Several stainless steel bolts used on a Titan Space Launch Vehicle broke at the shank and failure was attributed to stress-corrosion cracking. But results could not be duplicated in the laboratory with salt-solution immersion tests until the real culprit was established: the secondary effect of galvanic coupling, hydrogen embrittlement.

Aircraft missile launcher attachment bolts fabricated from cadmium-coated Hy-tuf steel were found broken. Subsequent analysis of the broken bolts indicated three causes of failure. First, the bolts had been carburized, which was not in conformance with the heat treating requirements. Second, macroetching showed that the bolts has been machined from stock rather than forged, and the threads cut rather than rolled. It was also determined that hydrogen-assisted stress-corrosion cracking also played a part in the failure of the high-strength bolts.

Failure occurred in two TOW flight missile cases in unrelated launchings. After an extensive investigation, it was concluded that stress corrosion was the most likely cause of failure. Subsequent to this conclusion, inner wall softening was observed in an unfired TOW flight missile case. Questions arose as to how the softening occurred and whether or not it had contributed to the failures. This report contains the results of a study which resolved that inner wall softening could not have been present in the failed missile cases.

Piping and structural components used in space launch facilities such as NASA's Kennedy Space Center and the Air Force's Cape Canaveral Air Station face extreme operating conditions. Launch effluent and residue from solid rocket boosters react with moisture to form hydrochloric acid that settles on exposed surfaces as they are being subjected to severe mechanical loads imparted during lift-off. Failure analyses were performed on 304 stainless steel tubing that ruptured under such conditions, while carrying various gases, including nitrogen, oxygen, and breathing air. Hydrostatic testing indicated a burst strength of 13,500 psi for the intact sections of tubing. Scanning electron microscopy and metallographic examination revealed that the tubing failed due to corrosion pitting exacerbated by stress-corrosion cracking (SCC). The pitting originated on the outer surface of the tube and ranged from superficial to severe, with some pits extending through 75% of the tube's wall thickness. The SCC emanated from the pits and further reduced the service strength of the component until it could no longer sustain the operating pressure and final catastrophic fracture occurred. Corrosion-resistant coatings added after the investigation have proven effective in preventing subsequent such failures.

A precipitation-hardened stainless steel poppet valve assembly used to shut off the flow of hydrazine fuel to an auxiliary power unit was found to leak. SEM and optical micrographs revealed that the final heat treatment designed for the AM-350 bellows material rendered the AM-355 poppet susceptible to intergranular corrosive attack (IGA) from a decontaminant containing hydroxy-acetic acid. This attack provided pathways for which fluid could leak across the sealing surface in the closed condition. It was concluded that the current design is flight worthy if the poppet valve assembly passes a preflight helium pressure test. However a future design should use the same material for the poppet and bellows so that the final heat treatment will produce an assembly not susceptible to IGA.

Pinhole defects were found in a main combustion chamber made from NARloy-Z after an unexpectedly short time in service. Analysis indicated that the throat section of the liner had been exposed to very severe environmental conditions of high temperature and high oxygen content, which caused ductility loss and grain-boundary separation. The excessive oxygen content in the liner was attributed to diffusion from an oxygen-rich environment that had resulted from nonuniform mixing of propellants. The internal oxygen embrittled the alloy and reduced its thermal conductivity, which resulted in a higher hot-gas wall temperature and associated degradation of mechanical properties.

A Stirling engine heat pipe failed after only 2h of operation in a test situation. Cracking at the leading edge of an evaporator fin allowed air to enter the system and react with the sodium coolant. The fin was fabricated from 0.8 mm (0.03 in.) thick Inconel 600 sheet. The wick material was type 316 stainless steel. Macro- and microexaminations of specimens from the failed heat pipe were conducted. The fin cracking was caused by overheating that produced intergranular corrosion in both the fin and the wick. Recommendations for alleviating the corrosion problem included reducing the heat flux, redesigning the wick, and reducing the oxygen content of the sodium.

An investigation of a Stirling engine after an aborted test run revealed that the regenerator screens had suffered substantial damage. During the run, the individual screens oscillated as the helium working fluid was shuttled through the regenerator. In localized areas, the 41 mu m (1600 mu in.) diam type 304 stainless steel wire screening had been torn and pieces were missing. Scanning electron microscope revealed that the fracture had occurred at wire crossover locations by a fatigue mechanism. The problem was solved by sintering the individual screens into a single unit.