A metallurgical failure analysis of the right rear inside tandem wheels on a tractor trailer was requested after a trailer wheel failure. The rear tandem wheels exited the trailer and struck an oncoming vehicle. The fifth axle hub, onto which the wheels had been mounted was lost, leaving only the failed wheel available for metallurgical failure analysis.
A metallurgical failure analysis was requested including both visual and stereomicroscopic examinations. Visual examination revealed the existence of a weld nugget on the wheel rim. This weld nugget was the result of an attempt to weld repair a previous crack and/or localized damage or defect in the wheel rim. The failed trailer wheel exhibited a circumferential fracture, as a result of metal fatigue, of the wheel rim traversing each of the ten wheel stud holes.
The fatigue crack, once initiated at the weld nugget on trailer wheel, grew circumferentially as a result of normal or possibly abnormal operating stress until final separation of the dual wheels from the subject vehicle's fifth axle hub.
Photograph A Failed truck wheel rim.
A close-up examination revealed remnants of a weld repair nugget between lug bolt holes denoted "C" and "D." A close-up view of the area containing the weld repair nugget is shown below in Photograph B.
Photograph B Remnants of weld repair nugget between lug holes C and D.
Microscopic view of the weld nugget area shown above confirms the presence of the repair weld nugget. Weld repair (which is strictly forbidden by the U.S. DOT codes and standards) was evidently attempted. The result of this truck wheel rim weld repair was initiation of a fatigue crack which propagated to the C and D lug holes. The fatigue crack then circumferentially propagated around the rim and through the adjacent lug stud holes as the truck was driven. A close-up view of the weld repair is shown in Photograph C.
Photograph C Microscopic view of repair weld nugget.
The matter was resolved when evidence of welding on the truck's heat-treated wheel rim was identified.