, comprising large trapezoidal members — technically, laminate-angled web members with structural-radius fllers between facesheets (see photo at top left) because it can be optimized to efciently take compressive launch loads and vented to prevent hydrogen buildup. Further, the skirt is a co-cured, unitized carbon fber/ epoxy laminate construction, so there are no adhesive bond- lines that could degrade at -253°C cryogenic test temperatures. Subscale futed-core components created during the coupon and joint-testing phase were shown to be stronger in shear and end-loaded compression, and were more damage-tolerant than conventional honeycomb designs. "With good bending stifness, especially in axially loaded applications, the futed core is consid- ered a viable candidate for next-generation launch vehicle shells and skirts," says McCarville, adding, "Tis type of construction, which resembles bridge truss structure, was determined to be the lowest weight construction option for the CCTD tank." During the build, tooling was assembled around the fber- placed tank so that skirt plies and futed core could be placed and Fluted core concept: Co-cured construction The CCTD skirt departs from conventional skirt sandwich constructions by linking its facesheets with a trapezoidal network of angled web members, and is fber- placed onto the tank layup and then co-cured, leaving no adhesive bondlines to weaken at cryogenic temperatures. The futed core also provides pathways to vent hydrogen in case of tank leakage. Source | Boeing R&T; Robotic AFP meets manufacturing test Boeing Research and Technology's AFP system's robotic arm and head-mounted fber-delivery system resulted in a compact system capable of the necessary range of motion, layup accuracy and speed to meet engineering and material out-time requirements. Source | Boeing R&T;