Rollover accidents in light trucks and cars involving an axle failure frequently raise the question of whether the axle broke causing the rollover or did the axle break as a result of the rollover. Axles in these vehicles are induction hardened medium carbon steel. Bearings ride directly on the axles. This article provides a fractography/fracture mechanic approach to making the determination of when the axle failed. Full scale tests on axle assemblies and suspensions provided data for fracture toughness in the induction hardened outer case on the axle. These tests also demonstrated that roller bearing indentions on the axle journal, cross pin indentation on the end of the axle, and axle bending can be accounted for by spring energy release following axle failure. Pre-existing cracks in the induction hardened axle are small and are often difficult to see without a microscope. The pre-existing crack morphology was intergranular fracture in the axles studied. An estimate of the force required to cause the axle fracture can be made using the measured crack size, fracture toughness determined from these tests, and linear elastic fracture mechanics. The axle can be reliably said to have failed prior to rollover if the estimated force for failure is equal to or less than forces imposed on the axle during events leading to the rollover.

An automobile manufacturer rejected several 1035 steel stub axles because of what appeared to be short longitudinal cracks in the surfaces of the pins. The cracks were found when six axles were examined for defects by magnetic-particle inspection. However, metallographic examination showed that these lines were not cracks but slag inclusions at and immediately below the surface. Analysis (visual inspection, metallographic examination, and 100x/500x magnetic-particle inspection) supported the conclusions that the inclusions consisted of pieces of fireclay from channel brick that were flushed into the ingot mold. Although no true cracks were present, rejection of the stub axles was nevertheless justified. Slag streaks could reduce the strength of the stub axles and lead to the formation of fatigue fractures during operation. No recommendations were made.

This article provides a background of friction-bearing failures due to overheating. The failures of locomotive axles caused by overheated traction-motor support bearings are discussed. The article also describes liquid-metal embrittlement (LME) in steel. It examines the results of various axle studies, with illustrations and concludes with information on the simulation of the LME mechanism.

An axle shaft in an open-pit mining truck hauling overburden failed after operating for 27,000 h. Previous failures had resulted from longitudinal shear, but this had not, bringing material quality into question. Chemical analysis verified that the part was SAE4340 Ni-Cr-Mo alloy steel and thus met material specification. The failure was a result of torsional fatigue in the tensile plane, originating from one of several gouges around the splined radius of the shaft. The fatigue crack progressed for a large number of cycles before final fracture. The shaft met metallurgical requirements and should have withstood normal operating conditions. The spacing of the gouge marks coincided with the spacing of the splines, indicative of careless assembly with the mating wheel gear.

One end of an axle shaft containing the integral spur pinion was submitted for examination, along with the report of a tooth pitting failure. The spur pinion, integral to the axle shaft, operated in a medium-size, off-highway truck at an open-pit mine, for “a relatively short time.” Only the pinion head had been returned. The shaft portion had been torch-cut away. Chemical analysis along with the microstructure confirmed the specified material was SAE 43BV12 Ni-Cr-Mo alloy steel. The mode of failure was surface contact fatigue through the shear plane subsurface at the lowest point of single-tooth contact. The cause of failure was tooth-tip interference from the mating gear teeth. Because the mating parts within the assembly had not been returned or examined, unanswered questions remained.

Following an accident in which a light pickup truck left the road and overturned, one of the rear axles, made of approximately 0.30C steel, was found to be fractured adjacent to the bearing lock nut. A keyway was present in the failed area, as were threads for the lock nut. Fracture surfaces of the failed axle and exemplar fractures obtained from simulation tests were studied using scanning electron microscope. The examination showed that the outer perimeter fracture in the axle was very flat and composed of cleavage and that the interior portion was composed of both cleavage and dimples. No evidence of prior cracking was found. The exemplar specimens from the simulation impact testing failed in a manner consistent with that observed in the axle. The examination confirmed that the failure was a one-time impact overload fracture and not the result of any prior crack in the material, indicating that the axle failure did not initiate the accident.

A cylindrical spiral gear, part of a locomotive axle assembly, cracked ten days after it had been press-fit onto a shaft, after which it sat in place as other repairs were made. Workers at the locomotive shop reported hearing a sound, and upon inspecting the gear, found a crack extending radially from the bore to the surface of one of the tooth flanks. The crack runs the entire width of the bore, passing through an oil hole in the hub, across the spoke plate and out to the tip of one of the teeth. Design requirements call for the gear teeth to be carburized, while the remaining surfaces, protected by an anti-carburizing coating, stay unchanged. Based on extensive testing, including metallographic examination, microstructural analysis, microhardness testing, and spectroscopy, the oil hole was not protected as required, evidenced by the presence of a case layer. This oversight combined with the observation of intergranular fracture surfaces and the presence of secondary microcracks in the case layer point to hydrogen embrittlement as the primary cause of failure. It is likely that hydrogen absorption occurred during the gas carburizing process.

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.

Failed portions of a 4140 steel axle from a prototype urban transit vehicle were examined to determine the cause of failure. The testing procedures included visual examination, macrofractography, metallography, chemical analysis, and hardness and tensile testing. The analysis showed that a salvage welding repair had resulted in the formation of martensite that developed cracks, leading to fatigue failure of the axle. It was recommended that no weld buildup or repair be permitted on the axles.

Metallography is an important component of failure analysis. In the case of a liquid metal embrittlement (LME) failure it is usually conclusive if a third phase constituent can be formed inside of the cracks after failure. In the case where it is necessary to characterize the third phase material, one can use various x-ray spectrographic techniques in conjunction with a scanning electron microscope (SEM). This study describes those metallographic and SEM analysis techniques for determining the mode of failure for a locomotive traction motor by LME.

A supplementary axle, which was used as an extension to a highway-trailer tractor to increase its load-bearing capacity, failed in service. The rolled steel channel extensions that secured the axle assembly to the tractor main-frame I-beams fractured transversely, with the crack in each instance initiating at a weld that joined the edge of the lower flange to the support bracket casting. The cracks propagated through the flange on each side until the effective cross-sectional area had been reduced sufficiently to bring about sudden and complete fracture of the remaining web and upper flange. Fatigue fracture was caused by a combination of high bending stresses in the bottom flanges of the channels due to the heavy load being carried, concentration of stresses due to the rapid change in section modulus of the channel at its point of attachment to the support-bracket casting, and brittleness of the high-hardness HAZ of the weld associated with the abnormally high carbon content in the central part of the channel. Welding of channel edges contributed to harmful gradients in section moduli and should be avoided in future assemblies.

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.

One main undercarriage axle made of high strength alloy steel was subjected to simulated fatigue test for 6000 h of service. After only 300 h it broke in two along the sharp radius. The fracture revealed a coarse, irregular, and brittle surface before final fracture by thick angular shear lip zone. The presence of micropores in the cleavage facets as well as at the grain boundaries and hairline type crack indications under SEM examination were all suggestive of hydrogen embrittlement. On the basis of investigation results and observations, it was concluded that the transverse breakage of the axle had occurred intergranularly in a brittle manner, possibly, initiated by a shallow zone of fatigue along the sharp radius acting as stress riser.

Two examples involved brittle fracture promoted by small fatigue cracks owing to welding deficiencies. Other parts involving inadequate welding were a ski-wheel axle flange, ski fitting (brackets), and undercarriage shock strut stub assembly. In an attach fitting for an engine mount, weld cracks (severe stress concentrations) formed during repair welding. Cracks were severely oxidized. The main cause was incorrect repair and inadequate inspection of the fitting. In a cast CrNi alloy ski wheel axle, brittle fatigue failure emanated from welding cracks (notches). These welding cracks formed during the fabrication of the axle mounting plate. So-called all-purpose electrodes were used. Thus, the main cause for failure was a manufacturing deficiency-fatigue failure developed because of improper welding during fabrication of the axle. The proper electrode should have been used.