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During a routine preflight inspection at Ft. Hood, an outboard barrel nut was found to be cracked on an Army helicopter. The part was fabricated from Inconel 718 (UNS N07718) according to AMS 5662F, “Alloy Bars, Forgings, and Rings, Corrosion and Heat Resistant”. Subsequent inspections at Ft. Hood and Ft. Rucker revealed an additional seven barrel nuts with large cracks. The components are used in many critical applications. The failures under investigation in this study were relegated to the vertical stabilizer of the aircraft. The failures were all attributed to hydrogen induced cracking. Galling between the unlubricated bolt and the nut threads provided the sustained hoop stress while galvanic corrosion of the carbon steel retaining clip in contact with the barrel nut generated hydrogen. Microstructural analysis of the nut revealed excessive banding consisting of a Widmanstatten phase and MC carbides which ran parallel to the fracture plane. The grains were almost completely surrounded by an undesirable acicular delta phase. No evidence of Laves phase was observed. Recommendations were made to utilize a corrosion inhibitive lubricant on the threads of the barrel nut and mating bolt to reduce galling and the consequential high stresses which result from metal to metal contact during torquing. A stress analysis of the part showed that the high strength level of the material could be reduced to increase fracture toughness and resistance to hydrogen cracking. The acicular delta phase should be avoided in accordance with AMS 5662F and the extrusion direction of the material should be parallel to the principal loading direction. Salt fog testing of the proposed barrel nut configuration revealed that the shoulder height base thickness should be increased. Future vendors should qualify their product by conducting a prescribed salt fog test incorporating the prescribed torque requirements. Finally, the material used to fabricate the retaining clip should be changed to prevent galvanic corrosion.
Failure analysis, fractography, hydrogen induced cracking, Inconel 718
Materials Engineer, U.S. Army Research Laboratory, MD