LAU-7/A Missile Launcher Attachment Bolt Failures
Abstract
A comprehensive metallurgical examination of failed LAU-7/A missile launcher attachment bolts was conducted at the U.S. Army Materials Technology Laboratory (MTL) to determine the probable cause of failure. These bolts were utilized to attach LAU-7/A missile launcher systems to Navy F-14 fighter jets. Light optical microscopy revealed that the fractures occurred at the head to shank radius near the hexagonal recess. Chemical analysis verified the parts were fabricated from the commercially-known Hy-Tuf steel. It was determined through metallographic examination that the parts consisted of a typical tempered martensitic structure. Metallography determined that a potentially detrimental surface layer of carburization had been formed during heat treatment. This layer was verified by chemical analysis and microhardness testing. In addition, it was revealed through examination of the macrostructure that the bolts had been machined and not forged as outlined in the governing specification. Fractographic analysis of the crack initiation sites utilizing scanning electron microscopy (SEM) revealed an intergranular mode of fracture, indicative of stress-corrosion cracking (SCC). It was determined that SCC was the probable cause of failure, and hydrogen charging as a result of surface corrosion was the failure mechanism. Contributing factors to SCC included surface carburization and an unacceptable grain flow pattern. Missile launcher bolts, Stress Corrosion Cracking (SCC), Failure, 4340 Steel, Metallography, Hy- Tuf steel.
Document Details
- Document Type
- Technical Report
- Publication Date
- Jul 01, 1992
- Accession Number
- ADA257982
Entities
People
- Gary Wechsler
- Marc S. Pepi
- Victor K. Champagne