High-Performance Composites

NOV 2014

High-Performance Composites is read by qualified composites industry professionals in the fields of continuous carbon fiber and other high-performance composites as well as the associated end-markets of aerospace, military, and automotive.

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APPLICATIONS N O V E M B E R 2 0 1 4 | 5 5 APPLICATIONS PMI foam core outperforms honeycomb in infused nose landing gear doors Composite Technology Centre GmbH (CTC, Stade, Germany) specializes in developing cost-effective serial-produc- tion composite technologies. A subsid- iary of Airbus (Toulouse, France), CTC undertook studies in 2013 to fnd more productive but less costly methods of fabricating carbon fber-reinforced poly- mer (CFRP) sandwich constructions. Evonik (Darmstadt, Germany) submitted samples of its new ROHACELL HERO polymethacrylimide (PMI) closed-cell foam to CTC for evaluation as a core- material alternative to traditional honey- comb. The new PMI foam is the fruit of an unusual Evonik effort to develop a new polymethacrylimide (PMI) foam product that can mimic honeycomb core's ten- dency to show visible laminate damage. Although foam is touted as a means to prevent water-ingress into surface-dam- aged aircraft parts, PMI foam's greatest asset in other applications — its supe- rior impact resistance compared to hon- eycomb core products — makes it diff- cult for aircraft maintenance and repair organizations (MROs) to visually identify damage on aircraft sandwich construc- tions during part inspections. Evonik's HERO product is specifcally formulated to accommodate this need in PMI foam performance properties. (Read more about the HERO formulation in "MRO- friendly PMI foam core" online at short. compositesworld.com/MRO-PMI.) CTC evaluated three CFRP sandwich construction design options: (1) auto- claved, honeycomb-cored, carbon/epoxy prepreg; (2) autoclaved PMI-cored, car- bon/epoxy prepreg; and (3) an out-of- autoclave (OOA) alternative featuring PMI core between faceskins of carbon fber infused with liquid epoxy resin. Op- tions 2 and 3 required signifcantly less time and cost than option 1. Option 3 was the best, at 19 percent lower weight and 25 percent lower cost than option 1. Further, it cut 18 hours off production time (a 43 percent savings) because the process eliminated the lengthy core pot- ting step and the two cure cycles typi- cally necessary to integrate the prepreg skins and honeycomb. "These signifcant savings in time and cost were achieved for a simple part," says Remo Hinz, CTC's R&D project leader, "As part complexity increases, the savings in processing time and costs exponentially increase in favor of an infused foam core over an autoclaved honeycomb sandwich design." On the basis of the CTC evaluation, nose landing-gear doors for a Dornier 728 aircraft were successfully fabricated as a technology demonstrator by INVENT GmbH (Braunschweig, Germany). The ROHACELL 71 HERO (75 kg/m 3 density) arrived preshaped, via CNC machining, by Evonik's ROHACELL SHAPES divi- sion and was sandwiched between dry carbon fber fabrics (see photo, above). The layup was infused with Hexcel's (Stamford, Conn.) Airbus-qualifed RTM 6 epoxy infusion resin. Evonik reports that the PMI core not only prevents problems seen in hon- eycomb-cored constructions, such as water ingress, but also eliminates skin debonding due to potting compound de- terioration. In addition, the core's greater elongation (9 to 10 percent) and impact resistance makes it viable for exterior air- craft parts. Impact tests were conducted at 35 Joule impact energy by the Fraunhofer IWM Institute (Halle, Germany), ac- cording to ASTM D7766/D7766M–11, on specimens cut from the PMI-cored in- fused laminate and the original honey- comb/prepreg laminate. Nondestructive testing confrmed that damage size and depth in each were comparable. Professor Axel Herrmann, CTC's man- aging director, concludes, "ROHACELL HERO is, technically, the better and safer material to specify for sandwich construc- tion, as water ingress issues are eliminat- ed and debonding failure risks are signif- cantly reduced." Evonik, therefore, is working on several structural applications with Airbus, and says that the frst Airbus aircraft part specifed with its PMI foam will enter service during 2015. Preshaped core The 75-kg/m 3 density PMI foam arrived at CTC GmbH preshaped via CNC machining by Evonik's ROHACELL SHAPES division. The photo shows the shaped foam core during layup between dry carbon fiber fabrics, prior to infusion. Finished door This finished nose landing-gear door, designed to replace a forward landing gear door on a Dornier 728 aircraft, was one of two (left and right side doors) successfully fabricated as technology demonstrators by INVENT GmbH (Braunschweig, Germany). Source: Evonik Source: Evonik

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