Investigation of the UH-60 Main Rotor Spindle Assembly Retaining Rods P/N 70102-08102/-103

摘要

The U.S. Army Research Laboratory (ARL) was tasked by the U.S. Army Aviation andTroop Command (ATCOM) to perform a metallurgical examination of main rotor spindle assembly retaining rods fabricated from precipitation hardened (PH 13-8 Mo) stainless steel by three different manufacturers. These components were subjected to prior spectrum load fatigue testing in order to qualify an alternate source. One of the manufacture's components exhibited only half the fatigue resistance of the other two. The results of fatigue testing (of coupons sectioned from the original rods) showed a dramatic difference between the rods. Metallography was utilized to examine the microstructure and grain size. The structure of each rod was consistent with the prior treatment, and the grain size met the governing requirement. The amount of delta (free) ferrite within the structure varied slightly from rod to rod, but was well within the specified limits. The threads of each rod were examined metallographically, since this was the area of failure as a result of the spectrum load fatigue testing. Although differences in the surface profile of the threads from the different manufacturers were noted, there was no evidence of gross abnormalities such as tear out or chatter. The chemical analysis of each rod varied, but each composition met the governing requirements. Based upon the results of reheat treating, it was concluded that an inadequate prior heat treatment sequence was performed by the manufacturer.