Metallurgical Examination of a Failed Blade Lag Shock Absorber (Part No. 114H6802) From a CH-47D Chinook 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 (Boeing Helicopters, Philadelphia, PA) performed a visual examination of the failed part, fluorescent penetrant inspection, fractographic evaluation, metallography, hardness testing, conductivity testing, and chemical analysis. It was concluded that the part failed due to fatigue from an area exhibiting intergranular attack. The corrosive attack was most likely caused by the processing fluids used during the rework process. In addition, the parts may not have been properly aged, as evidenced by the higher-than-nominal yield strength values. An improper aging treatment could have facilitated the intergranular attack.