The locking collar on a machine failed suddenly when the shaft it restrained was inadvertently subjected to an axial load slightly higher than the allowable working load. The locking collar fractured abruptly, producing four large fragments. This allowed the shaft to be propelled forcefully in the direction of the load, causing substantial damage to other machinery components in the vicinity. The failed component, which was 43 cm (17 in.) in diameter, was machined from 4140 plate and heat treated to 34 to 36 HRC. Analysis (visual inspection, composite micrographs, scanning electron microscopy, and mechanical-property analysis) supported the conclusions that the alloy steel plate used in this application contained significant brittle microstructural fibering or banding. This condition produced considerable anisotropy in ductility and toughness as revealed by mechanical testing. Unfortunately, the potential effects of anisotropy were apparently neglected when this component was designed and manufactured from the plate stock, because the loading was applied in a direction that stressed the weakest planes in the material, that is, a direction normal to the fibering. No recommendations were made.

A narrow bone plate made of type 316 stainless steel and used to stabilize an open midshaft femur fracture failed. A crack at a plate hole next to the fracture site had been revealed by a radiograph taken 13 weeks after the operation. The plate was revealed to be slightly bent in the horizontal plane, and the fracture gap was considerably open. The screws and plates supplied by different manufacturers were revealed to be different with respect to microcleanliness (primary inclusion content) of the materials and only one of them was found to be according to specifications. The local crack formation was influenced by the presence of larger inclusions. The screw failed was revealed to have failed through a fatigue mechanism by the presence of striations in the scanning electron micrograph. The crack in the plate was revealed to have originated at the upper, outer corner of the plate by the beach marks which indicated the action of asymmetric bending and rotational forces.

The heads of two AISI 8740 steel bolts severed while being installed into an Army tank recoil mechanism. Both broke into two pieces at the head-to-shank radius and the required torque value had not been attained nor exceeded prior to the failure. A total of 69 bolts from inventory and the field were tested by magnetic particle inspection. One inventory bolt failed because of a transverse crack near the head-to-shank radius. It was deduced that either a 100% magnetic particle inspection had not been conducted during bolt manufacturing, or the crack went undetected during the original inspection. Optical and electron microscopy of the broken bolts revealed topographies and the presence of black oxide consistent with quench cracking. The two bolts failed during installation due to the presence of pre-existing quench cracks. Recommendations to prevent future failures include: ensuring that 100% magnetic particle inspections are conducted after bolts are tempered; using dull cadmium plate or an alternative to the electrode position process, such as vacuum cadmium plate or ion-plate or ion-plated aluminum, to mitigate the potential for delayed failures due to hydrogen embrittlement or stress-corrosion cracking; ensuring that the radius at the shoulder/shank interface conforms to specifications; and replacing all existing bolts with new or reinspected inventory bolts.

Several case-hardened and zinc-plated carbon-manganese steel wheel studs fractured in a brittle manner after very limited service life. The fracture surfaces of both front and rear studs showed no sign of fatigue beach marks or deformation in the form of shear lips that would indicate either a fatigue mechanism or ductile overload failure. SEM analysis revealed that the mode of fracture was intergranular decohesion, which indicates an environmental influence in the fracture mechanism. The primary fracture initiated at a thread root and propagated by environmentally-assisted slow crack growth until final fracture. The natural stress concentration at the thread root, when tightened to the required clamp load concomitant with the presence of cracks in the carburized case, was sufficient to exceed the critical stress intensity for hydrogen-assisted stress cracking (HASC). The zinc plating exacerbated the situation by providing a strong local corrosion cell in the form of a sacrificial anode region adjacent to the cracked thread. The enhanced generation of hydrogen in a corrosive environment subsequently lead to HASC of the wheel studs.

Several tin plated, low-alloy steel couplings designed to connect sections of 180 mm (7 in.) diam casing for application in a gas well fractured under normal operating conditions. The couplings were purchased to American Petroleum Institute (API) specifications for P-110 material. Chemical analysis and mechanical testing of the failed couplings showed that they had been manufactured to the API specification for Q-125, more stringent specification than P-110, and met all requirements of the application. Fractographic examination showed that the point of initiation was an embrittled region approximately 25 mm (1 in.) from the end of the coupling. The source of the embrittlement was determined to be hydrogen charging during tin plating. Changes in the plating process were recommended.

Leakage was detected at the welds between stiffening plates and the pipe in a transfer line carrying butane and related petrochemical compounds. The line and reinforcing rings were of AISI 316 stainless steel, the pipe being of 508 mm diam and 6.25 mm wall thickness. The design temperature and pressure were 621 deg C and 2.75 kPa, respectively, while the operating conditions were 579 deg C and 1.03 kPa. The line was insulated. Failure occurred after approximately 90,000 h of operation, shutdowns being approximately two per annum. The cracking occurred at the toe of welds between the plates and the pipe. The creep damage failure was attributed to repeated relaxation cycles of very high thermal stresses of resulting from the periodic shutdowns, temperature fluctuations during service, or both. This failure emphasized the information available from an evaluation of the operative creep mechanism, namely grain boundary sliding, relating to the periodic nature of the loading, with high residual stresses being present.

A steel splice plate in a power transmission line tower cracked while in service. Metallographic analysis indicated the presence of a white hard martensite layer near the crack, which occurred in the heel of the plate. Mechanical property tests revealed localized hardening in the area of the crack, supporting the metallurgical findings. A substantial deterioration of the Charpy impact toughness of the material in the heel region was also observed which is believed to have caused the initiation and propagation of the cracks leading to the failure.

This article addresses some established protocols for characterizing thermoplastics and whether they are homogeneous resins, alloyed, or blended compositions or highly modified thermoplastic composites. It begins with a discussion on characterizing mechanical, rheological, and thermal properties of polymer. This is followed by a section describing molecular weight determination using viscosity measurements. Next, the article discusses the use of cone and plate and parallel plate geometries in melt rheology. It then reviews the processes involved in the analysis of thermoplastic resins by chromatography. Finally, the article covers three operations of thermoanalysis, namely differential scanning calorimetry, thermogravimetric analysis, and thermomechanical testing.

A ring clamp (8740 (AMS 6322), steel forged and cadmium plated) used for attaching ducts to an aircraft engine became loose after three hours of service. When the clamp was removed from the engine, the hinge tabs on one clamp half were found to be broken. Analysis (visual inspection and microscopic and metallographic examination) supported the conclusion that both hinge tabs on the clamp half fractured in a brittle manner as the result of gross overheating, or burning, during forging. The mechanical properties of the metal, especially toughness and ductility, were greatly reduced by burning. Evidence that burning was confined to the hinge end of the clamp indicated that the metal was overheated before or during the upset forging operation. Recommendations included notifying the supplier of the burned condition on the end of the clamp. The clamps should be macroetched before cadmium plating to detect overheating. The clamps in stock should be inspected to ensure that the metal had not been weakened by overheating during the upset forging operation.

The operator of an electric transit system purchased a large number of tin-plated copper connectors, putting some in service and others in reserve. Later, when some of the reserve connectors were inspected, the metal surfaces were covered with spots consisting of an ash-like powder and the plating material had separated from the substrate in many areas. Several connectors, including some that had been in service, were examined to determine what caused the change. The order stated that the connectors were to be coated with a layer of tin-bismuth (2% Bi) to guard against tin pest, a type of degradation that occurs at low temperatures. Based on the results of the investigation, which included SEM/EDS analysis, inductively coupled plasma spectroscopy, and x-ray diffraction, the metal surfaces contained less than 0.1% Bi and thus were not adequately protected against tin pest, which was confirmed as the failure mechanism in the investigation.

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.

The structural collapse of an iron-ore bucket-wheel stacker reclaimer at the beginning of operation was investigated by means of mechanical tests, microstructural characterization, and computational structural analysis. The mechanical failure was a consequence of a brittle fracture by cleavage. The crack followed the heat-affected zone of a welded joint connecting a rectangular hollow section member and a plate flange. The main factors contributing to failure were related with a combination of design-in and manufacturing-in factors like high load-strength ratio at the point of failure, local stress concentration as a result of geometry restrictions, and weld defects. This particular section was responsible for the load transfer between the front tie member and the boom extremity, and its failure was the main cause of the catastrophic failure of the equipment.

Fretting and fretting corrosion at the contact area between the screw hole of a type 316LR stainless steel bone plate and the corresponding screw head was studied. The attack on the 316LR stainless steel was only shallow. Mechanical grinding and polishing structures were exhibited by a large portion of the contact area. Fine corrosion pits in the periphery were observed and intense mechanical material transfer that can take place during fretting was revealed. Smearing of material layers over each other during wear was observed and attack by pitting corrosion was interpreted to be possible.

Following a fracture mechanics “fitness-for-purpose” analysis of petroleum industry cold service pressure vessels, using the British Standard PD 6493, it was realized that an analogous approach could be used for the failure analysis of a similar pressure vessel dome which had failed in service some years previously. The failed pressure vessel, with a diam of 2.5 m and several meters tall, had been made of 12 mm thick IZETT steel plate of the same type and heat treatment as used in the earlier fitness-for-purpose already measured. Examination of the fracture surfaces suggested, from fatigue striations manifested by SEM, that the vessel was subject to significant fatigue cracking, which was probably corrosion assisted. From COD measurements at the operating temperature of -130 deg C (-202 deg F), and a finite stress analysis, a fracture mechanics evaluation using BS PD6493 yielded realistic critical flaw sizes (in the range 51 to 150 mm). These sizes were consistent with the limited fracture surface observations and such flaws could well have been present in the vessel dome prior to catastrophic failure. For similar pressure vessels, an inspection program based on a leak-before-break philosophy was consequently regarded as acceptable.

A failed 25 x 32 mm (1 x 1 in.) cadmium-plated 1040 carbon steel countersunk head type nose gear door securing bolt with a common screwdriver slot was examined. Fracture originated at a thread root and propagated across the cross section. The topography of the fracture was excessively rough and more granular than would be expected from pure mechanical fatigue. This indicated an allied corrosion mechanism. Cracks other than the one leading to failure were observed. Metallographic examination of the bolt cross section showed many cracks typical of stress-corrosion damage. It was concluded that the bolt failed by a combination of SCC and fatigue. It was recommended that aerospace-quality fasteners meeting NAS 7104, NAS 7204, or NAS 7504 be used to replace the currently used fasteners.

A 230 mm (9 in.) diameter disk attrition mill was scheduled to grind 6.35 mm (0.25 in.) diameter quartz particles to a 0.075 mm (0.003 in.) diameter powder. Due to severe wear on the grinding plates, however, the unit was unable to complete the task of grinding the rock. The mill consisted of a heavy gray cast iron frame, a gravity feeder port, a runner, and a heavy-duty motor. The frame and gravity feeder weighed over 200 kg (440 lb) and, in some areas, was over 25 mm (1 in.) thick. To obtain the operating speed of 200 rpm, a gear system was used to transmit the torque from the 2-hp motor. The runner consisted of a 50 mm (2 in.) diameter shaft and two gray cast iron grinding plates. Investigation (visual inspection, historical review, photographs, model testing of new plates, chemical analysis, hardness testing, optical macrographs, and optical micrographs) supported the conclusion that the primary feed material was harder than the grinding plates, causing wear and eventual failure. Recommendations included reducing the clearance between the flutes and possible material changes.

A helicopter tail rotor blade spar failed in fatigue, allowing the outer section of the blade to separate in flight. The 7075-T7351 aluminum alloy blade had fiberglass pockets. The blade spar was a hollow “D” shape, and corrosion pits were present on the inner surface of the hollow spar A single corrosion pit, 0.38 mm (0.015 in.) deep, led to a fatigue failure of the spar The failure initiated on the pylon side of the blade. Dimensional analysis of the spar near the failure revealed measurements within engineering drawing tolerances. Though corrosion pitting was present, there was an absence of significant amounts of corrosion product and all of the pits were filled with corrosion-preventative primer. This indicated that the pitting occurred during spar manufacture, prior to the application of the primer The pitting resulted from multiple nickel plating and defective plating removal by acid etching. Post-plating baking operations subsequently reduced the fatigue strength of the spar.

Wear, a form of surface deterioration, is a factor in a majority of component failures. This article is primarily concerned with abrasive wear mechanisms such as plastic deformation, cutting, and fragmentation which, at their core, stem from a difference in hardness between contacting surfaces. Adhesive wear, the type of wear that occurs between two mutually soluble materials, is also discussed, as is erosive wear, liquid impingement, and cavitation wear. The article also presents a procedure for failure analysis and provides a number of detailed examples, including jaw-type rock crusher wear, electronic circuit board drill wear, grinding plate wear failure analysis, impact wear of disk cutters, and identification of abrasive wear modes in martensitic steels.

This investigation involved two AISI 304L acid storage tanks and one AISI 304L spent solvent tank from a sewage treatment facility. After installation, these tanks were hydrostatically tested using sewage effluent. No leaks were found and after a year or two, the tanks were drained and filled with nitric acid in preparation for service. Three weeks later the two acid tanks were found to be leaking from the bottom. Samples from the spent solvent tank revealed that pitting was located in a depressed area near a suction hole, beneath a black residue. It was concluded that the acid tanks failed by chloride-induced pitting initiated by microbial activity. Further, the spent solvent tank failed by a similar, but anaerobic mechanism. The use of the effluent for the hydrostatic test and the failure to remove it and clean and dry the tanks was the primary cause of failure. Localized carbide segregation in the original plate served as preferential corrosion sites. Had the tanks been hydrostatically tested in a proper manner, the pitting may not have occurred.