Fatigue Failure of a Locomotive Suspension Spring that Initiated at a Seam

A locomotive suspension spring with a bar diameter of 36 mm failed. Outdoor exposure of a hot-rolled hardened-and tempered 5160 bars for suspension springs resulted in rusting in the seam and on the fracture surface. A step due to a seam was visible on the surface. The thumb nail looked off-center from the step, but a smaller thumb-nail shape that is concentric with the step and a second stage of growth were found to be spread principally to the right of the step. The rapid stage of failure, which began at the edge of the thumb nail, was much rougher and exhibited rays that diverge approximately radially from it. The seam wall was revealed to have two zones among which the lower zone being mottled. Dozens of spearhead shaped areas (fatigue cracks) pointing away from the seam was revealed at the base of the seam. The orientation of these origins was normal to the direction of resultant tensile stress from torsional stressing of the spring material. It was concluded that the fatigue failure in the spring was initiated at the base of a seam.