During a routine inspection on an aircraft assembly line, an airframe attachment bolt was found to be broken. The bolt was one of 12 that attach the lower outboard longeron to the wing carry-through structure. Failure occurred on the right-hand forward bolt in this longeron splice attachment. The bolt was fabricated from PH13-8Mo stainless steel heat treated to have an ultimate tensile strength of 1517 to 1655 MPa (220 to 240 ksi). A water-soluble coolant was used in drilling the bolt hole where this fastener was inserted. Investigation (visual inspection, 265 SEM images, hardness testing, auger emission spectroscopy and secondary imaging spectroscopy, tensile testing, and chemical analysis) supported the conclusion that failure of the attachment bolt was caused by stress corrosion. The source of the corrosive media was the water-soluble coolant used in boring the bolt holes. Recommendations included inspecting for corrosion all the bolts that were installed using the water-soluble coolant at the spliced joint areas, rinsing all machined bolt holes with a noncorrosive agent, and installing new PH13-8Mo stainless steel bolts with a polysulfide wet sealant.

A preflight inspection found a broken diaphragm from a side controller fabricated from 17-7 PH stainless steel in the RH 950 heat treatment condition. Failure occurred by cracking of the base of the flange-like diaphragm. The crack traveled 360 deg around the diaphragm. Scanning electron microscopy (SEM) revealed that the failure occurred by a brittle intergranular mechanism and stress-corrosion cracking (SCC), and indicated a failure mode of selective grain-boundary separation. The diaphragms were heat treated in batches of 25. An improper heat treatment could have resulted in the formation of grain boundary precipitates, including chromium carbides. It was concluded that failure of the diaphragm was due to a combination of sensitization caused by improper heat treatment and subsequent SCC. It was recommended that the remaining 24 sensor diaphragms from the affected batch be removed from service. In addition, a sample from each heat treat batch should be submitted to the Strauss test (ASTM A262, practice E) to determine susceptibility to intergranular corrosion. Also, it was recommended that a stress analysis be performed on the system to determine whether a different heat treatment (which would offer lower strength but higher toughness) could be used for this part.

Several elbow subassemblies comprising segments of oil-line assemblies that recycled aircraft-engine oil from pump to filter broke in service. The components of the subassemblies were made of aluminum alloy 6061-T6. Two subassemblies were returned to the laboratory to determine cause of failure. In one, the threaded boss had separated from the elbow at the weld. In the other, the failure was by fracture of the elbow near the flange. The separation of the threaded boss from the elbow was due to a poor welding procedure. Crack propagation was accelerated by fatigue caused by cyclic service stresses. The fracture of the second elbow near the flange was caused by overaging during repair welding of the boss weld. Satisfactory weld penetration was achieved by improved training of the welders plus more careful inspection. Repair welding was prohibited, to avoid recurrence of overaging from the welding heat. Additional support for the oil line was installed to reduce vibration and minimize fatigue of the elbow.

The purpose of this investigation was to determine the root cause of the differences noted in the fatigue test data of main rotor spindle assembly retaining rods fabricated from three different vendors, as part of a Second Source evaluation process. ARL performed dimensional verification, accessed overall workmanship, and measured the respective surface roughness of the rods in an effort to identify any discrepancies. Next, mechanical testing was performed, followed by optical and electron microscopy, and chemical analysis. Finally, ARL performed laboratory heat treatments at the required aging temperature and follow-up mechanical testing.

During a routine inspection on an aircraft assembly line, an airframe attachment bolt was found to be broken. The bolt was one of 12 that attach the lower outboard longeron to the wing carry-through structure. Failure occurred on the right-hand forward bolt in this longeron splice attachment. The bolt was fabricated from PH13-8Mo stainless steel heat treated to have an ultimate tensile strength of 1517 to 1655 MPa (220 to 240 ksi). A water-soluble coolant was used in drilling the bolt hole where this fastener was inserted. Investigation (visual inspection, 265 SEM images, hardness testing, auger emission spectroscopy and secondary imaging spectroscopy, tensile testing, and chemical analysis) supported the conclusion that failure of the attachment bolt was caused by stress corrosion. The source of the corrosive media was the water-soluble coolant used in boring the bolt holes. Recommendations included inspecting for corrosion all the bolts that were installed using the water-soluble coolant at the spliced joint areas, rinsing all machined bolt holes with a noncorrosive agent, and installing new PH13-8Mo stainless steel bolts with a polysulfide wet sealant.

A valve stem made of 17-4 PH (AISI type 630) stainless steel, which was used for operating a gate valve in a steam power plant, failed after approximately four months of service, during which it had been exposed to high-purity water at approximately 175 deg C (350 deg F) and 11 MPa (1600 psi). The valve stem was reported to have been solution heat treated at 1040 +/-14 deg C (1900 +/-25 deg F) for 30 min and either air quenched or oil quenched to room temperature. The stem was then reportedly aged at 550 to 595 deg C (1025 to 1100 deg F) for four hours. Investigation (visual inspection, 0.7x/50x images, hardness testing, reheat treatment, and metallographic examination) supported the conclusion that failure was by progressive SCC that originated at a stress concentration. Also, the solution heat treatment had been either omitted or performed at too high of a temperature, and the aging treatment had been at too low of a temperature. Recommendations included the following heat treatments: after forging, solution heat treat at 1040 deg C (1900 deg F) for one hour, then oil quench; to avoid susceptibility to SCC, age at 595 deg C (1100 deg F) for four hours, then air cool.

Rivets from the longitudinal seam of the terminal shell ring of a 12 year old Lancashire boiler broke off easily during examination. Cleavage fractures indicated a brittle material. Microstructure of a sectioned rivet head was typical of a normal rimming steel except the ferrite crystals contained numerous nitride needles. Their existence indicated an abnormally high nitrogen content. If such a steel is heated for a lengthy period to a temperature of that prevailing in a boiler, precipitation of the nitrides may be expected, with consequent embrittlement. In this case, embrittlement of this type was the primary cause of the breaking off of the type rivet heads. Nothing was observed in the course of the examination that suggested caustic cracking.

When a roller-bearing assembly was removed from an aircraft for inspection after a short time in service, several areas of apparent galling were noticed around the inside surface of the inner cone of the bearing. These areas were roughly circular spots of built-up metal. The bearing had not seized, and there was no evidence of heat discoloration in the galled areas. The inner cone, made of modified 4720 steel and carburized for wear resistance, rode on an AISI type 630 (17-4 PH) stainless steel spacer. Consequently, it was desirable to determine whether the galled spots contained any stainless steel from the spacer. Other items for investigation were the nature of the bond between the galled spot and the inner cone and any evidence of overtempering or rehardening resulting from localized overheating. Analysis (visual inspection, electron probe x-ray microanalysis, microscopic examination, and hardness testing) supported the conclusions that galling had been caused by a combination of local overload and abnormal vibration of mating parts of the roller-bearing assembly. No recommendations were made.

Aluminum alloy BS.1476-HE.15 by virtue of its high strength and low density finds application in the form of bars or sections for cranes, bridges, and other such structures where a reduction in dead weight load and inertia stresses is advantageous. Bars and sections in H.15 alloy are mostly produced by extrusion. Some material processed this way has been prone to exfoliation corrosion. Extended aging for 24 h at a temperature of 185 deg C (365 deg F) virtually suppresses the tendency for exfoliation corrosion to develop. Also, the use of a sprayed coating, either of aluminum or Al-1Zn alloy, was effective in halting and preventing this form of attack. While alarming, the appearance of exfoliation corrosion provides a valuable warning to the engineer or inspector before a severe weakening of the particular sections has occurred.

The failures of two aircraft components, one from a landing gear and the other from an ejector rack mechanism, were investigated. Both were made from PH 13-8 Mo (UNS S13800) precipitation-hardening stainless steel which had been heat treated to the H1000 and H950 tempers respectively and then chromium plated. The parts were characterized metallographically and mechanically and were found to be compliant. Detailed fractographic examination revealed that the first stage of both failures was similar: subsurface initiation of numerous cracks with a wide range of orientations and cleavage like features. The cracking was followed by fatigue in one case and catastrophic failure in the other. Hydrogen embrittlement was identified as the most likely mechanism of failure.

A type 17-4PH stainless steel tube exhibited brown discoloration after a pickling operation. EDS analysis of the extracted substance revealed relatively high levels of iron and chromium, along with lower amounts of aluminum, silicon, sulfur, chlorine, calcium, manganese, and nickel. The iron, chromium, and nickel are likely in the form of dissolution products from the pickling solution. FTIR analysis revealed the presence of polypropylene and poly(ethylene:propylene). The EDS results showed that the discoloration of the tube was associated with oxidation products of the tube material, as well as adherent organic residue. Analysis by FTIR of the residue revealed detectable levels of two polymeric substances, which were later determined to be construction materials of the pickling tank. It was recommended that more frequent cleaning and/or replacement of the pickling solution be put into place and another type of tank material be considered.

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.

After several years' use, an eyebolt suffered brittle fracture in the first turn of the thread. The fracture started at the notch at the root of the thread. Neither localized material defect nor an old crack were present. The investigation showed that instead of the specified steel quality St 37-2 N, a steel with about 0.5% C had been used. The microstructure with the coarse ferrite network indicated that the forged eye bolt had been normalized either at too high a temperature or not at all. In any case the anneal at 900 deg C produced a considerably more finely grained structure. In addition, the nature of the fracture and the results of the notched bar impact tests showed that in spite of the high C-content, the eye bolt had become brittle as a result of aging.

A spherical carbon steel fixed-catalyst bed reactor, fabricated from French steel A42C-3S, approximately equivalent to ASTM A201 grade B, failed after 20 years of service while in a standby condition. The unit was found to contain primarily hydrogen at the time of failure. The vessel had a type 304 stainless steel shroud around the catalyst bed as protection against the overheating that was possible if the gas bypassed the bed through the refractory material. The failure was observed to have begun at the toe of the shroud-support ring weld. The ring was found to have a number of small cracks at the root of the weld. The cleavage mode of fracture was confirmed by SEM. The presence of extensive secondary cracking and twinning (Neumann bands) where the fracture followed the line of the shroud-support ring was revealed by metallography. It was revealed by refinery maintenance records that the ring had been removed for hydrotest and welded without any postweld heat treatment. The final cause of failure was concluded to be cracking that developed during the installation of the new shroud ring. Stress-relief heat treatments were recommended to be performed to reduce residual-stress levels after welding.

Two types of chromium-plated hydraulic cylinders failed by cracking on their outer surfaces. In one case, the parts had a history of cracking in the nominally unstressed, as-fabricated condition. In another, cracks were detected after the cylinders were subjected to a pressure impulse test. Both part types were made of 15-5 PH (UNS S15500) precipitation hardening stainless steel. Hydrogen embrittlement cracking was the likely cause of failure for both part types. Cracking of the as-fabricated parts was ultimately prevented by changing the manufacturing procedure to allow for a reheat treatment. For parts that cracked after pressure testing, excessive dimensional changes precluded the inclusion of a reheat treatment as a manufacturing step, and further failure was averted by carefully employing proper machining practices, avoiding abusive machining.

This article reviews the applied aspects of creep and stress-rupture failures. It discusses the microstructural changes and bulk mechanical behavior of classical and nonclassical creep behavior. The article provides a description of microstructural changes and damage from creep deformation, including stress-rupture fractures. It also describes metallurgical instabilities, such as aging and carbide reactions, and evaluates the complex effects of creep-fatigue interaction. The article concludes with a discussion on thermal fatigue and creep fatigue failures.

A 17-4 PH steering actuator rod end body broke during normal take-off. Results of failure analysis revealed that the wall thickness of the race was much below the design limits, thus causing the race to rest on the body's swaged edges rather than on the load carrying centerline of the body. This assembly condition generated abnormal high loads on the swaged edges, ultimately resulting in fatigue failure. To prevent a recurrence of similar failure in the future, the dimensions of the race in the spherical bearing were changed, no further failure occurred.

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.

Cracks were discovered in the cast 17-4 PH stainless steel outboard leading edge flap support of an aircraft wing during overhaul inspection. Failure analysis focused on an apparently intergranular area of fracture surface. It was determined that the original mode of crack growth was cleavage, probably caused by cast-in hydrogen. The intergranular appearance resulted from heat treatment of the already cracked part, which caused the formation of grain-boundary “growth figures” on the exposed crack surfaces. It was recommended that the castings be more closely inspected for defects before further processing and that foundry practices be reviewed to correct deficiencies leading to excessive hydrogen absorption during melting and casting.

A stud that was part of a man hole cover on an old steam boiler broke when it was dropped. The boiler was more than 100 years old, but still performed satisfactorily when fired up at regular intervals. Chemical analysis showed the steel to be a low-carbon EN2 type. Microstructural examination indicated brittle fracture caused by strain aging. Replacement of all studs on the man hole cover and examination of the boiler steel in other places were recommended.