Cracking of Erosion Strip of Main Rotor Blade of a Military Helicopter

Abstract

Erosion strip fitted on main rotor blade (MRB) of a military helicopter (flying in coastal/marine environment) had cracked in service. The erosion strip was made of austenitic stainless steel material of specification MIL-S-5059D Type 301 and was in quarter hard condition. The chemical composition, the hardness and the microstructure (condition) of the erosion strip met all the specified requirements. Two cracks were observed in the erosion strip. Scanning electron microscopy of the fracture surfaces showed the presence of fatigue striations amidst thick corrosion products indicating toward failure by corrosion fatigue. Energy-dispersive X-ray spectroscopy (EDS) showed that the corrosion products on the fracture surfaces were predominantly oxides of iron consisting of corrosive species like chlorine, which is typical of corrosion in marine/coastal environment (i.e., the service environment of the helicopter). Fractographic studies showed that fatigue failure had originated from the inner surface of the erosion strip that is bonded to the MRB. Intergranular and transgranular cracks and corrosion were observed in the erosion strip material from its inner surface at the failure origin. These appeared to be stress corrosion cracks as a result of crevice corrosion that had occurred on the inner surface of the erosion strip at the failure origin. Chlorides present in the environment (viz. marine/coastal) had apparently concentrated inside the crevice (at the bonding between the erosion strip and the MRB), attacked the protective oxide surface film on stainless steel and also prevented the re-forming of the protective oxide layer. Thus, the material in the crevice was attacked rapidly and the corrosion continued. Stress corrosion cracks can develop from the base of crevice where the stress concentration is greatest. Thus, the cracking of the erosion strip is a typical example of environment-induced mechanical failure of a fairly passive metallic material.