Failure Analysis of the Main Rotor Retention Nut from AH-64 Helicopter

摘要

An inspection of the entire fleet of Apache helicopters revealed that eight nutscontained cracks. The goals in this investigation were to determine the failure mode and cause,to establish the mechanical and corrosion properties of the nut material, to verify that cadmium and nickel coatings were applied in accordance with the specification requirements and to make appropriate recommendations to prevent further problems. Additional nuts from both the inventory and field were characterized and tested for comparison. The following analyses and tests were performed: visual examination; light and electron microscopy of the retention nuts and fracture surfaces; metallographic analysis of both failed and intact nuts; chemical analysis of the 18 Nickel maraging steel C-250 grade steel; mechanical property, stress corrosion, and electrochemical tests. The chemical composition of the steel was well within the contractors specification and met the industry standards for maraging C-250 grade steel. Mechanical properties were also within specified requirements. Hydrogen-assisted cracking was the cause of all cracks found in the retention nuts.