Octagonal cast ingots weighing 6.5 tons and made of unalloyed heat treated steel CK 45 according to DIN 17200, and crankshafts forged from these ingots showed internal separations during ultrasonic testing. To determine the cause of defect, an ingot slice and a crank arm were examined metallographically. Investigation showed this was a case where flaky forgings were made from cast ingots with primary grain boundary cracks. This parallelity supports the often expressed opinion that both occurrences have the same origin, i.e. that hydrogen precipitation was the driving force in the formation of primary grain boundary cracks in cast ingots.

Several crankshaft failures occurred in equipment that was being used in logging operations in subzero temperatures. Failure usually initiated at a cracked pin oil hole, and the failure origin was approximately 7.6 mm (0.3 in.) from the shaft surface. The holes were produced by gun drilling, giving rise to surface defects. The fracture surface was characteristic of fatigue in that it was flat, relatively shiny, and exhibited beach marks. The crack surface was at a 45 deg angle to the axis of the shaft, indicating dominant tensile stresses. The material was the French designation AFNOR 38CD4 (similar to AISI type 4140H) and was in the quenched-and-tempered condition, with a yield strength of about 760 MPa (110 ksi). It was treated to have compressive surface stresses, and the prior-austenite grain size was ASTM 8. Analysis (visual inspection, stress analyses, and macrographs) supported the conclusion that failure was caused by fatigue stress caused by surface defects in the oil holes. Recommendation includes drilling the oil holes by a technique that essentially eliminates surface defects.

This report covers case histories of failures in fixed-wing light aeroplane and helicopter components. A crankshaft of AISI 4340 Ni-Cr-Mo alloy steel, heat treated and nitrided all over, failed in bending fatigue. The nitrided layer was ground too rapidly causing excessive heat generation which induced grinding cracks and grinding burn. Tensional stresses resulting from grinding developed in a thin surface layer. On another crankshaft, chromium plating introduced undesirable residual tensile stresses. Such plating is an unsatisfactory finish for crankshafts of aircraft engines. Aircraft engine manufacturers and aeronautical standards require magnetic particle inspection to detect grinding cracks after reconditioning. Renitriding after any grinding is needed also, regardless of the amount of undersize as it introduces beneficial residual compressive stresses.

Two diesel engine crankshafts of similar dimensions, the journal diam being approximately 7 in., failed due to cracking originating in the fillet at the junction between the crankpin and the web nearest to the flywheel. The cracks were discovered before rupture occurred. Several small cracks originated in the fillet, ran together and developed as two main crack fronts that ultimately merged into one, a typical example of a fatigue failure. Electromagnetic crack detection revealed the presence of a number of discontinuities which were located at a position that would correspond to the vertical axis of the original ingot. The crankshaft had not been stress-relieved after a welding operation had been carried out. The only satisfactory course to follow when dealing with a highly stressed part in which defects of the type in question are revealed during machining is to scrap the forging.

A broken aircraft crankshaft and a severely damaged main brass bearing were examined to determine whether engine failure was initiated in the bearing or in the crankshaft. The steel crankshaft failure was a classical fatigue fracture. The bearing had been subjected to extremely high temperatures, as indicated by melting in the brass components and the extreme distortion in the rollers. Microscopic examination on the crankshaft material showed it to be a good quality steel. On the other hand, the chromium plate was thick, porous, and cracked in many places, including the point of the main fatigue crack. It was concluded that the over-all failure was initiated in the crankshaft, and the failure of the bearing resulted from that failure.

The AISI 1080 steel crankshaft of a large-capacity double-action stamping press broke in service and was repair welded. Shortly after the crankshaft was returned to service, the repair weld fractured. The repair-weld fracture was examined ultrasonically which revealed many internal reflectors, indicating the presence of slag inclusions and porosity. A low-carbon steel flux-cored filler metal was used in repair welding the crankshaft, without any preweld or postweld heating. This resulted in the formation of martensite in the HAZ. The repair weld failed by brittle fracture, which was attributed to the combination of weld porosity, many slag inclusions and the formation of brittle martensite in the HAZ. A new repair weld was made using an E312 stainless steel electrode, which provides a weld deposit that contains considerable ferrite to prevent hot cracking. Before welding, the crankshaft was preheated to a temperature above which martensite would form. After completion, the weld was covered with an asbestos blanket, and heating was continued for 24 h. During the next 24 h, the temperature was slowly lowered. The result was a crack-free weld.

The crankshaft of a 37.5-hp, 3-cylinder oil engine was examined. The engine had been dismantled for the purpose of a general overhaul and in the course of this work the crankpins were chromium-plated before regrinding. The engine was returned to service and after running for 290 h the crankshaft broke at the junction of the No. 3 crankpin and the crankweb nearest to the flywheel. A typical fatigue crack had originated at a number of points in the root of the fillet to the web. In its early stages it ran slightly into the web but turned back to the pin when it encountered the oil hole. The shaft had been made from a heat-treated alloy steel. The thickness of the plating was approximately 0.025 in. and numerous cracks were visible in it, several of which had given rise to cracks in the steel below. The primary cause of the crankshaft failure was the plating of the crankpins. The presence of the grooves alone would result in considerable intensification of stress in zones which are normally highly stressed, while the crazy cracking introduced a multiplicity of stress-raisers of a type almost ideal from the point of view of initiating fatigue cracks.

A crankshaft flange from a marine diesel engine illustrated a less-common case of fretting-fatigue cracking. The crankshaft was from a main engine of a sea-going passenger/vehicle ferry. The afterface of the flange was bolted to the flange of a shaft driving the gearbox. Cracks observed were sharp, transgranular, and not associated with any decarburization or other microstructural anomalies in the steel. Cracking of this main engine crankshaft flange was very likely a consequence of fatigue cracking initiated at fretting damage. The cause of the fretting was from loosening of the bolts.

A crankshaft was overloaded on a test stand and suffered an incipient crack in the crank pin. The crack run generally parallel to the longitudinal axis and branched off at the entrance into the two fillets at the transition to the crank arm. It consisted of many small cracks, all of which propagated at an angle of approximately 45 deg to the longitudinal axis, and therefore were caused by torsion stresses. Neither macroscopic nor microscopic examination determined any material or processing faults. Experience has shown that torsion vibration fractures of this kind usually appear in comparatively short journal pins at high stresses. This crankshaft fracture was an example of the damage that is caused or promoted neither by material nor heat treatment mistakes nor by defects of design or machining, but solely by overstressing.

The crankshaft of a compressor fractured through the web remote from the driving end after about three years of service. The fracture ran diagonally across the web into the crankpin. It passed through the centers of two screwed plugs inserted into the web from opposite faces approximately in line with the crankpin center line. The fracture was of the fatigue type, slowly developing cracks having started from opposite sides of each tapped hole and crept across the section. Microstructure of the crankshaft indicated the material was a plain carbon steel, the carbon content being of the order of 0.3%. The failure resulted principally from the stress-raising effects of the screw holes combined with the cracks in the welds. If the screw holes had been left unfilled or if some form of mechanical locking had been used if plugged, failure would have been postponed if not averted.

Nodular cast iron crankshafts and their main-bearing inserts were causing premature failures in engines within the first 1600 km (1000 mi) of operation. The failures were indicated by internal noise, operation at low pressure, and total seizing. Concurrent with the incidence of engine field failures was a manufacturing problem: the inability to maintain a similar microfinish on the cope and drag sides of a cast main-bearing journal. Investigation supported the conclusion that the root cause of the failure was carbon flotation due to the crankshafts involved in the failures showing a higher-than-normal carbon content and/or carbon equivalent. Larger and more numerous cope side graphite nodules broke open, causing ferrite caps or burrs. They then became the mechanism of failure by breaking down the oil film and eroding the beating material. A byproduct was heat, which assisted the failure. Recommendations included establishing closer control of chemical composition and foundry casting practices to alleviate the carbon-flotation form of segregation. Additionally, some nonmetallurgical practices in journal-finishing techniques were suggested to ensure optimal surface finish.

The fracture cause had to be determined in a three-cylinder crankshaft made of chrome steel 34Cr4 (Material No. 1.7033) according to DIN 17200. The fracture occurred after only 150 h of operation. The fracture was of the bend fatigue type which originated in the fillet of the main bearing and ran across the jaw almost to the opposite fillet of the adjoining connecting rod bearing. The fillet was well rounded and smoothly machined. Thus, no reason for the fracture of the crankshaft could be found externally. No material defects were discernible in the origin or anywhere else. No cause for the crank fracture could be established from material testing. Probably the load was too high for the strength of the crank. Tensile strength could have been increased for the same material by tempering at lower temperature. Additionally, the resistance against high bend fatigue stresses or torsion fatigue stresses could have been increased substantially by including the fillet in the case hardening process.

The crankshaft of a six cylinder, 225-hp diesel engine driving a small locomotive was examined. About nine months after installation a fall in oil pressure was traced to damage to No. 5 crank pin bearing. A small lip present on one side of the discontinuity apparently served to scrape the bearing material. The defect was stoned smooth, a new bearing fitted, and the engine returned to service. The engine performed satisfactorily for a further twelve months until fracture of the crankshaft through the No. 5 crank pin supervened. The fracture revealed a complex torsional fatigue failure. Microscopic examination revealed that the pin had been hard chromium plated and that the plating followed the curved edge of the outer extremity of the defect. This crank pin contained an inherent defect in the form of a slag inclusion or crack situated at the surface. That the crack only showed itself after a period of service suggests that initially it may have been slightly below the surface of the machined pin and some slight extension outwards took place in service.

In the course of a general overhaul, the crankpins and main journals (3 in. diam) of the crankshaft of a four-cylinder oil engine were built up by metal spraying. Four weeks later, the shaft broke through the pin remote from the flywheel (driving) end. The fracture was of the fatigue type. A creeping crack originated in the fillet at the inside surface of the pin and extended parallel to the plane of the web across practically the entire section before complete rupture occurred. The sprayed metal on the fractured pin had very poor adhesion. The surfaces of the main journals had not been grooved but appeared to have been roughened by shot or grit-blasting prior to spraying and the deposit was more firmly adherent to these surfaces than in the case of the pins. It is doubtful, however, whether the adhesion of sprayed metal to a surface prepared even in this manner would always be satisfactory under severe loading conditions, such as those to which a crankpin is subjected in service.

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 bolt breaks along a change in cross section well below its rated capacity. An anchoring screw spins freely in place, having snapped at its first supporting thread. A motor unexpectedly disengages its load, its driveshaft having fractured near a keyway. Such failures – involving axles, leaf springs, engine rods, wing struts, bearings, gears, and more – can occur, seemingly without cause, due to vibrational fracture. Vibrational fractures begin as cracks that form under cyclic loading at nominal stresses which may be considerably lower than the yield point of the material. The fracture is proceeded by local gliding and the development of cracks along lattice planes favorably orientated with respect to the principal stress. This non-reversible process is often misleadingly called “fatigue” and presents significant challenges to engineering teams that ill-advisedly take to searching for material faults. Several examples of notch-induced vibrational fractures are presented along with guidelines for investigating their cause.

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 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.

The propeller from a small private airplane came off in flight. The head ends of all six attachment bolts remained in the propeller hub when it was found. Two threaded shanks with nuts remained with the engine, while the remaining four shank ends with their nuts were missing. Parts available for examination, in addition to the hub and attachment bolts, were the two propeller blades and the engine crankshaft. The purpose of this examination was to determine the nature and probable cause of failure in the six attachment bolts. Indications of fatigue failure and wear were the major findings in visual and low power microscopic examination. Fracture surfaces indicated failure was initiated in the threads in four bolts and in the shanks in two. The group of four bolts failed primarily due to tensile loads, while the other two bolts failed primarily due to bending loads. It was concluded that failure was due to improper installation torqueing of the bolts.

This case study involves two continuously cast steel crankshaft failures. Three parties performed their own failure analyses: (1) the engine manufacturer responsible for component design, specification, and application; (2) the steel supplier and forging supplier responsible for making the steel, forging the shape, and preliminary heat treatment; and (3) a supplier that provided induction hardening, finish machining, and inspection. An independent engineering firm was subsequently involved, but because each party had its own agenda, there was no agreement on the metallurgical source of the failure and thus no continued analysis to pin down and eliminate the root cause.