Failure Analysis of a Missile Locking Hook from the F-14 Jet.

摘要

A comprehensive metallurgical examination of the missile hook was conducted at the U.S. Army Materials Technology Laboratory (MTL) to determine the probable cause of failure. The component is one of two launcher housing support points for the Sparrow Missile and is located on the F-14 jet. The missile hook failed while in service. Chemical analysis verified that the part was fabricated from 300M steel. It was determined by metallographic examination that the microstructure was martensitic. Visual inspection and light optical microscopy of the load-bearing area showed that the surface coating 'Microlube' had been completely worn away during service, allowing corrosion to take place. Extensive pitting was found on surface regions near the fracture. Further analysis of the 'Microlube' coating revealed that required thickness specifications were exceeded. The machined radius where the fracture initiated was measured and found to be sharper than specified. The mechanical properties of the material compared favorably with requirements. Scanning electron microscopy (SEM) and energy dispersing spectroscopy (EDS) were used to characterize the fracture surface and the surface coatings. The entire manufacturing history of the missile hook was conducted in order to calculate the operating stresses, the critical crack size, and the critical stress intensity factor (KIC) for the conditions observed. The failure was attributed to a fatigue crack that had initiated at the base of an undercut radius on the machined lip of the component. Keywords: High strength steels; Fatigue(mechanics); Missile hook; Corrosion; Failure mechanics. (SDW)