Failure Analysis of an ICC Underride Bar

Underride bars are required to be installed on the rear of large trucks. They are intended to prevent cars from running under the rear of the trucks and injuring the occupants. Unfortunately, these bars are often damaged in normal service because they can easily hit loading docks or suffer from other handling issues. In addition, there are currently no required or code repair specifications.

This case involved a poorly repaired underride bar which was subsequently hit by a vehicle side swiping the rear of the truck. The accident resulted in serious injuries to the right front passenger. The injuries were alleged to have been caused by the bar breaking away because of poorly made repair welds. It was alleged that the welds were so poor that it broke away with no resistance. Had a properly repaired or new bar been in place, the serious injuries therefore would have to been avoided.

Metallurgical Consulting was called in because of their expertise in failure analysis of welds. The fracture occurred in the poorly made weld that joined the new outboard bar to the rest of the assembly. Tire marks left on the bar from the impacting vehicle provided the point of loading. Calculations on stresses in the weld were made using the instantaneous center of load method for eccentrically loaded welds, which is found in the AISC Steel Construction Manual. This method is a recognized procedure used to size welds in steel frame construction.

Using the AISC method required identifying and removing safety factors from the procedure to determine actual failure stresses. The calculated tire forces required for failure of the poor welds were slightly greater than those required by FMVSS 223 to validate the guard strength. A different European approach to the problem also gave similar results. The impacting vehicle had a crash data event recorder from which the actual crash pulse data was retrieved. The nature of the damage to the vehicle and the data itself enabled separation of the crash pulse data into tire strike and sheet metal damage in the first 40 milliseconds of the crash. The tire strike data analysis actually provided about 20% higher force on the bar than the weld calculations. This extra stress could be accounted for by bar deformation, which was very apparent. Therefore, the analysis of the poor quality weld showed it actually could meet or exceed FMVSS 223 requirements.

Images From The Metallurgical Services Provided For This Failure Analysis of an Underride Bar

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