This article first provides an overview of the types of mechanical fasteners. This is followed by sections providing information on fastener quality and counterfeit fasteners, as well as fastener loads. Then, the article discusses common causes of fastener failures, namely environmental effects, manufacturing discrepancies, improper use, or incorrect installation. Next, it describes fastener failure origins and fretting. Types of corrosion in threaded fasteners and their preventive measures are then covered. The performance of fasteners at elevated temperatures is addressed. Further, the article discusses the types of rivet, blind fastener, and pin fastener failures. Finally, it provides information on the mechanism of fastener failures in composites.

Nineteen out of 26 bolts in a multistage water pump corroded and cracked after a short time in a severe working environment containing saline water, CO 2 , and H 2 S. The failed bolts and intact nuts were to be made from a special type of stainless steel as per ASTM A 193 B8S and A 194. However, the investigation (which included visual, macroscopic, metallographic, SEM, and chemical analysis) showed that austenitic stainless steel and a nickel-base alloy were used instead. The unspecified materials are more prone to corrosion, particularly galvanic corrosion, which proved to be the primary failure mechanism in the areas of the bolts directly exposed to the working environment. Corrosion damage on surfaces facing away from the work environment was caused primarily by chloride stress-corrosion cracking, aided by loose fitting threads. Thread gaps constitute a crevice where an aggressive chemistry is allowed to develop and attack local surfaces.

An 18-MW gas turbine exploded unexpectedly after three hours of normal operation. The catastrophic failure caused extensive damage to the rotor, casing, and nearly all turbo-compressor components. Based on their initial review, investigators believed that the failure originated at the interface between two shaft sections held together by 24 marriage bolts. Visual and SEM examination of several bolts revealed extensive deterioration of the coating layer and the presence of deep corrosion pits. It was also learned that the bolts were nearing the end of their operating life, suggesting that the effects of fatigue-assisted corrosion had advanced to the point where one of the bolts fractured and broke free. The inertial unbalance produced excessive vibration, subjecting the remaining bolts to overload failure.

Wind turbine blades are secured by a number of high-strength bolts. The failure of one such bolt, which caused a turbine blade to detach, was investigated to determine why it fractured. Based on the results of a detailed analysis, consisting of stress calculations, chemical composition testing, metallurgical examination, mechanical property testing, and fractographic analysis, it was determined that the bolt failed by fatigue accelerated by stress concentration at low temperatures. The investigation also provided suggestions for avoiding similar failures.

During the pre-test inspection following the stress calculation check on a 7-ton capacity Scotch derrick crane, it was noted that threads on the back stay anchorage bolts were of unusually fine pitch (11 tpi) and that the machined faces of the nuts showed irregular pits or depressions disposed in an annular manner. When sectioned, the nuts showed a surprising method of construction. The nuts for the bolts had been made by using conventional pipe couplings inserted into sleeves made from hexagonal bar and the coupling secured to the sleeve by welding at each outer face. The ends of the sleeve bore were chamfered to form a weld preparation. After welding, the faces were machined which resulted in the removal of most of the weld metal and revealed a pronounced lack of penetration. All bolts used to anchor derrick crane back stays should be designed in accordance with the recommendations of British Standard 327:1964 (Clauses 10 and 18).

A 140 ft. (42.7 m) long boom on a dragline crane used in coal strip-mining operations failed. One of the principal load-bearing longitudinal beams or chords of the trussed boom had fractured adjacent to a bolt hole at a location about halfway along the length of the boom. Over the lifetime of the crane, several repairs had been made to the boom. At least a year before the failure, a reinforcing gusset plate had been bolted and welded to this chord at this location. Stereomicroscopy revealed microcracks in the weld metal. A fatigue crack 45 mm (1.8 in.) long was observed to emanate from this microcrack. Scanning electron microscopy showed an overload crack extended across the remaining cross section of the chord. It was concluded that the presence of the bolt hole used to attach the gusset plate to the chord created a stress riser adjacent to the hole. Repeated high tensile stresses on the chord during the lifting of enormous loads initiated a fatigue crack in the weld region adjacent to the bolt hole.

SAE grade 5 U-bolts were used to fasten auxiliary dual wheels to the axles on a farm tractor. Under typical farm usage, the bolts are expected to have infinite life. However, several U-bolts made of 29 mm diam rod broke after less than 100 h of service. The bolt legs in which the failures occurred were all in the same position relative to the direction of wheel rotation. Visual examination showed the break was a fairly flat transverse fracture in the threaded section between the washer and the nut. The appearance of the fracture surfaces was characteristic of failure by low-cycle fatigue, with a smooth matte fatigue failure region showing beach marks and generally extending over about 40 to 60% of the fracture surface, which indicated severe overload. The point of initiation of fatigue was at the root of the last thread at the edge of the nut on the side toward this washer. The U-bolts fractured in fatigue because the bolt material had poor hardenability relative to the diam of the bolts. The bolt material was changed from 1045 steel to 1527 steel, a warm-finished low-alloy steel. The diameter of the bolts was reduced to 27.2 mm and the threads were rolled rather than cut.

A portion of a 19 mm (0.75 in.) diam structural steel bolt was found on the floor of a manufacturing shop. This shop contained an overhead crane system that ran on rails supported by girders and columns. Inspection of the crane system revealed that the bolt had come from a joint in the supporting girders and could be considered one of the principal fasteners in the track system. Analysis (visual inspection, metallographic exam, and hardness testing) supported the conclusions that fatigue induced by the overhead movement of the crane produced failure of the bolt. The bolt was deficient in strength for the cyclic applied loads in this case and probably was not tightened sufficiently. Recommendations included removing the remaining bolts in the crane support assembly and replacing them with a higher-strength, more fatigue-resistant bolt, for example, SAE grade F, 104 to 108 HRB. The bolts should be tightened according to the specifications of the manufacturer, and the system should be periodically inspected for correct tightness.

Two bolts from the stressed structure of a church building that had broken during stressing were examined to establish the cause of fracture. The fracture of one of the first bolt occurred in a double-vee groove weld whose root was not completely welded. The second bolt had cracked outside of the weld seam closely under the head. Neither one had been particularly deformed before fracture. The composition of the head pieces corresponded approximately to manganese steel (Material No. 1 0845), a weldable construction steel with increased yield point and strength, while the shafts were made from Cr-Mo steel (Material No. 1.7225) according to DIN 17200. It was found that the bolts were not made from a suitable alloy steel, but were welded together from two unsuitable steels, one of which lacked sufficient strength. The austenitic weld seams showed hot tears and were not welded through to the root. Also, the pieces were not preheated before welding, so that stress cracks occurred in the transition zones. The second bolt was overstressed during the impact caused by the breaking of the first bolt.

The cover of a feed cock fitted to an economic boiler suddenly blew off and the plug lifted sufficiently to permit the escape of water. It was found that all four of the studs securing the cover had fractured. In each case fracture had occurred at the end of one of the screwed portions adjacent to the shank. The fractures were of a short nature, with no evidence of progressive cracking by fatigue, nor was there any sign of stretching prior to failure. The fractured faces and the shanks of the studs were of rusted appearance. Microscopic examination of the material showed it to be an austenitic nickel-chromium stainless steel, stabilized by titanium and of the free machining type. Multiple transgranular cracking characteristic of failure from stress-corrosion cracking, was present to an extensive degree. It was considered probable that there had been slight leakage of water from the valve over a period and evaporation resulted in a solution which favored failure from stress-corrosion cracking. If corrosion resistant studs were desired, those of bronze or Monel metal are to be preferred.

This article discusses the failure of cylinder clamping rods in single cylinder diesel engines. The AISI 4140 hardened and tempered steel clamping rods were failing after 200 to 250 h of operation. The fatigue failures initiated at the root of the last thread on the clamping rod that was engaged in a blind hole in the cylinder block. The failures were caused by loose tolerances on the threads that resulted in a non-uniform distribution of load. The load was concentrated on the last threads to engage, thus causing fatigue crack nucleation at the thread root and propagation until the rod broke by overload. Changing the tolerance on the threads virtually eliminated the fatigue problem.

A bolt manufacturer observed that products made from certain shipments of steel 41 Cr4 wire were prone to the formation of quench cracks in their rolled threads. The affected wire was tested and found to be highly sensitive to overheating because of the metallurgical method by which it was produced. A stronger decarburization of the case was a contributing factor that could not be prevented by working because the thread was rolled. Hardening tests conducted by the bolt manufacturer showed that quench cracks did not occur in specimens that were turned down before hardening and when notches were machined instead of beaten with a chisel.

Socket head cap screws used in a naval application were failing in service due to delayed fracture. The standard ASTM A 574 screws were zinc plated and dichromate coated. Investigation (visual inspection, 1187 SEM images, chemical analysis, and tension testing) of both the failed screws and two unused, exemplar fasteners from the same lot supported the conclusion that the cap screws appear to have failed due to hydrogen embrittlement, as revealed by delayed cracking and intergranular fracture morphology. Static brittle overload fracture occurred due to the tension preload, and prior hydrogen charging that occurred during manufacturing. The probable source of charging was the electroplating, although postplating baking was reportedly performed as well. Recommendations included examining the manufacturing process in detail.

The stub-shaft assembly which was part of the agitator shaft in a polyvinyl chloride reactor, fractured in service after a nut that retained a loose sleeve around the smaller-diam section of the shaft had been tightened several times to reduce leakage. The shaft was made of ASTM A105, grade 2 steel, and the larger-diam section was covered with a type 316 stainless steel end cap. The cap was welded to each end using type ER316 stainless steel filler metal. The forged steel shaft was revealed to have fractured at approximately 90 deg to the shaft axis in the weld metal and not in the heat-affected zone of the forged steel shaft. Microscopic investigation and chemical analysis of the steel shaft revealed presence of martensite (offered a path of easy crack propagation) around the fusion line and dilution of the weld metal by the carbon steel shaft. The microstructure was found to be martensitic as the fusion line was approached. The forged steel shaft was concluded to have failed by ductile fracture and possible reasons were discussed. Corrective measures adopted in the replacement shaft were specified.

Two titanium alloy wing attachment bolts from a commercial jetliner failed during the course of a routine service operation. Failure of the bolts occurred during the re-torque process as the wing was being reattached. Metallurgical failure analysis indicated that the fracture mechanism was ductile overload and that the mechanical properties of the bolts were consistent with exemplar bolts that had been supplied. After eliminating other sources of excessive load application, the most probable cause of failure was ascribed to variances between the frictional characteristics of the bolt at the time of re-torque and at the time of initial torque application several years earlier.

The bolt in a bolt and thimble assembly used to connect a wire rope to a crane hanger bracket was worn excessively. Two worn bolts, one new bolt, and a new thimble were examined. Specifications required the bolts to be made of 4140 steel heat treated to a hardness of 277 to 321 HRB. Thimbles were to be made of cast 8625 steel, but no heat treatment or hardness were specified. Analysis (visual inspection, hardness testing, and metallographic examination) supported the conclusion that the wear was due to strikingly difference hardness measurements in the bolt and thimble. Recommendations included hardening and tempering the bolts to the hardness range of 375 to 430 HRB. The thimbles should be heat treated to a similar microstructure and the same hardness range as those of the bolt. Molybdenum disulfide lubricant can be liberally applied during the initial installation of the bolts. A maintenance lubrication program was not suggested, but galling could be reduced by periodic application of a solid lubricant.

Crane collapse due to bolt fatigue and fatigue failure of a crane support column, crane tower, overhead yard crane, hoist rope, and overhead crane drive shaft are described. The first four examples relate to the structural integrity of cranes. However, equipment such as drive and hoist-train components are often subject to severe fatigue loading and are perhaps even more prone to fatigue failure. In all instances, the presence of fatigue cracks at least contributed to the failure. In most instances, fatigue was the sole cause. Further, in each case, with regular inspection, fatigue cracks probably would have been detected well before final failure.

A detailed investigative failure analysis was conducted on an autoclave which blew apart in a furnace for no apparent reason. Bolt failure resulted in separation of the autoclave lid and subsequent destruction of the furnace. Analysis using metallography, fractography, mechanical testing and exemplar tests were performed on the bolt material. Mechanical engineering analysis and leak-before-break criteria were extensively analyzed. Results led to only one possible conclusion: that an explosion occurred within the autoclave. Suggestions for autoclave design are presented as a result of the analysis.

A drive-line assembly failed during vehicle testing. The vehicle had traveled 9022 km (5606 mi) before the failure occurred. Both the intact and fractured parts of the assembly were analyzed to determine the cause and sequence of failure. Visual examination of the assembly showed three of four bearing caps, two cap screws, and one universal-joint spider had fractured. Examination of the three fractured bearing caps and the spider showed no evidence of fatigue but showed that fracture occurred in a brittle manner. The bearing cap that was not destroyed still contained portions of the two fractured cap screws. It was found that the two cap screws failed in fatigue under service stresses. The three bearing caps and the universal-joint spider broke in a brittle manner. The properties of the material in the cap screws did not fulfill the specifications. The modified 1035 steel was of insufficient alloy content. Also, the tensile strength and endurance limit were lower than specified and were inadequate for the application. The material for the cap screw was changed from modified 1035 steel to 5140 steel.

Six galvanized high-tensile steel bolts were used to hold the wheels of a four-wheel drive vehicle. The right hand rear wheel of this vehicle detached causing the vehicle to roll and resulting in considerable damage to the body. The wheel was detached by shearing of four of the bolts and stripping the nuts from the other two bolts, which remained unbroken. SEM fractography of the fracture surfaces of the four broken bolts indicated that the failure was due to reversed bending fatigue. Optical microscopy indicated that the bolts were heat treated to a tempered martensite structure and that the nuts were manufactured from low carbon steel. The paper discusses the influence of the microstructure on the failure process the events surrounding the nature of incident and the analysis of in-service failure of the failed components utilizing conventional metallurgical techniques.