Corrosion-Initiated Rotating Bending Fatigue Failure of a Fertilizer Conveyor Belt Head Roller

Abstract

This paper presents the experimental process and findings of a forensic investigation of a failed fertilizer plant conveyor belt head roller shaft. A 25-year service life was expected but failure occurred following only 10 years. A gearbox which provided direct power to the roller had been replaced only several weeks earlier. The primary aim for the plant owner was to determine if the gearbox replacement had caused failure of the shaft, or, if the two incidents were unrelated. If incorrect installment of the gearbox had caused failure, liability would fall on the gearbox suppliers in terms of mitigation. However, it was revealed that the shaft had failed due to low-stress rotating bending fatigue, over an extended period of time. A premeditated change of material selection at the manufacturing stage, substituting carbon steel for 304L stainless steel, resulted in reduced corrosion resistance. Fertilizer ingredients tend to pose limited problems in their dry form in terms of corrosion; in the presence of moisture, however, can change the ingredients into aggressive corrosion species. Thus, corrosion allowed multiple fatigue cracks initiated from corrosion pitting under rotating bending load. The cracks then eventually joined together to form a single fatigue crack which propagated through the cross section.