Failure and Cracking of Inconel 718 Air Supply Manifold Support Rods. A Service-211 Related Contribution to IEPG TA 31

摘要

In 1995 an air supply manifold support rod from a high-performance turbofan211u001eengine failed by bending fatique. The rod material was Inconel 718, operating at 211u001eabout 550 degrees. in an air environment. Diagnostic aspects of the failure 211u001e(fatigue in bending, Stage I and Stage II fatigue fracture characteristics, and 211u001einterference colors due to fracture surface oxidation) were aided by test results 211u001efrom the European collaborative Program IEPG TA 31: Lifting Concepts for Military 211u001eAero-Engine Components. Bending of the rod was caused by relative movement of the 211u001einner and outer sections of a connection in between the diffuser case air supply 211u001emanifold, whereby accomodatory swivelling of a spherical ends of the rod in the 211u001espherical seatings of the connection was prevented by contact pressure, fretting 211u001eand binding. A contributory factor is likely to have been decomposition of an 211u001eanti-galling compound applied to the spherical contact surfaces. This 211u001edecomposition, and hence loss of function, could have begun at temperatures well 211u001ebelow the normal operating temperatures of the rod. Recommendations were made for 211u001ethe alternative anti-galling compounds or a plasma-sprayed coating, which is a 211u001eless suitable option. It was also recommended to thoroughly inspect the rods in 211u001eother engines, as an addition to the specified maintenance procedure.