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Fatigue fracture
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Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.auto.c0047846
EISBN: 978-1-62708-218-1
... was increased to 760 MPA by nitriding for 10 h at 515 deg C. Shock loading Stresses Vibration 8640 UNS G86400 Fatigue fracture The shaft shown in Fig. 1 was from a fuel-injection-pump governor that controlled the speed of a diesel engine used in trucks and tractors. Shafts in newly installed...
Abstract
An 8640 steel shaft installed in a fuel-injection-pump governor that controlled the speed of a diesel engine used in trucks and tractors broke after few days of operation. The mechanism that drove the shaft was designed to include a slip clutch to protect the governor shaft from shock loading. It was revealed by visual examination that the fracture had initiated in the sharp corner at the bottom of a longitudinal hole which was part of a force feed lubricating system. Beach marks were observed on the fracture surfaces. It was revealed by further examination that the slip clutch was removed in an effort to reduce cost and hence the shaft was subjected to increased vibration and shock loading. Insufficient fatigue limit of the shaft was revealed by fatigue testing of the shafts taken from stock in a rotating-beam machine. As a corrective measure, the fatigue limit of shafts was increased to 760 MPA by nitriding for 10 h at 515 deg C.
Series: ASM Handbook
Volume: 11
Publisher: ASM International
Published: 15 January 2021
DOI: 10.31399/asm.hb.v11.a0006776
EISBN: 978-1-62708-295-2
... Abstract Fatigue failure of engineering components and structures results from progressive fracture caused by cyclic or fluctuating loads. Fatigue is an important potential cause of mechanical failure, because most engineering components or structures are or can be subjected to cyclic loads...
Abstract
Fatigue failure of engineering components and structures results from progressive fracture caused by cyclic or fluctuating loads. Fatigue is an important potential cause of mechanical failure, because most engineering components or structures are or can be subjected to cyclic loads during their lifetime. This article focuses on fractography of fatigue. It provides an abbreviated summary of fatigue processes and mechanisms: fatigue crack initiation, fatigue crack propagation, and final fracture,. Characteristic fatigue fracture features that can be discerned visually or under low magnification are then described. Typical microscopic features observed on structural metals are presented subsequently, followed by a brief discussion on fatigue in polymers and polymer-matrix composites.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.matlhand.c0046160
EISBN: 978-1-62708-224-2
... shifting during operation. Axles Cranes Forgings Torsional fatigue 4150 UNS G41500 Fatigue fracture A stepped drive axle used in a high-speed electric overhead crane broke after 15 months of service. The axle ( Fig. 1a ) was made from a hardened-and-tempered resulfurized 4150 steel forging...
Abstract
A stepped drive axle (hardened and tempered resulfurized 4150 steel forging) used in a high-speed electric overhead crane (rated at 6800 kg, or 7 tons, and handling about 220 lifts/day with each lift averaging 3625 to 5440 kg, or 4 to 6 tons) broke after 15 months of service. Visual examination of the fracture surface revealed three fracture regions. The primary fracture occurred approximately 50 mm (2 in.) from the driven end of the large-diam keywayed section on the stepped axle and approximately 38 mm (1 in.) from one end of the keyway where the crane wheel was keyed to the axle. Macroscopic, microscopic, and chemical examination revealed composition that was basically within the normal range for 4150 steel. This evidence supports the conclusion that cracking initiated at a location approximately opposite the keyway, and final fracture was due to mixed ductile and brittle fracture. Axial shift of the crane wheel during operation, because of insufficient interference fit, was the major cause of fatigue cracking. Recommendations included redesigning the axle to increase the critical diameter from 140 to 150 mm (5.5 to 6 in.) and to add a narrow shoulder to keep the drive wheel from shifting during operation.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.matlhand.c0048060
EISBN: 978-1-62708-224-2
... which indicated that the material was in the as-forged condition (which meant lower fatigue strength). The fracture was concluded to have occurred due to stress concentration in the root of the last thread. Normalizing of the crane hook after forging was suggested as a corrective measure. A stress...
Abstract
The crane hook (rated for 13000 kg) failed in the threaded shank while lifting a load of 9072 kg. The metal in the hook was revealed by chemical analysis to be killed 1020 steel. It was disclosed by visual examination that the fracture had at the last thread on the shank and rough machining and chatter marks were evident on the threads. Beach marks that emanated from the thread-root locations on opposite sides of the fracture surface identified these locations to be the origins of the fracture. A medium-coarse slightly acicular structure was revealed by metallographic examination which indicated that the material was in the as-forged condition (which meant lower fatigue strength). The fracture was concluded to have occurred due to stress concentration in the root of the last thread. Normalizing of the crane hook after forging was suggested as a corrective measure. A stress-relief groove with a diam slightly smaller than the root diam was placed at the end of the thread and a large-radius fillet was machined at the change in diameter of the shank.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.matlhand.c0048064
EISBN: 978-1-62708-224-2
... radius of the hook. Beach marks (typical of fatigue fracture) were found extending over approximately 20% of the fracture surface. Numerous cracks were revealed by macroscopic examination of the torch-cut surfaces. It was revealed by macrograph of an etched specimen that the cracks had initiated...
Abstract
A 10,890-kg coil hook torch cut from 1040 steel plate failed while lifting a load of 13,600 kg after eight years of service. The normal ironing (wear) marks were exhibited by the inner surface of the hook. It was revealed by visual examination that cracking had originated at the inside radius of the hook. Beach marks (typical of fatigue fracture) were found extending over approximately 20% of the fracture surface. Numerous cracks were revealed by macroscopic examination of the torch-cut surfaces. It was revealed by macrograph of an etched specimen that the cracks had initiated in a hardened martensitic zone at the torch-cut surface and had extended up to the coarse pearlite structure beneath the martensitic zone. The fatigue fracture was concluded to have initiated in the brittle martensitic surface while failure was contributed by the 25% overload. As a corrective measure, the coil hooks were flame cut from ASTM A242 fine-grain steel plate, ground to remove the material damaged by flame cutting and stress relieved at 620 deg C.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.matlhand.c0048102
EISBN: 978-1-62708-224-2
... Abstract A bridge wheel from a 272,160 kg stripper crane fractured in the web near the rim after one year of service. The wheel was forged from 1055 steel, and the tread, hub faces, and hub bore were machined. Beach marks indicative of fatigue at ten locations were revealed by macroscopic...
Abstract
A bridge wheel from a 272,160 kg stripper crane fractured in the web near the rim after one year of service. The wheel was forged from 1055 steel, and the tread, hub faces, and hub bore were machined. Beach marks indicative of fatigue at ten locations were revealed by macroscopic examination of the fracture surfaces. The surface of the web was heavily scaled and decarburized. A gross forging defect extending about 1.8 mm along the fracture surface was disclosed by examination of a micrograph of a section through one of the fatigue origins. Shallower forging defects were visible along the web surface. Fatigue cracking of the wheel was initiated at forging defects in the web. Replacement wheels were machined all over and were magnetic particle inspected to detect any cracks that could act as stress raisers.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.matlhand.c0048073
EISBN: 978-1-62708-224-2
... confirmed failure of the shaft by fatigue fracture. As a corrective measure the shaft was replaced with one made of 4140 steel, quenched and tempered to a hardness of 286 to 319 HRB. The keyway was moved away from the change in section and was machined with a 1.6-mm radius in the bottom corners and a larger...
Abstract
A drum pinion shaft (1030 steel) which was part of the hoisting gear of a crane (capacity 18,140-kg) operating in a blooming mill failed while lifting a 9070 kg load. Chatter marks, rough-machining marks, and sharp corner radii were revealed in the keyway which extended into a shoulder at a change in diam. A circular recess below the keyway surface was revealed at each end of the keyway. A sharp corner at the end of the keyway was revealed by examination to be the origin of fracture. Beach marks were found radiating from the origin over a large portion of the fracture surface which confirmed failure of the shaft by fatigue fracture. As a corrective measure the shaft was replaced with one made of 4140 steel, quenched and tempered to a hardness of 286 to 319 HRB. The keyway was moved away from the change in section and was machined with a 1.6-mm radius in the bottom corners and a larger-radius fillet was machined at the change in section.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.matlhand.c0048031
EISBN: 978-1-62708-224-2
..., this wire rope was substituted by the more abrasion resistant 6 x 19 rope. Carbon Chemical composition Cranes Martensite Specifications Tensile Strength Fe-0.73C Fatigue fracture The wire rope on a crane in a scrapyard broke after 2 weeks of service under normal loading conditions...
Abstract
The 16 mm diam 6 x 37 fiber-core improved plow steel wire rope on a scrapyard crane failed after two weeks of service under normal loading conditions. This type of rope was made of 0.71 to 0.75% carbon steel wires and a tensile strength of 1696 to 1917 MPa. The rope broke when it was attached to a chain for pulling jammed scrap from the baler. The rope was heavily abraded and several of the individual wires were broken. a uniform cold-drawn microstructure, with patches of untempered martensite in regions of severe abrasion and crown wear was revealed by metallographic examination. As a result of abrasion, a hard layer of martensite was formed on the wire. The wire was made susceptible to fatigue cracking, while bending around the sheave, by this brittle surface layer. The carbon content and tensile strength of the wire was found lower than specifications. As a corrective measure, this wire rope was substituted by the more abrasion resistant 6 x 19 rope.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.chem.c0089633
EISBN: 978-1-62708-220-4
... castings Stress concentraion Stuffing box 60-45-10 Casting-related failures Fatigue fracture The stuffing box shown in Fig. 1(a) was sand cast from ASTM A 536, grade 60-45-10, ductile iron and began leaking water after two weeks of service. The machine was operating at 326 rpm, with discharge...
Abstract
A stuffing box (sand cast from ASTM A 536, grade 60-45-10, ductile iron) began leaking water after two weeks of service. The machine was operating at 326 rpm with a discharge water pressure of 21.4 MPa (3100 psi). Investigation (visual inspection, mechanical analysis, and nital etched 100x magnification) supported the conclusion that the crack initiated at the inner edge of a lubrication hole and had propagated toward both the threaded and flange ends of the casting. An appreciable residual-stress concentration must have been present and caused propagation of the crack. The residual stress might have been caused when a fitting was tightly screwed into the lubrication hole, and it might have been concentrated by notches at the inner end of the hole created when the drill broke through the sidewall to the stuffing box.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.auto.c0048586
EISBN: 978-1-62708-218-1
... beach marks, indicative of fatigue cracking, starting at opposite sides of each stud, with final fracture occurring across the stud. Each failure occurred in the first thread of the stud. It was concluded that the wheel studs fractured by reversed-bending fatigue. To minimize the possibility...
Abstract
Each of the ten studs on one wheel of a semitrailer used to haul coal broke in half while the trailer was in operation. Both halves of each of three studs were sent to the laboratory to determine the cause of failure. Visual examination of the fracture surfaces of the studs disclosed beach marks, indicative of fatigue cracking, starting at opposite sides of each stud, with final fracture occurring across the stud. Each failure occurred in the first thread of the stud. It was concluded that the wheel studs fractured by reversed-bending fatigue. To minimize the possibility of a recurrence, the wheel nuts were tightened with an air impact wrench to a torque of 610 to 678 J (450 to 500 ft · lb) dry. All wheel studs were checked at normal maintenance periods to ensure uniform and proper loading, and no further failures occurred.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.auto.c0046182
EISBN: 978-1-62708-218-1
... of the cap. The defect extended approximately 9.5 mm (3/8 in.) along the side of the cap. The fracture surface exhibited beach marks typical of fatigue. The surface of the defect was stained, indicating that oxidation occurred either in heat treatment or in heating during forging. Deep etching...
Abstract
A connecting cap from a truck engine fractured after 65,200 km (40,500 mi) of normal service. The cap was made from a 15B41 steel forging and was hardened to 29 to 35 HRC. Visual examination of the fracture surface disclosed an open forging defect across one of the outer corners of the cap. The defect extended approximately 9.5 mm (3/8 in.) along the side of the cap. The fracture surface exhibited beach marks typical of fatigue. The surface of the defect was stained, indicating that oxidation occurred either in heat treatment or in heating during forging. Deep etching of the fracture surface revealed grain flow normal for this type of forging, but no visible defects. 400x metallographic examination of a section through the fracture surface showed that the microstructure was an acceptable tempered martensite. However, oxide inclusions were present at the fracture surface. This evidence supported the conclusion that fatigue fracture initiated at a corner of the cap from a forging defect that extended to the surface. Fatigue cracking was propagated by cyclic loading inherent in the part. Recommendations included more careful fluorescent magnetic-particle inspection of the forged surfaces before machining and before putting the part into service.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.auto.c0048592
EISBN: 978-1-62708-218-1
... 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...
Abstract
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.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.auto.c0089551
EISBN: 978-1-62708-218-1
... of the casting. The mass of a feeder-riser system located near the tear retarded cooling in this region, creating a hot spot. This supported the conclusion that the tear causing the fracture of the axle housing was formed during solidification by hindered contraction and was enlarged in service by fatigue...
Abstract
A sand-cast medium-carbon steel heavy-duty axle housing, which had been quenched and tempered to about 30 HRC, fractured after almost 5000 h of service. Investigation (0.4x magnification) revealed that the fracture had been initiated by a hot tear that formed during solidification of the casting. The mass of a feeder-riser system located near the tear retarded cooling in this region, creating a hot spot. This supported the conclusion that the tear causing the fracture of the axle housing was formed during solidification by hindered contraction and was enlarged in service by fatigue. Recommendations were to change the feeder location to eliminate the hot spot and thus the occurrence of hot tearing.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.power.c0047621
EISBN: 978-1-62708-229-7
..., and flange and more skillful welding techniques to avoid undercutting and unfused interfaces. Arc welding Combustion chamber Flanges Gas turbine engines Pipe fitting Undercuts Welding defects Inconel 718 (Nickel-base superalloy) UNS N07718 Fatigue fracture Joining-related failures...
Abstract
The case and stiffener of an inner-combustion-chamber case assembly failed by completely fracturing circumferentially around the edge of a groove arc weld joining the case and stiffener to the flange. The assembly consisted of a cylindrical stiffener inserted into a cylindrical case that were both welded to a flange. The case, stiffener, flange, and weld deposit were all of nickel-base alloy 718. It was observed that a manual arc weld repair had been made along almost the entire circumference of the original weld. Investigation (visual inspection, 0.5x macrographs, and 10x etched with 2% chromic acid plus HCl views) supported the conclusions that failure was by fatigue from multiple origins caused by welding defects. Ultimate failure was by tensile overload of the sections partly separated by the fatigue cracks. Recommendations included correct fit-up of the case, stiffener, and flange and more skillful welding techniques to avoid undercutting and unfused interfaces.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.power.c0047813
EISBN: 978-1-62708-229-7
... by fatigue. (Left) Section through pulverizer showing the inner main shaft that fractured, repaired itself by friction welding, and fractured a second time. (Right) Photograph of the friction welded surface The lower grinding ring of the pulverizer was attached to the outer main shaft ( Fig. 1 , left...
Abstract
After being in service for ten years the ball-and-race coal pulverizer was investigated after noises were noted in it. Its lower grinding ring was attached to the 6150 normalized steel outer main shaft while the upper grinding ring was suspended by springs from a spider attached to the shaft. A circumferential crack in the main shaft at an abrupt change in shaft diam just below the upper radial bearing was revealed by visual examination. The smaller end of the shaft was found to be slightly eccentric with the remainder when the shaft was set up in a lathe to machine out the crack for repair welding. The crack was opened by striking the small end of the shaft and the shaft was broken 1.3 cm away from the crack in the process. A previous fracture that resulted from torsional loading acting along a plane of maximum shear was revealed almost perpendicular to the axis of the shaft. Faint lines parallel to the visible crack thought to be fatigue cracks were revealed on examination of the machined surface. The shaft was repaired by welding a new section and machined to required diameters and tapers to avoid abrupt changes.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.rail.c0046238
EISBN: 978-1-62708-231-0
... had been magnetic-particle inspected by inserting a probe into the longitudinal hole. Evidence found supports the conclusions that the knuckle pins failed by fatigue fracture. The circular cracks at the longitudinal holes were the result of improper technique in magnetic-particle inspection. Thermal...
Abstract
Within about one month, several knuckle pins (AMS 6470 steel failed, and required to have a minimum case hardness of 92 h15N, a case depth of 0.4 to 0.5 mm (0.017 to 0.022 in.), and a core hardness of 285 to 341 HRB) used in engines failed over a range of 218 to 463 h in operation. Visual examination revealed beach marks typical of fatigue cracks that had nucleated at the base of the longitudinal oil hole. Micrographs of sections revealed a remelt zone and an area of untempered martensite within the region of the cracks. However, review of inspection procedures disclosed the pins had been magnetic-particle inspected by inserting a probe into the longitudinal hole. Evidence found supports the conclusions that the knuckle pins failed by fatigue fracture. The circular cracks at the longitudinal holes were the result of improper technique in magnetic-particle inspection. Thermal transformation of the metal also causes a stress concentration that may lead to fatigue failure. Recommendations included insulating the conductor to prevent arc burning at the base of the longitudinal oil hole. Also, a borescope or metal monitor could be used to inspect the hole for evidence of arc burning from magnetic-particle inspection.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.bldgs.c0048039
EISBN: 978-1-62708-219-8
... of fractures: flat-type fractures were observed in 1.2 and 1 mm diam wires and cup-and-cone fractures were observed in 0.6 mm diam wires. A nick observed in the side of one of the larger wires was found to be rusted. Beach marks radiating inward, indicative of fatigue cracking, were also revealed. The smaller...
Abstract
One of six cables on a passenger elevator was found fractured during a routine inspection. The cable is made of 16-mm steel wire rope designated 8 x 19 G Preformed Extra High Strength Special Traction Elevator Cable with fiber core. Samples of wire from the cable revealed two types of fractures: flat-type fractures were observed in 1.2 and 1 mm diam wires and cup-and-cone fractures were observed in 0.6 mm diam wires. A nick observed in the side of one of the larger wires was found to be rusted. Beach marks radiating inward, indicative of fatigue cracking, were also revealed. The smaller wires were found to be slightly oxidized and behaved in a ductile manner under excessive loads before ultimate failure. Flat-type fractures were believed to have resulted from cyclic torsional stresses along with longitudinal cracking. Restriction of free movement of the socket-end in the shackle was found to have promoted fracture due to increased magnitude of stresses. Mechanical damage to surfaces of wires was concluded to be sufficient to cause fatigue cracking under the stresses encountered in service.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.usage.c0046242
EISBN: 978-1-62708-236-5
... teeth (found at visual inspection) was composed of the same material as the metal in the coupling. Beach marks and evidence of cold work, typical of fatigue failure, were found on the fracture surface. Chips remaining in the analysis cut were difficult to remove, indicating a strong magnetic field...
Abstract
An integral coupling and gear (Cr-Mo steel), used on a turbine-driven main boiler-feed pump, was removed from service after one year of operation because of excessive vibration. Spectrographic analysis and metallographic examination revealed the fact that gritty material in the gear teeth (found at visual inspection) was composed of the same material as the metal in the coupling. Beach marks and evidence of cold work, typical of fatigue failure, were found on the fracture surface. Chips remaining in the analysis cut were difficult to remove, indicating a strong magnetic field in the part. Evidence found supports the conclusions that failure of the coupling was by fatigue and that incomplete demagnetization of the coupling following magnetic-particle inspection caused retention of metal chips in the roots of the teeth. Improper lubrication caused gear teeth to overheat and spall, producing chips that eventually overstressed the gear, causing failure. Because the oil circulation system was not operating properly, metal chips were not removed from the coupling. Recommendations included checking the replacement coupling for residual magnetism and changing or filtering the pump oil to remove any debris.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.usage.c0047558
EISBN: 978-1-62708-236-5
... Abstract The shaft of an exciter that was used with a diesel-driven electric generator broke at a fillet after ten hours of service following resurfacing of the shaft by welding. The fracture surface contained a dull off-center region of final ductile fracture surrounded by regions of fatigue...
Abstract
The shaft of an exciter that was used with a diesel-driven electric generator broke at a fillet after ten hours of service following resurfacing of the shaft by welding. The fracture surface contained a dull off-center region of final ductile fracture surrounded by regions of fatigue that had been subjected to appreciable rubbing. The fracture appeared to be typical of rotary bending fatigue under conditions of a low nominal stress with a severe stress concentration. It appeared that the fatigue cracks initiated in the surface-weld layer. The weld deposit in the original keyway displays a lack of fusion at the bottom corner. Fatigue fracture of the shaft resulted from stresses that were created by vibration acting on a crack or cracks formed in the weld deposit because of the lack of preheating and postheating. Rebuilding of exciter shafts should be discontinued, and the support plate of the exciter should be braced to reduce the amount of transmitted vibration. Also, the fillet in the exciter shaft should be carefully machined to provide an adequate radius.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.usage.c9001435
EISBN: 978-1-62708-236-5
... inclusions Shafts (power) Slags Weld defects Weld deposited coatings Welded steel Joining-related failures Fatigue fracture The fracture of the shaft of a 10 h.p. squirrel-cage motor took place at the driving end just outside the roller bearing and not at an abrupt change of section behind...
Abstract
Shaft fracture of a 10 hp squirrel cage motor took place at the driving end just outside the roller bearing and not at an abrupt change of section behind the bearing where it might be expected to occur. A portion of shaft to the right of the fracture was deeply grooved. About a year prior to failure the inner race of the roller bearing became slack on the shaft and the seating was built up by the metal-spray process. The shaft was machined to form a rough thread to provide the requisite mechanical key for the sprayed-on metal. Part of this sprayed-on layer became detached after the fatigue failure occurred. The quality of the welding was poor. Slag inclusions were present adjacent to the sides of the keyway, which had been re-cut shorter than the original one after the welding repair. Failure at the unusual location was caused by the presence of the weld deposit.
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