A United Arab Emirates F-16E alights at Red Flag exercises in Nevada in 2009. (Image source: USAF/Michael R. Holzworth)

Fokker uses ZwickRoell creep testing machines to test for hydrogen embrittlement on landing gear components

Aerospace manufacturers walk a metaphorical balance beam to continually develop and produce stronger, more efficient materials and components, while addressing all potential failure modes. While many people may begin to think about airframes, wings, and engines, this is also true for safety-critical aircraft components like landing gear systems. Helmond, Netherlands-based Fokker Landing Gear B.V., a subsidiary of GKN Aerospace of the United Kingdom, recently equipped its mechanical laboratory with three creep testing machines to verify its manufacturing process control of zinc-plated bolts for aircraft landing gear systems.

The creep testing machines – acquired from Messphysik Materials Testing GmbH, a Zwick Roell Group (ZwickRoell) subsidiary based out of Austria – were used to test for hydrogen embrittlement of zinc-plated bolts. If the manufacturing process was left unchecked and these bolts fail during landing, it could result in landing gear collapse and catastrophe for the aircraft and its occupants.

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As with most aerospace fasteners and load-carrying landing gear components, the bolts used by Fokker are made from high-strength, lightweight, aerospace-grade steel. These steels, and the components made from them, are especially rigid – resultant of a high martensite content, or a significant amount of needle-like, crystalline structure – making them especially susceptible to damage due to hydrogen embrittlement.

Hydrogen embrittlement – or the change in brittleness of metals due to hydrogen penetration and absorption into the metal’s crystal lattice – can occur during the manufacturing process or during surface treatment of the steel, such as during galvanic zinc plating of a component.

During a hard landing, if the load on the landing gear system is high enough, a brittle fracture can occur on a component, fastener, or bolt without any recognizable indications of the imminent failure.

To address the potential safety risk according to the international ASTM F519 Standard Test Method for Mechanical Hydrogen Embrittlement Evaluation of Plating/Coating Processes and Service Environments and NADCAP requirements in the aviation industry, Fokker Landing Gear tests for hydrogen embrittlement mechanically with three ZwickRoell Kappa 50 LA lever-arm creep testing machines. The creep testing systems are rated for International Electrotechnical Commission (IEC) Class-1 measurement accuracy and are equipped with ZwickRoell’s standard testControl machine electronics and advanced testXpert testing software.

The test specimens were produced and underwent surface treatment in parallel with the manufacturing process for original components. Three testing systems were installed, each of which is capable of testing four specimens simultaneously. Over the course of 200 test hours, Fokker Landing Gears was able to successfully validate its manufacturing process control.

The company develops and produces landing gear systems for aircraft like the fifth-generation Lockheed Martin F-35 Lightning II Joint Strike Fighter, Lockheed Martin F-16 Fighting Falcon, General Atomics Predator B, and Boeing Apache AH-64 attack helicopter.

ZwickRoell’s materials testing machines are modular and capable of filling many testing needs. They are used to test almost every type of industrial and consumer product, from aerospace composites and medical devices to building materials and food packaging.

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William Kucinski is content editor at SAE International, Aerospace Products Group in Warrendale, Pa. Previously, he worked as a writer at the NASA Safety Center in Cleveland, Ohio and was responsible for writing the agency’s System Failure Case Studies. His interests include literally anything that has to do with space, past and present military aircraft, and propulsion technology.

Contact him regarding any article or collaboration ideas by e-mail at william.kucinski@sae.org.
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