Investigation of main rotor spindle assembly retaining rods for an army utility helicopter fabricated by three different contractors

摘要

The US Army Research Laboratory (ARL) was tasked by the US Army Aviation and Troop 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 ("A", "B" and "C"). These components were subjected to prior spectrum load fatigue testing in order to qualify an alternate source. Manufacturer "A" was the current source, and this testing was performed to compare the results of "B" and "C" versus "A". The "C" components exhibited only half the fatigue resistance of the "A" and "B" components. Upon dimensional examination by ARL, it was determined that the root radius of an "A" and "C" rod did not conform to the governing requirements. Although the surface roughness of the parts from each manufacturer varied, all readings met the governing requirement. Hardness results differed only slightly between the rods, however, the tensile properties (of coupons sectioned from the original rods) were significantly different. Nevertheless, the hardness and tensile results met the minimum requirements specified. 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 witin the specified limits. Banding was more prevalent witin the "C" material than the "A" and "B" material. 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 was 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.