An Exploration of Failure Modes in Rolled, Ductile, Cast-Iron Crankshafts Using a Resonant Bending Testing Rig

This report explores the relationship of different failure criteria - specifically, surface cracks, stiffness changes, and two-piece failures - on rolled, ductile, cast-iron crankshafts. Crankshaft samples were closely monitored throughout resonant bending fatigue testing and were taken to near complete fracture. By monitoring resonance shifts of the samples during testing, stiffness changes and cracks were monitored. These data showed that an accelerating frequency shift was sufficient to indicate imminent two-piece failure and that this condition can be used as a failure criterion. Fatigue studies on two different crankshafts using this failure criterion were compared to those using a surface crack failure criterion. This comparison showed that using the surface crack failure criterion erroneously decreased the apparent fatigue life of the crankshaft significantly. These results support the theory of crack arrest in the fillet area of rolled crankshafts and raise a significant challenge in the use of the surface crack failure criterion due to its inconsistent correlation to two-piece failure.