In addition to failures in shafts, this article discusses failures in connecting rods, which translate rotary motion to linear motion (and conversely), and in piston rods, which translate the action of fluid power to linear motion. It begins by discussing the origins of fracture. Next, the article describes the background information about the shaft used for examination. Then, it focuses on various failures in shafts, namely bending fatigue, torsional fatigue, axial fatigue, contact fatigue, wear, brittle fracture, and ductile fracture. Further, the article discusses the effects of distortion and corrosion on shafts. Finally, it discusses the types of stress raisers and the influence of changes in shaft diameter.

The types of metal components used in lifting equipment include gears, shafts, drums and sheaves, brakes, brake wheels, couplings, bearings, wheels, electrical switchgear, chains, wire rope, and hooks. This article primarily deals with many of these metal components of lifting equipment in three categories: cranes and bridges, attachments used for direct lifting, and built-in members of lifting equipment. It first reviews the mechanisms, origins, and investigation of failures. Then the article describes the materials used for lifting equipment, followed by a section explaining the failure analysis of wire ropes and the failure of wire ropes due to corrosion, a common cause of wire-rope failure. Further, it reviews the characteristics of shock loading, abrasive wear, and stress-corrosion cracking of a wire rope. Then, the article provides information on the failure analysis of chains, hooks, shafts, and cranes and related members.

This article focuses on common failures of the components associated with the flow path of industrial gas turbines. Examples of steam turbine blade failures are also discussed, because these components share some similarities with gas turbine blading. Some of the analytical methods used in the laboratory portion of the failure investigation are mentioned in the failure examples. The topics covered are creep, localized overheating, thermal-mechanical fatigue, high-cycle fatigue, fretting wear, erosive wear, high-temperature oxidation, hot corrosion, liquid metal embrittlement, and manufacturing and repair deficiencies.

This article focuses on failure analyses of aircraft components from a metallurgical and materials engineering standpoint, which considers the interdependence of processing, structure, properties, and performance of materials. It discusses methodologies for conducting aircraft investigations and inspections and emphasizes cases where metallurgical or materials contributions were causal to an accident event. The article highlights how the failure of a component or system can affect the associated systems and the overall aircraft. The case studies in this article provide examples of aircraft component and system-level failures that resulted from various factors, including operational stresses, environmental effects, improper maintenance/inspection/repair, construction and installation issues, manufacturing issues, and inadequate design.

Results of failure analyses of two aircraft crankshafts are described. These crankshafts were forged from AMS 6414 (similar composition to AISI 4340) vacuum arc remelted steels with sulfur contents of 0.003% (low sulfur) and 0.0005% (ultra-low sulfur). A grain boundary sulfide precipitate was caused by overheat of the low sulfur steel, and an incipient melting of grain boundary junctions was caused by overheat of the ultra-low sulfur steel. The precipitates and incipient melting in these two failed crankshafts were observed during the examination. As expected, impact fractures from the low sulfur steel crankshaft contained planar dimpled facets along separated grain boundaries with a small spherical manganese sulfide precipitates within each dimple. In contrast, planar dimpled facets along separated grain boundaries of impact fractures from the ultra-low sulfur crankshaft steel contained a majority of small spherical particles consisting of nitrogen, boron, iron, carbon, and a small amount of oxygen. Some other dimples contained manganese sulfide precipitates. Fatigue samples machined from the ultra-low sulfur steel crankshaft failed internally at planar grain boundary facets. Some of the facets were covered with nitrogen, boron, iron, and carbon film, while other facets were relatively free of such coverage. Results of experimental forging studies defined the times and temperatures required to produce incipient melting overheat and facets at grain boundary junctions of ultra-low sulfur AMS 6414 steels.

A failure analysis investigation was conducted on a fractured aluminum tailwheel fork which failed moments after the landing of a privately owned, 1955 twin-engine airplane. Nondestructive evaluation via dye-penetrant inspection revealed no discernible surface cracks. The chemical composition of the sand-cast component was identified via optical emission spectroscopy and is comparable to an aluminum sand-cast alloy, AA 712.0. Metallographic evaluation via optical microscopy and scanning electron microscopy revealed a high degree of porosity in the microstructure as well as the presence of deleterious intermetallic compounds within interdendritic regions. Macrohardness testing produced hardness values which are noticeably higher than standard hardness values for 712.0. The primary fracture surfaces indicate evidence of mixed-mode fracture, via intergranular cracking, cleaved intermetallic particles, and dimpled cellular regions in the matrix. The secondary fracture surface demonstrates similar features of intergranular fracture.

The case study presented in this article details the failure investigation of an M50 alloy steel bearing used in a jet engine gearbox drive assembly. It discusses the investigative steps and analytic tools used to determine the root cause, highlighting the importance of continuous, thorough questioning by the investigating activity. The combined analyses demonstrated that the bearing failed by a single event overload as evidenced by bulk deformation and traces of foreign material on the rolling elements. The anomalous transferred metal found on the rolling elements subsequently led to the discovery of overlooked debris in an engine chip detector, and thus resulted in a review of several maintenance practices.

An aero engine failed due to the misalignment of the ball bearing fitted on the main shaft of the engine. The aero engine incorporates two independent compressors: a six-stage axial flow LP compressor and a nine-stage axial flow HP compressor. The bearing under consideration is a HP location bearing and is fitted at the rear of the nine-stage compressor. It was supposed to operate for at least 5000 h, but failed catastrophically after 1300 h, rendering the engine unserviceable. Unusually high stresses caused by misalignment and uneven axial loading resulted in the generation of fatigue crack(s) in the inner race. When the crack reached the critical size, the collar of the race fractured, causing subsequent damage. The cage also failed due to excessive stresses in the axial direction, and its material was smeared on the steel balls and the outer race.

The failure of HP turbine blades in a low bypass turbofan engine was analyzed to determine the root cause. Forensic and metallurgical investigations were conducted on all failed blades as well as failed downstream components. It was found that one of the blades fractured in the dovetail region, causing extensive damage throughout the turbine. Remedial measures were suggested to prevent such failures in the future.

This article describes the visual, fractographic, and metallographic evidence typically encountered when analyzing stress rupture of turbine airfoils. Stress-rupture fractures are generally heavily oxidized, tend to be rough in texture, and are primarily intergranular and/or interdendritic in appearance compared to smoother, transgranular fatigue type fractures. Often, gross plastic yielding is visible on a macroscopic scale. Commonly observed microstructural characteristics include creep voiding along grain boundaries and/or interdendritic regions. Internal voids can also nucleate at carbides and other microconstituents, especially in single crystal castings that do not possess grain boundaries.

An investigation of a damaged crankshaft from a horizontal, six-cylinder, in-line diesel engine of a public bus was conducted after several failure cases were reported by the bus company. All crankshafts were made from forged and nitrided steel. Each crankshaft was sent for grinding, after a life of approximately 300,000 km of service, as requested by the engine manufacturer. After grinding and assembling in the engine, some crankshafts lasted barely 15,000 km before serious fractures took place. Few other crankshafts demonstrated higher lives. Several vital components were damaged as a result of crankshaft failures. It was then decided to send the crankshafts for laboratory investigation to determine the cause of failure. The depth of the nitrided layer near fracture locations in the crankshaft, particularly at the fillet region where cracks were initiated, was determined by scanning electron microscope (SEM) equipped with electron-dispersive X-ray analysis (EDAX). Microhardness gradient through the nitrided layer close to fracture, surface hardness, and macrohardness at the journals were all measured. Fractographic analysis indicated that fatigue was the dominant mechanism of failure of the crankshaft. The partial absence of the nitrided layer in the fillet region, due to over-grinding, caused a decrease in the fatigue strength which, in turn, led to crack initiation and propagation, and eventually premature fracture. Signs of crankshaft misalignment during installation were also suspected as a possible cause of failure. In order to prevent fillet fatigue failure, final grinding should be done carefully and the grinding amount must be controlled to avoid substantial removal of the nitrided layer. Crankshaft alignment during assembly and proper bearing selection should be done carefully.

The shafts on two centrifugal pumps failed during use in a petroleum refinery. Light optical microscopy and scanning electron microscopy were used to analyze the damaged materials to determine the cause of failure. The results showed that one shaft, made of duplex stainless steel, failed by fatigue fracture, and the other, made of 316 austenitic stainless steel, experienced a similar fracture, which was promoted by the presence of nonmetallic inclusion particles.

A shaft which carried the diverter sheave wheel of an electric goods lift failed, resulting in the cage failing to the bottom of the well. Failure had taken place at a reduction in diam at which no filet radius existed. Metallurgical examination did not disclose any abnormal features. The material was a mild steel in the normalized condition. The appearance of the fracture indicated failure was due to bending stresses. The absence of any fillet radius at the reduction in diam provided a region of stress concentration from which fatigue cracks developed.

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.

Two oil-pump gears broke after four months of service in a gas compressor that operated at 1000 rpm and provided a discharge pressure of 7240 kPa (1050 psi). The compressor ran intermittently with sudden starts and stops. The large gear was sand cast from class 40 gray iron with a tensile strength of 290 MPa (42 ksi) at 207 HRB. The smaller gear was sand cast from ASTM A536, grade 100-70-03, ductile iron with a tensile strength of 696 MPa (101 ksi) at 241 HRB. Analysis (metallographic examination) supported the conclusion that excessive beam loading and a lack of ductility in the gray iron gear teeth were the primary causes of fracture. During subsequent rotation, fragments of gray iron damaged the mating ductile iron gear. Recommendations included replacing the large gear material with ASTM A536, grade 100-70-03, ductile iron normalized at 925 deg C (1700 deg F), air cooled, reheated to 870 deg C (1600 deg F), and oil quenched. The larger gear should be tempered to 200 to 240 HRB, and the smaller gear to 240 to 280 HRB.

There was a large incidence of surface defects on the crank pins and journals and other areas of crank shafts of a high power automotive engine. The steel used was a Cr-Mo type of nitriding steel. Metallographic observations conclusively proved that the defective areas were entrapment of foreign bodies, resulting from steel making/deoxidizing/teeming stages. The occasionally globular nature of the foreign particles suggested these were formed at the liquid condition of the steel. The ratio of Mn-Si as seen on electron probe microanalysis also suggested the globules high in Mn content might have resulted in deoxidizing stage. Particularly the absence of Fe in some areas in the inclusion was indicative of precipitation deoxidation by ferromanganese/ferrosilicon. The defects apparently did not have time to coalesce and rise up to the top.

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.

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.

The 1040 steel crankshaft in a reciprocating engine cracked within one year of operation. The journals of the main and crankpin bearings were inspected by the magnetic-particle method. Three to six indications of 1.5 to 9.5 mm long discontinuities were observed in at least four of the main-bearing journals. A crack along the fillet, almost entirely through the web, was observed in one of the main-bearing journals. Numerous coarse segregates, identified as sulfide inclusions, were identified by macroetching the surface during metallographic examination of a section taken through the main-bearing journal at the primary crack. Fatigue cracking with low-stress high-cycle characteristics was disclosed during macroscopic examination of the crack surface. Sulfide inclusions, which acted as stress raisers, were found to be present in the region where cracking originated. As a corrective measure, ultrasonic inspection was used in addition to magnetic-particle inspection to detect discontinuities.

A 1050 steel crankshaft with 6.4 cm (2.5 in.) diam journals that measured 87 cm (34.25 in.) in length and weighed 31 kg (69 lb) fractured in service. The shaft had been quenched and tempered to a hardness of 19 to 26 HRC, then selectively hardened on the journals to a surface hardness of 40 to 46 HRC. Visual inspection and 100x micrographs showed the fracture surface as having a complex type of fatigue failure initiated from subsurface inclusions in the transition zone between the induction-hardened surface and the softer core. The fractured shaft was examined for chemical composition and hardness, both of which were found to be within prescribed limits. This evidence supports the conclusions that the failure was caused by fatigue cracks that initiated in an area having an excessive amount of inclusions. The inclusions were located in a transition zone, which is a region of high stress. No recommendations were made.