Fatigue Failure of Ductile Iron Crankshafts

Abstract

Automobile crankshafts can be produced as either steel forgings or ductile-iron castings. In the latter case, because of the high structure-sensitivity of fatigue strength, mechanical test requirements must be very specific. A detailed investigation of two series of crankshafts produced from pearlitic nodular-graphite iron, grade SG 60, is described. Normal tensile tests and microstructural examinations were supplemented by determination of fatigue life and fatigue limit. Fatigue tests under axial bending conditions were carried out on segments taken from two series (I, II) of machined crankshaft castings. Satisfactory bending moment and performance data were obtained for one series of samples. In the other series, sub-surface microporosity in the cast structure was exposed during subsequent machining and led to premature fatigue failure after less than 2 × 10⁵ cycles of testing.