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.

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.

The A2 tool steel mandrel, part of a rolling tool used for mechanically joining two tubes was fractured after making five rolled joints. A 6.4 mm diam hole was drilled by EDM through the square end of the hardened mandrel due to difficulty was experienced in withdrawing the tool. The fracture progressed into the threaded section and formed a pyramid-shape fragment after it was initiated at approximately 45 deg through the hole in the square end. An irregular zone of untempered martensite with cracks radiating from the surface of the hole (result of melting around hole) was revealed by metallographic examination. A microstructure of fine tempered martensite containing some carbide particles was exhibited by the core material away from the hole. Brittle fracture characteristics with beach marks were exhibited by the fracture surfaces which is characteristic of a torsional fatigue fracture. As a corrective measure, the hole through the square end of the mandrel was incorporated into the design of the tool and was drilled and reamed before heat treatment and specified hardness of the threaded portion and square end of the mandrel was reduced.

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.

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.

A type 316L stainless steel angled plate failed. The fatigue fracture was found to have occurred at a plate hole. Symmetric cyclic bending forces were revealed by the fatigue damage at the fracture edge at the top surface of the plate. Fatigue striations and slip bands produced on the surface during cyclic loading were observed. The material was showed by the deformation structure to be in the cold-worked condition and was termed to not be the cause of the implant failure.

A structure had been undergoing fatigue testing for several months when a post-like member heat treated to a tensile strength of 1517 to 1655 MPa (220 to 240 ksi) ruptured. The fracture occurred in the fillet of the post that contacted the edge of a carry-through box bolted to the member. At failure, the part was receiving a second set of loads up to 103.6% of design load. Visual investigations showed rubbing and galling of the fillet. Microscopic and metallographic examination revealed beach marks on the fracture surface and evidence of cold work and secondary cracking in the rubbed and galled area. Electron fractography confirmed that cracking had initiated at a region of tearing and that the cracks had propagated by fatigue. Mechanical properties of all specimens exceeded the minimum values specified for the post. This evidence supports the conclusion that fatigue was the primary cause of failure. Rubbing of the faying surfaces worked the interference area on the post until small tears developed. These small tears became stress-concentration points that nucleated fatigue cracks. Recommendations included rounding the edge of the box in the area of contact with the post to ensure a tangency fit.

A pushrod made by inertia welding two rough bored pieces of bar stock installed in a mud pump fractured after two weeks in service. The flange portion was made of 94B17 steel, and the shaft was made of 8620 steel. It was disclosed by visual examination that the fracture occurred in the shaft portion at the intersection of a 1.3 cm thick wall and a tapered surface at the bottom of the hole. The fatigue crack was influenced by one-way bending stresses initiated at the inner surface and progressed around the entire inner circumference. A heavily decarburized layer was detected on the inner surface of the flange portion and sharp corner was found at the intersection of the sidewall and bottom of the hole. It was concluded that the stress raiser due to the abrupt section change was accentuated by decarburized layer. As a corrective measure, the design of the pushrod was changed to a one-piece forging and circulation of atmosphere during heat treatment was permitted through a hole drilled in the flange end of the rod to avoid decarburization.

Plunger shafts machined from 4150 steel bar stock were involved in a series of fatigue failures. The fractures consistently occurred at two locations on the shafts: the shaft fillet and either side of a machined notch. The material specification for the shafts required 41xx series steel with a carbon content of 0.38 to 0.53%, a hardness of 35 to 40 HRC for the shaft, and a hardness of 50 to 55 HRC for the notch (which was case hardened). Analysis (visual inspection, chemical analysis, hardness testing, and magnetic particle inspection) supported the conclusions that all the fractures were fatigue-induced failures due to sharp radii in the fillets. The stress-concentrating effects of the fillets caused fatigue cracks to initiate and grow under cyclic loading until the crack depth was critical, causing the shaft to fail and rendering the assembly inoperative. Recommendations included increasing the radii of the notch and shaft fillets. If fatigue cracking had continued to be a problem with this component, shot peening of the subject radii would be appropriate. This process produces residual compressive stresses in the surface of the part, thereby retarding initiation of fatigue cracks.

Stainless steel liners (AISI type 321) used in bellows-type expansion joints in a duct assembly installed in a low-pressure nitrogen gas system failed in service. The duct assembly consisted of two expansion joints connected by a 32 cm (12 in.) OD pipe of ASTM A106 grade B steel. Elbows made of ASTM A234 grade B steel were attached to each end of the assembly, 180 deg apart. A 1.3 mm (0.050 in.) thick liner with an OD of 29 cm (11 in.) was welded inside each joint. The upstream ends were stable, but the downstream ends of the liners remained free, allowing the components to move with the expansion and contraction of the bellows. Investigation (visual inspection, hardness testing, and 30x fractographs) supported the conclusion that the liners failed in fatigue initiated at the intersection of the longitudinal weld forming the liner and the circumferential weld by which it attached to the bellows assembly. Recommendations included increasing the thickness of the liners from 1.3 to 1.9 mm (0.050 to 0.075 in.) in order to damp some of the stress-producing vibrations.

The power-type counterbalance spring, formed from hardened-and-tempered carbon steel strip and subsequently subjected to phosphating treatment, fractured at the two locations during fatigue testing. A rust colored dark band at the inside edge of the fracture surface was disclosed during investigation. Etch pits were revealed by the cleaned surface which were never observed on properly phosphated coating. It was interpreted that the spring had been subjected to an abnormal acid attack in pickling or phosphating which had resulted in considerable absorption of hydrogen by the metal and hence embrittlement. The part was concluded to have cracked during phosphating or excessive acid pickling before phosphating.

Thermal and transformation stresses, resulting from welding, adding up with operational stresses can result in failure. Examples involving the crankshaft of a shaft-drive to produce artificial waves in a swimming pool, the joint bar of a dredger cast out of a running non-alloyed steel with 39 kg/sq mm tensile strength, which had been strengthened by welding plate strips on both sides had fractured in service; an axle tube out of 40 Mn 4 after DIN 17 200 from a paper fabrication machine, which had three short longitudinal slits distributed uniformly over its surface; welding to repair worn out bearing or fits, and a broken rear axle tube of a bus are described.

A 4340 steel shaft, the driving member of a large rotor subject to cyclic loading and frequent overloads, broke after three weeks of operation. The driving shaft contained a shear groove at which the shaft should break if a sudden high overload occurred, thus preventing damage to an expensive gear mechanism. The rotor was subjected to severe chatter, which was an abnormal condition resulting from a series of continuous small overloads occurring at a frequency of around three per second. Investigation (visual inspection, hardness testing, and hot acid etch images) supported the conclusion that the basic failure mechanism was fracture by torsional fatigue, which started at numerous surface shear cracks, both longitudinal and transverse, that developed in the periphery of the root of the shear groove. These shear cracks resulted from high peak loads caused by chatter. The shear groove in the shaft had performed its function, but at a lower overload level than intended. Recommendations included increasing the fatigue strength of the shaft by shot peening the shear groove to minimize chatter.

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.

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.

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.

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.

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.

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.

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.