Metallurgical Examination of a Failed Blade Lag Shock Absorber from an Army Cargo Helicopter

A metallurgical examination was performed on a failed blade lag shock absorber from the aft red rotor blade of an Army cargo helicopter. The U.S. Army Research Laboratory (ARL) and the primary contractor performed a visual examination of the failed part, fluorescent penetrate inspection, fractographic evaluation, metallography, hardness testing, conductivity testing and chemical analysis. It was concluded that the part failed in fatigue from an area exhibiting intergranular attack. The corrosive attack was most likely caused by the processing fluids used during the rework process, as it was later determined that an unapproved aqueous alkaline immersion cleaner was being used. Failure to meet the required yield strength may have been an indication that the parts were not properly aged. An improper aging treatment could have facilitated the intergranular attack.