CompositesWorld
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NEWS CompositesWorld.com 87 and Airbus, with support from the Dutch government as well as European Union-supported projects. ese activities were vital in evolving the FML concept to a level where it ultimately became a new structural material with design principles that gave it distinct advantages over aluminum, says van Mourik. FMLs generated considerable interest in the aircraft industry when introduced in the late 1980s, due to their performance and weight savings potential. ey were ultimately certified and adopted by several airframers in primary structure. First known as Arall (Aramid Reinforced Aluminum laminate) aimed at wing applications, then as Glare (Glass fiber-reinforced aluminum) for fuselage structure, and more recently as High Static Strength Glare, FMLs have been investigated by numerous OEMs, including Boeing, which considered FMLs as reinforcement material for the longeron of the A10 close air-support aircraft. Lockheed Martin (Bethesda, MD, US) successfully investigated the potential of Glare FMLs for weight savings and increased life for lower wingskins on Hercules C130 aircraft. FMLs also found use in the C130 flaps, in the aft cargo door of the McDonnell Douglas C-17, in the Airbus A400M frames, and, beginning in 2000, in the Airbus A380's upper fuselage and tail leading edges. But van Mourik points out that the original purpose for which these hybrid laminates were developed is still a concern: "Fuselage structure, as shown by the Aloha Airlines Flight 243 accident (see Learn More, p. 93), is susceptible to fatigue damage and failure due to pressur- ization cycles, and can really benefit from FMLs." And as OEMs begin to evaluate materials for new narrowbody commercial aircraft, interest in this material has re-ignited. In response, GKN Aerospace's Fokker business is developing, with partner- firms Airbus, Premium Aerotec GmbH (Augsburg, Germany) and Stelia Aerospace (Toulouse, France, see Learn More), both of which also produce FML panels for Airbus, automated manu- facturing methods to meet the anticipated demand. A synergistic combination Fokker's FMLs address fatigue by bonding alternating layers of treated aluminum sheet and fiber/epoxy prepreg (see Fig. 1, above right). When TU Delft researchers substituted glass fiber for the aramid fiber employed in the first-generation Arall panels of the 1980s to create Glare, the choice proved serendipitous. e S-2 Glass, sourced from AGY (Aiken, SC, US), was more compat- ible with aluminum properties, and improved the laminate's fatigue performance as well as impact, corrosion and fire resis- tance, without hindering the aluminum's good lightning strike properties. e glass layers stopped crack growth, eliminating the need for expensive titanium crack-stopper straps previously used at highly loaded airframe locations. And, the glass fibers increased the material's elastic strain capacity, enabling it to absorb more impact energy. Further, impact damage resulting from ground operations or abrasion would be visible on the surface, something not always true in the case of composites- only fabrication. Fiber-metal Laminates FIG. 1 An advantageous multi-material mix FMLs address fatigue, a serious structural integrity issue on aircraft, by bonding together alternating layers of treated aluminum sheet (AL) and fiber/epoxy UD prepreg as shown in this diagram typical of GLARE construction. FIG. 2 Building big panels seamlessly This representation shows how splicing connects layers, where edges meet, to create larger panels. FIG. 3 Doubling up for doors and fasteners A close-up of a completed panel that contains a door opening shows how the FML is built up with doublers to meet projected loads and fastener locations. AL layer 0° direction = rolling direction of aluminum layers 90° direction UD prepeg layer with fibers in 0° direction UD prepeg layer with fibers in 90° direction AL layer UD prepeg 90° UD prepeg 0° AL layer